ALBERT

Description: ABSTRACT The Miocene Searchlight pluton, exposed in the Colorado River extensional corridor of southern Nevada, tilted up to 90° on its side in the footwall of the east‐directed Dupont Mountain detachment fault system. Rapid extension and rotation occurred immediately after the ca. 17‐16 Ma emplacement of this 10×10 km granite‐monzogranite body. To constrain the mechanism and timing of rapid footwall exhumation, we conducted detailed field, microstructural, electron backscatter diffraction (EBSD), and zircon (U‐Th)/He (ZHe) and 40Ar/39Ar hornblende thermochronological analyses. Steeply dipping fabrics across this pluton formed over a range of temperature conditions, from magmatic to high‐ to low‐temperature sub‐solidus strain, and display distributed eastside‐up shear. ZHe cooling ages are consistent with constraints from tilted volcanic strata and cross‐cutting dikes that suggest initial rapid rotation (~75° Myr‐1) at 16.2‐15.7 Ma followed by more modest exhumation rates until ca. 13 Ma. Our observations are used to test tilting models for the Searchlight pluton, including rigid‐body rotation, antithetic imbrication, or flow‐like rotation. Available observations are most consistent with a flow‐like tilting mechanism. We present scaling analyses that highlight how footwall tilting‐dominated extension more effectively cools the upper crust than pure‐shear extension because the hottest deep materials exhumed rapidly toward the cooler surface. This extensional mechanism efficiently cools the upper crust, causing a negative feedback whereby the rapidly cooled crust becomes strong enough to halt further fast simple‐shear extension. This may explain why rapid extension was transient and further extension is mostly accommodated by high‐angle low‐offset magnitude normal faults that developed in a colder stronger crust.

Description: Abstract Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three possible cross‐section geometries and kinematics along a transect through the eastern Bhutan Himalaya with a thermal model (Pecube‐D) to calculate the resulting thermal field and predict potential ages. We compare predicted ages to a suite of new and published cooling ages. Our results argue for ramp‐focused exhumation of the Main Central Thrust (MCT) from 16 to 14 Ma at shortening rates of 40‐55 mm/yr, followed by slower rates (25 mm/yr) during the last 50 km of MCT displacement and growth of the Lesser Himalayan (LH) duplex from 14‐11 Ma. Emplacement of frontal LH thrust sheets occurred rapidly (55‐70 mm/yr) between ~11 and 9 Ma, followed by a decrease in shortening rates to ~10 mm/yr during motion on the Main Boundary Thrust (MBT). Modern shortening rates (17 mm/yr) and out‐of‐sequence motion on the MBT from 0.5 Ma to present reproduce the young cooling ages near the MBT. We show that the dominant control on exhumation patterns in a fold‐thrust belt results from the evolution of ramps and emphasize that the geometry and kinematics of structures driving hinterland exhumation need to be evaluated with their linked foreland structures to ensure the viability of the proposed geometry, kinematics and thus cooling history.

Description: Abstract Contractional deformation in the outer parts of fold‐and‐thrust belts is in part controlled by the presence of syntectonic sediments and multiple décollements (e.g. the Apennines, the Appalachians, the Pyrenees, the Zagros, or the Sub‐Andean and Kuqa fold‐and‐thrust belts). To better understand the influence of these parameters in the kinematic evolution of fold‐and‐thrust systems, we carried out an experimental study including four 3D sandbox models inspired by one of the previously mentioned prototypes, the Kuqa fold‐and‐thrust belt. This belt contains two décollements: a weak synorogenic salt layer; and a deeper, preorogenic and frictionless décollement (i.e., organic‐rich shales) showing along strike variations of rheology. The experimental results show that increasing synkinematic sedimentation rate: (i) generates a progressive change from distributed to localized deformation and, (ii) delays the development of frontal contractional structures detached on the salt, favoring the formation and reactivation of more hinterland thrusts and backthrusts. With respect to the rheology, our study reveals that as the viscosity of the prekinematic décollement increases: (i) the deformation propagates more slowly towards the foreland and, (ii) the underlying thrust stack becomes broader and lower and has a gentler thrust taper angle. The rheology of the prekinematic décollement defines the distribution and geometry of the structures detached on it that in turn influence the development of overlying, salt‐detached structures. Subsalt structures can: (i) determine the areal extent of the salt and therefore of any fold‐and‐thrust system detached on it and, (ii) hamper or even prevent the progressive foreland propagation of deformation above the salt.

Description: Abstract We investigate the late Miocene–Pleistocene offshore sedimentary record of the Yakutat microplate to evaluate the spatial and temporal variations in rock exhumation and sediment routing patterns at the heavily glaciated and actively converging plate boundary in southeast Alaska. We present 1456 new fission track ages and 1372 new U‐Pb ages from double‐dated detrital zircons derived from fourteen samples collected from offshore. We integrate our results with published geo‐ and thermochronology data onland and offshore in order to constrain grain provenance. We find that offshore strata deposited east of the fold and thrust belt are sourced from the rapidly exhuming areas along the entire Fairweather Fault, the northeastern part of the syntaxial region, as well as the slowly exhuming Insular superterrane. In contrast, the western strata are sourced from the emerging fold and thrust belt and the Chugach Metamorphic Complex located north of the plate boundary. In these sediments we identified a change in sediment provenance, which we suggest marks the capture of the Bagley Ice Valley by the proto‐Bering Glacier at the transition from the early to late Pliocene. This implies that the modern Bagley‐Bering Glacier System is much older than previously known. Strata deposited at ~8.6 Ma suggest that extreme rapid exhumation was already ongoing in the late Miocene, which supports previous findings in deep‐sea deposits. Overall, the data help discern several stages in the evolution of sediment routing patterns in response to dynamic tectonic and surficial processes along this active convergent margin.

Description: Abstract Mesozoic‐Cenozoic rifting between Greenland and North America created the Labrador Sea and Baffin Bay, while leaving preserved continental lithosphere in the Davis Strait, which lies between them. Inherited crustal structures from a Palaeoproterozoic collision have been hypothesized to account for the tectonic features of this rift system. However, the role of mantle lithosphere heterogeneities in continental suturing has not been fully explored. Our study uses 3‐D numerical models to analyze the role of crustal and subcrustal heterogeneities in controlling deformation. We implement continental extension in the presence of mantle lithosphere suture zones and deformed crustal structures and present a suite of models analyzing the role of local inheritance related to the region. In particular, we investigate the respective roles of crust and mantle lithospheric scarring during an evolving stress regime in keeping with plate tectonic reconstructions of the Davis Strait. Numerical simulations, for the first time, can reproduce first‐order features that resemble the Labrador Sea, Davis Strait, Baffin Bay continental margins, and ocean basins. The positioning of a mantle lithosphere suture, hypothesized to exist from ancient orogenic activity, produces a more appropriate tectonic evolution of the region than the previously proposed crustal inheritance. Indeed, the obliquity of the continental mantle suture with respect to extension direction is shown here to be important in the preservation of the Davis Strait. Mantle lithosphere heterogeneities are often overlooked as a control of crustal‐scale deformation. Here, we highlight the subcrust as an avenue of exploration in the understanding of rift system evolution.

Description: Abstract As one of the most important Early Paleozoic arc systems in the Central Asian orogenic belt, the tectonic affinity of the Bainaimiao arc belt is highly controversial. This study performed paleocurrent, petrography, trace elements of the whole rocks and U‐Pb geochronology of tuff and detrital zircons of the Early Devonian deposits, to constrain the tectonic affinity of the Bainaimiao arc belt. New zircon U‐Pb age constrains the Xibiehe Formation to the Early Devonian. Both the Xibiehe and Qiankuntougou formations show similar framework mineral and trace element components, suggesting a dissected ensialic island arc source. However, the U‐Pb age spectra of detrital zircons for the two formations are totally different. The Xibiehe Formation reflects continuous ages ranging 1977‐597 Ma, indicating the Bainaimiao arc origin. The Qiankuntougou Formation has a mixed source displaying the North China craton derived ages (~1.85 Ga, ~2.52 Ga) and Bainaimiao arc ages (~1.4 Ga, ~1.2 Ga, ~0.97 Ga and 0.55 Ga 0.5~0.43 Ga). The comparable age components indicate that the Bainaimiao arc is an exotic terrane accreted to the North China craton in an arc‐continent collision way at ca. 430‐410 Ma. The arc‐continent collision and the following post‐orogenic magmatism (410‐360 Ma), sedimentation during extensional collapse and south‐dipping subduction complex evolution imply a subduction polarity flip from the northward subduction to the southward subduction.

Description: Abstract Rocks with a well‐developed lineation but weak or no foliation (L‐tectonites) commonly occur as isolated volumes dispersed in other tectonites. We consider L‐tectonites that reflect constrictional finite strains here and use a multiscale approach to investigate the conditions for constrictional strain fields. The approach combines the strength of kinematic and mechanical analyses in large strain 3D deformations. Our modeling shows that, in simple shearing and thinning zone progressive deformations, constrictional strains develop only in rheological heterogeneities that are moderately stronger than the bulk material as a whole. Stronger elements never accumulate enough internal strains for any fabric to develop. Inclusions weaker than the bulk material will develop flattening strains. L‐tectonites are most likely developed in macroscale simple shearing, simple‐shearing‐dominated plane‐strain general shearing, or simple‐shearing‐dominated Sanderson and Marchini transpression. The lineations of the L‐tectonites are always nearly parallel to the lineations in the bulk material. Where the lineations are nearly 90° degree from the vorticity axis, the macroscale flow is close to a plane‐strain general shearing. Where the lineations are oblique to the vorticity axis or more variable, a simple‐shearing‐dominated triclinic thinning zone with mainly uniaxial boundary stretching is likely. The concept of homogeneous transtension deformation, combining a homogeneous pure shearing and a transcurrent simple shearing, is unsupported by fabric evidence and is likely unrealistic. Under an oblique divergence boundary condition, the upper lithosphere deforms by folding and fracturing and the ductile lithosphere develops simple‐shearing‐dominated detachment shear zones. Constrictional strains (hence L‐tectonites) can develop in these detachment zones due to flow partitioning.

Description: Abstract This paleomagnetic study is located at the north‐west extremity of the Tarim Basin and has aimed to constrain the style of Neotectonic deformation where indentation of the Pamir Orogen into the southward‐verging Tian Shan frontal zone has produced a complex zone of thrusting, folding and strike‐slip. Sampling focused on two Pliocene to Pleistocene sedimentary formations folded across the Mingyaole Anticline, the major structural feature between the two frontal zones, has yielded well‐grouped characteristic remanent magnetizations at 18 of 24 sites and a positive fold test. Together with fabric evidence, the results indicate a probable post‐depositional detrital origin for the remanence. The results show that only small inter‐locational vertical‐axis rotations have occurred within the Kashi‐Atushi fold and thrust system since the Miocene and imply that the Kashi depression has behaved as a quasi‐rigid block. A common 15‐30° counterclockwise (CCW) rotation relative to Eurasia since the Miocene of the Kashi Depression and the bordering Tian Shan range proves to be unrelated to the right lateral motion along the Talas‐Ferghana intracontinental transform fault to the north west. This contrast is provisionally interpreted as taking place along a transfer fault between different segments of the thrust belt. Ongoing CCW rotation of the Tarim Basin is interpreted as a regional response to impingement by northward movement of the larger Tibetan Block to the south east.

Description: Abstract At the latitude of 32°‐33°S, the orogenic front of the eastern flank of the Andes is located in the Southern Precordillera, where two major thrusts concentrate Quaternary deformation at the surface. We estimate the shortening rate for one of these, the Las Peñas thrust, for the last ~200 ka, at the site of one of the best exposures of Quaternary thrusting along the Andes. Shortening was estimated across two prominent splays of the deformation zone at the mouth of the Las Peñas River through balanced cross‐sections by using a terrace surface as a marker of cumulative deformation. Terrace ages were constrained through cosmogenic isotopes (10Be) analysis on quartz cobbles collected from the surface and three depth profiles. Results indicate a mean shortening rate across the Las Peñas thrust of 0.27 + 0.11 mm/a. Our results are lower than Holocene shortening rates (1.9‐2.4 mm/a) obtained for individual splays of the Las Peñas thrust nearby. We discuss these discrepancies and implications for thrust evolution since the Late Pleistocene, in particular regarding along‐strike rate variations which highlight the complexity of the Quaternary‐active thrust deformation zone. We argue that ongoing stream incision rates largely prevail with respect to thrust activity at the study site. This observation might be connected with a recent (Holocene?) basinward shift of blind thrust activity.

Description: Abstract We revisit the nature, structure and evolution of the Ionian Basin and its surrounding passive margins (Apulia, Eastern Sicily/Malta, and Cyrenaica margins). Relying on geological observations (wells, dredges and dives) and seismic calibrations from the surrounding platforms and escarpments, we present age correlations for the deepest sedimentary sequences of the Ionian Basin. Two‐ship deep refraction seismic data combined with reprocessed reflection seismic data enable us to identify, characterize and map these thick sedimentary sequences and to present a consistent seismic stratigraphy across the basin. At a larger scale, we present new geological transects illustrating the tectono‐stratigraphic relationships between the Ionian Basin and its surrounding rifted margins. On this basis, we suggest that a Late Triassic‐Early Jurassic rifting preceded the late Early Jurassic to Middle Jurassic formation of the Ionian Basin. The combination of geophysical and geological arguments suggests that the entire deep basin is floored with oceanic crust of normal thickness, but with high seismic velocity in the upper part. Upscaling our results in the framework of the Eastern Mediterranean, we propose that the Ionian Basin represents the remnant of a short‐lived oceanic basin resulting from the interaction between two propagating oceans: the Central Atlantic and the Neo‐Tethys.

Description: Abstract We present the largest dataset of anisotropic high‐quality parameters (~12000) for the Amatrice‐Visso‐Norcia seismic sequence and investigate the physical mechanisms causing crustal anisotropy and its relation with crustal deformation, stress field, fluids and earthquake generation. We performed shear wave splitting analysis on ~40000 aftershocks recorded at 31 seismic stations during the first six months of the sequence following the August 24th 2016 Mw 6.0, Amatrice mainshock. Automatic and manual‐revised P‐ and S‐ picking and high‐precision locations are used to delineate the fracturing pattern and spatio‐temporal variations in the anisotropic parameters: fast direction polarization (φ) and delay time (δt). The mean φ strikes N146°, parallel to the extensional Quaternary fault systems, and to the NW‐SE local active SHmax as proposed by the EDA model. Locally φ directions outline the pattern of micro and mesoscale structures that we relate to structural‐controlled anisotropy. Temporal variations of anisotropic parameters allowed us to deduce stress‐induced and pervasive fluid‐filled stress‐aligned crack systems as the prevalent anisotropic mechanisms in some sectors. Higher δt (〉 0.072 s) and higher anisotropy percentage (〉 1.5%‐2.0%) are found at the boundaries and in the western side of the activated fault systems, where heavily fractured carbonates are present. The fault networks along with the lithological heterogeneities present in the area may act as a structural barrier along which fluids are channeled or trapped, thus causing over‐pressured fluid zones. Observations of shear‐wave splitting parameters during a seismic sequence can monitor the buildup of stress before earthquakes and the stress release as earthquakes occur.

Description: Abstract The Rif belt (northern Morocco) is a mountain chain located at the junction between the Mediterranean and Central Atlantic Domains. Although the Rif belt underwent important Cenozoic (i.e., Alpine) compression, remnants of the Mesozoic North African rifted margin are preserved in its external zones. This contribution aims to characterize the Mesozoic architecture and poly‐phase rifting history of this rifted margin. We present detailed field evidence and geochronological data from two palaeogeographic zones (Mesorif and Intrarif) preserving remnants of the former North African distal margin. The Mesorif conserves lithostratigraphic associations characterized by mafic intrusive rocks overlain by dismembered and discontinuous blocks of Lower Jurassic carbonates covered by Middle to Upper Jurassic sediments. U‐Pb zircon dating of 4 samples from this gabbroic complex shows ages close to the Triassic‐Jurassic boundary (195‐200 Ma). The gabbro was emplaced within the continental crust at the end of the first Triassic rift event and exhumed shortly after during a second Middle Jurassic rift event, which presents exceptional rift‐related structures. The most distal part of the margin is exposed in the Intrarif. In this unit, the Beni‐Malek serpentinized peridotites exhibit ophicalcites with uppermost Jurassic limestones resting conformably on top, suggesting that exhumation of the mantle occurred at the distal part of the North African margin at this time. Together, these observations enable us to discuss the evolution of the western part of the North African rifted margin and its relations with the Moroccan Atlantic margin and Tethys system.

Description: Abstract U‐Th dating of carbonate veins in connection with active tectonics has recently been used as an attractive tool for constraining the absolute timing of late Quaternary crustal deformations. In this study, for the first time we correlate U‐Th ages of travertine deposits in coseismic fissures along the North Anatolian Fault Zone with records of paleoseismological studies supported by historical earthquake catalogued data. U‐Th ages are assessed in relation to the recurrence interval and the size and epicenter distance of major Holocene earthquakes. Our statistical evaluations on age correlations indicate that the carbonate vein precipitation is concentrated in eight different periods along the North Anatolian Fault Zone. The periods are well correlated with historical earthquake records and with previous dating results of the nearby trench studies. At least six of the periods correspond to the earthquakes reported in the historical catalogues. The age correlations of carbonate precipitation intervals for the last millennium show a recurrence along the eastern North Anatolian Fault Zone with a mode at 130–330 years that is consistent with a previously proposed paleoseismic recurrence interval of the fault. Recorded events in carbonate veins indicate a close‐epicenter (d 〈 200 km) and high‐intensity (I 〉 VI) paleoearthquakes. Our results suggest that coseismic carbonate veins could be used to determine paleoseismic records as a supplementary tool to augment paleoseismological techniques. This tool has advantages over traditional paleoseismological methods for the understanding of long‐term earthquake behavior, particularly for prehistoric late Pleistocene events which cannot be dated easily by traditional paleoseismological methods.

Description: Abstract The Ordovician intra‐oceanic Macquarie Arc terrane is faulted against coeval, quartz‐rich turbidites of the Adaminaby Group within the Lachlan Orogen of eastern Australia. Debates exist concerning the polarity of subduction beneath the Macquarie Arc and the nature of its emplacement, given it is juxtaposed against the Adaminaby Group to both the west and east. We present new provenance and zircon analyses of the Triangle Formation, which consists of interleaved quartz‐rich passive margin sandstones and island arc volcaniclastic rocks. In contrast, the structurally underlying Adaminaby Group contains no volcaniclastic detritus and displays a strong passive margin affinity. One sample from the Triangle Formation yielded a youngest zircon age of 456 ± 16 Ma indicating a subtle Macquarie Arc signature amongst an overwhelmingly Neoproterozoic and older Gondwanan provenance. The Adaminaby Group yielded a youngest zircon age of 481 ± 6 Ma and a strong Gondwanan zircon signature. We compared these results with volcaniclastic rocks from the Weemalla Formation stratigraphically higher in the Macquarie Arc, which yielded a distinctly unimodal zircon age of 451 ± 8 Ma, which is indistinguishable from the youngest zircon in the Triangle Formation. We suggest the Triangle Formation represents trench‐fill material sourced predominantly from the Gondwana margin but including some younger Macquarie Arc detritus. This constrains the initiation of this arc‐continent collision to between ~448 – 462 Ma (Late Ordovician).

Description: Abstract The Pollino Massif is the most southeastern outcrop of the Apennine core. It marks the transition between Apenninic shortening and extension, respectively SE and NW of the massif, and is also the cusp of a SE‐ward plunge that characterizes the submerged Apennines. The SE limit of NE‐SW extension merges with the east limit of Tyrrhenian extension in Calabria. This strategic position is expected to transition SEward in the progressive oblique collision of the Calabrian forearc and Apulia. We test this hypothesis using published results and new field data. The time‐transgressive emergence of basins on the Apennine thrust wedge is quantitatively consistent with the ESE rollback of the Calabrian arc. Specifically, a thrust‐normal slip reversal on a SW‐dipping fault is responsible for the tectonic collapse that lead to the Mercure Basin along strike NW of the Pollino Massif and to an east‐to‐west reversal of drainage. This reversal is timed by an intermediate stage of trapped internal drainage with mid Pleistocene lacustrine sedimentation, but it may young to SE as the normal displacement on the border fault decreases gradually to SE and vanishes near the apex of the massif. On the SE side of the massif, contractional tectonics persists at least into the Mid‐Pleistocene and likely later, while NE‐SW extension is absent. Prominent normal faults in that area accommodate range‐parallel extension and are coupled with the thrust faults. The combination of longitudinal extension with a CCW rotation of hanging‐wall units and thrust directivity can account for the final setting in the Apennines.

Description: No abstract is available for this article.

Description: Abstract A significant issue in the study of orogenic systems concerns the roles played by frontal and basal accretion in the construction of orogenic wedges. These different accretion mechanisms result in different thermal histories, with underplated materials experiencing significant heating and deformation during tectonic burial. This work provides new thermal data from Raman spectroscopy of carbonaceous material (RSCM) in combination with structural and stratigraphic observations of the northern Taiwan slate belt to address these questions of wedge development. Sedimentary rocks of the Northern slate belt were deposited on the Chinese continental margin immediately before the onset of the Neogene Taiwan arc‐continent collision. In the slates of the northern Hsüehshan Range, a large‐scale pop‐up structure on the prowedge of the Taiwan Orogen, syn‐orogenic metamorphism has been investigated through analyses of peak temperatures and metamorphic field gradients. Results indicate underthrusting of the margin sediments to ~8 km depth with significant folding in two major duplexes occurring before underplating. Such basal accretion is considered responsible for the distinct culmination of the Hsüehshan Range in central Taiwan and its relative uplift with respect to the Backbone Range to the east along the Lishan Fault. A similar underthrusting scenario is also suggested for the Backbone Range Slate Belt. We propose that basal accretion is the predominant mechanism in the growth and evolution of the Taiwan orogenic wedge, and may have been achieved through inversion of a graben system on the ancient passive margin during continental subduction.

Description: Abstract We constrain the fault pattern and the kinematics of faulting that facilitated the uplift of the Troodos Massif in Cyprus in the eastern Mediterranean. The fault pattern consists of E‐W striking reverse faults, N‐S striking normal faults, and NW striking dextral and NE striking sinistral strike/oblique‐slip faults. Fault‐slip analysis reveals that this overall pattern resulted from subhorizontal NNW directed shortening and coeval, subhorizontal ENE directed extension. Dated sediments affected by faulting reveal that the N‐S striking normal faults are, at least in part, younger than 2.14 Ma. This suggests that the entire fault pattern, or at least a large part of it, resulted or was reworked during post‐2.14‐Ma deformation. Published work further shows that the uplift of the Troodos Massif was controlled by underthrusting of the Eratosthenes Seamount, which entered the Cyprus subduction zone by about 2 Ma. The alleged E‐W extent of the underthrust segment of the seamount approximately matches the size of the high‐topography area of the Troodos Mountains. We interpret this geometry to have caused spatially limited crustal thickening underneath the high‐topography area by subhorizontal N‐S shortening in front of and above the underthrusting seamount and coeval, subhorizontal E‐W extension at its flanks, and that the uplift of the high‐topography area occurred in the footwall of the N‐S striking normal faults. This model is broadly in line with seismicity and may also explain why serpentinized mantle rocks and dense gabbro are now exposed on top of the Troodos Massif.

Description: Abstract This study aims at further documenting the mechanisms of shortening at the front of fold‐and‐thrust belts. We focus on an actively growing anticline located at the deformation front of the Taiwan fold‐and‐thrust belt. Based on a multidisciplinary approach combining mainly subsurface data and geodetic techniques, we show that the Tainan anticline is a pure‐shear fault‐bend‐fold growing above a 38‐45° west‐dipping backthrust, the Houchiali fault, rooted on a 3.8‐km‐deep detachment. The cumulative shortening is estimated at 2‐3 km since 310 ± 50 ka, including ~30‐50% of horizontal compaction shortening. The significance of the fold is little in terms of total shortening at the scale of the mountain piedmont, yet the Holocene shortening rate of 10.3 ± 1.0 mm/a accounts for 25% of the present‐day shortening rate across the piedmont. Earthquake scaling relationships applied to the Houchiali fault predict Mw~6 earthquakes that would occur a lot more frequently than indicated from historical earthquake catalogues. Hence, the aseismic slip behavior observed from geodetic measurements since two decades is a representative behavior of the fault at least at the scale of a few centuries. Our results bear out the dominance of pure‐shear folding at the front of fold‐and‐thrust belts and support horizontal compaction as a significant shortening mechanism. In contrast, the backthrust wedge structure and the aseismic slip are peculiar characteristics that likely arise from the combination of low friction and high‐pore pressure related to the thick mudstone formation hosting the wedge and of high syntectonic sedimentation rates.

Description: Abstract The Dras Arc in NW India Himalaya is a belt of basaltic‐andesites intercalated with arkose‐dominated volcaniclastic rocks of the Nindam Formation situated along the Indus Suture between India and Eurasia. Debates exist as to whether these rocks developed in a forearc basin to the Eurasian margin or as part of an intraoceanic island arc system that collided with either India or Eurasia before final continental collision. Detrital zircons from the Nindam Formation yield U‐Pb age spectra with dominant youngest age populations of ~84–125 Ma, corresponding with arc magmatism. Sandstone provenance analysis from the Nindam Formation indicates that the Dras Arc evolved from an undissected arc to dissected arc over a period of ~41 Myr. Slightly older, smaller populations occur at ~135–185 Ma, corresponding with reported ages of Neotethyan ophiolites (e.g., Spongtang). The basal section of the Nindam Formation reveals the presence of arc‐derived basaltic‐andesite and tonalite clasts, plus ophiolitic components sourced from an adjacent accretionary complex. There is a distinct absence of quartz or felsic granitic clasts, suggesting that the Nindam Formation did not develop as a forearc basin to the Ladakh Batholith of southern Eurasia but rather as separate intraoceanic island arc. A distinct “Gondwanan” signature occurs in all samples, with zircon age peaks at ~514–988, ~1000–1588, ~1627–2444, and ~2500 Ma. We suggest that the Dras and Spong arcs are the same intraoceanic island arc system that developed as a result of subduction initiation along NNE‐SSW transform faults perpendicular to the Indian and Eurasia continents.

Description: Abstract Recent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold‐and‐thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold‐and‐thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate‐P metamorphism (M1) at circa 620 Ma and a low‐P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2‐related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right‐lateral strike‐slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.

Description: Abstract Deciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between 〉~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.

Description: Abstract We present new, high‐resolution constraints on crustal and uppermost mantle structure of the southeastern, eastern, and northeastern Tibetan plateau and adjacent regions by combining extensive data from three regional‐scale dense seismic arrays. Our results show significant differences in the crustal and uppermost mantle structure within the Songpan‐Ganzi terrane. The southern part has a thick crust and high Poisson's ratio, while the crustal thickness becomes less and Poisson's ratio is nearly normal in the northeast. Prominent low‐velocity anomalies appear beneath both the southern and northeastern Songpan‐Ganzi terrane at depths of 20–40 km, but they are not interconnected. Seismic velocities of the uppermost mantle are slow in the northeastern Songpan‐Ganzi terrane relative to the south. We further find that the crust of the central Qilian orogen is thickened but with remarkable low Poisson's ratio. Low velocities are visible in both the mid‐lower crust and uppermost mantle, similar to the northeastern Songpan‐Ganzi terrane. For comparison, the crustal low velocities are less pronounced beneath the central Qilian orogen. High Poisson's ratio and the mid‐lower crustal anomaly of the southern Songpan‐Ganzi terrane strongly indicate partial melting of the crust. Localized asthenospheric upwelling, however, can account for the coincident anomalies in both the mid‐lower crust and uppermost mantle beneath the northeastern Songpan‐Ganzi terrane and central Qilian orogen. We conclude that outward expansion of the Tibetan plateau has evolved by a combination of crustal flow on its southeastern margin and mantle upwelling likely induced by removal of thickened lithosphere on its easternmost and northeastern edges.

Description: Abstract In the past 200 years, the Kachchh paleorift in western India, a plate interior region, has witnessed unusually large number of strong earthquakes, namely, the 2001 Bhuj (Mw 7.6), 1956 Anjar (Mw 6.0), 1845 Lakhpat (Mw ~6), and 1819 Allah Bund (Mw ~7.7) earthquakes. We report continuous GPS measurements from the Kachchh and adjoining region. We find that the ongoing deformation during 2009–2016 is inconsistent with the models of postseismic deformation due to the 2001 Bhuj earthquake. The current widespread deformation pattern implies that the paleorift, bounded by the Kachchh Mainland fault and several other faults along its southern flank and the Allah Bund fault and Nagar Parker fault along its northern flank, is under long‐term compression from both sides at a rate of ~4–5 mm/year. We show that most of this compression is released seismically, making this one of the most seismically active paleorifts in the world.

Description: Abstract The 〉10km thick Paleozoic Guacha Corral shear zone in Central Argentina is arguably one of the thickest thrust zones anywhere. It played a key role during the two early Paleozoic orogenies of W Gondwana and currently separates two terranes: one dominated by events related to the 〉510 Ma Pampean Orogeny, and the other by the 〈500Ma events of the Famatinian Orogeny. The combination of structures and geochronometry suggests that the shear zone was active for over 100 Ma. Thrusting started during migmatization at 520 Ma as recorded by U‐Pb ages in zircon and monazite, and was demonstrably active also at 480 and 470 Ma. The age coincidence between Ar‐ plateau ages of muscovite and biotite at 420 Ma, in the upper part of the shear zone, and at 405 Ma in the lower part, suggests two intense cooling pulses. These are ascribed to thrusting, rapid exhumation and cooling. Its anomalous width is likely a result of this prolonged kinematic history related to its role in transferring shortening to the Pampean fore‐arc possibly as a basal detachment, and later by acting as the main boundary between the Pampean and Famatinian tectono‐metamorphic terranes. The two low‐temperature shearing events inferred from the Ar‐ages suggest that deformation continued during the period between the end of the Famatinian cycle at ~440 Ma and the start of the Chanic or Achalian Orogeny and its magmatism after 400 Ma.

Description: Abstract New marine geophysical data acquired across the partly ice‐covered northern East Greenland continental margin highlight a complex interaction between tectonic and magmatic events. Breakup‐related lava flows are imaged in reflection seismic data as seaward dipping reflectors, which are found to decrease in size both northward and southward from a central point at 75°N. We provide evidence that the magnetic anomaly pattern in the shelf area is related to volcanic phases and not to the presence of oceanic crust. The remnant magnetization of the individual lava flows is used to deduce a relative timing of the emplacement of the volcanic wedges. We find that the seaward dipping reflectors have been emplaced over a period of 2–4 Ma progressively from north to south and from landward to seaward. The new data indicate a major post‐middle Eocene magmatic phase around the landward termination of the West Jan Mayen Fracture Zone. This post‐40‐Ma volcanism likely was associated with the progressive separation of the Jan Mayen microcontinent from East Greenland. The breakup of the Greenland Sea started at several isolated seafloor spreading cells whose location was controlled by rift structures and led to the present‐day segmentation of the margin. The original rift basins were subsequently connected by steady‐state seafloor spreading that propagated southward, from the Greenland Fracture Zone to the Jan Mayen Fracture Zone.

Description: ABSTRACT The Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the Attic‐Cycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modelling, U–Pb geochronology and whole rock geochemistry, resulting in a tectono‐thermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by U‐Pb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a supra‐subduction zone setting, comparable to the “East‐Vardar Ophiolites”, and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at P–T conditions of ca. 8.5 kbar 850‐600 °C, down‐structural section to greenschist‐facies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a U–Pb zircon protolith age of ca. 190 Ma and a high‐grade metamorphic‐anatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myrs after it formed during initiation of a NE‐dipping intra‐oceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in high‐pressure metamorphism of the Cycladic continental margin ~25 Myrs later.

Description: Abstract Significant tectonic clockwise rotations were evidenced in the Tell Atlas (Neogene Algerian Cheliff basin) by previous paleomagnetic studies. For northwestern Africa and in the context of the Africa‐Eurasia convergence, they provided a new argument validating a kinematic model, based on transpression with shortening accommodated by clockwise block rotations. To corroborate this deformation pattern at a larger scale, new paleomagnetic studies were performed on 349 cores in the Mitidja basin, of Mio‐Plio‐Quaternary age, in the Algerian Tell Atlas. This intramontaneous basin is structured by two regional major E‐W to WNW‐ESE dextral shear zones. Primary magnetization data were obtained in 43 out of the 49 sampled sites. This magnetization is carried by Ti‐poor titanomagnetite. Its direction shows that significant tectonic block rotations affected this basin since 16 Ma. Zones located between the E‐W to WNW‐ESE major structures are affected by coherent clockwise rotations (average magnitude of 48°) of large blocks, compartmented by presently associated sinistral NE‐SW faults. Along the shearing structures, smaller blocks, resulting from the fragmentation of the large blocks, show various rotations, many of which are of large magnitude. These rotations, similar to those highlighted previously in the Cheliff basin, are interpreted as resulting from bookshelf, consequence of the Africa‐Eurasia plates convergence in the Tell Atlas.

Description: Abstract Shear zones typically interact to form connected systems or networks to accommodate crustal deformation, but our knowledge of how this happens is fragmentary. Understanding branching and interacting shear zones requires knowledge of timing, deformation kinematics, and rheology. The Senador Pompeu, Tauá, and Cococi strike‐slip shear zones of the Borborema Province (NE Brazil) have a central role and location in the Neoproterozoic assembly of Gondwana and provide a means to understand shear zone interaction. We apply (i) U‐Pb in situ SHRIMP analysis of zircons from syntectonic plutons and dykes to constrain the timing of shearing and (ii) vorticity and strain analysis on pluton's megacrystic facies deformed in the magmatic state and during final stages of crystallization. Obtained ages show that the shear zone pair was active under high temperature at 583.5±4.6 Ma, while felsic dykes were emplaced in the brittle regime in the wall rocks. Average vorticity estimates of 0.70 indicate a strong component of pure shear in the shear zones. Despite the transpressional character, the dispersion in estimates of thinning and thickening for the Senador Pompeu shear zone highlights variations of offset rate for the interacting branches that leads to localized transtension. We conclude that the kinematic framework of the Senador Pompeu and Tauá conjugate pair involves the formation of a dextrally closing zipper structure involving the trailing Cococi shear zone to the southwest, which in turn caused the northeastward extrusion of the enclosed crustal wedge and possibly activation of the nappe system of the Ceará Central domain.

Description: Abstract Although plate tectonics has pushed the frontiers of geosciences in the past 50 years, it has legitimate limitations, and among them we focus on both the absence of dynamics in the theory and the difficulty of reconstructing tectonics when data are sparse. In this manuscript, we propose an anticipation experiment, proposing a singular outlook on plate tectonics in the digital era. We hypothesize that mantle convection models producing self‐consistently plate‐like behavior will capture the essence of the self‐organization of plate boundaries. Such models exist today in a preliminary fashion, and we use them here to build a database of mid‐ocean ridge and trench configurations. To extract knowledge from it, we develop a machine learning framework based on Generative Adversarial Networks (GANs) that learns the regularities of the self‐organization in order to fill gaps of observations when working on reconstructing a plate configuration. The user provides the distribution of known ridges and trenches, the location of the region where observations lack, and our digital architecture proposes a horizontal divergence map from which missing plate boundaries are extracted. Our framework is able to prolongate and interpolate plate boundaries within an unresolved region but fails to retrieve a plate boundary that would be completely contained inside of it. The attempt we make is certainly too early because geodynamic models need improvement and a larger amount of geodynamic model outputs, as independent as possible, is required. However, this work suggests applying such an approach to expand the capabilities of plate tectonics is within reach.

Description: Abstract The Eocene (~50‐45 Ma) major absolute plate motion change of the Pacific plate forming the Hawaii‐Emperor bend is thought to result from inception of Pacific plate subduction along one of its modern western trenches. Subduction is suggested to have started either spontaneously, or result from subduction of the Izanagi‐Pacific mid‐ocean ridge, or from subduction polarity reversal after collision of the Olyutorsky arc that was built on the Pacific plate with NE Asia. Here we provide a detailed plate‐kinematic reconstruction of back‐arc basins and accreted terranes in the northwest Pacific region, from Japan to the Bering Sea, since the Late Cretaceous. We present a new tectonic reconstruction of the intra‐oceanic Olyutorsky and Kronotsky arcs, which formed above two adjacent, oppositely‐dipping subduction zones at ~85 Ma within the north Pacific region, during another Pacific‐wide plate reorganization. We use our reconstruction to explain the formation of the submarine Shirshov and Bowers Ridges, and show that if marine magnetic anomalies reported from the Aleutian Basin represent magnetic polarity reversals, its crust most likely formed in an ~85‐60 Ma back‐arc basin behind the Olyutorsky arc. The Olyutorsky arc was then separated from the Pacific plate by a spreading ridge, so that the ~55‐50 Ma subduction polarity reversal that followed upon Olyutorsky‐NE Asia collision initiated subduction of a plate that was not the Pacific. Hence, this polarity reversal may not be a straightforward driver of the Eocene Pacific plate motion change, whose causes remain enigmatic.

Description: Abstract The SW Ecuador‐NW Peru forearc region is the southernmost location, where the Caribbean Large Igneous Province (CLIP) interacted with the South American margin since the Late Cretaceous. The accretion of the CLIP to the margin led to the entrapment of the North Andean crustal Sliver, conforming the underlying basement of the forearc region in Ecuador, whereas in NW Peru, forearc depocenters involve rocks of continental affinity. Many existing tectonic reconstructions have treated these two areas independently, largely based on their crustal affinities. In contrast, this study integrates previous studies into an analysis of unpublished seismic profiles, potential field data, outcrop stratigraphy, and recent studies dealing with the dynamics of allochthonous terrane accretion along continental margins. Our integrated approach shows that SW Ecuador was dominated by a Late Cretaceous deforming outer wedge, which may have constituted a remnant of a northeast or northwest‐dipping obliquely obducted oceanic block at the edge of the CLIP. This tectonic phase was governed by plate instability, affecting NW Peru and SW Ecuador, followed by re‐establishment of the margin by early Eocene. The resulting margin configuration and the spatial distribution of the different tectonic elements seems to have played a key role into the further Cenozoic development of the forearc region. The model presented in this study proposes that the accretion of buoyant oceanic terranes may have had a profound impact on the early margin configuration of SW Ecuador and NW Peru and led to the development of localized but genetically related forearc depocenters.

Description: Abstract The Tanzania‐North Mozambique continental margin is a transform segment associated with Davie Fracture Zone (DFZ). The DFZ is described as an elongated linear oceanic fracture zone, commonly linked with the breakup between Eastern and Western Gondwana. We conducted a synthesized study using gravity, magnetic and seismic data presenting the crustal architecture, geometry and the kinematic nature of continental breakup along a transform margin. The Crustal nature of DFZ, its role in forming kinematic linkage between two extensional margins during continental breakup processes is focus of our study. The two extensional margins, Somalia‐Majunga and North Mozambique‐Antarctica were linked via a 2600 km long dextral transform segment, partially overlapping with DFZ. Absence of classical rift indicators, weak signs of hyperextension, abrupt ocean‐continent boundary (OCB) suggests transform margin architecture. We redefined this feature as the Davie Transform System (DTS). The nature of deformation varies form transtensional pull‐apart in Tanzania to almost pure strike‐slip in North Mozambique. The southern transform segment exhibits abrupt change in ocean continent transition with a narrow zone of continental extension. This variation is recognized through the newly interpreted OCB along this entire transform segment. Notably, within large pull‐apart systems in the north, presence of fossilized incipient spreading center suggest that the extension had reached at quite advanced stages, characterized by significant thermal weakening as a consequence of strong magmatic activity. Through a series of reconstruction snapshots, we show the geodynamic evolution along the Tanzania‐North Mozambique margin explaining the role of DTS in the southward movement of Madagascar.

Description: Abstract Preserved sets of marine terraces and palaeoshorelines above subduction zones provide an opportunity to explore the long‐term deformation that occurs as a result of upper‐plate extension. We investigate uplifted palaeoshorelines along the South Central Crete Fault (SCCF) and over its western tip, located above the Hellenic Subduction Zone, in order to derive uplift rates and examine the role that known extensional faults contribute to observed coastal uplift. We have mapped palaeoshorelines and successfully dated four Late‐Quaternary wave‐cut platforms using in‐situ 36Cl exposure dating. These absolute ages are used to guide a correlation of palaeoshorelines with Quaternary sea‐level highstands from 76.5 to ~900 ka; the results of which suggest that uplift rates vary along fault strikes but have been constant for up to 600 ka in places. Correlation of palaeoshorelines across the SCCF results in a throw‐rate of 0.41 mm/yr and, assuming repetition of 1.1 m slip events, a fault‐specific earthquake recurrence interval of approximately 2700 years. Elastic‐half space modelling implies that coastal uplift is related to offshore upper‐plate extensional faults. These faults may be responsible for perturbing the uplift rate signals in the south central Crete area. Our findings suggest that where uplifted marine terraces are used to make inferences about the mechanisms responsible for uplift throughout the Hellenic Subduction Zone, and other subduction zones worldwide, the impact of upper‐plate extensional faults over multiple seismic cycles should also be considered.

Description: Abstract I perform multicycle dynamic simulations of a restraining double bend that is simplified from the Aksay bend along the Altyn Tagh Fault in northwest China. The earthquake cycle includes a coseismic dynamic rupture phase and an interseismic stress loading and relaxation phase. The double bend fault system has two fault strands and each strand comprises a stem segment that strikes parallel to the maximum shear loading direction and a bend segment that deviates from the loading direction. I find that the restraining double bend is an effective barrier to dynamically propagating ruptures over multiple earthquake cycles, with a probability of rupture jumping from one strand to the other varying from 0% to 10%, depending on parameter values of the rate dependence of friction during the dynamic phase and the viscosity for stress relaxation during the interseismic phase. The critical condition for the primary rupture on the first strand to jump onto the second strand is that the pre‐shear stress is close enough to the shear strength over a certain size of the fault patch on the second strand. These results on the simplified double bend provide important insights into rupture behavior of geometrically complex faults over multiple earthquake cycles in general, and a reference to explore rupture behavior of the Aksay bend in particular.

Description: Abstract Strain fabric and monazite microstructure were studied and dated by EPMA in situ Th‐U‐total Pb monazite geochronology in the Ambaji granulite, South Delhi terrane, NW India. The Ambaji granulite comprises pelitic, calcareous and mafic granulites with several phases of granite intrusions, G0‐3. The granulites were deformed by three phases of folding, F1‐3 during South Delhi orogeny, and marked by a subhorizontal pervasive fabric, S1, axial planar to isoclinal‐recumbent F1 folds that developed during granulite facies metamorphism. S1 is overprinted by discrete sets of subvertical shear zones associated with a mylonitic fabric, S2, that were developed axial planar to NE‐SW striking upright F2 folds and facilitated exhumation of granulite facies rocks to the upper crust. The shear zones show early history of high temperature thrust sense shear and late stage low temperature sinistral shear. The NW‐SE striking F3 folds also affected the granulite facies rocks resulting in interference patterns and variations in regional structural trends. Post F3 folding, brittle strike‐slip and normal fault (Sf fabric) that led the final exhumation of the granulite facies rocks to the surface. The S1 monazites are Y‐depleted and recrystallized through dislocation creep and the S2‐Sf monazites are Y‐enriched and recrystallized through dissolution‐precipitation creep. Different monazite population yielded distinct ages of ca. 875‐857 Ma, 838‐778Ma and 764‐650 Ma for S1, S2 and Sf strain respectively indicating that the South Delhi orogeny spanned 875‐650 Ma overlapping with the early phase of the Pan‐African orogeny or representing a transition between Grenvillian and Pan‐African orogeny.

Description: ABSTRACT We integrate structural, geophysical and geodetic studies showing that the Dinarides‐Hellenides orogen along the Adria‐Europe plate boundary in the Western Balkan peninsula has experienced clockwise oroclinal bending since Eo‐Oligocene time. Rotation of the Hellenic segment of this orogen has accelerated since the middle Miocene and is associated with a north‐to‐south increase in shortening along the orogenic front. Within the Paleogene nappe pile, bending was accommodated by orogen‐parallel extension, clockwise block rotation and thrusting in the hanging wall of the Skhoder‐Peja Normal Fault (SPNF). The SPNF and related faults cut the older Skhoder‐Peja Transfer Zone with its pre‐Neogene dextral offset of the West Vardar ophiolite nappe. Rotation of the SPNF hanging wall involved Miocene‐to‐recent, out‐of‐sequence thrusting that was transferred to the Hellenic orogenic front via lateral ramps on dextral transfer zones. Along strike of the Dinarides‐Hellenides and coincident with the southward increase in Neogene shortening, the depth of the Adriatic slab increases from ~160 km north of the SPNF to ~200 km just to the south thereof, to several hundreds of km to the south of the Kefalonia Transfer Zone. The geodynamic driver of tectonics since the early Miocene has been enhanced rollback of the Hellenic segment of the Adriatic slab in the aftermath of Eo‐Oligocene slab tearing and breakoff beneath the Dinarides, which focused slab pull in the south. The SW‐retreating Hellenic slab segment induced clockwise bending of the southern Dinarides and northern Hellenides, including their Adriatic foreland, about a rotation pole in the vicinity of the Mid‐Adriatic Ridge.

Description: Abstract Combined field structural analysis with in situ EPMA (electron probe microanalysis) Th‐U‐Pb monazite dating, petrologic and microstructural data provide a reconstruction of the pressure‐temperature‐deformation‐time (P‐T‐D‐t) path of the Gondwanide basement of the North Patagonian Cordillera. For samples from the Challhuaco hill, the timing of development of the metamorphic S2 foliation and associated L2 lineation and tight to isoclinal F2 folds is constrained by monazite ages of 299 ± 8 and 302 ± 16 Ma during peak metamorphic conditions of ca. 650 °C and 11 kbar, achieved during prograde metamorphism and progressive deformation. Metamorphism and deformation of metamorphic complexes of the North Patagonian Andes seem to record Late Paleozoic crustal thickening and are coeval with metamorphism of accretionary complexes exposed further west in Chile, suggesting a coupled Late Devonian‐Carboniferous evolution. Instead of the result of continental collision, the Gondwanide orogeny might thus be essentially linked to transpression due to advancing subduction along the proto‐Pacific margin of Gondwana. On the other hand, a second generation of monazite ages of 171 ± 9 and 170 ± 7 Ma constrain the timing of low‐grade metamorphism related to kink band and F3 open fold development during Jurassic transtension and emplacement of granitoids. Finally, a Cretaceous overprint, likely resulting from hydrothermal processes, is recorded by monazite ages of 110 ± 10 and 80 ± 20 Ma, which might be coeval with deformation along low‐grade shear zones during the onset of Andean transpression.

Description: Abstract Results from thermochronological studies have multiple applications to various problems in tectonics and landform evolution. However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectono‐thermal evolution of the region. Here, we use both new and previously published multi‐mineral 40Ar/39Ar data (amphibole, mica and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900‐360 Ma. Using the apatite fission track method as well as a numerical model of the heating‐cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360‐0 Ma. According to our model, since Lapland‐Kola orogenesis (1930‐1905 Ma) northeastern Fennoscandia experienced a quasi‐monotonous cooling with the average rate of ~0.15°C/Myr, which is equal to an exhumation rate of ~1‐2 m/Myr. New apatite fission track data and time‐temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360‐300 Ma. This we attribute to an elevated geothermal gradient due to Baltica's drift over the African Large Low Shear‐wave Velocity Province in the lowest mantle and/or thermal blanketing by insulating Devonian‐Carboniferous sedimentary/volcanic cover. Our model is further supported by evidence of Late Devonian‐Carboniferous rifting in the East and South‐Western Barents Basin, as well as various 360‐300 Ma magmatic events within SW Fennoscandia and the Baltic countries.

Description: Abstract It is widely assumed that the India–Asia collision initiated at 55 ± 5 Ma along the Indus–Yarlung suture zone (IYSZ). Paradoxically, however, the majority of tectonic responses to collision as well as the accelerated faunal exchange between the two continents did not occur until ca. 35–30 Ma and intense deformation and uplift occurred along the southern side of the Tibetan Himalaya rather than the IYSZ. Additionally, this widely accepted scenario requires large‐scale subduction of buoyant continental crust into the dense mantle, which is inconsistent with tectonic reconstructions of Gondwana as well as the general knowledge of physics. Here we present evidence from crustal volume budget calculations to show that (1) at 55 ± 5 Ma 2100 ± 840 km of oceanic lithosphere existed between India and the Lhasa Block and hence India–Asia collision is unlikely to have occurred at that time, (2) substantial India–Asia collision should have occurred after this large region of oceanic lithosphere had been consumed by subduction, along the south side of Tibetan Himalaya diachronously at 32 ± 2 Ma in the west, 23 ± 2 Ma in the east and 18 ± 2 Ma in the centre, and (3) no large‐scale subduction of continental crust occurred during India–Asia collision. These findings resolve the problematic issues that have not been satisfactorily explained by many existing models for the system and suggest that the evolution of the Himalaya–Tibet orogen and many existing geodynamic models of India–Asia collision require critical reassessment.

Description: Abstract Structural observations and 40Ar/39Ar geochronology on pseudotachylyte, mylonite, and other fault zone materials from Fiordland, New Zealand reveal a multistage history of fault reactivation and uplift above an incipient ocean‐continent subduction zone. The integrated data allow us to distinguish true fault reactivations from cases where different styles of brittle and ductile deformation happen together. Five stages of faulting record the initiation and evolution of subduction at the Puysegur Trench. Stage 1 normal faults (40–25 Ma) formed during continental rifting prior to subduction. These faults were reactivated as dextral strike‐slip shear zones when subduction began at ~25 Ma. The dextral shear zones formed part of a transpressional regime (stage 2, 25–10 Ma) that included minor reverse motion and modest uplift above the leading edge of the subducting slab as it propagated below Fiordland. At 8–7 Ma (stage 3), trench‐parallel faults accommodated the first and only episode of pure reverse motion. Reconstructions confirm that these faults formed when the slab reached mantle depths and collided with previously subducted crust. At 5–4 Ma (stage 4), trench‐parallel faults accommodated oblique‐reverse motion when an oceanic ridge collided obliquely with the Puysegur Trench. Stages 3 and 4 both accelerated rock uplift and topographic growth in short pulses. A return to strike‐slip motion occurred after ~4 Ma (stage 5) when deformation localized onto the Alpine Fault. These results highlight how the rock record of faulting links displacements occurring at Earth's surface to events occurring both at the trench and deep within the lithosphere during subduction.

Description: Abstract The Longmen Shan orogenic belt is the landform boundary between the eastern Tibetan Plateau and the Sichuan Basin. However, there are significant differences in lithospheric deformation across its southern and northern segments. We established numerical models to investigate the effect of rheological heterogeneity on lithospheric deformation across the eastern plateau margin. The results show that the rheological heterogeneity of the Longmen Shan controls the type of lithospheric deformation and the strain partitioning. When the Longmen Shan is weak or narrow, high strain is predicted between the plateau and the Sichuan Basin along the Longmen Shan, which helps to explain the lack of a Cenozoic foreland basin along the southwestern part of the Sichuan Basin. However, when the upper crust of the Sichuan Basin is weak, the deformation should extend into the interior of the basin, such as the Longmen Shan‐Longquan Shan thrust system, which provides sedimentary space for the Cenozoic sediments. In contrast, when the Longmen Shan is strong or wide, the deformation is diffuse over a broad transition zone. The strain is mainly localized along the boundary between the Songpan‐Ganzi terrane and the Longmen Shan. This result can be used to understand the uplift of the Min Shan. Compared to the observations, the high strength of the Longmen Shan corresponds to the distribution of the Precambrian crystalline basement. The rheological heterogeneity of the Longmen Shan is from the result of the amalgamation of ancient continents, which illustrates the control of the ancient continental margin on recent tectonics.

Description: Abstract Zircon (U‐Th)/He (ZHe) and zircon fission‐track (ZFT) thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier fold‐thrust belt record enhanced cooling and exhumation during major thrust slip spanning ca. 125–90 Ma. ZHe and ZFT age‐paleodepth patterns along structural transects and age‐distance relations along stratigraphic‐parallel traverses, combined with thermo‐kinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/myr from 125–105 Ma, increasing to ~3 km/myr from 105–92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/myr. Local cooling ages of ca. 160–150 Ma may record a period of regional erosion, or alternatively an early phase of limited (〈10 km) thrust slip. Propagation of the Sevier wedge front and major thrust slip during the late Early to mid‐Cretaceous were synchronous with increasing subsidence and deposition of thick synorogenic strata in the foreland, crustal thickening in the hinterland, growing igneous activity in the Sierran magmatic arc, and increasing plate convergence rates. Along‐strike, other parts of the Cordilleran retroarc fold‐thrust belt also experienced major shortening during the late Early to mid‐Cretaceous, following a period of earlier Cretaceous quiescence. Late Jurassic shortening was concentrated nearer the arc, and thus mostly in the hinterland at the latitude of northern Utah, related to width and location of the passive‐margin sedimentary wedge relative to the plate margin.

Description: Abstract The October 15, 2013 magnitude (MW) 7.2 Bohol Earthquake produced an ~50‐km‐long, ~12 km wide northeast‐trending zone of uplift with an ~8‐km‐long discontinuous ground rupture indicating predominantly reverse‐slip movement on a southeast‐dipping fault. Documentation of the nearly‐continuous northern terminus of the 2013 Bohol earthquake ground rupture revealed its association to pre‐existing scarps of the previously‐unmapped, Quaternary‐active North Bohol Fault (NBF). Trenching across the rupture at four sites not only reveals the geometry and kinematics of the fault, but also shows at least one or two pre‐2013 surface rupturing events. Onshore geologic mapping and offshore seismic reflection profiles demonstrates the presence of an island‐wide, northeast‐southwest trending fold‐and‐thrust belt through which deformation related to the regional shortening across the Visayan Sea Basin in the central Philippines is likely distributed.

Description: Abstract Contractional deformation in the transition between the Iberian and Catalan Coastal Ranges (Linking Zone) generated both thin‐skinned structures detached in low‐strength Triassic units, and basement‐involved structures. To evaluate their extent and relative contribution to the overall structure, we carried out a study combining structural geology and gravimetry. New gravity data (938 stations) and density determinations (827 samples) were acquired and combined with previous existing databases to obtain Bouguer anomaly and residual Bouguer anomaly maps of the study area. Seven serial and balanced cross sections were constructed, their depth geometries being constrained through the 2.5D gravity modelling and the 3D gravity inversion that we accomplished. The residual Bouguer anomaly map shows a good correlation between basement antiforms and gravity highs whereas negative anomalies mostly correspond to (i) Meso‐Cenozoic synclines and (ii) Neogene‐Quaternary basins. Cross sections depict a southern, thick‐skinned domain where extensional, basement faults inherited from Late Jurassic‐Early Cretaceous times were inverted during the Cenozoic. To the North, we interpret the existence of both Triassic‐detached and basement‐involved deformation domains. The two deformation styles are vertically overlapped in the southernmost part of the Catalan Coastal Ranges but relay both across and along strike in the Eastern Iberian Range. These basement and cover relationships and their along‐strike variations are analyzed in terms of the interplay between structural inheritance, its obliquity to the shortening direction and the continuity and effectiveness of Triassic décollements in the study area.

Description: Abstract I suggest that the Earth Sciences in the mid‐1950's entered a state of supercooling where the smallest input could lead to the simultaneous crystallization of new ideas. I joined in 1959 the Lamont Geological Observatory, one of the hotbeds where the Plate Tectonic revolution germinated. This paper is not an exhaustive history from an unbiased outside observer. It is a report of one of the participants who interacted with quite a few of the main actors of this revolution and who, fifty years later, revisits these extraordinary times. I emphasize the state of confusion and contradiction but also of extraordinary excitement in which we, earth scientists, lived at this time. I will identify several cases of what I consider to be simultaneous appearances of new ideas and will describe what now appear to be incomprehensible failures to jump on apparently obvious conclusions, based on my own experience.

Description: Abstract Exhumation and landscape evolution along strike‐slip fault systems reflect tectonic processes that accommodate and partition deformation in orogenic settings. We present seventeen new apatite (U‐Th)/He (He), zircon He, apatite fission‐track (FT), and zircon FT dates from the eastern Denali fault zone (EDFZ) that bounds the Kluane Ranges in Yukon, Canada. The dates elucidate patterns of deformation along the EDFZ. Mean apatite He, apatite FT, zircon He, and zircon FT sample dates range from ~26‐4 Ma, ~110‐12 Ma, ~94‐28 Ma, and ~137‐83 Ma, respectively. A new zircon U‐Pb date of 113.9 ± 1.7 Ma (2σ) complements existing geochronology and aids in interpretation of low‐temperature thermochronometry data patterns. Samples ≤2 km southwest of the EDFZ trace yield the youngest thermochronometry dates. Multi‐method thermochronometry, zircon He date‐effective U patterns, and thermal history modeling reveal rapid cooling ~95‐75 Ma, slow cooling ~75‐30 Ma, and renewed rapid cooling ~30 Ma‐present. The magnitude of net surface uplift constrained by published paleobotanical data, exhumation, and total surface uplift from ~30 Ma‐present are ~1 km, ~2‐6 km, and ~1‐7 km, respectively. Exhumation is highest closest to the EDFZ trace, but substantially lower than reported for the central Denali fault zone. We infer exhumation and elevation changes associated with ~95‐75 Ma terrane accretion and EDFZ activity, relief degradation from ~75‐30 Ma, and ~30 Ma‐present exhumation and surface uplift as a response to flat‐slab subduction and transpressional deformation. Integrated results reveal new constraints on landscape evolution within the Kluane Ranges directly tied to the EDFZ during the last ~100 Myr.

Description: Abstract Volcanic‐Intrusive complexes often formed along lateral slab‐edges as a consequence of subduction‐induced mantle flow. We investigate this process in the southern Tyrrhenian Sea by integrating multibeam bathymetry, seismic‐reflection data, regional magnetic anomalies data, and seismological data. The interpretation of the data highlights the presence of magmatic intrusions that locally reach the seafloor forming volcanic edifices. Chimneys, lava flows, and laccoliths are observed beneath and surrounding the volcanoes. The emplacement and cooling of the magma occurred during the Brunhes Chron. The volcanoes are not active even if the hydrothermal activity occurs. The volcanic‐intrusive complex can be subdivided in a western domain (Diamante and Enotrio seamounts) where strike‐slip transpressional faults deform the volcanic edifices, and an eastern domain (Ovidio volcanic seamounts) characterized by flat‐topped volcanic edifices. The flat‐topped morphology is the result of the interplay between volcanism, erosion, sedimentation and sea‐level change. The Ovidio volcanic seamounts formed in an area that experienced at least 60 m of subsidence. Magnetic signatures over the northern side of the Ovidio and Diamante seamounts highlight the presence of a deep‐rooted, magnetized feeding system remnant. Volcanic edifices extend above a magma feeding system, characterized by low Vp/Vs ratios. The Diamante‐Enotrio‐Ovidio volcanic‐intrusive complex formed as a consequence of the ascent of subduction‐induced mantle flow originated in the northern‐western edge of the retreating Ionian slab. We speculate that the magma ascent was controlled by a strike‐slip deformation belt, which accommodated the bulk of the shear strain resulting from the formation of a roughly E‐W trending, Subduction‐Transform Edges Propagator fault.

Description: Abstract The Tien Shan accommodates a significant portion of the India‐Eurasia N‐S convergence. In its northern part a zig‐zag pattern of mountain ranges bounds the western Ili Basin. The role of this basin in the overall shortening and the regional kinematics is not well understood. Geodetic data and instrumental seismicity are not sufficient to infer the role of individual faults and fault systems. We analyze GPS data and earthquake slip vectors and present the results of fault mapping based on remote sensing and field campaigns in the western Ili Basin. These observations indicate that E‐W thrust faults are active at the basin margins, and oblique and strike‐slip faults, both in the basin and in the Paleozoic rocks within the mountain ranges, have been active in the Late Quaternary. We propose a regional tectonic model in which the left‐lateral strike‐slip faults at the NW margin of the basin accommodate ~3 mm/yr NE‐SW shear. Smaller right‐lateral oblique faults transfer the motion in between the left‐lateral faults, and also take up shortening by rotations about vertical axes. We see the onset of internal deformation within the Ili Basin, although it has a strong basement. Our kinematic model is consistent with geodetic data, earthquake seismology, historical, and pre‐historical surface faulting, and describes the first‐order features of active deformation that can be observed in the Northern Tien Shan. Our study illustrates the importance of combining these different data sets to understand the regional tectonics.

Description: Abstract Assessing the seismic hazard of a fault is usually based on its record of strong earthquakes. Earthquake records with long periods of quiescence for active faults can lead to underestimates of seismic hazards, such as for the Longmenshan fault zone which produced the unanticipated 2008 Mw 7.9 Wenchuan earthquake. The Daliangshan fault zone has a low slip rate and has not produced any strong earthquakes in history. As a result, little is known about its paleoearthquake history, including the behavior of any strong earthquakes it might produce and the seismic hazards posed by the Daliangshan fault zone. To solve this problem, we excavated four trenches across the Jiaojihe and Butuo faults along the southern segment of the Daliangshan fault zone. The paleoseismic investigations revealed six paleoearthquakes on the Jiaojihe fault in ~20000 years, and determined another seven rupturing events on the Butuo fault in ~42000 years. The strong earthquake history of the Jiaojihe fault has evidence of temporal clustering, while the Butuo fault exhibits a relatively periodic recurrence pattern with intervals of 1710‐2460 years. Based on its surface rupture length and the magnitude of observed displacement, the southern segment of the Daliangshan fault zone is capable of producing M 〉6.5 earthquakes. Furthermore, based on their respective slip rates and the elapsed times since the most recent events along the Jiaojihe and Butuo faults, they have accumulated seismic energy equivalent to M ~7.6, suggesting they pose a significant seismic hazard to the southeastern Tibetan Plateau.

Description: Abstract Madagascar is a key area for unraveling the geodynamic evolution of the transition between the Rodinia and Gondwana supercontinents as it contains several suites of c. 850–700 Ma magmatic rocks that have been postulated to correlate with other Rodinian terranes. The Bemarivo Domain of northern Madagascar contains the youngest of these units that date to c. 750–700 Ma. We present zircon Hf and O isotope data to understand northern Madagascar's place in the Neoproterozoic plate tectonic reconfiguration. We demonstrate that the northern component of the Bemarivo Domain is distinct from the southern part of the Bemarivo Domain and have therefore assigned new names—the Bobakindro Terrane and Marojejy Terrane, respectively. Magmatic rocks of the Marojejy Terrane and Anaboriana Belt are characterized by evolved εHf(t) signatures and a range of δ18O values, similar to the Imorona‐Itsindro Suite of central Madagascar. These magmatic suites likely formed together in the same long‐lived volcanic arc. In contrast, the Bobakindro Terrane contains juvenile εHf(t) and mantle‐like δ18O values, with no probable link to the rest of Madagascar. We propose that the Bobakindro Terrane formed in a juvenile arc system that included the Seychelles, the Malani Igneous Suite of northwest India, Oman, and the Yangtze Belt of south China, which at the time were all outboard from continental India and south China. The final assembly of northern Madagascar and amalgamation of the Bobakindro Terrane and Marojejy Terrane occurred along the Antsaba subduction zone, with collision occurring at c. 540 Ma.

Description: Abstract During the terminal stages of Wilson cycles, remnants of magma‐poor margins may be incorporated into the orogens, whereas the magma‐rich margins often are lost in subduction due to low buoyancy. The understanding of magma‐rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma‐rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dike Complex was emplaced into a sedimentary basin during the initial breakup and opening of the Iapetus Ocean 615 to 590 million years ago. The dike complex constitutes 70–90% of the magma‐rich, syn‐rift basins and is locally well preserved despite the complex Caledonian history. This contribution provides new observations about the geometry, relative timing, and development of the margin. Jadeite‐in‐clinopyroxene geothermobarometry, titanium‐in‐biotite geothermometry, and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dike complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to approximately 700 °C. In the northernmost part of the study area, U‐Pb dating of magmatic zircon shows that partial melting of the sedimentary host rock, at relatively shallow levels, occurred at 612 Ma. This shows that the crust was molten already 6 million years before the northernmost dike swarm was emplaced at 605.7 ± 1.8 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradients and introduction of mafic melt in the lower crust. These processes significantly reduced the strength of the crust, eventually facilitating continental breakup.

Description: Abstract The Anatolian block preserves remnants of Tethyan oceanic basins consumed by north dipping subduction zones until the Late Cretaceous prior to Paleogene collision. The Sivas Basin, which belongs to the Maastrichtian to Cenozoic Central Anatolian basins, is located in a key position limited to the north and the south by respectively the Pontides and Tauride ophiolitic bodies and to the west by the Kırşehir block. This study focuses on the southern margin of the Sivas Basin, where an obducted ophiolite is capped by Maastrichtian‐Paleocene sediments. We present new field observations, with U‐Pb zircon dating on magmatic rocks and geochemical analyses to (1) unravel the pre‐obduction nature and origin of the ophiolitic basement and (2) describe the post‐obduction tectonosedimentary evolution. The pre‐obduction evolution shows that (i) the Sivas ophiolite is characterized by serpentinized peridotites, with minor magmatic intrusions, (ii) the top of the serpentinized mantle is characterized by a cataclastic deformation with ophicalcites interpreted as an extensional detachment fault, (iii) the U‐Pb zircon dating of two magmatic intrusions yield age of 91.49 ± 0.8 Ma and 72.7 ± 0.5 Ma, and (iv) petrological and geochemical data show that the magmatic intrusions were affected by hydrothermal metamorphism. These data suggest that the Sivas ophiolite may have recorded forearc hyperextension in frame of a Late Cretaceous suprasubduction zone. The post‐obduction evolution is characterized by the deposition of a Maastrichtian‐Paleocene carbonate platform on the ophiolite, followed by clastic sediments containing reworked ophiolitic and Tauride Mesozoic clasts.

Description: Abstract Integrated 2‐D and 3‐D seismic data sets reveal that a WNW trending complex anticlinal belt (the Tazhong Uplift) was cut roughly perpendicularly by a series of NNE oriented strike‐slip faults in the central Tarim Basin, NW China. Through detailed interpretation of the internal architecture of the Tazhong Uplift and the structural characteristics of the strike‐slip faults, the timing of their movements was determined. Based upon their geometric relationship and coeval movement, we propose an integrated model for the evolution of both the Tazhong Uplift and the strike‐slip fault system. Initially, the Tazhong Uplift formed perpendicular to the NNE oriented compression that responded to the collision between the Tarim block and the western Kunlun terrane in the latest Middle Ordovician. Meanwhile, regional joints formed parallel to the maximum compression, that is, perpendicular to the Tazhong Uplift. In the latest Ordovician, northward compression caused the uplifting climax of the Tazhong Uplift in response to the collision between the Tarim block and the coherent eastern Kunlun‐Altyn Tagh‐Qaidam‐Qilian terrane. Under such a northward compression, the resolved shear stress on the NNE trending joints formed the transpressional strike‐slip faults. Afterward, the Tazhong Uplift tilted northward during the ongoing subduction from the Kunlun Ocean, which lasted until the latest Middle Devonian. The negative flower structures formed along the preexisting transpressional strike‐slip faults due to the dragging of the overlying covers in the latest Middle Devonian. The data and evolution model not only unveil the formation mechanism of the Tazhong Uplift and the associated strike‐slip faults but also provide further constraints on the evolutionary age of the Proto‐Tethys Ocean.

Description: Abstract We investigate the crustal structure of the Dangerous Ground (South China Sea) through processing and interpretation of coincident wide‐angle reflection and refraction seismic data. Continental crust of Dangerous Ground has been moderately thinned, down to 15 km, so that most of the structures accompanying the early opening of the South China Sea from Cretaceous to Miocene have been preserved. Subbasement reflectors as well as refraction velocities image an interpreted dismantled Mesozoic metamorphic unit in the southernmost section of our study area. A rollover structure indicates that the reflective base of the unit was used as a décollement where low‐angle normal faults root and blocks rafted. The metamorphic unit is discontinued in a nearby basin located immediately to the north, where the refraction velocity model shows thinning of the crust from 20 to 15 km, with the presence of a 5‐km‐high mantle dome. In this deeper basin, mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an ~15 km offset crustal detachment surface that we link down to the mantle dome. We infer that the detachment reactivated an inherited low‐angle contact most probably related to the Yanshanian belt. In map view, the reactivated structure forms a half‐graben basin oriented NNE‐SSW oblique to the generally accepted direction of extension. This orientation follows the general trend of a granitic belt that spanned the South China margin prior to extension, related to the subduction of the Paleo‐Pacific.

Description: Abstract Obduction is the tectonic process that results from thrusting of an oceanic lithosphere section (ophiolite) onto a continent. In contrast, thrusting of subcontinental mantle, observed in several mountain belts, remains a major unknown of plate tectonics. In the western Mediterranean, the Ronda and Beni Bousera peridotites are the largest worldwide subcontinental mantle exposure. From a geological point of view, the Ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back‐arc immediately followed by thrusting, explaining their present‐day position inside the Alboran crust. Using 2‐D and 3‐D modeling of new gravimetric data combined with local seismic tomography, we show that the Ronda peridotites are rooted inside the Alboran mantle along the entire Gibraltar arc. On these bases, we propose that the emplacement of the Ronda peridotites occurred in a back‐arc setting and corresponds to the thrusting of an entire hyper‐stretched continental margin onto a continent, a process that we define as continental margin obduction. This results from two successive deformation events: continental upper plate extension driven by slab roll‐back, immediately followed by upper plate shortening, likely triggered when a buoyant continental domain enters the subduction. We propose that this process affected the entire western Alboran domain, with peridotite bodies embedded within the crust along the whole Gibraltar Arc. We suggest that other examples such as the Alps Ivrea mantle body could likely represent continental margin obduction at the onset of continental collision.

Description: Abstract Curvature of fold‐and‐thrust belts (FTBs) is a rather common feature in foreland basins. The problem of how curves in FTBs originate is essential for understanding the propagation of deformation and tectonic history of orogens. In this study, we carried out systematic paleomagnetic studies in the westernmost part of the Qiulitage FTB, southern Tian Shan foreland, where thrusting, strike‐slip fault, and tectonic boundary coexist. Our new results suggest that the study area has been subjected to ~20° clockwise rotations after ~5 Ma. Oroclinal test of the paleomagnetic data across the FTB suggests that oroclinal bending caused by the formation of the curved Qiulitage FTB is the dominate reason for these tectonic rotations. The Kalayuergun dextral strike‐slip transfer fault delimiting the western boundary of the Qiulitage FTB is a thin‐skinned structure accommodating the discrepancy in horizontal displacement on both sides of it. These results also suggest that the formation of the curved Qiulitage FTB should not be older than ~5 Ma, indicating that the southern Tian Shan foreland has experienced significant tectonic shortening since the latest Miocene to early Pliocene. Our new results, together with previous studies on deformation history, estimates of crustal shortening, GPS observations and earthquake records, suggest that the southern Tian Shan foreland has been subjected to significant deformation during the past ~5 million years, and the crustal shortening is still ongoing.

Description: Abstract We have investigated subduction‐exhumation processes in the Pelagonian zone, exposed on the Northern Sporades islands (Aegean Sea) related to successive episodes of ocean continent and continent‐continent convergence through integrating multiscale structural analysis, metamorphic petrology, and white mica 40Ar/39Ar dating. Two major progressive phases of ductile deformation are documented, which are related to distinct episodes of tectonic burial and exhumation of the Pelagonian continental margin, which was facing the Neotethys/Vardar ocean. Review of existing data sets from neighboring regions shows that both deformation phases can be correlated along strike up to the Dinarides. The first phase of tectonic burial and exhumation (D1) is characterized by NW‐SE tectonic transport, greenschist facies metamorphism, and Early Cretaceous (~105‐135Ma) 40Ar/39 Ar white mica single fusion ages. D1 is correlated with the initial closure of the Vardar ocean by top‐to‐the‐W to NW ophiolite obduction and the underthrusting of the Pelagonian margin below the oceanic upper plate. Underthrusting was followed by exhumation and the deposition of Late Cretaceous‐Paleogene sediments. The second phase of burial and exhumation (D2) is characterized by NE‐SW tectonic transport, greenschist to blueschist facies metamorphism, and latest Cretaceous‐Early Eocene 40Ar/39 Ar white mica ages of S2 fabrics. Top‐to‐the‐SW shearing is correlated with the tectonic burial of the Pelagonian zone below the Eurasian continent (Rhodopia), while top‐to‐the‐NE shearing is attributed to subsequent extensional exhumation. D2 fabrics record low‐grade P‐T conditions suggesting that the decoupled cover formations exposed on Skopelos were incorporated in an accretionary wedge that formed above the subducting Pelagonian basement during Paleogene times.

Description: Abstract The Menderes Massif, Turkey, is a type locality for deciphering the plate tectonic response from collision‐ to extension‐driven exhumation. Conventional thermobarometry and garnet pressure‐temperature (P‐T) paths from isochemical phase diagrams were calculated across a major fault (Selimiye Shear Zone, SSZ) bounding the southern edge of the Menderes Massif. Both approaches yield similar garnet rim temperatures (from 555 to 671 °C), but estimated P differs by between 8 and 15 kbar. Three garnets north of the SSZ reveal N‐shaped P‐T paths, whereas paths from three samples south of the SSZ show a simple increase in P‐T. Monazite and zircon were dated in thin section from the same rocks using Secondary Ion Mass Spectrometry and Laser Ablation Inductively Coupled Plasma‐Mass Spectrometry, respectively. Textural relationships of monazite within garnet appears indicative of post‐garnet growth. The amount of monazite common 204Pb and 137Ba+/Th+ significantly exceeds what is observed for the monazite age standard, suggesting their ages mark fluid‐driven events, loosely constrained to Late Eocene‐Early Miocene. Some zircon ages are consistent with Cambro‐Ordovician ages reported elsewhere in the region, and other ages are Neoproterozoic and Permian‐Triassic, a period not previously recognized in this area. Despite the lack of age constraints for the duration of garnet growth, we present a thermal model to understand the meaning of the N‐shaped path. These paths are best reproduced by thermal models incorporating SSZ thrusting before and after denudation. This paper presents an example of the insight from high‐resolution P‐T paths, and an example of denudation within a prograde metamorphic event.

Description: Abstract Orogenesis in the Variscan belt of Western Europe was followed by a major magmatic event during the Permian that formed a mafic lower crust by crystallising pyroxenite and gabbros from mantle‐derived melts at the base of the continental crust. Partial melting of the asthenosphere left a significantly depleted mantle that was progressively incorporated into the subcontinental lithospheric mantle as the orogenic domain cooled. The potential impact of such large‐scale thermal and lithologic layering has never been taken into account in the study of the Alpine Tethys and North Atlantic rift systems that developed subsequently in Western Europe. Here we investigate via numerical modeling how a mafic heterogeneity within the lowermost part of a quartzo‐feldspathic continental crust and/or a zone of depleted mantle within the lithospheric mantle beneath a former orogenic domain could have influenced subsequent rifting. Our numerical modeling results indicate that, in a thermally equilibrated lithosphere, a mafic body within the lower continental crust or a zone of shallow depleted mantle prevents rifting of overlying weaknesses (e.g. faults, suture zones). We propose that the regional mafic lower crust beneath the Variscan orogenic domain may explain why the Tethyan and southern North‐Atlantic rift systems did not localize at former suture zones of the Western European orogenic lithosphere.

Description: Abstract Global deep‐time plate motion models have traditionally followed a classical rigid plate approach, even though plate deformation is known to be significant. Here we present a global Mesozoic–Cenozoic deforming plate motion model that captures the progressive extension of all continental margins since the initiation of rifting within Pangea at ~240 Ma. The model also includes major failed continental rifts and compressional deformation along collision zones. The outlines and timing of regional deformation episodes are reconstructed from a wealth of published regional tectonic models and associated geological and geophysical data. We reconstruct absolute plate motions in a mantle reference frame with a joint global inversion using hot spot tracks for the last 80 million years and minimizing global trench migration velocities and net lithospheric rotation. In our optimized model, net rotation is consistently below 0.2°/Myr, and trench migration scatter is substantially reduced. Distributed plate deformation reaches a Mesozoic peak of 30 × 106 km2 in the Late Jurassic (~160–155 Ma), driven by a vast network of rift systems. After a mid‐Cretaceous drop in deformation, it reaches a high of 48 x 106 km2 in the Late Eocene (~35 Ma), driven by the progressive growth of plate collisions and the formation of new rift systems. About a third of the continental crustal area has been deformed since 240 Ma, partitioned roughly into 65% extension and 35% compression. This community plate model provides a framework for building detailed regional deforming plate networks and form a constraint for models of basin evolution and the plate‐mantle system.

Description: Abstract We have modeled the propagation of the perturbation induced by the 30 October 2016 Norcia (Central Italy) earthquake by a two‐dimensional finite element procedure based on strain diffusion in the crust‐mantle system. This approach simulates the excitation of an elastic‐viscous medium by the coseismic slip related to the above seismic event. We have analyzed the temporal evolution of the expected planar strain and strain rate at selected points of the model, located near active fault systems of the Central and Northern Apennines. Moreover, the strain style (i.e., the ratio of the principal axes of the planar strain tensor) computed at each selected point is compared with available tectonic and seismological information concerning the active strain regime. Finally, we discuss the possible influence of postseismic strain perturbation on major fault systems of the Central and Northern Apennines.

Description: Abstract The 2008 Wenchuan earthquake (Mw 7.9) initiated at the southern part of the Yingxiu‐Beichuan fault of the Longmen Shan fault zone, eastern Tibet Plateau. The earthquake rupture propagated northeast for ~300 km, producing dextral‐thrust slip along the fault zone. Numerous studies have documented the characteristics of the rupture zone, however, the exact endpoints and the termination structures at both ends remain poorly constrained. On the northeastern‐most rupture section, the aftershock distribution extends ~50 km along strike past the end of the observed surface rupture, suggesting that the rupture may extend northeastward at depth. Our field observations show that the aftershock zone follows a NE‐trending linear trace indicative of an active fault at the surface, and confirm that the Qingchuan fault, Which is one of the faults of the northeastern Longmen Shan fault zone and is reported to have ruptured during the 2008 earthquake by some studies, did not rupture during the Wenchuan earthquake. Detailed investigations show that the rupture terminated at an area between the Qingchuan and the Bahai‐Yuquanba faults where these two faults bound a 10‐to‐20‐km‐wide mountain block. To the southwest, the southwestern‐most end of the Wenchuan earthquake rupture zone terminated south of Sanjiang, where a NE‐trending 50‐cm‐high fault scarp marks the endpoint of the surface rupture. Our study suggests that fault step‐overs at the northeastern and southwestern segments may have acted as a barrier to rupture propagation and thus play an important role in controlling the rupture propagation, but were not critical in terminating the rupture, whereas the cross structures and/or stratigraphy (complex or massifs) in the northeastern and southwestern rupture endpoints area appear to have controlled the rupture termination.

Description: Abstract This study investigates the influence of the 3‐D geometry of a down‐going plate, the rheological structure of the upper plate, and the migration of the overriding plate toward the trench in relation to the overall subduction velocity on the exhumation pattern in orogen syntaxes. Using a thermomechanical numerical code (DOUAR), we analyze the strain localization, rock uplift, and exhumation response of a rheologically stratified continental lithosphere to subduction of a convex‐upward‐shaped indenter. The models consider three thermorheological lithospheric profiles that determine the degree of mechanical coupling between the upper crust and lithospheric mantle. These models include a strong, cratonic lithosphere; a weaker, younger (and hotter) continental plate; and an intermediate case. The strongly coupled case predicts a localization of high rock uplift rates along narrow linear bands crossing the entire model domain parallel to the trench. In contrast, in a weakly coupled lithosphere, rock uplift is concentrated within a curved ellipse region of anomalously high exhumation rates located above the indenter apex. The aspect ratio of the localized area of rapid rock uplift is controlled by the initial width of the rigid indenter and the relationship between boundary velocities. In particular, the combination of little or no upper plate migration with a narrow indenter causes a nearly circular region (~100‐km diameter) of rapid exhumation that resembles the pattern of thermochronometer ages observed in orogen syntaxes such as the Southeast Alaska and the Olympic Mountains of the Cascadia subduction zone (western USA).

Description: Abstract The kinematic and exhumational evolution of the Lesser Himalaya (LH) remains a topic of debate. In NW India, the stratigraphically diverse LH is separated into the inner LH (iLH) of late Paleo‐Mesoproterozoic rocks and the outer LH (oLH) of Cryogenian to Cambrian rocks. Contradictory models regarding the age and structural affinity of the Tons thrust—a prominent structure bounding the oLH and iLH—are grounded in conflicting positions of the oLH prior to Himalayan orogenesis. This study presents new zircon (U‐Th)/He and U‐Pb ages from the thrust belt and foreland basin of NW India that refine the kinematic and exhumational evolution of the LH. Combined cooling ages and foreland provenance data support emplacement and unroofing of the oLH via southward in‐sequence propagation of the Tons thrust by middle Miocene time. This requires that, before India–Asia collision, the oLH was positioned as the southernmost succession of Neoproterozoic–Cambrian strata along the north Indian margin. This is further supported by detrital zircon U‐Pb ages from Cretaceous–Paleogene strata (Singtali Formation) unconformably overlying the oLH, which yield diagnostic Cretaceous detrital zircons correlative with coeval strata in the frontal Himalaya of Nepal. A pulse of rapid exhumation along the Tons thrust front at ~16 Ma was followed by east‐to‐west development of a midcrustal ramp at ~12 Ma which facilitated diachronous iLH duplexing. This duplexing shifted the locus of maximum exhumation northward, eroding away Main Central Thrust hanging wall rocks until the iLH breached the surface at ~9–11 Ma near Nepal and by ~3–7 Ma within the Kullu‐Rampur window.

Description: Abstract We report, for the first time, evidence of seismically induced soft‐sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine‐transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1‐km‐long tunnel in the Pozzuoli area. The successions host several soft‐sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55‐ to 4.28‐kyr BP interval. The volcano‐sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors; from the base progressing upward we recognize (1) a massive tuff, (2) marine‐transitional sands of the La Starza unit, and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano‐sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic, and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft‐sediment deformation within the volcano‐sedimentary record suggests that moderate earthquakes could occur during future volcano‐seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises.

Description: Abstract The Maidan fault, which is an east‐northeast‐trending fault in the southwestern Tian Shan, is a sinistral reverse fault that extends more than 400 km in length and constitutes the boundary between the southwestern Tian Shan and the Tarim Basin. Here, we quantify its late Quaternary activity based on the interpretations of high‐resolution remote sensing images and detailed field investigations. In the Aheqi valley, an ~150‐km‐long active fault can be divided into northeastern and southwestern segments based on variations in its strike and geometry. Based on the analysis of its offset geomorphological features and the dating of Quaternary sediments, we estimate the late Quaternary shortening rate across the fault to be 1.19 ± 0.25 mm/yr, the sinistral strike‐slip rate to be 1.56 ± 0.64 mm/yr, and the oblique thrust rate to be 1.96 ± 0.69 mm/yr. Active tectonics, GPS crustal deformation data, and seismic activity indicate that the deformation in the southwestern Tian Shan is characterized by out‐of‐sequence thrust faulting and folding. Late Quaternary deformation has been partitioned into low‐angle thrust faulting along the Kalpin Tagh foreland fold and thrust system and sinistral reverse faulting along the high‐angle range‐front Maidan fault. The sinistral Maidan fault acts as a nucleation point for slip partitioning system, which can be viewed as positive flower structure with its surrounding thrust faults.

Description: ABSTRACT The Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists of an arched footwall of foliated crystalline rocks bounded above by the generally outward‐dipping, Oligocene‐Miocene San Pedro extensional detachment fault. The southwest‐trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation and inferred top‐southwest displacement on the fault. An upper‐plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an under‐appreciated influence on extensional tectonics in cratonic southwestern North America.

Description: Abstract The rupture behavior of large‐scale thrust faults has become an increasingly important topic in active tectonic and seismic studies. This is especially true for the 2008 Wenchuan earthquake (Mw 7.9), which occurred along the Longmenshan thrust fault (LMS) in China. While this earthquake has been the subject of many studies to date, paleoseismic studies along the LMS are still lacking. In this study, results from trenching at the town of Bailu, as well as radiocarbon and Optical‐Stimulated Luminescence dating, are used to constrain the timing of the penultimate and earlier events, respectively: 3.3±0.6 to 4.4±0.6 ka and 4.8±0.6 to 5.8±0.8 ka. Further, coseismic displacement produced by the two events is similar to that produced by the 2008 event. The results suggest that fault segments along the Yingxiu‐Beichuan and Jiangyou‐Guanxian faults likely ruptured coseismically with similar offsets and that the northern segment of the LMS possesses independent rupture behavior. The magnitude of the penultimate seismic event was likely lower than that of the 2008 event, which indicates a characteristic slip‐rupture pattern along the LMS.

Description: Abstract The Turkana Depression (TD) is a NW‐trending topographic corridor within the East African Rift System (EARS) between the Ethiopia – Yemen plateau in the northeast and the East African plateau. The Anza rift within the TD is a NW‐trending failed arm of a late Jurassic rift‐rift‐rift triple junction. This rift has been correlated with the Sudan and South Sudan rifts. The Anza rift is intersected by the EARS represented by the N‐S trending Turkana rifted zone. We imaged the lithospheric structure beneath the TD using satellite gravity data. We also used these data to model crustal density distribution beneath the Kino Sogo fault belt; part of the Turkana rifted zone. Our results show thinner crust (23 km ‐ 28 km) and lithosphere (140 km ‐ 150 km) beneath the TD. We interpret this thinning as due to tectonic extension that resulted in the formation of the Kenya – Sudan and South Sudan rifts. Our results also show that crustal depth between 0 km and 4.8 km is dominated by N‐S density contrast anomalies and between 4.8 km and 14.5 km by E‐W anomalies. We interpret the N‐S anomalies as due to the presence of Precambrian structure that might have facilitated strain localization during the initiation of the Kino Sogo fault belt. Differently, we interpret the E‐W anomalies as due to the presence E‐W trending faults that were formed in association with the development of the Anza rift and/or Turkana rifted zone and were later filled with Cenozoic and Mesozoic mafic dikes.

Description: Abstract The Yanshan fold‐thrust belt is a significant intraplate deformation belt along the northern margin of North China Craton, and records multiple phases of deformation during the Mesozoic. However, the specific tectonic evolution of the Yanshan belt during the Mesozoic has long been debated. In particular, many arguments exit regarding whether a contractional or extensional context dominated the Yanshan belt in the Late Jurassic‐Early Cretaceous. One reason for this argument is the poor understanding of the tectono‐sedimentary relationship preserved in the sedimentary records. A lack of evidence of syntectonic sediments makes it more difficult to reconstruct this relationship. The Chicheng basin in the western Yanshan belt contains complete Upper Jurassic‐Lower Cretaceous succession and is bounded by marginal faults. New regional geological mapping, detailed sedimentary facies analysis, and recognition of two growth strata packages help reconstruct the tectono‐sedimentary relationship in the Chicheng basin. Additionally, zircon U‐Pb dating provides constraints on the timing of the deformation. Our results suggest that the dynamic sediment dispersal system in the Chicheng basin during the Late Jurassic‐Early Cretaceous were controlled by two phases of thrusting, including the activities of the Shangyi‐Pingquan fault during 162‐147 Ma and the Fengning‐Longhua fault during 140‐135 Ma. Therefore, we propose that the western Yanshan belt was in a contractional context during the Late Jurassic‐Early Cretaceous. Our findings indicate that the tectonic setting of the Yanshan belt experienced a prominent transition from E‐W‐trending structures to NE‐SW‐trending structures and that this transition was probably related to subduction of the Paleo‐Pacific plate beneath the East Asian continent.

Description: Abstract The age and distribution of the syn‐rift and early post‐rift infill records the spatial and temporal distribution of extension and breakup processes in a rift basin. The Eastern Black Sea Basin (EBSB) is thought to have formed by back‐arc extension during Cretaceous to Early Cenozoic time. However, a lack of direct constraints on its deep stratigraphy leaves uncertainties over the time, duration and location for rifting and breakup processes in the basin. Here we use the enhanced imaging provided by 2D long‐offset seismic reflection profiles to analyze the deep structural and stratigraphic elements of the EBSB. Based on these elements, we infer the presence of two distinct Late Cretaceous syn‐rift units, recording initial extension (rift stage 1) over the continental highs (Shatsky Ridge and the Mid Black Sea High), followed by strain localization along the major basin‐bounding faults and rift migration towards the basin axis (rift stage 2). Overlying these units, Palaeocene(?)‐Eocene and Oligocene units show a syn‐kinematic character in the NW, with evidence for ongoing extension until Oligocene time. Towards the SE, these sequences are instead post‐kinematic, directly overlaying a basement emplaced during breakup. We interpret the Palaeocene(?)‐Oligocene units to record the time spanning from the initiation of breakup (Late Cretaceous‐Palaeocene, in the SE) to the end of extension (Oligocene, in the NW). The first ubiquitously post‐rift infill is the Lower Miocene Maykop Formation. Our results highlight the along‐strike temporal variability of extension and breakup processes in the EBSB.

Description: Abstract The Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back‐arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono‐metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new LA‐ICP‐MS bedrock and detrital zircon (DZ) U‐Pb ages that place robust constraints on the pre‐subduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U‐Pb ages of crystalline CB are ~306‐330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo‐Tethys subduction. The plutons intruded the CB metasedimentary host‐rock sequence, characterized by distinct Gondwanan DZ U‐Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and syn‐magmatic, contact metamorphic zircon rims (~300‐330 Ma). DZ U‐Pb dating revealed post‐magmatic Permian metasedimentary rocks (~270‐295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U‐Pb results clarify the late Paleozoic‐early Mesozoic evolution of the CB as a peri‐Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction‐related juxtaposition within the CB metasedimentary sequence.

Description: Abstract The Cretan Detachment represents a major tectonic contact between an Alpine high‐pressure Lower Nappe System in the footwall and an Alpine unmetamorphosed Upper Nappe System in the hanging wall. Interestingly, the kinematics and the tectonic nature of this contact is still highly controversial and has been interpreted either as a top‐to‐the S thrust or as a normal fault with top‐to‐the N and/or top‐to‐the S displacement sense. Whereas some models suggest exhumation of the high‐pressure rocks synchronous with out‐of‐sequence thrusting or Himalayan‐type extrusion tectonics, other models propose that Crete represents a metamorphic core complex that exhumed below the extensional Cretan Detachment. In this work, we show that the Cretan Detachment in eastern Crete is an up to 100 m thick cataclastic low‐angle fault zone, which localized below the Tripolitza Unit (Upper Nappe System) within the upper part of the Upper Violet Slates (top of Lower Nappe System) cutting down‐section to lower structural levels. The unequivocal kinematic indicators in this fault zone clearly suggest a top‐to‐the N displacement of the hanging wall. New low‐temperature geochronological data and numerical modeling suggest rapid cooling of the rocks in the footwall of the Cretan Detachment at c. 15 Ma. Top‐to‐the S ductile shear sense towards lower structural level in the Lower Nappe System support the idea that the Cretan Detachment facilitated the exhumation of the high‐pressure rocks in an extrusion tectonic setting. Therefore, in the early to middle Miocene, subduction‐related extrusion in Crete was simultaneously active with back‐arc extension‐related metamorphic core formation in the Cyclades.

Description: Abstract Information on structure, stress, and their inter‐relationship is essential for understanding structurally controlled geothermal permeability. Active fault mapping, borehole image analysis, and well testing in the Te Mihi geothermal area, New Zealand allows us to refine structural and fluid flow architecture of this resource. The Te Mihi area is structurally complex, comprising a set of NW dipping master faults containing pervasive SE dipping antithetic and splay structures in their hangingwalls. These faults are also intersected by E‐W striking faults. A localized, N‐S striking structural trend is also observed at Te Mihi. In consideration with Global Navigation Satellite System velocity vectors, both active NE‐SW, and E‐W striking faults create biaxial extension at Te Mihi, though the observed NE‐SW SHmax direction suggests contemporary extension is NW‐SE dominated. Stress field perturbations coincide with structural complexities like fault splays and intersections, and/or proximity to recently active E‐W and NE‐SW striking structures. Borehole fluid flow at Te Mihi is concentrated at NW dipping master fault intersections, travel time fractures on acoustic image logs, halo fractures on resistivity image logs, NE‐SW and E‐W striking fractures, intervals of high fracture density, and spatial concentrations of wide aperture fractures and recently active NE‐SW and E‐W striking fractures. This study suggests Te Mihi geothermal expression results from biaxial extension evident from active structural trend intersections and the predominance of NE‐SW and E‐W striking structures within permeable well zones. Biaxial extension is therefore an important control on crustal fluid flow within the Taupo Volcanic Zone and thus geothermal resource delineation.

Description: Abstract The Semail Gap/Semail Gap Fault Zone is one of the most prominent tectonic features in the Oman Mountains and is located between the Jabal Akhdar Dome in the west and the Saih Hatat Dome to the east. The two domes were uplifted during the Late Eocene to Early Miocene. Uplift led to monoclinal bending with a “fold axis” parallel to the Semail Gap. Subsequently, dextral transtensional faulting along the Semail Gap Fault Zone ensued. This fault zone follows a major preexisting fault, which explains the distribution differences of Neoproterozoic and Phanerozoic sedimentary formations (partly omissions) across this structure. The vertical and horizontal displacements along the Semail Gap amount to up to ~5.5 and ≤2km, respectively. Vertical displacement may be a combination of monoclinal bending and later faulting along the Semail Gap Fault Zone. Its latest activity mainly preceded the Quaternary because there is only minor faulting of Quaternary deposits. Despite ongoing uplift of the Oman Mountains, which may amount to ≤50m during the Quaternary based on incision of wadi/alluvial terraces, only insignificant seismicity is observed and attributed to minor movements along this fault zone. The transtensional deformation is a result of uplift and orogenic/gravitational collapse of the Jabal Akhdar/Saih Hatat domes. The neighboring two hanging‐wall blocks of each dome moved downward, ESE/SE and SSW/SW, respectively, which explains dextral transtension along the Semail Gap Fault Zone. The Semail Gap Fault Zone has a sinistral transtensional counterpart at the southwestern margin of the Nakhl Subdome/Saih Hatat Dome.

Description: Abstract We analyzed a set of mantle xenoliths from Fernando de Noronha (FN) archipelago to constrain the role of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber‐[100] tendency; these patterns imply dominant activation of [100](010) slip system during dislocation creep. Olivine fiber‐[010] patterns are less common; they probably result from recrystallization or melt‐rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post‐deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid‐Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt‐rock interaction. An early refertilization of the base of the lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization, due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the FN mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.

Description: Abstract Triassic flysch in the Songpan‐Ganzi Complex (SGC), eastern Tibet, is an important and rich record of tectonism associated with evolution and closure of the eastern Paleo‐Tethys Ocean. However, current models for tectonic evolution of the ocean remain controversial, in large part due to ambiguity of the origin of SGC deposits. We constrain provenance of Middle‐Upper Triassic turbidites from central SGC and the adjacent Yidun and West Qinling terranes by using detrital zircon U‐Pb geochronology. The results show that the detrital zircon ages mainly comprise five populations: 240–310 Ma; 400–480 Ma; 750–1,000 Ma; 1,700–2,000 Ma; and 2,300–2,600 Ma. These ages indicate the East Kunlun and North China block served as major sediment sources for most of central SGC and West Qinling. The predominance of Paleozoic zircons in Upper Triassic turbidites indicates uplift and exhumation of the East Kunlun orogen during the Late Triassic. The southern SGC and Yidun turbidites display quite similar zircon age spectra (significant age peaks at approximately 1,850 Ma), implying that Songpan‐Ganzi and Yidun depocenters were adjacent to each other by the Middle Triassic. This finding suggests that the Ganzi‐Litang Ocean, as a part of eastern Paleo‐Tethys Ocean, probably had been closed by the Middle Triassic. We favor subsequent deformation and uplift of the Triassic flysch was coeval with or possibly predated latest Triassic shortening within the Longmenshan thrust belt, resulting in recycling of the flysch into surrounding sedimentary basins (e.g., Sichuan and Qamdo basins) since latest Triassic time.

Description: Abstract The Paleogene Lulehe Formation marks the onset of deposition in the Qaidam basin and preserves evidence of the initial topographic growth of northern Tibet. However, limited outcrops impede understanding of the sedimentary features of the Lulehe Formation as well as the tectonic relationship between the basin and surrounding topography. To fill this gap, we investigated core samples along the basin margin and conducted flexural modeling to estimate the topographic load of the Qilian Shan and Eastern Kunlun Shan during the deposition of the Lulehe Formation. Core samples reveal that the Lulehe Formation mainly consists of distal fluvial to marginal lacustrine deposits and proximal fluvial deposits along the southern margin of the basin while characterized by proximal alluvial fan deposits along the northern margin of the basin. Together with evidence for faulting shown on the seismic profiles, we infer that simultaneous deformation within the Qilian Shan and Altyn Tagh Shan during the Paleogene resulted in accumulation of coarse detrital deposits in the northwestern and northeastern Qaidam basin. The simultaneous deformation within the Altyn Tagh Shan and Qilian Shan since the Paleogene supports the idea that deformation in these two regions is kinematically linked. One‐load and two‐load beam flexural modeling indicate that the topographic load generated by both the Eastern Kunlun Shan and the Qilian Shan is responsible for the subsidence of the Qaidam basin during deposition of the Lulehe Formation. Our results highlight the initial relative high topography in the northern Tibetan plateau during the early Cenozoic.

Description: Abstract We examine the intra‐arc crustal seismicity of the Andean Southern Volcanic Zone. Our aim is to resolve interseismic deformation in an active magmatic arc dominated by both margin‐parallel (Liquiñe‐Ofqui fault system, LOFS) and Andean transverse faults. Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique‐subduction. Here, we present local seismicity based on 16‐month data collected from 34 seismometers monitoring a ~200‐km‐long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between Mw 0.6 and Mw 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike‐slip faulting consistent with ENE‐WSW shortening in line with the long‐term deformation history revealed by structural geology studies. However, we find regional to local‐scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE‐ and WNW‐striking discrete faults, spatially associated with, hitherto seismic Andean transverse faults. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra‐arc crust, where variable strike‐slip faulting dominates over dip‐slip movements.

Description: Abstract Erosion and deposition redistribute mass as a continental rift evolves, which modifies crustal loads and influences subsequent deformation. Surface processes therefore impact both the architecture and the evolution of passive margins. Here we use coupled numerical models to explore the interactions between the surface, crust, and lithosphere. This interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink. If transport is efficient, there are two possible outcomes: (1) Faulting within the zone of extension is longer lived and has larger offsets. This implies a reduction of the number of faults and the width of the proximal domain. (2) Efficient transport of sediment leads to significant deposition and hence thermal blanketing. This will induce a switch from brittle to ductile deformation of the upper crust in the distal domains. The feedbacks between these two outcomes depend on the extension history, the underlying lithospheric rheology, and the influence of submarine deposition on sediment transport. High erosion/sedimentation during early faulting leads to abrupt crustal necking, while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains. In models where rheological conditions favor the formation of asymmetric conjugate margins, only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization. These models suggest that passive margin architecture can be strongly shaped by the solid Earth structure, sea level, and climatic conditions during breakup.

Description: Abstract The Qilian Shan, at the northeastern frontier of the Tibetan Plateau, is a key area for studying the expansion mechanism of the Tibetan Plateau. Although previous thermochronology and paleomagnetic studies indicate Neogene northward expansion of the northern Qilian Shan, there is a distinct temporal gap in knowledge relative to the tectonic history of the southern Qilian Shan. This has hindered a complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan. To study the growth history of the southern Qilian Shan, apatite fission track (AFT) data have been acquired from Zongwulong Shan and the Huaitoutala section. AFT Thermal history modeling from the former shows a rapid cooling episode occurred at ~18‐11 Ma, which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan. Within the Huaitoutala section, detrital grains up‐section show progressively decreasing peak AFT ages followed by an age increase from mid‐section, implying that a sediment‐recycling event occurred at ca. 7±2 Ma. Together with a shift in paleocurrent directions, this change marks the onset of Late Miocene deformation of the northern Qaidam Basin. Combined with previous studies on the deformation time of the Qilian Shan, our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions during the Neogene. This resulted in the formation of a flower structure which had an important impact on the deformation pattern of north Tibet. The synchronous outward expansion may have been triggered by the removal of mantle beneath north Tibet.

Description: Abstract The A newly recognized Upper Cretaceous (~87 Ma) olistostrome belt in Central Turkey west of Ankara extends for more than 112 km subparallel to the Izmir‐Ankara suture with a width of 10 km. The Alacaatlı Olistostromes are stratigraphically underlain by a Triassic basement, and are up to 2‐km thick. Over 80% of the blocks in the olistostromes consist of pelagic limestones, which reach up to 300‐m in size, other blocks include basalt, chert, serpentinite, tuff and sandstone. The limestone blocks are Jurassic and Cretaceous in age with micropaleontology documenting the presence of Callovian‐Oxfordian, Tithonian, Berriasian, Aptian, Albian, Cenomanian and Turonian stages. The flows are separated by intrabasinal sediments of shale, siltstone, volcaniclastic sandstone with Albian (108‐101 Ma) detrital zircons. The olistostromes show minor tectonic deformation, and are unconformably overlain by Santonian pelagic limestones. The deposition of the Alacaatlı Olistostromes was followed by arc magmatism, which started in the Campanian (~78 Ma) after a period of shortening and uplift, and the region became a forearc basin with deposition of shale and volcaniclastic sandstone with Campanian (78‐72 Ma) detrital zircons. A number of peculiar features of these olistostromes, including rapid uplift and erosion before the creation of a deep, short‐lived (89‐86 Ma) ephemeral basin, dominance of deep marine limestone blocks, and inception of arc magmatism ca. 9 Myr after their deposition indicate a major tectonic event involving the disruption of the continental margin prior to the onset of arc magmatism. This event is interpreted as a change from transform margin to subduction.

Description: Abstract The Broadly Rifted Zone (BRZ) of southern Ethiopia is a long‐lived and structurally complex segment of the East African Rift System (EARS). However, due to poor surface exposure of early syn‐rift strata and a dearth of subsurface data, the evolution of the BRZ remains poorly understood. We present new apatite (U‐Th‐Sm)/He and augmented apatite fission track low‐temperature thermochronology data from the Beto and Galana basin boundary fault systems to constrain the tectono‐thermal evolution of the western and eastern BRZ, respectively. Time‐temperature reconstructions suggest that EARS‐related extension began concurrently across the BRZ in the early Miocene (20‐17 Ma), at least 6 Myr prior to faulting in the Main Ethiopian Rift further north. Increased time‐temperature resolution provided by multi‐thermochronometer analyses reveals contrasting along‐strike spatiotemporal variations in Beto and Galana margin cooling histories, which appear to mirror the disparate structural geometries of their basin‐bounding normal fault arrays. Longitudinal contrasts in basin architecture and rift‐related cooling histories across the BRZ may reflect the region's heterogeneous distribution of pre‐existing basement fabrics, namely the presence of a previously reported N‐NNE trending Neoproterozoic suture zone beneath the eastern BRZ. Its influence may explain both the development of long, curvilinear faults and the gradual basin‐wards migration of strain exhibited by the easternmost BRZ, absent further west. The anomalous evolution of the BRZ compared to the greater Ethiopian Rift, both in its earlier onset and its wider deformation zone, likely results from its inheritance of pre‐attenuated lithosphere, thermo‐mechanically modified by earlier Cretaceous‐Paleogene Anza‐South Sudan rifting and/or Eocene plume impingement.

Description: Abstract Tectonic and magmatic activity may couple at volcanic arcs, even though any relationship is less defined in smaller arcs, experiencing limited activity. Here we use gas geochemistry data collected during the 2011‐2012 unrest at Santorini (Greece) to understand better the dynamics of the Aegean Volcanic Arc with regard to its tectonic setting. Since the most recent eruption in 1950 and before the unrest, minor seismicity and CO2 degassing (mainly from the fumaroles of Nea Kameni islet) were observed at Santorini. On January 2011, anomalous seismicity along the NE‐SW trending Kameni Line was accompanied by an inflation north of Nea Kameni. Fumarolic gas composition changed and gas release notably increased. We carried out geochemical study both on Kameni and Thera islands from January 2012 to June 2013. We repeated surveys of diffuse soil CO2 degassing and of in‐soil gas concentration and we analyzed fumaroles and gas dissolved in thermal waters for chemical and isotopic composition. In agreement with previous studies, our geochemical data, particularly the diffuse soil CO2 flux increase, the increase of H2 content and of CO2/CH4 and 3He/4He ratios in fumarolic gases, support geophysical data in indicating that unrest was associated with the emplacement of new mafic magma. This unrest had limited effect on the regional setting, with gas emissions focusing along the regional NE‐SW structures, without triggering by any seismic event, conversely to the 1950 eruption, which probably occurred in a frame of general tectonic reorganization of the Aegean microplate.

Description: Abstract The Lower and Upper Cretaceous turbidites cover large areas in the Central Pontides (north‐central Turkey). The Lower Cretaceous turbidites are over 2 km thick and are exposed in an area of 400x90 km. The Upper Cretaceous (Campanian‐Maastrichtian) forearc turbidites are up to 1200 m thick and crop out in NW‐SE trending elongate basins about 40 km wide, extending along the Central and Eastern Pontides. We present new detrital zircon U‐Pb ages, petrography and paleocurrent measurements from the Campanian‐Maastrichtian turbidites and compare them to those from the Lower Cretaceous turbidites. The Campanian‐Maastrichtian sandstones are dominated by carbonate and magmatic lithic grains. The paleocurrents indicate paleoflow directions parallel to the axis of the basin. The sandstones are dominated by Late Cretaceous zircons indicating derivation mainly from the coeval magmatic arc. In contrast, the Lower Cretaceous sandstones are dominated by quartz and feldspar, and paleocurrents indicate southward transport. The detrital zircons in the Lower Cretaceous sandstones are mainly Archean and Paleoproterozoic, indicating that in the Early Cretaceous, the source of turbidites was from the Archean‐Paleoproterozoic Ukrainian Shield to the north. After the opening of the Black Sea in the Late Cretaceous, the connection between the Pontides and the East European Platform was severed and the Campanian‐Maastrichtian turbidites were sourced principally from the magmatic arc in the Pontides. Our results indicate that the Campanian‐Maastrichtian turbidites represent the first turbiditic sequence deposited after the opening of the Black Sea.

Description: Abstract The Rukwa and North Malawi Rift Segments (RNMRS) both define a major rift‐oblique segment of the East African Rift System (EARS) and are often regarded as discrete rifts due to the presence of the uplifted Mbozi block between them. Here, we investigate the influence of basement fabrics on the coupling and linkage of border faults across an inter‐rift transfer zone between discrete juvenile rift segments. We utilized satellite DEM to investigate the morphological architecture of the rift domains; and aeromagnetic data to assess the relationships (plan‐view) between the rift structures and the pre‐rift basement fabrics. Our results show that the present‐day morphology of the RNMRS is characterized by along‐rift alternation of rift shoulder polarity, characteristic of coupled rift segments. Interpretation of filtered aeromagnetic maps along the boundaries of the RNMRS reveal striking alignment of the rift‐bounding faults with colinear NW‐SE‐trending pre‐existing basement fabrics. We find that rift‐coupling along the NE boundary of the Mbozi transfer zone is accommodated by fault‐assisted magma‐plumbing, whereas, coupling along the SW boundary is accommodated by strike‐slip and oblique‐normal faulting that reactivated the Proterozoic Mughese shear zone, within the collisional boundary between the Tanzania craton and the Bangweulu cratonic block. Further, we show how the configuration of the basement fabrics may influence the formation of rift bifurcation across basement transfer zones. We suggest that the structural connectivity of the boundary faults along the RNMRS, and their alignment with colinear basement fabrics demonstrate the influence of structural inheritance on the amalgamation of approaching rift segments.

Description: Abstract The Yao Shan complex, located at the southwestern margin of the South China plate, to the east of the Sanjiang‐Southeast Asia Tethyan domain, and at the northern extension of the Day Nui Con Voi massif of the Ailao Shan‐Red River shear zone, provides a reasonable example to study the multistage tectonic remobilization of continental plates. In this paper, we present new zircon LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) U‐Pb dating results and Hf isotopic data of the granitic plutons and dikes from the Yao Shan complex. Zircon U‐Pb ages of the granitic intrusions mainly concentrate on three periods: 825–780, 82–69, and 31–29 Ma. The zircons are characterized by mainly negative εHf(t) values with TDMC ages of 2637–1645, 1964–1610, and 2077–1400 Ma, respectively. The results show that the Yao Shan complex record evidences for three episodes of extensive partial melting of middle and lower crustal Precambrian crystalline basement rocks. They contributed to the granitic magmatism in relation to tectonic processes and structural deformation from Neoproterozoic, Late Cretaceous to Oligo‐Miocene. Therefore, we argue that the southwestern margin of the South China plate is the weakness for multistage remobilization throughout the tectonic evolution of the plate after its primary cratonization. Remobilization of the plate margin is related to oceanic plate subduction, triggered by rifting in a passive continental margin setting, induced by extension at back‐arc setting, or resulted from continental extrusion.

Description: Abstract The Mt. Massico ridge (central‐southern Apennines, Italy) is characterized by a ~150 m thick tectonic mélange located at the base of a Tortonian‐lower Messinian heterogeneous clastic succession consisting of layered sandstones, limestones, marls, and claystones with intercalated mass wasting deposits and isolated olistoliths, which deposited above Meso‐Cenozoic limestones. Geological mapping and structural analyses, integrated with illite/smectite paleothermal indicators and U‐Pb dating of syn‐tectonic calcite veins and slickenfibers, allowed us to unravel: (1) the tectonic evolution of the Mt. Massico ridge and (2) the development of the intra‐wedge tectonic mélange in the framework of the Apennine accretionary wedge evolution. Results show that after thrusting and folding of Meso‐Cenozoic limestones during late Tortonian times (7.0 ± 1.6 Ma), late Messinian‐early Pliocene out‐of‐sequence thrusting (5.1 ± 3.7 Ma) juxtaposed ~3,300 m thick, imbricate thrust sheets above the Tortonian‐lower Messinian clastic succession. During out‐of‐sequence thrusting, the base of the weak clastic deposits acted as a décollement horizon due to the rheological contrast and mechanical buttress with the underlying competent Mesozoic‐Cenozoic limestones. Heterogeneous deformation along the base of the clastic succession was accomodated by ductile pressure‐solution of claystones and marls, by brittle stratal disruption and fracturing/veining of competent olistoliths and primary foliation (i.e. sandstones and limestones strata), thus leading to the development of a tectonic mélange. The compressional phase was followed by extensional tectonics after the late Pliocene (minimum age 2.85 ± 0.5 Ma). We conclude that out‐of‐sequence thrusting, buttressing, and intraformational rheological contrast can be fundamental factors for the development of intra‐wedge tectonic mélange.

Description: Abstract This work focuses on how the progress in earthquake science that follows a large, deeply studied earthquake might be promptly combined with updated approaches of seismic hazard analysis to guide applicative choices for seismic risk reduction, such as postevent seismic microzoning and building design. Both seismic microzoning and seismic design of structures require strong motion records to perform numerical site response analyses. These records have to be related to the seismotectonic context and historical seismicity of the investigation area. We first performed a fault‐based probabilistic seismic hazard analysis in the area struck by the 2016 central Italy seismic sequence to individuate reference uniform hazard spectra at rock conditions. We used two different seismic hazard models, one considering 27 individual seismogenic sources (ISSs), and the second one involving grid point seismicity, using a fixed‐radius smoothing approach. The geological and seismotectonic data of the 2016 seismic sequence were used to update the model of ISSs. We performed a deaggregation analysis to evaluate the contribution of the ISS in the hazard of four representative sites and to select the magnitude‐distance pairs useful in the selection of the real accelerograms. The deaggregation analysis has been performed to identify which source and magnitude most contribute to the hazard for each site, and for different periods of spectral accelerations. Finally, we select, for each site, a set of natural accelerograms, from both nonimpulsive and pulse‐like records, based on the magnitude‐distance pairs that are compatible on average with target uniform hazard spectra.

Description: Abstract Although hyperextended rifted margins have been the focus of many recent studies, the potential interaction between tectonic deformation and sedimentation during extreme crustal thinning is poorly understood. In this study, we aim to explore the tectono‐sedimentary evolution during final rifting and lithospheric breakup in the hyperextended Liwan sag basin. We demonstrate the presence of an oceanward inclined shale sequence, which acts as a decoupling level separating a subshale from a suprashale layer. The suprashale layer is characterized by progradational deltaic facies with a high sediment supply to the north and basal facies farther south. Extensional faults together with local gravitational structures are syndepositional and formed during hyperextension. The shale is observed to overlie a highly attenuated basement comprising high‐reflective sequences that may correspond to meta‐sediments. We propose a new model to explain the interaction of tectonic deformation, gravity sliding, and sedimentation. A first stage includes the crustal necking that goes along with the deposition of synrift deltaic sequences decoupled above prodelta shales. During hyperextension, a high sediment supply is linked to migration of depocenters and extension oceanward. The prodelta shale layer acts as the main decoupling level, gravity sliding affects synhyperextensional sediments, and dome‐shape structures start to form. During a third stage, deformation migrates outboard, while formation of dome structures continues in the Liwan sag basin. Although the process controlling these dome‐shape structures remains unclear, a magmatic origin is likely. Diffuse deformation related to differential compaction and minor magmatic additions continued to affect the margin after the onset of seafloor spreading.

Description: Abstract We focus on the coseismic surface faulting exposed along the Mt. Vettore‐Mt. Bove fault system (VBF, central Italy), that activated during the 24 August 2016, Amatrice earthquake (Mw 6.0) and soon after reactivated during the 26 October Visso (Mw 5.9) and 30 October Norcia events (Mw 6.5 mainshock). We systematically recognized the coseismic surface ruptures of the aforesaid earthquakes, which document the repeated surface faulting on the same seismogenic structure in close temporal succession. We surveyed 1,747 evidence of coseismic ruptures, 325 fault plane attitudes along the Vettoretto‐Redentore segment, and over 4,000 data along the entire VBF that were organized in a GIS‐database. This data set allowed us to estimate the coseismic surface rupture length (SRL), maximum (MD) and average (AD) displacement associated with the Mw 6.0 and Mw 6.5 events. We found that the SRL and MD associated with the former are respectively 5.8 km and 28.5 cm and AD reaches 12.7 cm. For the mainshock, the values of SRL ≥ 22 km and MD = 222 cm were measured. The cumulative, post‐30 October parameters are SRL = 30 km, MD = 240 cm, AD = 36 cm. Despite that the MD of the Mw 6.0 event differs by ~1 order of magnitude respect to the mainshock MD, the two slip profiles display a similar multiscale sinuosity showing a significant control of the long‐term fault segmentation on the coseismic rupturing. Comparing the obtained coseismic parameters with literature global earthquakes data highlights some peculiarities of the 2016 central Italy surface rupture pattern, which suggest caution in applying empirical relationships to highly segmented seismogenic faults.

Description: Abstract The burial and exhumation of continental crust during collisional orogeny exerts a strong control on the dynamics of mountain belts and plateaus. Constraining the rates and style of exhumation of deeply buried crust has proven difficult due to complexities in the local geology and thermochronometric methods typically used. To advance this field, we applied trace element and U‐Pb LA‐ICPMS analyses to rutile from eclogite and amphibolite samples from the Western Gneiss Complex of Norway—an archetypal continental (ultra)high‐pressure (UHP) terrane. Peak temperature and timing of mid‐crustal cooling was constrained for samples collected along a subduction‐ and exhumation‐parallel transect, using Zr‐in‐rutile thermometry and U‐Pb rutile geochronology, respectively. Peak temperatures decrease from 830 °C in the UHP domain to 730 °C at the UHP‐HP transition, remains constant at 730 °C across most of the terrane, and decreases to 620 °C at the eclogite‐out boundary. U‐Pb results show that most of the terrane cooled through 500 °C at 380‐375 Ma except for the lowest‐grade region, where cooling occurred c. 20 million years earlier. The results indicate that exhumation was a two stage process, possibly involving 1) flexural rebound and slab flattening at depth combined with foreland‐directed extrusion, followed by 2) synchronous cooling below 500 °C across the largely flat‐lying WGC. The latter is consistent with mass removal across a large area, consistent with erosion of an overlying orogenic plateau. The Caledonides were at near‐equatorial latitudes at the time. A Caledonian paleo‐plateau thus may have strongly impacted climate during the Devonian and Carboniferous.

Description: Abstract Constraining the way in which continental deformation is accommodated in time and space is essential to reconcile past plate movements with geological observations. Kinematic reconstructions of the Iberia‐Europe plate boundary are still debated. Here we focus on an inverted Mesozoic rift basin, the Cameros basin, which is part of the Iberian chain. We use a combination of detrital low‐temperature thermochronological techniques to define the time‐temperature evolution of the basin from Mesozoic rifting to Cenozoic collision. Zircon fission‐track analyses of Oligocene–Miocene sedimentary rocks yield two main age populations at ~170 ± 10 and ~100 ± 10 Ma, reflecting (i) an Early Jurassic thermal event related to the Atlantic‐Alpine Tethys opening and (ii) an Albo‐Cenomanian thermal event related to the Bay of Biscay opening. Thermal modeling of combined zircon fission‐track, apatite fission‐track, and apatite (U‐Th‐Sm)/He data reveals that collision‐related cooling of the Cameros basin started during the Paleocene (~60 Ma). A second cooling/exhumation phase of the basin is recorded from 35 to 25 Ma. Initial cooling occurred after a protracted postrift period characterized by persistence of high geothermal gradients, a feature also recognized in the Pyrenees. Our results show that the Iberian chain shared the same Early to Late Jurassic tectonothermal evolution with the Atlantic‐Alpine Tethyan rifted margins. From the Albian onward, the thermal evolution of the Cameros basin was very similar to that of the Pyrenees. This study shows that the preservation in mountain ranges of a succession of rifting events provide important clues for plate reconstructions.

Description: Abstract Spatiotemporal patterns of deformation and exhumation in the central Andes are key parameters for reconstructing the kinematic history of the orogenic belt. Previous studies of the retroarc thrust belt document overall eastward propagation of deformation since the late Eocene, but the amount and timing of exhumation during the early phase of Andean orogeny remains largely unconstrained, particularly in the modern forearc region. In order to determine the timing and amount of exhumation prior to the late Eocene, we employed a multidating approach combining zircon U‐Pb geochronology with apatite fission track and apatite (U‐Th)/He thermochronology. We focus on the low‐temperature cooling history of the Cordillera de Domeyko thrust belt and synorogenic deposits in the Salar de Atacama basin. Our results show Late Cretaceous to Oligocene cooling and exhumation in the Cordillera de Domeyko. The distribution of cooling ages in the forearc indicates three periods of exhumation: ~86–65, ~65–50, and 50–28 Ma. The amount of cooling was variable in space and time but requires total exhumation of ~2.5–3.3 km of rocks above major structures in the thrust belt. Regional unconformities in the Salar de Atacama basin correlate with periods of eastward migration of the orogenic front at ~65 Ma and ~50–40 Ma. Pulses of deformation at the front of the thrust belt alternated with periods of out‐of‐sequence hinterland deformation and exhumation. Overall, our data show that shortening in the central Andes commenced during the Late Cretaceous (as early as ~86 Ma) and that deformation (shortening) and exhumation were coupled in space and time.

Description: Abstract This paper discusses the framework for comparing independent study means (SMs) with averages of aggregated study mean paleomagnetic poles, hereafter referred to as reference poles, contributing to continental or global apparent polar wander paths (GAPWP). Paleomagnetists typically compare individual SMs to these population means to compute latitudinal displacements or rotations with uncertainties estimated by comparing A95's. Using this comparison statistic, more than half of the population of reference SMs contributing to a GAPWP fall at an angular distance that is greater than their combined uncertainties. Two approaches are presented that yield similar results based on different rationales and comparison statistics. Common to both approaches is K95,GAPWP, the angular distance within which 95% of poles contributing to an APWP mean fall. One approach combines K95,GAPWP and the A95,SM of the independent study mean that can only be judged to be significantly displaced if the angular difference with the GAPWP mean exceeds their combined uncertainty. The second approach uses K95,GAPWP as the best estimate of total error of each study mean, which when combined with uncertainty of the GAPWP mean, A95,GAPWP, defines the angular difference that the independent study mean needs to exceed to be judged to be significantly displaced from a GAPWP mean. These comparison statistics are used to assess paleomagnetic support for Greater Indian Basin hypothesis for the paleogeographic evolution of the northern margin of India prior to collision with Asia. The Greater Indian Basin hypothesis is not supported based on these more rigorous criteria.

Description: Abstract The first low‐temperature thermochronological data from Thurston Island, West Antarctica, provide insights into the poorly constrained thermo‐tectonic evolution of the paleo‐Pacific margin of Gondwana since the Late Paleozoic. Here we present the first apatite fission track (AFT) and apatite (U‐Th‐Sm)/He (AHe) data from Carboniferous to mid‐Cretaceous (meta‐) igneous rocks from the Thurston Island area. Thermal history modeling of AFT dates of 145–92 Ma and AHe dates of 112–71 Ma, in combination with kinematic indicators, geological information and thermobarometrical measurements, indicate a complex thermal history with at least six episodes of cooling and reheating. Thermal history models are interpreted to reflect Late Paleozoic to Early Mesozoic tectonic uplift of pre‐Jurassic arc sequences, prior to the formation of an extensional Jurassic–Early Cretaceous back‐arc basin up to 4.5 km deep, which was deepened during intrusion and rapid exhumation of rocks of the Late Jurassic granite suite. Overall Early to mid‐Cretaceous exhumation and basin inversion coincided with an episode of intensive magmatism and crustal thickening and was followed by exhumation during formation of the Zealandia‐West Antarctica rift and continental break‐up. Final exhumation since the Oligocene was likely triggered by activity of the West Antarctic rift system and by glacial erosion.

Description: Abstract A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60–70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume‐fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.

Description: Abstract Transform margins are first‐order tectonic features that accommodate oceanic spreading. Uncertainties remain about their evolution, genetic relationship to oceanic spreading, and general structural character. When the relative motion of the plates changes during the margin evolution, further structural complexity is added. This work investigates the evolution of transform margins, and associated rift‐transform intersections, using an analogue modelling approach that simulates changing plate motions. We investigate the effects of different crustal rheologies by using either a) a two‐layer brittle‐ductile configuration to simulate upper and lower continental crust, or b) a single layer brittle configuration to simulate oceanic crust. The modeled rifting is initially orthogonal, followed by an imposed plate vector change of 7o that results in oblique rifting and plate overlap (transpression) or underlap (transtension) along each transform margin. This oblique deformation reactivates and overprints earlier orthogonal structures, and is representative of natural examples. We find that: a) a transtensional shift in the plate direction produces a large strike‐slip principal displacement zone, accompanied by en‐echelon oblique‐normal faults that accommodate the horizontal displacement until the new plate motion vector is stabilized, while b) a transpressional shift produces compressional structures such as thrust fronts in a triangular zone in the area of overlap. These observations are in good agreement with natural examples from the Gulf of California (transtensional) and Tanzania Coastal Basin (transpressional) shear margins, and illustrate that when these deformation patterns are present, a component of plate vector change should be considered in the evolution of transform margins.

Description: Abstract Most of the permanent deformation in the Pampean Flat slab segment of the Central Andes is taken up at the Andean Orogenic Front in Argentina, a narrow zone between the Eastern Precordillera and Sierras Pampeanas that comprises one of the world's most seismically active thrust zones. Active faults and folds in the region have been extensively mapped but still largely lack information on style and rates of deformation, which is essential for understanding the distribution of regional strain and estimating the seismic potential of individual faults. Structural, geomorphic, and 36Cl cosmogenic radionuclide surface exposure ages methods are used to focus on key sites along the 30‐km‐long La Rinconada Fault Zone in this region of west‐central Argentina, which is ~15 km away from the highly‐populated (~500,000) city of San Juan, to define a late Quaternary average shortening rate of 0.41±0.01 mm/yr. This slip rate is the same order of magnitude, but slightly lower than nearby similar east‐dipping Eastern Precordillera faults including the La Laja and Las Tapias faults. Relatively low slip‐rates are interpreted as being a consequence of distributed deformation between the latitude of the La Rinconada Fault Zone (31 and 32°S), as compared to between latitudes 32 to 33°S where deformation appears to be focused on fewer structures, including the Las Peñas and La Cal Thrust faults. The La Rinconada Fault Zone is capable of generating earthquakes of MW 6.6–7.2, but further investigations are required to determine timing and recurrence intervals of discrete events.