ALBERT

Beschreibung: The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed, or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics. We combine existing tectono-stratigraphic data with new structural and geophysical datasets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the c. 2960–2750 Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded and unconformably overlain by a c. 2730 Ma, syntectonic clastic sequence, deposited in shallow marine to subaerial conditions, and derived from the erosion of the underlying greenstones. This c. 2730 Ma regional unconformity predates the oldest-known Yilgarn structures, therefore its tectonic significance is so far unknown. At around the same time, at deeper crustal levels, the c. 2728 Ma Yarraquin pluton was being emplaced along an active, large-scale shear zone network. Our meso- and microstructural analysis shows that the bulk of the fabric in the granite and its country rocks developed during pluton emplacement, and was largely assisted by magma-present shearing. Overall, these structures reflect an important event of syn-emplacement crustal shortening. The regional unconformity and the syndeformational emplacement of the Yarraquin pluton are both expressions of a c. 2730 Ma regional deformation event associated with significant crustal thickening, marking the onset of the Neoarchean Yilgarn Orogeny.

Beschreibung: In this paper we investigate the tectonics of the central Swiss Molasse Basin (SMB), situated between the central European Alps and the Jura fold-and-thrust belt. Based on (re-)interpreted 2D reflection seismic data and geological 3D modeling we present new insights into the structural configuration and evolution of the central SMB. During the thin-skinned, main deformation phase of the Jura fold-and-thrust belt (~12-4 Ma), the central SMB was riding passively above a major detachment horizon within Triassic evaporites, which roots beneath the External Crystalline Massifs of the central Alps. In the SMB, low amplitude detachment folds and detachment-rooted WSW-ENE striking thrust and reverse faults are related to this event. Younger, major NNE-SSW and NNW-SSE trending strike-slip fault zones cross the basin over a length of several kilometers, affecting the Cenozoic and Mesozoic cover units and extending into the pre-Mesozoic crystalline basement. In cases, the fault zones apparently reactivate inherited, Permo-Carboniferous trough (PCT) bounding, Paleozoic normal faults. Seismic data further reveals mild inversion of WSW-ENE striking PCTs. These structures clearly cut the basement-cover contact thus postdating the main deformation phase of the Jura fold-and-thrust belt. Present-day earthquake focal mechanisms and epicenter distribution show strike-slip faulting and basement-seated seismicity, respectively, thus corroborating our observations from seismic interpretation. Our data suggests a change from Late Miocene thin-skinned to Pliocene to present-day incipient thick-skinned tectonics. This major change reflects the establishment of a mid-crustal detachment and may be related to changes in slab dynamics of the subducting European lithosphere.

Beschreibung: Rocks in the continental crust are long-lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations (CPOs) influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction (EBSD) and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to mid-lower crustal depths. Tensor symmetry is a function of mineral modal composition, with mica-rich samples trending towards hexagonal symmetry, amphibole-rich samples trending towards an increased orthorhombic symmetry component, and quartz-feldspar-rich samples showing a larger component of lower symmetries. 77% of samples have a best-fit hexagonal tensor with slow-axis symmetry, as opposed to mantle deformation fabric that usually has fast-axis symmetry. The best-fit hexagonal approximation for crustal tensors is not elliptical, but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out-of-plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for non-elliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.

Beschreibung: Xenoliths that erupted in the SE Pamir of Tajikistan from 1000–1050°C and 90 km depth illuminate what happens when crust founders into the mantle. 40 Ar/ 39 Ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic Dunkeldik pipe and dike field indicates eruption at 11.2 ± 0.2 Ma. U-Pb and trace-element laser-ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded: the Jurassic–Cretaceous Andean-style magmatic arc and its Proterozoic–Mesozoic host rocks along the southern margin of Asia. Recrystallization of these zircons was extensive, yielding a range of dates down to 11 Ma. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations compatible with garnet growth and minimal heating at 22−20 Ma, and then 200–300°C of heating, ~25 km of burial, and alkali−carbonate melt injection at 14−11 Ma. These changes are interpreted to coincide with: i) heat input due to Indian slab breakoff at ~22–20 Ma; ii) rapid thickening and foundering of the Pamir lithosphere at 14–11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

Beschreibung: HP/LT chloritoid-bearing micaschists crop out widely in the central part of northern Turkey and represent deep-seated subduction–accretionary complexes. Three peak metamorphic assemblages are identified in the area studied: 1) garnet–chloritoid–glaucophane with pseudomorphs after lawsonite 2) chloritoid with pseudomorphs after glaucophane and 3) chloritoid with pseudomorphs after jadeite in addition to phengite, paragonite, quartz, chlorite, rutile, and apatite. The latter is interpreted as transformation of a chloritoid + glaucophane assemblage to chloritoid + jadeite with increasing pressure; PT modeling indicates ~17 and 22–25 kbars for the two peak parageneses. The diversity of peak metamorphic assemblages and the PT estimates suggest that basal accretion occurred at different depths within the wedge. The depth of the basal accretion is possibly controlled by the slab–mantle decoupling depth. Stretching and thinning of the lithospheric forearc induced by the slab roll-back possibly caused shallowing of the slab–mantle decoupling depth which limited depth of the basal accretion from 70–80 km to ~55 km within the subduction channel. A slab–mantle coupling depth controlled basal accretion may also explain the scarcity of eclogite and high-grade blueschist facies metamorphic rocks in active intra-oceanic subduction zones. Because the overriding plate is young and hot in intra-oceanic subductions, the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust. This prevents accretion and exhumation of eclogite along the subduction channel.

Beschreibung: The Musgrave Block in central Australia exposes numerous large-scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560-520 Ma. The most prominent structure is the crustal-scale, over 600 km long, E-W-trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximate top-to-N sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation in the direction of thrusting from predominantly mid-crustal (ca. 520-620 °C and 0.8-1.1 GPa) to lower-crustal (ca. 650 °C and 1.0-1.3 GPa) levels, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south-dipping thrust plane, but is only small, implying that a ≥ 60 km long N-S segment of the Woodroffe Thrust was originally shallow-dipping at an average estimated angle of ≤ 6°. The reconstructed geometry suggests that basement-cored, thick-skinned, mid-crustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g. large volumes of syn-tectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low-angle near-planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers.

Beschreibung: No abstract is available for this article.

Beschreibung: In the southern Central Andes at 37-38 °S latitude, the Chos Malal fold-and-thrust belt (FTB), which results from the Late Cretaceous closure of the Neuquén Basin, has generated increasing interest because of its potential for hydrocarbon exploration. Using detailed field mapping, seismic reflection and well data analysis, cross-section balancing, and detrital apatite fission-track thermochronology from 18 samples distributed along the FTB, we bring new quantitative constraints on the chronology of the Neogene structural development of the Chos Malal FTB. Our dataset reveals that extensive tectonically-driven exhumation through basement-involved thrusting, occurred at ~15-7 Ma in both the inner and outer sectors of the FTB. This age range is corroborated by indirect dating of the Chos Malal Formation, a syntectonic unit also deformed after deposition, which we find was deposited after 7.2 Ma ± 1.6 / 2.0 Ma, an age much younger than the Miocene age it was previously assigned. We document for the first time an Early Pliocene rapid cooling stage that was ongoing ~ 5.5 to 5.1 Ma ago in the interior of the orogenic wedge, which we attribute to out-of-sequence thrusting along basement-involved thrust faults bounding the Cordillera del Viento, driving the surface uplift of this prominent topographic feature. Finally, our sequential reconstruction of the thick and thin-skinned structures of the Chos Malal FTB shows that most of the horizontal shortening was accommodated after the Middle Miocene, which we thus consider as being the main phase of orogenic building in this sector of the Andes.

Beschreibung: The Yakima folds of central Washington, USA are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north-vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8-0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest-striking frontal thrust faults and shorter north- to northeast-striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north-trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west-trending Yakima folds and west-striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low-strain-rate environment in the backarc of the Cascadia Subduction Zone.

Beschreibung: Temporal variation of crustal structure is key to our understanding of Earth processes on human time-scales. Often, we expect that the most significant structural variations are caused by strong ground shaking associated with large earthquakes and recent studies seem to confirm this. Here, we test the possibility of using P receiver functions (PRF) to isolate structural variations over time. Synthetic receiver function tests indicate that structural variation could produce PRF changes on the same order of magnitude as random noise or contamination by local earthquakes. Nonetheless, we find significant variability in observed receiver functions over time at several stations located in northeastern Honshu. Immediately following the Tohoku-oki earthquake, we observe high PRF variation clustering spatially, especially in two regions near the beginning and end of the rupture plane. Due to the depth sensitivity of PRF and the time-scales over which this variability is observed, we infer this effect is primarily due to fluid migration in volcanic regions and shear stress/strength reorganization. While the noise levels in PRF are high for this type of analysis, by sampling small datasets, the computational cost is lower than other methods, such as ambient noise, thereby making PRF a useful tool for estimating temporal variations in crustal structure.

Beschreibung: We perform the finite-extent fault inversion of the three main events of the 2016 Central Italy seismic sequence using near-source strong-motion records. We demonstrate that both earthquakes nucleation and rupture propagation were controlled by segmentation of the (N)NW–(S)SE-trending Quaternary normal faults. The first shock of the sequence (August 24 th , Mw 6.0) ruptured at the relay zone between the Laga Mts (LF) and the Cordone del Vettore (CVF) normal faults. The second shock (October 26 th , Mw 5.9) nucleated at a minor relay zone within the Mt. Vettore–Mt. Bove fault (VBF), while the third and largest one (October 30 th , Mw 6.5) initiated at the relay zone between the VBF and CVF, triggering the multiple rupture of the VBF, CVF and probably LF. We show that this latter relay zone corresponds to the deeper, high-angle, fault-zone of the Sibillini Mts cross-structure, a thrust-ramp inherited from the Miocene-Pliocene contractional phase of the Apennines. This structure acted as a barrier to rupture propagation of the first two events thus defining an area of large stress concentration until it acted as the initiator of the rupture originating the largest Mw 6.5 event that crossed the barrier itself. We suggest that the “young” CVF have started to cut through the barrier acting as a soft-linkage between the two long-lived LF and VBF. The evidence that coseismic cumulative slip shows a maximum at the CVF, provided by both slip inversion and original surface rupture data, suggests that the CVF is growing faster than the adjacent faults.

Beschreibung: The tectono-sedimentary evolution of asymmetric extensional systems driven by the activity of major normal faults or detachments associated with footwall exhumation is often characterized by a sequence of slower, faster and ultimately again slower subsidence rates in the center of hanging-wall half-grabens during their syn- and post- kinematic evolution. We have studied this specific evolution by the means of 3D stratigraphic numerical modeling that accounts for the variability of the sediment and water flux combined with climatic and sea-level variations, and sediment compaction. The model setup is constrained by observations from the Pannonian back-arc basin of Central Europe. Our modeling predicts the formation of low-order tectonic and higher order sea-level and climate-driven transgressive-regressive sedimentary cycles. Furthermore, we model and analyze the auto-cyclic nature of the depositional systems. Retrograding-prograding cycles are visible on the proximal flank of the half-grabens by their different spatial and temporal expressions, while depocenters record large water depth variations linked to the specific and episodic activity of normal faults and their migration with time. The application to a system of multiple half-grabens in the Pannonian Basin, which are activated in different locations, at different times and with different kinematics, demonstrates a complex interplay between direct sediment sourcing and the sediments ability to

Beschreibung: Magmatic rift systems are composed of discrete spreading segments defined by morphologic, structural, and volcanic features that vary systematically along strike. In Iceland, structural features mapped in the glaciated and exhumed Miocene age upper crust correlate with analogous features in the seismically and volcanically active Neovolcanic Zone. Integrating information from both the active rift zones and ancient crust provides a three-dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments. Crustal exposures in the Skagi region of northern Iceland reveal significant along-strike variations in geologic structure. The upper crust at exhumed magmatic centers (segment centers) is characterized by a variety of intrusive rocks, high temperature hydrothermal alteration, and geologic evidence for kilometer-scale subsidence. In contrast, the upper crust along segment limbs, which extend along strike from magmatic centers, is characterized by thick sections of gently dipping lava flows, cut by varying proportions of sub-vertical dikes. This structure implies relatively minor upper crustal subsidence and lateral dike intrusion. The differing modes of subsidence beneath segment centers and segment limbs require along-axis mass redistribution in the underlying upper, middle, and lower crust during crustal construction. This along-axis material transport is accomplished through lateral dike intrusion in the upper crust, and by along-axis flow of magmatic to high-temperature solid-state gabbroic material in the middle and lower crust. These processes, inferred from outcrop evidence in Skagi, are consistent with processes inferred to be important during active rifting in Iceland and at analogous magmatic oceanic and continental rifts.

Beschreibung: The Yishu fault zone (YSFZ) is the most active fault in eastern China, and it is the location of an M 8.5 earthquake that occurred in 1668. However, topography and landforms in this area have been greatly altered by human activity during the previous three to four decades, which makes assessment of past earthquakes difficult. We therefore used a set of remote sensing images captured in the 1960s to evaluate the single-event and multi-event cumulative offsets of streams along the fault. The local landforms have preserved the Holocene seismic offsets well despite the moderately humid climate in which the local landforms have evolved rapidly. Geomorphic markers associated with 401 systematically dextrally offset streams were analyzed and measured. These offset markers are considered to record seismic rupture processes. Cumulative offset probability densities were analyzed according to slip distributions along various subsections of the YSFZ. Four strong paleoearthquakes in addition to the 1668 earthquake were revealed by offset density peaks within the YSFZ, a finding that differs from those of previous studies. The average offset of the 1668 event is approximately 9 m. The cumulative offsets of the other four paleoseismic events are essentially multiples of the latest event. A single event ruptured more than 200 km of discontinuous fault traces. The similarity of the previous five earthquakes implies that rupturing of the YSFZ has followed a characteristic slip model during the Holocene, with a right-lateral slip rate of 2.2-2.6 mm/yr. We expect that the YSFZ is not due for another large earthquake for several thousand years.

Beschreibung: S K S , S K K S , and P K S splitting parameters measured at 34 seismic stations that we deployed in the vicinity of the Cenozoic Malawi Rift Zone (MRZ) of the East African Rift System demonstrate systematic spatial variations with an average splitting time of 1.0±0.3 s. The overall NE-SW fast orientations are consistent with absolute plate motion (APM) models of the African Plate constructed under the assumption of no-net rotation of the global lithosphere, and are inconsistent with predicted APM directions from models employing a fixed hotspot reference frame. They also depart considerably from the trend of most of the major tectonic features. These observations, together with the results of anisotropy depth estimation using the spatial coherency of the splitting parameters, suggest a mostly asthenospheric origin of the observed azimuthal anisotropy. The single-layered anisotropy observed at 30 and two-layered anisotropy observed at four of the 34 stations can be explained by APM-related simple shear within the rheologically transitional layer between the lithosphere and asthenosphere, as well as by the horizontal deflection of asthenospheric flow along the southern and western edges of a continental block with relatively thick lithosphere revealed by previous seismic tomography and receiver function investigations. This first regional-scale shear wave splitting investigation of the MRZ suggests the absence of rifting-related active mantle upwelling or small-scale mantle convection, and supports a passive-rifting process for the MRZ.

Beschreibung: Craton cores far from plate boundaries have traditionally been viewed as stable features that experience minimal vertical motion over 100-1000 Ma time scales. Here, we show that the Fennoscandian Shield in southeastern Sweden experienced several episodes of burial and exhumation from ~1800 Ma to the present. Apatite, titanite, and zircon (U-Th)/He ages from surface samples and drill-cores constrain the long-term, low-temperature history of the Laxemar region. Single grain titanite and zircon (U-Th)/He ages are negatively correlated (104-838 Ma for zircon, 160-945 Ma for titanite) with effective uranium (eU=U+0.235×Th), a measurement proportional to radiation damage. Apatite ages are 102-258 Ma and are positively correlated with eU. These correlations are interpreted with damage-diffusivity models, and the modeled zircon He age-eU correlations constrain multiple episodes of heating and cooling from 1800 Ma to the present, which we interpret in the context of foreland basin systems related to the Neoproterozoic Sveconorwegian and Paleozoic Caledonian orogens. Inverse time-temperature models constrain an average burial temperature of ~217 °C during the Sveconorwegian, achieved between 944 Ma and 851 Ma, and ~154 °C during the Caledonian, achieved between 366 Ma and 224 Ma. Subsequent cooling to near-surface temperatures in both cases could be related to long-term exhumation caused by either post-orogenic collapse, or mantle dynamics related to the final assembly of Rodinia and Pangea. Our titanite He age-eU correlations cannot currently be interpreted in the same fashion; however, this study represents one of the first examples of a damage-diffusivity relationship in this system, which deserves further research attention.

Beschreibung: Studies of mountain belts worldwide show that along-strike changes are common in their foreland fold-and-thrust belts. These are typically caused by processes related to fault reactivation and/or fault focusing along changes in sedimentary sequences. The study of active orogens, like Taiwan, can also provide insights into how these processes influence transient features such as seismicity and topography. In this paper, we trace regional-scale features from the Eurasian continental margin in the Taiwan Strait into the south-central Taiwan fold-and-thrust belt. We then present newly mapped surface geology, P-wave velocity maps and sections, seismicity, and topography data to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, and the distribution of seismicity and topography in this part of the Taiwan fold-and-thrust belt. These data show that the most important along-strike change takes place at the eastward prolongation of the upper part of the margins necking zone, where there is a causal link between fault reactivation, involvement of basement in the thrusting, concentration of seismicity, and the formation of high topography. On the area correlated with the necking zone, the strike-slip reactivation of east-northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones. Here, basement is not involved in the thrusting, there is weak focusing of seismicity, and localized development of topography. We also show that there are important differences in structure, seismicity, and topography between the margins shelf and its necking zone.

Beschreibung: Detrital zircon surface microtextures, geochronologic U-Pb data, and tectonic subsidence analysis from Upper Jurassic to Paleocene strata in the Black Hills of South Dakota reveal provenance variations in the distal portion of the Cordillera foreland basin in response to tectonic events along the outboard margin of western North America. During Late Jurassic to Early Cretaceous time, nonmarine strata record initially low rates of tectonic subsidence that facilitated widespread recycling of older foreland basin strata in eolian and fluvial systems that dispersed sediment to the northeast, with minimal sediment derived from the thrust belt. By middle Cretaceous time, marine inundation reflects increased subsidence rates along with a change to eastern sediment sources. Lowstand Albian fluvial systems in the Black Hills may have been linked to fluvial systems upstream in the midcontinent and downstream in the Bighorn Basin in Wyoming. During latest Cretaceous time, tectonic uplift in the study area reflects dynamic processes related to Laramide low-angle subduction that, relative to other basins to the west, was more influential due to the greater distance from the thrust load. Provenance data from Maastrichtian and Lower Paleocene strata indicate a change back to western sources that included the Idaho-Montana batholith and exhumed Belt Supergroup. This study provides a significant contribution to the growing database that is refining the tectonics and continental-scale sediment dispersal patterns in North America during Late Jurassic-Early Paleocene time. In addition, it demonstrates the merit of using detrital zircon grain shape and surface microtextures to aid in provenance interpretations.

Beschreibung: This study describes the strain geometry, crystal-plastic deformational features, isotopic age of metamorphism, and calculated seismic properties of two medium-temperature eclogite types from the Malpica-Tui Allochthonous Complex of Variscan NW Iberia. The eclogite types are eclogites with coronitic garnets and eclogites with a planolinear fabric. Both of them were buried, deformed and recrystallized under maximum P and T of 2.6 GPa and 610-640 °C, and subsequently exhumed in a late Devonian subduction channel. The metamorphic peak of the subduction-exhumation cycle occurred ca. 375 Ma ago. Omphacite petrofabric ties eclogites with coronitic garnet to non-coaxial constrictional strain and eclogites with planolinear fabrics to non-coaxial flattening strain and stretching along the lineation. We also used omphacite crystallographic preferred orientations to calculate and constrain the seismic properties of the eclogites. The slight variations in petrophysical properties observed are interpreted to result from variations in the strain regime recorded by pristine eclogites, or from variations in the modal proportions of the constituent high-pressure minerals. We foresee that eclogite in subduction metamorphic complexes might be either seismically undetectable or detected as planar features with high impedance contrasts relative to their host rocks.

Beschreibung: The fragmentary Phanerozoic geological record of the anomalously elevated Zimbabwe Craton makes reconstructing its history difficult using conventional field methods. Here, we constrain the cryptic Phanerozoic evolution of the Zimbabwe Craton using a spatially extensive apatite (U-Th-Sm)/He (AHe), apatite fission track (AFT) and zircon (U-Th)/He (ZHe) dataset. Joint thermal history modelling reveals that the region experienced two cooling episodes inferred to be the denudational response to surface uplift. The first and most significant protracted denudation period was triggered by stress transmission from the adjacent ~750-500 Ma Pan-African orogenesis during the amalgamation of Gondwana. The spatial extent of this rejuvenation signature, encompassing the current broad topographic high, could indicate the possible longevity of an ancient topographic feature. The ZHe data reveal a second, minor denudation phase which began in the Paleogene and removed a km-scale Karoo cover from the craton. Within our dataset, the majority of ZHe ages are younger than their corresponding AHe and AFT ages, even at relatively low eU. This unexpectedly recurrent age ‘inversion’ suggests that in certain environments, moderately, as well as extremely, damaged zircons have the potential to act as ultra-low-temperature thermochronometers. Thermal history modelling results reveal that the zircon radiation damage accumulation and annealing model (ZRDAAM) frequently over-predicts the ZHe age. However, the opposite is true for extremely damaged zircons where the ZHe and AHe data are also seemingly incompatible. This suggests that modification of the ZRDAAM may be required for moderate to extreme damage levels.

Beschreibung: How freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere, which includes information regarding the magnitude of driving/resisting forces of plate tectonics. We investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes (OCEQs) and numerical simulations. The stress release due to OCEQs in young oceanic lithosphere (5–15 Ma) shows significant differences between the spreading-directional component and the ridge-parallel component. The extensional stress release of the ridge-parallel component is six times as large as that of the spreading-directional component, while the compressional stress release of the ridge-parallel component is one-seventh that of the spreading-directional component. We conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge-parallel direction can explain the observed anisotropic stress release. The result indicates that young oceanic lithosphere (5–15 Ma) barely contracts in the ridge-parallel direction (only 0–30% of the spreading-directional contraction rate), while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere. From the results, we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0.44 MPa.

Beschreibung: Uranium-Lead (U-Pb) geochronology and Hafnium (Hf) isotope geochemistry of detrital zircons of the Harmony Formation of north-central Nevada provide new insights into the tectonic evolution of the late Paleozoic western Laurentian margin. Using laser-ablation inductively coupled plasma mass spectrometry, ten arenite samples were analyzed for U-Pb ages, and eight of these samples were further analyzed for Hf-isotope ratios. Three of the sampled units have similar U-Pb age peaks and Hf-isotope ratios, including a 1.0-1.4 Ga peak with εHf values of +12 to -3 and a 2.5-2.7 Ga peak with εHf values of +7 to -5. The remaining seven samples differ significantly from these three, but are similar to one another; having age peaks of 1.7-1.9 Ga with εHf of +10 to -20 and age peaks of 2.3-2.7 Ga with εHf of +6 to -8. The data confirm the subdivision of the Harmony Formation into two petrofacies: quartzose (Harmony A) and feldspathic (Harmony B). The three samples with 1.0–1.4 and 2.5–2.7 Ga peaks are the Harmony A, which originated in the central Laurentian craton. The other seven samples are the Harmony B, which originated in eastern Alberta-western Saskatchewan, north of the Harmony A source. We propose that all Harmony Formation strata were deposited near eastern Alberta and subsequently tectonically interleaved with Roberts Mountains allochthon strata. We interpret that the entire package was tectonically transported south along the western Laurentian margin and then emplaced eastward onto the craton during the Late Devonian – Early Mississippian Antler orogeny.

Beschreibung: 2D seismic reflection data tied to biostratigraphical and log information from wells in the central and south-eastern Alboran Sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion. Normal faults, tilted basement blocks and growth wedges reveal a thinned continental crust that formed in response to NW-SE extension. To the east a secondary SW-NE trend of extension affects the transitional crust adjacent to the oceanic Algerian Basin. The maximum thickness of syn-rift sediments is ~3.5 km and the oldest recorded deposits are Serravallian. The WNW-ESE Yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains: the thinned continental crust of Alboran and the oceanic spreading of the Algerian Basin. Late Tortonian to present-day NW-SE Africa/Eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike-slip faults, forming complex, compartmentalised sub-basins. Tectonic inversion coexisted with the formation of new faults and folds. Inversion was partial along the Habibas Basin and Al-Idrisi fault, but complete along the Alboran Ridge, where some SW-NE trending faults were perpendicular to the recent NW-SE plate convergence and were reactivated as thrusts. The WNW-ESE Yusuf fault is oblique to the convergence vector and therefore, reactivation is mainly expressed as transpressional deformation. Volcanic rocks intruded along the Alboran Ridge and Yusuf faults during the latest stages of extension, formed rheological anisotropies that localised the later inversion.

Beschreibung: Inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW-SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present-day N-S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in sub-parallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along-axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right-lateral stepover and avoiding a direct link to form a continuous N-S depression. The models reveal five potential types of rift linkage across the pre-existing basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.

Beschreibung: Oblique plate convergence is common, but it is not clear how the obliquity is achieved by continental fold-and-thrust belts. We address this problem in the Qilian Shan, northeastern Tibetan Plateau, using fieldwork observations, geomorphic analysis and elastic dislocation modeling of published geodetic data. A thrust dips SSW from the northern range front, and underlies steeper thrusts in the interior. Cenozoic thrust-related shortening across the Qilian Shan is ~155-175 km, based on two transects. Elastic dislocation modeling indicates that horizontal strain in the interseismic period is consistent with oblique slip on a single low angle detachment thrust below ~26 km depth, dipping SSW at ~17 o . We suggest this detachment is located above North China Block crust, originally underthrust during Paleozoic orogeny. Horizontal shear strain is localized directly above the up-dip limit of creep on the detachment, and is coincident with the left-lateral Haiyuan Fault. This configuration implies oblique slip on the detachment below seismogenic depths is partitioned in the shallow crust onto separate strike-slip and thrust faults. This is consistent with strain partitioning in oceanic subduction zones, but has not previously been found by dislocation models of continental interiors. The marginal, strike-slip, Altyn Tagh Fault influences thrusting within the Qilian Shan for 100-200 km from the fault, but does not control the regional structure, where Paleozoic basement faults have been reactivated. The Qilian Shan resembles the main Tibetan Plateau in nascent form: active thrusts are marginal to an interior that is developing plateau characteristics, involving low relief, and low seismicity.

Beschreibung: No abstract is available for this article.

Beschreibung: Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative contributions of retroarc underthrusting and trench-side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U-Pb zircon geochronology, Sm-Nd thermochronology, and field-based studies to document the protoliths and early burial history of amphibolite- to granulite-facies metasedimentary rocks in the Central Gneiss Complex. U-Pb detrital zircon data from the structurally highest sample localities yielded ~190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust-related burial and metamorphism at 〉25 km depths prior to ~80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench-side underplating or intra-arc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.

Beschreibung: Subducting slabs commonly acquire complex geometries from the migration of subduction trenches, slab–mantle interaction, slab tearing, and collision of slabs at depth. Although it is possible to construct three-dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models, it is often not possible to rigorously test their accuracy. Here we present a methodology for performing such a test, by ‘reverse-engineering’ the pre-subduction configuration of a slab of oceanic lithosphere from interpretations of its present-day morphology. We illustrate our approach for the Ryukyu and Shikoku slabs, northwest Philippine Sea, having simulated them as viscoelastic sheets that we unfolded and ‘floated’ to the surface. The net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable (minimal net strain), and which parts of the mesh required additional tears and/or zones of localized ductile extension to have enabled the slab to deform during subduction. In the instance of the Ryukyu and Shikoku slabs, the Palau-Kyushu and Gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears. These subducted ridges are connected by a trench-parallel tear that represented the former contact between the Huatung and West Philippine basins. The fossil spreading centre of the Shikoku Basin formed a separate slab window upon subduction along the Nankai Trough. The methodology presented herein is a powerful tool to evaluate complex slab morphologies, infer the locations of slab tears, and therefore reconstruct intricate configurations of subducted oceanic lithosphere.

Beschreibung: We use lithospheric-scale analogue models to study the reactivation of preexisting heterogeneities under oblique shortening, and its relation to the origin of arcuate orogens. Reactivation of inherited rheological heterogeneities is an important mechanism for localization of deformation in compressional settings and consequent initiation of contractional structures during orogenesis. However, the presence of an inherited heterogeneity in the lithosphere is in itself not sufficient for its reactivation once the continental lithosphere is shortened. The heterogeneity orientation is important in determining if reactivation occurs and to which extent. This study aims at giving insights on this process by means of analogue experiments in which a linear lithospheric heterogeneity trends with various angles to the shortening direction. In particular, the key parameter investigated is the orientation (angle α) of a strong domain (SD) with respect to the shortening direction. Experimental results show that angles α ≥ 75° (high obliquity) allow for reactivation along the entire SD and the development of a linear orogen. For α ≤ 60° (low obliquity) the models are characterized by the development of an arcuate orogen, with the SD remaining partially non-reactivated. These results provide a new mechanism for the origin of some arcuate orogens, in which orocline formation was not driven by indentation or subduction processes, but by oblique shortening of inherited heterogeneities, as exemplified by the Ouachita orogen of the southern U.S.

Beschreibung: No abstract is available for this article.

Beschreibung: The northern North Sea rift basin developed on a heterogeneous crust comprising structures inherited from the Caledonian orogeny and Devonian post-orogenic extension. Integrating two-dimensional regional seismic reflection data and information from basement wells we investigate the pre-rift structural configuration in the northern North Sea rift. Three seismic facies have been defined below the base rift surface: 1) relatively low-amplitude and low-frequency reflections, interpreted as pre-Caledonian metasediments, Caledonian nappes and/or Devonian clastic sediments; 2) packages of high-amplitude dipping reflections (〉500 ms thick), interpreted as basement shear zones; and 3) medium-amplitude and high frequency reflections interpreted as less sheared crystalline basement of Proterozoic and Paleozoic (Caledonian) origin. Some zones of Seismic Facies 2 can be linked to onshore Devonian shear zones whereas others are restricted to the offshore rift area. Interpreted offshore shear zones dip S, ESE and WNW in contrast to W to NW dipping shear zones onshore W Norway. Our results indicate that Devonian strain and ductile deformation was distributed throughout the Caledonian orogenic belt from central South Norway to the Shetland Platform. Most of the Devonian basins related to this extension are, however, removed by erosion during subsequent exhumation. Basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters, e.g. in the Stord and East Shetland basins. Pre-rift structures with present-day dips 〉15° were reactivated, although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction.

Beschreibung: Study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the Central Atlantic conjugate margins are compared , based on compilation of wide-angle seismic profiles from NW-Africa Nova Scotian and US passive margins. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone . suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern Central Atlantic image differences in the continental crustal thickness. While profiles on the eastern US margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the African continental margin. In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Canadian side, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. After isochron M25, a large-scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides.

Beschreibung: The composition of the continental lower crust is not well known. High seismic wavespeeds may indicate mafic or garnet-bearing material, with implications for emplacement history, evolution, and rheological and dynamic behavior. In this contribution, we use recent seismic results from the EarthScope Transportable Array, compilations of active source studies, and selected xenolith studies to attempt to map the distribution of high-velocity lower crust across the continental U.S. and assess its relationship to proposed emplacement and destruction-related mechanisms such as under-and intraplating, collision, extension, heating, cooling, hydration, and delamination. Thin layers of high-velocity lower crust related to regional processes are found scattered throughout the continent. Thicker layers in large areas are found in the central and eastern U.S. in areas with thick crust, bounded roughly by the Rocky Mountain Front. Emplacement processes likely originally spanned this boundary, and the difference between the two domains may reflect garnet growth with cooling and aging of continental crust in much of the central and eastern U.S., while crustal thickness and lithospheric temperatures in the western U.S. are unfavorable for growth and maintenance of thick layers of high-velocity garnet-bearing lower crust.

Beschreibung: We present a new approach for evaluating existing crustal models using ambient noise datasets and its associated uncertainties. We use a transdimensional hierarchical Bayesian inversion (THBI) approach to invert ambient noise surface wave phase dispersion maps for Love and Rayleigh waves using measurements obtained from Ekstrom (2014). Spatiospectral analysis show that our results are comparable to a linear least-squares inverse approach (except at higher harmonic degrees), but the procedure has additional advantages: (1) it yields an auto-adaptive parameterization that follows earth structure without making restricting assumptions on model resolution (regularization or damping) and data errors (2) it can recover non-Gaussian phase velocity probability distributions while quantifying and separating the sources of uncertainties in the data measurements and modeling procedure and (3) it enables statistical assessments of different crustal models (e.g., CRUST1.0, LITHO1.0 and NACr14) using variable resolution residual and standard deviation maps estimated from the ensemble. These assessments show that in the stable old crust of the Archean, the misfits are statistically negligible requiring no significant update to crustal models from the ambient noise dataset. In other regions of the US, significant updates to regionalization and crustal structure are expected especially in the shallow sedimentary basins and the tectonically active regions, where the differences between model predictions and data are statistically significant.

Beschreibung: To decipher the Miocene evolution of the Shimanto belt of southwestern Japan, structural and paleo-thermal studies we carried out in the western area of Shikoku Island. All units constituting the belt, both in its Cretaceous and Tertiary domains, are in average strongly dipping to the NW or SE, while shortening directions deduced from fault kinematics are consistently orientated NNW-SSE. Peak paleo-temperatures estimated with Raman spectra of organic matter increase strongly across the southern, Tertiary portion of the belt, in tandem with the development of a steeply-dipping metamorphic cleavage. Near the southern tip of Ashizuri Peninsula, the unconformity between accreted strata and fore-arc basin, present along the whole belt, corresponds to a large paleo-temperature gap, supporting the occurrence of a major collision in Early Miocene. This tectonic event occurred before the magmatic event that affected the whole belt at ~15 Ma. The associated shortening was accommodated in two opposite modes, either localized on regional-scale faults such as the Nobeoka Tectonic Line in Kyushu, or distributed through the whole belt as in Shikoku. The reappraisal of this collision leads to reinterpret large-scale seismic refraction profiles of the margins, where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the Shimanto belt. When integrated into reconstructions of Philippine Sea Plate motion, the collision corresponds to the oblique collision of a paleo Izu-Bonin-Mariana Arc with Japan in Early Miocene.

Beschreibung: We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west-directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.

Beschreibung: The mechanisms that drove Permian – Triassic orogenesis in Australia and throughout the Cordilleran-type Gondwanan margin is a subject of debate. Here we present field-based results on the structural evolution of the Gympie Terrane (eastern Australia), with the aim of evaluating its possible role in triggering widespread orogenesis. We document several deformation events (D 1 -D 3 ) in the Gympie Terrane, and show that the earliest deformation, D 1 , occurred only during the final pulse of orogenesis (235-230 Ma) within the broader Gondwanide Orogeny. In addition, we found no evidence for a crustal suture, suggesting that terrane accretion was not the main mechanism behind deformation. Rather, the similar spatio-temporal evolution of Permian – Triassic orogenic belts in Australia, Antarctica, South Africa, and South America suggest that the Gondwanide Orogeny was more likely linked to large-scale tectonic processes such as plate-reorganization. In the context of previous work, our results highlight a number of spatial and temporal variations in pulses of deformation in eastern Australia, suggesting that shorter cycles of deformation occurred at a regional scale within the broader episode of the Gondwanide Orogeny. Similarly to the Cenozoic evolution of the central and southern Andes, we suggest that plate coupling and orogenic cycles in the late Paleozoic to early Mesozoic Gondwanide Orogeny have resulted from the superposition of mechanisms acting at a range of scales, perhaps contributing to the observed variations in the intensity, timing, and duration of deformation phases within the orogenic belt.

Beschreibung: This study presents detrital zircon analyses of 23 samples of the Cycladic Blueschist Unit (CBU) from Kea, Kythnos, and Serifos islands, as well as the Lavrion Peninsula of SE Attica. The maximum depositional ages (MDA) and age distributions of detrital zircon U-Pb dates are used to correlate metasediments between the islands considered herein and infer their provenance. Two distinct detrital zircon U-Pb age distributions are found in CBU metasediments: ‘Proterozoic’, comprised of 〉40% Neoproterozoic zircons with Triassic-Early Jurassic maximum depositional ages and ‘Paleozoic’, containing 〉30% Paleozoic zircons and yielding Late Jurassic-Cretaceous MDAs. Proterozoic affinity metasediments are rift-margin deposits derived from the northern Gondwanan margin. ‘Paleozoic’ metasediments are flysch sediments most probably sourced from the Internal Hellenides. This metamorphosed flysch forms a distinct marker horizon found in a similar structural position in Lavrion, Kythnos, and Serifos. Based on lithologic correlation, sediment provenance, and MDA estimates, the CBU of Kythnos is correlative to the Lavrion Schists of Attica. On the islands of Serifos and Kythnos and within the Lavrion Schists only young-on-old relationships exist between rocks based on MDA estimates.

Beschreibung: The Talesh Mountains at the NW margin of the Iranian Plateau curve around the southwestern corner of the south Caspian Block and developed in response to the collision of the Arabian-Eurasian plates. The timing, rates, and regional changes in late Cenozoic deformation of the Talesh Mountains are not fully understood. In this study, we integrate 23 new apatite and zircon bedrock U-Th/He ages and structurally restored geologic cross-sections with previously published detrital apatite fission track data to reconstruct the deformation history of the Talesh Mountains. Our results reveal that slow rock exhumation initiated during the late Oligocene (~27–23 Ma) and then accelerated in the middle Miocene (~12 Ma). These events resulted in the present day high elevation and curved geometry of the Mountains. The spatial and temporal distribution of cooling ages suggest that the Oligocene bending of the Talash Mountains was earlier than in the eastern Alborz, Kopeh- Dagh, and central Alborz Mountains that initiated during the late Cenozoic. Late Oligocene and middle Miocene deformation episodes recorded in the Talesh Mountains can be related to the collisional phases of the Arabian and Eurasian plates. The lower rate of exhumation recorded in the Talesh Mountains occurred during the initial soft collision of the Arabia – Eurasia plates in the late Oligocene. The accelerated exhumation that occurred during final collision since the middle Miocene resulted from collision of the harder continental margin.

Beschreibung: The Pamir-Tian Shan collision zone in the western Tarim Basin, northwest China, formed from rapid and ongoing convergence in response to the Indo-Eurasian collision. The arid landscape preserves suites of fluvial terraces crossing structures active since the Late Neogene that create fault and fold scarps recording Quaternary deformation. Using geologic and geomorphic mapping, dGPS surveys of deformed terraces, and OSL dating, we create a synthesis of the active structures that delineate the timing, rate, and migration of Quaternary deformation during ongoing convergence. New deformation rates on eight faults and folds, when combined with previous studies, highlight the spatial and temporal patterns of deformation within the Pamir-Tian Shan convergence zone during the Quaternary. Terraces spanning ~130 to ~8 ka record deformation rates between ~0.1 and 5.6 mm/yr on individual structures. In the westernmost Tarim Basin, where the Pamir and Tian Shan are already juxtaposed, the fastest rates occur on actively deforming structures at the interface of the Pamir-Tian Shan orogens. Farther east, as the separation between the Pamir-Tian Shan orogens increases, the deformation has not been concentrated on a single structure, but rather has been concurrently distributed across a zone of faults and folds in the Kashi-Atushi fold-and-thrust belt and along the NE Pamir margin, where shortening rates vary on individual structures during the Quaternary. Although numerous structures accommodate the shortening and the locus of deformation shifts during the Quaternary, the total shortening across the western Tarim Basin has remained steady and approximately matches the current geodetic rate of 6-9 mm/yr.

Beschreibung: No abstract is available for this article.

Beschreibung: In this study we present microstructural data from hydrothermal veins in the sedimentary cover and the igneous basement recovered from Hole U1414A, IODP Expedition 344 (Costa Rica Seismogenesis Project, CRISP), to constrain deformation mechanism operating in the subducting Cocos Plate. Cathodoluminescence studies, mechanical e-twin piezometry and electron backscatter diffraction analyses (EBSD) of carbonate veins were used to give insights into the deformation conditions and to help to understand the tectonic deformation history of the Cocos Plate offshore Costa Rica. Analyses of microstructures in the sedimentary rocks and in the basalt of the igneous basement reveal brittle deformation, as well as crystal-plastic deformation of the host rock and the vein material. CL images showed that in the basalt fluid flow and related precipitation occurred over several episodes. The differential stresses, obtained from two different piezometers using the same parameter (twin density), indicate various mean differential stresses of 49 ± 11 and 69 ± 30 MPa and EBSD mapping of calcite veins reveals low angle subgrain boundaries. Deformation temperatures are restricted to the range from 170 °C to 220 °C, due to the characteristics of the existing twins and the lack of high temperature intracrystalline deformation mechanisms (〉 220 °C). The obtained results suggest that deformation occurred over a period associated with changes of ambient temperatures, occurrence of fluids and hydrofracturing, induced differential stresses due to the bending of the plate at the trench and related seismic activity.

Beschreibung: Accretionary orogens are major sites of modern continental growth, yet their role in the development of Archean continental crust remains enigmatic. Diverse granitoid suites from tonalite-trondhjemite-granodiorite (TTG) to potassic granitoids appeared during late Archean, representing a period of major continental formation and stabilization. In this study, whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data are reported for Neoarchean granitoid gneisses from the Northern Liaoning Terrane, northeastern North China Craton (NCC). Older granitoid gneisses (~2592-2537 Ma) define three magmatic zones migrating from southeast to northwest, each showing a common magmatic evolution from high-pressure TTGs to medium-/low-pressure TTGs and potassic granitoids. They have depleted zircon ƐHf(t) of +0.5 to +8.7. Younger ~2529-2503 Ma potassic granitoids and TTGs occur throughout the terrane, which are marked by variable zircon ƐHf(t) of -4.7 to +8.1, and are coeval with regional high grade metamorphism. Petrogenetic modeling and changing Sr/Y and (La/Yb) N of the granitoids suggest that the crust experienced episodic thickening and thinning, and became progressively evolved through development of potassic granitoids and sedimentary successions. The metavolcanic basement to the granitoids display tholeiitic to calc-alkaline affinities, together with the top-to-the-northwest thrusting and associated VMS-type Cu-Zn deposits, suggesting cyclic crustal formation of Northern Liaoning within an accretionary orogen with a SE-dipping subduction polarity. Cyclic crustal thickening and thinning is related to tectonic switching from advancing to retreating relations between the downgoing and overriding plate. After ~2530 Ma, this accretionary system accreted to the ancient continental nucleus of NCC (Anshan-Benxi Terrane), signifying final lithosphere stabilization.

Beschreibung: Detrital thermochronology can be used as a tool to quantitatively constrain exhumation rates and its spatial variability from active mountain belts. Commonly usedmethods for this purpose assume a steady-state relationship between tectonic uplift and erosion. However, this assumption does not account for the transitory response of a dynamic orogenic system to changes in the boundary conditions.We propose a different approach that uses the observed detrital age distributions as “markers” of the pastexhumation, and of the present-day erosion and mixing occurring in a river system. In this paper, we present new 40 Ar/ 39 Ar biotite and white mica age distributions for nineteen modern river sands from the Eastern Alps north of the Periadriaticline. The results present three main clusters of ages at ~0.5-50, ~60-120, ~250-350 Ma that record the main orogenic phases in this sector of the Alps. We have applied two numerical methods to the cooling agesto a) linearly compute the spatial variability of the relative present-day erosion of a set of 4 detrital mineral samplesfrom drainage basins along the Inn river, andb) quantify the rates of the cooling and erosion inthe Tauern Window during Paleocene-Miocene time of the Alpine orogeny. Our results suggest a 0.34-0.84mm/yr range ofexhumation rates for the Tauern Window since the Miocene. Our estimates of exhumation rates of the western Tauern Window are higher than those for the eastern Tauern Window, which is consistent with the previous studies.

Beschreibung: In the Central Mediterranean the Africa-Eurasia convergence led to the development of complex orogenic systems and backarc basins. Throughout Pliocene-Quaternary times the Southern Apennines accretionary prism migrated towards the Apulia foreland and, contemporaneously, the Tyrrhenian Sea backarc basin opened. In this study, we investigated the offshore of the southern Apulia foreland and the Southern Apennines. Using seismic reflection profiles and well data in a dedicated GIS software, we made a detailed stratigraphic analysis, reconstructed the structural pattern and built 2D geological models of the main geological surfaces. The structure of the Apulia region formed during two main tectonic stages: lower Pleistocene complex extensional faulting developed in a transtensional regime and middle-upper Pleistocene transpression/shortening. The Southern Apennines accretionary prism developed through the Pliocene-early lower Pleistocene and was overprinted by late lower Pleistocene-middle Pleistocene NW-SE sinistral faults. Through linking new data from the accretionary prism and the foreland with previous information from the Tyrrhenian Sea back-arc, we provide an original interpretation for the relationships between the various provinces that make the Central Mediterranean crustal puzzle and reconstruct the main phases of the evolution of its Pliocene-Quaternary orogenic cycle. We recognized collisional stages of uncoupled plates, followed by late lower-middle Pleistocene post-collisional stages of coupled continental plates. This Pleistocene plates re-organization of Central Mediterranean was probably due to the rupture of the Apulia/Ionian slab or to NNW intraplate shortening transmitted from Africa.

Beschreibung: Our understanding of the style and rate of Quaternary tectonic deformation in the forearc of the Central Andes is hampered by a lack of field observations and constraints on neotectonic structures. Here, we present a detailed analysis of the Purgatorio fault, a recently recognized active fault located in the forearc of southern Peru. Based on field and remote sensing analysis (Pléiades DEM), we define the Purgatorio fault as a subvertical structure trending NW-SE to W-E along its 60 km length, connecting, on its eastern end, to the crustal Incapuquio Fault System. The Purgatorio fault accommodates right lateral transpressional deformation, as shown by the numerous lateral and vertical plurimetric offsets recorded along strike. In particular, scarp with a 5-m cumulative throw is preserved and displays cobbles that are cut and covered by slickensides. Cosmogenic radionuclide exposure dating ( 10 Be) of quartzite cobbles along the vertical fault scarp yield young exposure ages that can be bracketed between 0 to 6 ka, depending on the inheritance model that is applied. Our preferred scenario, which takes in account our geomorphic observations, implies at least two distinct rupture events, each associated with ~3 and ~2 m of vertical offset. These two events plausibly occurred during the last thousand years. Nevertheless, an interpretation invoking more tectonic events along the fault cannot be ruled out. This work affirms crustal deformation along active faults in the Andean forearc of southern Peru during the last thousand years.

Beschreibung: The timing and location of eclogite metamorphism is central to understanding subduction events responsible for the assembly of eastern Gondwana. The Attunga eclogite is one of only six eclogites in Australia, and occurs as small blocks within a schistose serpentinite mélange known as the Weraerai terrane, along the Peel Fault of the southern New England Orogen. Our zircon data reveals the presence of high Th/U oscillatory zoned magmatic zircon with a weighted mean 206 Pb/ 238 U age of 534 ± 14 Ma, and recrystallized metamorphic domains with an age of 490 ± 14 Ma. The latter have lower Th/U ratios, mostly no Eu anomalies and heavy rare earth element (HREE) depleted patterns. Garnet rims demonstrate that the final stages of garnet growth occurred in a HREE depleted environment, due to coeval formation with metamorphic zircon. Direct application of the Ti–in–zircon thermometer to metamorphic zircon yields temperatures of 770-610°C. Hf isotopic analyses of the zircons have an average εHf (t) of +13, indicating a juvenile crustal signature. We interpret the Attunga eclogite to be an indicator of late Cambrian subduction beneath an oceanic supra-subduction zone prior to accretion against eastern Gondwanan in the latest Devonian. Phillips et al. (2015) suggest two metamorphic age populations within the Attunga eclogite, based on U–Pb zircon and 40 Ar/ 39 Ar phengite data. These are ~ 515 Ma and ~ 480 Ma. We confirm this data but our zircon trace element chemistry data indicates the early Cambrian age (530 Ma) represents igneous protolith formation rather than eclogite metamorphism.

Beschreibung: NA

Beschreibung: The Zailisky Alatau is a 〉250-km-long mountain range in Southern Kazakhstan. Its northern rangefront around the major city of Almaty has more than 4 km topographic relief, yet in contrast to other large mountain fronts in the Tien Shan, little is known about its Late Quaternary tectonic activity despite several destructive earthquakes in the historical record. We analyse the tectonic geomorphology of the rangefront fault using field observations, differential GPS measurements of fault scarps, historical and recent satellite imagery, metre-scale topography derived from stereo satellite images, and decimetre-scale elevation models from UAV surveys. Fault scarps ranging in height from ~2 m to 〉20 m in alluvial fans indicate surface rupturing earthquakes occurred along the rangefront fault since the Last Glacial Maximum (LGM). Minimum estimated magnitudes for those earthquakes are M6.8-7. Radiocarbon dating results from charcoal layers in uplifted river terraces indicate a Holocene slip rate of ~1.2-2.2 mm/a. We find additional evidence for active tectonic deformation all along the Almaty rangefront, basinward in the Kazakh platform, and in the interior of the Zailisky mountain range. Our data indicate the seismic hazard faced by Almaty comes from a variety of sources, and we emphasize the problems related to urban growth into the loess-covered foothills and secondary earthquake effects. With our structural and geochronologic framework we present a schematic evolution of the Almaty rangefront that may be applicable to similar settings of tectonic shortening in the mountain ranges of Central Asia.

Beschreibung: The central Italy Apennines were rocket in 2016 by the strongest earthquakes of the past 35 years. Two mainshocks (Mw 6.2 and Mw 6.6) between the end of August and October caused the death of almost 300 people, and the destruction of 50 villages and small towns scattered along 40 km in the hanging wall of the N165° striking Mount Vettore fault system, i.e., the structure responsible for the earthquakes. The August 24 southern earthquake, besides causing all the casualties, razed to the ground the small medieval town of Amatrice, and dozens of hamlets around it. The October 30 mainshock crushed definitely all the villages of the whole epicentral area (up to 11 intensity degree), extending northward the level of destruction, and inducing heavy damage even to the 30-km-far Camerino town. The survey of the macroseismic effects started the same day of the first mainshock, and continued during the whole seismic sequence, even during and after the strong earthquakes at the end of October, allowing the definition of a detailed picture of the damage distribution, day by day. Here we present the results of the final survey in term of Mercalli-Cancani-Sieberg (MCS) intensity, which account for the cumulative effects of the whole 2016 sequence (465 intensity datapoints, besides 435 related to the August 24 and 54 to the October 26 events, respectively). The distribution of the highest intensity datapoints evidenced the lack of any possible overlap between the 2016 earthquakes and the strongest earthquakes of the region, making this sequence a unique case in the seismic history of Italy. In turn, the cross-matching with published paleoseismic data provided some interesting insights concerning the seismogenic behavior of the Mount Vettore fault in comparison with other active normal faults of the region.

Beschreibung: The Pamir orogen, Central Asia, is the result of the ongoing northward advance of the Indian continent causing shortening inside Asia. Geodetic and seismic data place the most intense deformation along the northern rim of the Pamir, but the recent December 7, 2015 Mw7.2 Sarez earthquake occurred in the Pamir's interior. We present a distributed slip model of this earthquake using co-seismic geodetic data and post-seismic field observations. The earthquake ruptured an ∼80km-long, sub-vertical, sinistral fault consisting of three right-stepping segments from the surface to ∼30km depth with a maximum slip of three meters in the upper 10km of the crust. The co-seismic slip model agrees well with en-échelon secondary surface breaks that are partly influenced by liquefaction-induced mass movements. These structures reveal up to 2m of sinistral offs et along the northern, low-offset segment of modeled rupture. The 2015 event initiated close to the presumed epicenter of the 1911 Mw7.3 Lake Sarez earthquake, which had a similar strike-slip mechanism. These earthquakes highlight the importance of NE trending sinistral faults in the active tectonics of the Pamir. Strike-slip deformation accommodates shear between the rapidly northward moving eastern Pamir and the Tajik basin in the west, and is part of the westward (lateral) extrusion of thickened Pamir-plateau crust into the Tajik basin. The Sarez-Karakul fault system and the two large Sarez earthquakes likely are crustal expressions of the underthrusting of the northwestern leading edge of the Indian mantle lithosphere beneath the Pamir.

Beschreibung: The Sikhote-Alin orogenic belt in Russian Far East is comprised of several N-S trending belts, including the Late Jurassic to Early Cretaceous accretionary prisms and turbidite basin which are now separated by thrusts and strike-slip faults. The origin and collage of the belts have been studied for decades. However, the provenance of the belts remains unclear. Six sandstone samples were collected along a 200-km long east-west traverse across the major belts in the southern Sikhote-Alin for U-Pb dating and Lu-Hf isotope analysis to constrain the provenance and evaluate the evolution of the northwest Pacific margin at this time. The result reveals that the sediments from the main Samarka belt was mainly from the adjacent Bureya-Jiamusi-Khanka Block (BJKB); the eastern Samarka belt and the Zhuravlevka turbidite basin were supplied by detritus from both the North China Craton (NCC) and the BJKB; the Taukha belt was mainly fed by sediments from the NCC; whereas the data from the Sergeevka nappes are insufficient to resolve their provenance. In the Late Jurassic to Early Cretaceous, collision and subduction was important in the initial collage of most belts in Sikhote-Alin. However, merely E-W trending collage cannot explain the increasing importance of the NCC provenance from west to east. It is proposed that the main Samarka belt was located adjacent to the BJKB when deposited, whereas the other belts were father south to accept the materials from the NCC. Sinistral strike-slip faulting transported the eastern belts northwards after their initial collage by thrusting.

Beschreibung: Yokelson et al. (2015) present U-Pb and Hf isotope analyses of detrital zircons from the Gravina belt showing that the western facies was derived from the west and the eastern facies was derived from the east, and concluded that this is evidence that a west dipping subduction zone did not exist along the inboard margin of the Alexander-Wrangellia terrane during Late Jurassic-Early Cretaceous time as proposed by Sigloch and Mihalynuk (2013). However, the tectonic affinity of the eastern Gravina belt is open to debate, and Yokelson et al.'s (2015) claim that the Sigloch and Mihalynuk (2013) model of west dipping subduction was implausible is based on an incorrect assumption.

Beschreibung: A peculiar feature of the Himalaya is the occurrence of a system of low-angle-normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during syn-convergent tectonics. We describe the STDS-related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body cross-cutting the STDS, occurs. A detailed multidisciplinary study, integrating field-based, microstructural, petrographic and geochronological analyses was carried out on rocks along this valley. We demonstrate how the non-coaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P) – temperature (T) conditions of 0.9-1.1 GPa and ≥ 750°C at ≥ 24 Ma. This migmatite has been reworked structurally upwards leading to the formation of high-T sillimanite-bearing mylonites. Further upward, medium-T shearing deformed the Malari granite and leucogranite dykes, forming medium-T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar-Ar geochronology, between ~20-15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement and deformation along low-angle-normal faults.

Beschreibung: Classically, deepwater fold-and-thrust belts are classified in two main types, depending if they result from near- or far-field stresses and the understanding of their driving and triggering mechanism is poorly known. We present a geophysical dataset off the western margin of New Caledonia (SW Pacific) that reveals deformed structures of a deepwater fold-and-thrust belt that we interpret as a near-field gravity-driven system, which is not located at a rifted passive margin. The main factor triggering deformation is inferred to be oversteepening of the margin slope by post-obduction isostatic rebound. Onshore erosion of abnormally-dense obducted material, combined with sediment loading in the adjacent basin, has induced vertical motions that have caused oversteepening of the margin. Detailed morpho-bathymetric, seismic stratigraphic and structural analysis reveals that the fold-and-thrust belt extends 200 km along the margin, and 50 km into the New Caledonia Trough. Deformation is rooted at depths greater than 5 km beneath the seafloor, affects an area of 3500 km 2 , and involves a sediment volume of approximately 13 000 km 3 . This deformed belt is organized into an imbricate fan system of faults, and one out-of-sequence thrust fault affects the seabed. The thrust faults are deeply rooted in the basin along a low-angle floor thrust and connected to New Caledonia Island along a major detachment. This study not only provides a better knowledge of the New Caledonia margin, but also provides new insight into the mechanisms that trigger deepwater fold-and-thrust belts.

Beschreibung: The easternmost Tianshan forms the eastern extremity of the modern Central Asian Orogenic Belt and represents a key locality to investigate strain propagation from the Meso-Cenozoic plate margins to the Eurasian interior. The Tianshan as a whole has been reactivated multiple times throughout the Meso-Cenozoic, but the extent of these reactivation events is yet to be fully understood. This study applies apatite fission track and apatite (U-Th-Sm)/He thermochronology to the mountain ranges of the easternmost Tianshan. Our new results suggest that the area experienced two phases of rapid cooling in the Mesozoic - during the early-middle Triassic and the late Cretaceous. These cooling phases are linked to tectonic events at the distant plate margins such as the Permian-mid Triassic closure of the Paleoasian Ocean and the late Jurassic – early Cretaceous Mongol Okhotsk orogeny. Fault controlled differential exhumation and block tilting are recorded in the distribution of apatite fission track ages across the region. Finally, we show through a combination of multiple thermochronometers and the integration of structural analysis that the easternmost Tianshan has experienced insufficient exhumation to constrain the timing of reactivation in response to the Cenozoic collision of India with Eurasia, and instead records older, Mesozoic tectonic events.

Beschreibung: The Alpine Tethys rifted margins were generated by a Mesozoic polyphase magma-poor rifting leading to the opening of the Piedmont-Ligurian “Ocean”. This latter developed through different phases of rifting that terminated with the exhumation of sub-continental mantle along an extensional detachment system. At the onset of simple shear detachment faulting, two margin-types were generated: an upper and a lower plate corresponding to the hanging-wall and footwall of the final detachment system, respectively. The two margin architectures were markedly different and characterized by a specific asymmetry. In this study the detailed analysis of the Adriatic margin, exposed in the Serie dei Laghi, Ivrea-Verbano and Canavese Zone, enabled to recognize the diagnostic elements of an upper plate rifted margin. This thesis contrasts with the classic interpretation of the Southalpine units, previously compared with the adjacent fossil margin preserved in the Austroalpine nappes and considered as part of a lower plate. The proposed scenario suggests the segmentation and flip of the Alpine rifting system along strike, and the passage from a lower to an upper plate. Following this interpretation, the European and Southern Adria margins are coevally developed upper plate margins, respectively resting NE and SW of a major transform zone that accommodates a flip in the polarity of the rift system. This new explanation has important implications for the study of the pre-Alpine rift-related structures, for the comprehension of their role during the reactivation of the margin and for the palaeogeographic evolution of the Alpine orogen.

Beschreibung: Observations and modeling show that temperature controls crustal rheology and therefore also the orogenic evolution of continent-continent collision zones and the associated tectonic style. In order to explore the effect of temperature in a natural environment, we compare eroded sections through the unusually cold lower-Paleozoic North Atlantic Caledonian (Scandian) collision zone and the very hot Brasiliano/Pan-African Araçuaí-West Congo orogen. A cold and stiff subducting Caledonian continental margin was able to subduct as a rather coherent unit to ultrahigh pressure conditions, twice as deep as the Pan-African/Brasiliano crust that was quickly heated and softened and got involved in partial melting. Furthermore, the Caledonian collision developed large coherent thrust sheets that were transported hundreds of kilometers toward the foreland. This was never achieved in the hot Araçuaí-West Congo orogen, where much of the tectonic stress was absorbed by the partially molten central part of the orogen through magmatic state deformation. Major mylonite zones (thrusts) such as those seen in the Caledonides are therefore less common in the Araçuaí-West Congo orogen. Further, the deep continental subduction in the Caledonides developed a strongly asymmetric collision zone, with rapid variations in pressure and temperature. In contrast, the Araçuaí-West Congo orogen soon developed into a more symmetric geometry due to its easily flowing hot crust, with a relatively flat base and a corresponding plateau in its upper part. Deformation of the cold Caledonian crust was controlled by plate-tectonic stress, while gravitational forces more strongly influenced the hot Brasiliano/Pan-African example.

Beschreibung: Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca . 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.

Beschreibung: The Andes between 28°-30°S represent a transition between the Puna-Altiplano Plateau and the Frontal/Principal Cordillera fold-and-thrust belts to the south. While significant early Cenozoic deformation documented in the Andean Plateau, deciphering the early episodes of deformation during Andean mountain building in the transition area is largely unstudied. Apatite fission track (AFT) and (U-Th-Sm)/He (AHe) thermochronology from a vertical and a horizontal transect reveal the exhumation history of the High Andes at 30°S, an area at the heart of this major transition. Interpretation of the age-elevation profile, combined with inverse thermal modeling indicate that the onset of rapid cooling was underway by ~35Ma, followed by a significant decrease in cooling rate at ~30-25Ma. AFT thermal models also reveal a second episode of rapid cooling in the early Miocene (ca. 18Ma) related to rock exhumation to its present position. Low exhumation between the rapid cooling events allowed for the development of a partial annealing zone (PAZ). We interpret the observed Eocene rapid exhumation as the product of a previously unrecognized compressive event in this part of the Andes that reflects a southern extension of Eocene orogenesis recognized in the Puna/Altiplano. Renewed early-Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also impacted the core of the range in this transitional sector. The major episode of Eocene exhumation suggests the creation of significant topographic relief in the High Andes earlier than previously thought.

Beschreibung: The Meso-Cenozoic alpine belts of the Mediterranean area are characterized by complex architectures, result of a complex subduction and collision evolution that preserve also a legacy of the rifting-related configuration of the continental margins. The Northern Apennines is a segment of these belts originated during closure of the Ligure-Piemontese ocean, and collision between the Europe and Adria plates. The different configuration of the Adria and Europe margins, inherited from asymmetric rifting, is recorded in the Ligurian Units, that preserve incorporation into the subduction factory of fragments of the oceanic domain (Internal Ligurian Units), and portions of the Ocean-Continent Transition Zone (OCTZ) toward Adria (External Ligurian Units). We provide here unpublished data on the stratigraphy and sedimentology of these units, together with a review of what is already established in literature. Both datasets combined testify that at 80 M.a., an accretionary prism was growing between the deposition basins of the two groups of units, and fed both basins with clasts from the ocean realm, the continental crust and the subcontinental mantle. We propose that closure of the Ligure-Piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned Adria margin, and developed a double-vergent prism by accreting oceanic material and continental extensional allochthons from the OCTZ. We believe the revised site of subduction initiation, and the pre-collisional architecture, inherited from the rifting and spreading phases, allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre-collisional evolution of the Apennines.

Beschreibung: The evolution of the continental intra-arc Taupo Rift in the North Island, New Zealand is rapid, significantly faster than comparative intra-continental rifts such as the African Rifts. Based on our faulting data and published geological, geophysical and borehole data, we show that activity in the ~2 Ma Taupo Rift has rapidly and asymmetrically narrowed via inward and eastward migration of faulting (at rates of ca. 30 km My -1 and 15 km My -1 , respectively) and has propagated southwards along its axis ~70 km in 350 kyr. The loci of voluminous volcanic eruptions and active faulting are correlated in time and space, suggesting that a controlling factor in the rapid rift narrowing is the presence of large shallow heterogeneities in the crust, such as large rhyolitic magma bodies generated by subduction processes, which weaken the crust and localize deformation. Eastward migration of faulting also follows the eastward migration of the volcanic arc which may be related to rollback of the Pacific crust slab at the Hikurangi subduction zone. Southward propagation of the rift is linked with southward migration of the Hikurangi plateau/Chatham Rise subduction point and occurs episodically aided by stress changes associated with voluminous local volcanism. The large magma supply during early continental intra-arc rift stages explains faster evolution (from tectonic to magmatic) than intracontinental rifts. However, the fast changes in magma supply from the subduction zone can also lead to evolution reversals (more evolved magmatic stages reverting to less evolved tectonic stages), rift cessation, and thus failed continental break-up.

Beschreibung: Seismic reflection data along volcanic passive margins frequently provide imaging of strong and laterally continuous reflections in the middle and lower crust. We have completed a detailed 2D seismic interpretation of the deep crustal structure of the Vøring Margin, offshore mid-Norway, where high-quality seismic data allow the identification of high-amplitude reflections, locally referred to as the T-Reflection. Using a dense seismic grid we have mapped the geometry of the T-Reflection in order to compare it with filtered Bouguer gravity anomalies and seismic refraction data. The T-Reflection is identified between 7 and 10 s. Sometimes it consists of one single smooth reflection. However, it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries, amplitudes and contact relationships. The T-Reflection seems to be connected to deep sill networks and is locally identified at the continuation of basement high structures or terminates over fractures and faults. The spatial correlation between the filtered positive Bouguer gravity anomalies and the deep dome-shaped reflections indicates that the latter represent a high impedance boundary contrast associated with a high density and velocity body. In ~50% of the outer Vøring Margin, the depth of the mapped T-Reflection is found to correspond to the depth of the top of the lower crustal body (LCB) which is characterized by high P-wave velocities (〉 7 km/s). We present a tectonic scenario, where a large part of the deep crustal structure is composed of preserved upper continental crustal blocks and middle to lower crustal lenses of inherited high-grade metamorphic rocks. Deep intrusions into the faulted crustal blocks are responsible for the rough character of the T-Reflection, whereas intrusions into the ductile lower crust and detachment faults are likely responsible for its smoother character. Deep magma intrusions can be responsible for regional metamorphic processes leading to an increasing velocity of the lower crust to more than 7 km/s. The result is a heterogeneous LCB that likely represents a complex mixture of pre- to syn-breakup mafic and ultramafic rocks (cumulates and sills) and old metamorphic rocks such as granulites and eclogites. An increasing degree of melting towards the breakup axis is responsible for an increasing proportion of cumulates and sill intrusions in the lower crust.

Beschreibung: The geometric transformation of a descending plate, such as from steep to flat subduction in response to a change from normal to overthickened oceanic crust during subduction, is a common and important geological process at modern or fossil convergent margins. However, the links between this process and the metamorphic evolution of the exhumation of oceanic (ultra)high pressure eclogites are poorly understood. Here, we report detailed petrological, mineralogical, phase equilibria, and secondary ion mass spectrometry (SIMS) zircon and rutile U-Pb age data for the Dong Co eclogites at the western segment of the Bangong–Nujiang suture zone, central Tibet. Our data reveal that the Dong Co eclogites experienced peak eclogite-facies metamorphism (T = 610–630 °C, P = 2.4–2.6 GPa) and underwent multiple stages of retrograde metamorphism. P–T pseudosections and compositional isopleths of garnet define a complex clockwise P–T–t path (including two stages of decompression-dominated P–T path and one of isobaric heating), suggesting varying exhumation velocities. Combining previous studies with our new results, we suggest that the transformation from rapid to slow exhumation is dominated by the transition from steep to flat subduction. The flat-slab segment, caused by subduction of buoyant oceanic plateau, led to an extremely slow exhumation and a strong overprinting of HP granulite facies at a depth of ~50 km at ~177 Ma. The slab roll-back that followed in response to a substantial density increase of the eclogitized oceanic plateau resulted in another rapid exhumation process at ~168 Ma and triggered the formation of abundant near-simultaneous or later magmatic rocks.

Beschreibung: This work firstly presents field structural analysis, Anisotropy of Magnetic Susceptibility (AMS) measurements, kinematic and microstructural studies on the Neoproterozoic Pengguan complex located in the middle segment of the Longmenshan thrust belt (LMTB), NE Tibet. These investigations indicate that the Pengguan complex is a heterogeneous unit with a ductilely deformed NW domain and an undeformed SE domain, rather than a single homogeneous body as previously thought. The NW part of the Pengguan complex is constrained by top-to-the-NW shearing along its NW boundary and top-to-the-SE shearing along its SE boundary, where it imbricates and overrides the SE domain. Two orogen-perpendicular gravity models not only support the imbricated shape of the Pengguan complex, but also reveal an imbrication of high density material hidden below the Paleozoic rocks to the west of the LMTB. Regionally, this suggests a basement-slice-imbricated structure that developed along the margin of the Yangtze Block, as shown by the regional gravity anomaly map, together with the published nearby seismic profile and the distribution of orogen-parallel Neoproterozoic complexes. Integrating the previously published ages of the NW-normal faulting and of the SE-directed thrusting, the locally fast exhumation rate, and the lithological characteristics of the sediments in the LMTB front, we interpret the basement-slice-imbricated structure as the result of SE-ward thrusting of the basement slices during the Late Jurassic-Early Cretaceous. This architecture makes a significant contribution to the crustal thickening of the LMTB during the Mesozoic, and therefore the Cenozoic thickening of the Longmenshan belt might be less important than often suggested.

Beschreibung: Abastract At the eastern Qilian Shan mountain front in the NE Tibetan Plateau, the Minle-Damaying Fault (MDF), the southernmost fault of the North Frontal Thrust (NFT) system, has previously been proposed as an inactive structure during the Holocene. Here we present a detailed record of six strath terraces of the Xie River, that document the history of active deformation of the MDF. One optically stimulated luminescence dating sample constrains abandonment of the highest terrace T 6 at 12.7 ± 1.4 ka. The formation ages of the lower terraces (T 4 -T 1 ) are dated by AMS 14 C dating. The cumulative vertical offsets of the MDF recorded by these terraces are determined as 12.2 ± 0.4 m (T 6 ), 8.0 ± 0.4 m (T 5 ), 6.4 ± 0.4 m (T 4 ), 4.6 ± 0.1 m (T 3 ), and 3.2 ± 0.2 m (T 1c ) by an unmanned aerial vehicle system, respectively. A long-term vertical slip rate of the MDF of 0.9 ± 0.2 mm/yr is then estimated from the above data of terrace age and vertical offset by a linear regression. Assuming that the fault dip of 35 ± 5 ° measured at the surface is representative for the depth-averaged fault dip, horizontal shortening rates of 0.83 – 1.91 mm/yr are inferred for the MDF. Our new data show that, the proximal fault (the MDF) of the NFT system at the eastern Qilian Shan mountain front has remained active when the deformation propagated basinwards, a different scenario from that observed at both the western and central Qilian Shan mountain front.

Beschreibung: Mantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole-bearing spinel lherzolites with microstructures attesting for extensive annealing. Olivine and pyroxene CPO (crystal preferred orientation) preserve nevertheless the record of coeval deformation during a pre-annealing tectonic event. Olivine shows transitional CPO symmetry from [010]-fiber to orthogonal type. In most samples with [010]-fiber olivine CPO symmetry, the [001] axes of the pyroxenes are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed most probably during the transpressional phase after the Penninic slab break-off in the Eastern Alps. The calculated seismic properties of the xenoliths indicate that a significant portion of shear-wave splitting delay times in the Styrian Basin (0.5 s out of ca. 1.3 s) may originate in the a highly annealed subcontinental lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher concentrations in the more annealed textures. Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large-scale geodynamic events in the region - the Miocene extension then tectonic inversion of the Pannonian Basin.

Beschreibung: Combined zircon geochemistry and geochronology of Mesozoic volcaniclastic sediments of the central Transantarctic Mountains, Antarctica, yield a comprehensive record of both the timing and geochemical evolution of the magmatic arc along the Antarctic sector of the paleo–Pacific margin of Gondwana. Zircon age populations at 266–183 Ma, 367–328 Ma, and 550–490 Ma correspond to episodic arc activity from the Ediacaran to the Jurassic. Zircon trace element geochemistry indicates a temporal shift from granitoid–dominated source(s) during Ediacaran to Early Ordovician times to mafic sources in the Devonian through Early Jurassic. Zircon initial ε Hf shifts to more radiogenic Hf isotope compositions following the Ross Orogeny, and is inferred to reflect juvenile crustal growth within an extensional arc system during progressive slab rollback. These new ages and Hf isotopic record is similar to that from the Australian sector, indicating these regions constituted an ~3000 km laterally continuous extensional arc from at least the Carboniferous to the Permian. Conversely, the South American sector records enriched zircon Hf isotopic compositions and compressional/advancing arc tectonics during the same time period. Our new data constrain the location of this profound along–arc geochemical and geodynamic ‘switch’ to the vicinity of the Thurston Island block of West Antarctica.

Beschreibung: A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the Central High Atlas (Morocco). These gabbros were emplaced in the core of pre-existing structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to post-emplacement tectonic movements. The absence of paleo-position markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small-circles with horizontal axes, indicating horizontal-axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. AMS axes using the rotation routine inferred from paleomagnetic results, resulting in more tightly clustered magnetic lineations which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures.

Beschreibung: Reactivation of extensional structures is commonly inferred during rift evolution. In that context, we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the Coral Sea region, Papua New Guinea. The first event (R 1 ), poorly documented, occurred during the Triassic along an older N-S Permian structural fabric. During the Jurassic, extensional faults were reactivated through a second extensional episode (R 2 ) which formed small (~10/20km) basins bounded by N-S, NE-SW and E-W listric faults. Extension prolonged during the Lower Cretaceous with seafloor spreading in the Owen Stanley Oceanic Basin, now incorporated in the Papuan fold and thrust belts. A third Late Cretaceous extensional phase (R 3 ) gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent-ocean transition. Seafloor spreading in the Coral Sea followed from Danian to Ypresian. This extensional system is sealed by unequally-preserved Eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from Oligocene onward. This overall evolution suggests various extensional systems that are geographically and temporarily defined the one another. The early rifting of the crust is controlled by pre-existing continental features resulting in the local Pangaea breakup. In contrast, the Coral Sea propagator cuts through the rifted margin, is controlled by a subduction complex in accordance with the Tasman Sea opening. This evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around Australia.

Beschreibung: Key Points •Oversimplified the structure of the Manning Basin. •Ignored critical geological elements in their model.

Beschreibung: n/a

Beschreibung: Contractional deformation in Costa Rica is usually attributed to the subduction of the aseismic Cocos ridge. In this work, we review the evidences for contraction in the middle to late Miocene, prior to the arrival of the Cocos ridge at the Middle America Trench. We find that the Miocene phase of contractional deformation is found in all Costa Rica, probably extending to Nicaragua as well. The widespread distribution of this event requires a regional or plate geodynamic trigger. We analyze the possible mechanisms that could produce the onset of contractional deformation, using the better known case of subduction orogeny, the Andes, as an analogue. We propose that a change in the direction of the Cocos plate since ∼19 Ma led to a change from oblique to orthogonal convergence, producing contractional deformation of the upper plate.

Beschreibung: The southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use eleven broadband seismometers to implement receiver function analysis using H-κ stacking, a method sensitive to crustal thickness and V P /V S ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance and phase-weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by V P /V S ratios of 1.75-1.77, implying a felsic bulk composition. Crust beneath the fault bounded basins has high V P /V S (〉1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high V P /V S . However, elevated V P /V S values within fault-bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively un-extended Tanzanian craton and Bangweulu Block are 41.6-42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault-bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early-stage continental rifting in the Western rift, Africa.

Beschreibung: The continental crust involved in the Alpine orogeny was largely shaped by Paleozoic tectono-metamorphic and igneous events during oblique collision between Gondwana and Laurussia. In order to shed light on the pre-Alpine basement puzzle disrupted and re-amalgamated during the Tethyan rifting and the Alpine orogeny, we provide SHRIMP U-Pb zircon and geochemical whole rock data from selected basement units of the Grand St Bernard – Briançonnais nappe system in the Western Alps, and from the Penninic and Lower Austroalpine units in the Central Alps. Zircon U-Pb ages, ranging from 459.0 ± 2.3 Ma to 279.1 ± 1.1 Ma, provide evidence of a complex evolution along the northern margin of Gondwana including Ordovician transtension, Devonian subduction and Carboniferous-to-Permian tectonic reorganization. Original zircon U-Pb ages of 371 ± 0.9 Ma and 369.3 ± 1.5 Ma, from calcalkaline granitoids of the Grand Nomenon and Gneiss del Monte Canale units, provide the first compelling evidence of Late Devonian orogenic magmatism in the Alps. We propose that rocks belonging to these units were originally part of the Moldanubian domain, and were displaced towards the SW by Late Carboniferous strike-slip faulting. The resulting assemblage of basement units was disrupted by Permian tectonics and by Mesozoic opening of the Alpine Tethys. Remnants of the Moldanubian domain became either part of the European paleomargin (Grand Nomenon unit) or part of the Adriatic paleomargin (Gneiss del Monte Canale unit), to be finally accreted into the Alpine orogenic wedge during the Cenozoic.

Beschreibung: The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust sills, and intruded as sills and dikes at shallower depths. In the Main Ethiopian rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward-dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km-thick crust. In the 30Ma-Recent CVL, which lacks a hotspot age-progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hotspot towards the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology and contributing significantly to global carbon and water fluxes.

Beschreibung: The thickness and bulk composition of continental crust provide important constraints on the evolution and dynamics of continents. Crustal mineralogy and thickness both may influence gravity anomalies, topographic elevation and lithospheric strength, but prior to the inception of EarthScope's USArray, seismic measurements of crustal thickness and properties useful for inferring lithology are sparse. Here we improve upon a previously-published methodology for joint inversion of Bouguer gravity anomalies and seismic receiver functions by using parameter-space stacking of cross-correlations of modeled synthetic and observed receiver functions instead of standard H-κ amplitude stacking. The new method is applied to estimation of thickness and bulk seismic velocity ratio, v P / v S , of continental crust in the conterminous United States using USArray and other broadband network data. Crustal thickness variations are reasonably consistent with those found in other studies and show interesting relationships to the history of North American continental formation. Seismic velocity ratios derived in this study are more robust than in other analyses, and hint at large-scale variations in composition of continental crust. To interpret the results, we model the pressure/temperature-dependent thermodynamics of mineral formation for various crustal chemistries, with and without volatile constituents. Our results suggest that hydration lowers bulk crustal v P / v S and density, and releases heat in the shallow crust but absorbs heat in the lowermost crust (where plagioclase breaks down to pyroxene and garnet, resulting in higher seismic velocity). Hence, v P / v S variations may provide a useful proxy for hydration state in the crust.

Beschreibung: The Junggar–Balkhash Ocean was a major branch of the southern Paleo-Asian Ocean. The timing of its closure is important for understanding the history of the Central Asian Orogenic Belt. New sedimentological and geochronological data from the Late Paleozoic volcano-sedimentary sequences in the Barleik Mountains of West Junggar, NW China, help to constrain the closure time of the Junggar–Balkhash Ocean. Tielieketi Formation (Fm) is dominated by littoral sediments, but its upper glauconite-bearing sandstone is interpreted to deposit rapidly in a shallow-water shelf setting. By contrast, Heishantou Fm consists chiefly of volcanic rocks, conformably overlying or in fault contact with Tielieketi Fm. Molaoba Fm is composed of parallel-stratified fine sandstone and sandy conglomerate with graded bedding, typical of non-marine, fluvial deposition. This formation unconformably overlies the Tielieketi and Heishantou formations and is conformably covered by Kalagang Fm characterized by a continental bimodal volcanic association. The youngest U–Pb ages of detrital zircons from sandstones and zircon U–Pb ages from volcanic rocks suggest that the Tielieketi, Heishantou, Molaoba, and Kalagang formations were deposited during the Famennian–Tournaisian, Tournaisian–early Bashkirian, Gzhelian, and Asselian–Sakmarian, respectively. The absence of upper Bashkirian to Kasimovian was likely caused by tectonic uplifting of the West Junggar terrane. This is compatible with the occurrence of coeval stitching plutons in the West Junggar and adjacent areas. The Junggar–Balkhash Ocean should be finally closed before the Gzhelian, slightly later or concurrent with that of other ocean domains of the southern Paleo-Asian Ocean.

Beschreibung: No abstract is available for this article.

Beschreibung: Recent models support the view that the Pyrenees were formed after the inversion of a previously highly extended continental crust which included exhumed upper mantle rocks. Mantle rocks remain near to the surface after compression and mountain building, covered by the latest Cretaceous to Paleogene sequences. 3D lithospheric-scale gravity inversion demands the presence of a high-density mantle body placed within the crust in order to justify the observed anomalies. Exhumed mantle, having ~50 kilometers of maximum width, continuously extends beneath the Basque-Cantabrian Basin and along the northern side of the Pyrenees. The association of this body with rift, post-rift and inversion structural geometries are tested in a balanced cross-section across the Basque-Cantabrian Basin that incorporates a major south-dipping ramp-flat-ramp extensional detachment active between Valanginian and early Cenomanian times. Results indicate that horizontal extension progressed circa 48 km at variable strain rates which increased from 1 to ~4 mm/yr in middle Albian times. Low-strength Triassic Keuper evaporites and mudstones above the basement favor the decoupling of the cover with formation of minibasins, expulsion roll-overs and diapirs. The inversion of the extensional system is accommodated by doubly-verging basement thrusts due to the reactivation of the former basin bounding faults in Eocene-Oligocene times. Total shortening is estimated in ~34 km and produced the partial subduction of the continental lithosphere beneath the two sides of the exhumed mantle. Obtained results help to pinpoint the original architecture of the North-Iberian Margin and the evolution of the hyperextended aborted intracontinental basins.

Beschreibung: The Cenozoic deformation history of Central Iran has been dominantly accommodated by the activation of major intra-continental strike-slip fault zones, developed in the hinterland domain of the Arabia-Eurasia convergent margin. Few quantitative temporal and kinematic constraints are available from these strike-slip deformation zones, hampering a full assessment of the style and timing of intraplate deformation in Iran and the understanding of the possible linkage to the tectonic re-organization of the Zagros collisional zone. This study focuses on the region to the north of the active trace of the sinistral Doruneh Fault. By combing structural and low-temperature apatite fission track (AFT) and (U-Th)/He (AHe) thermochronology investigations, we provide new kinematic and temporal constraints to the deformation history of Central Iran. Our results document a post-Eocene polyphase tectonic evolution dominated by dextral strike-slip tectonics, whose activity is constrained since the early Miocene in response to an early, NW-SE oriented paleo-σ1 direction. A major phase of enhanced cooling/exhumation is constrained at the Miocene/Pliocene boundary, caused by a switch of the maximum paleo-σ1 direction to N-S. When integrated into the regional scenario, these data are framed into a new tectonic reconstruction for the Miocene-Quaternary time lapse, where strike-slip deformation in the intracontinental domain of Central Iran is interpreted as guided by the re-organization of the Zagros collisional zone in the transition from an immature to a mature stage of continental collision.

Beschreibung: In carbonate terranes, rocks types that provide apatite are not available to effectively use apatite fission track (AFT) or (U/Th)-He chronometry (AHe). Here we suggest that calcite cave spar can be an effective chronometer and complimentary to AFT and AHe thermochronometers in carbonate regions such as our study area, the Guadalupe Mountains of southeastern New Mexico and west Texas. Our measured depth of cave spar deposition is 500 ± 250 meters beneath the regional water table, formed at temperatures of 40° to 80° C, indicating these caves and their spar crystals form near the supercritical CO 2 -subcritical CO 2 boundary where we interpret the origin of both the caves and spar to occur. This depth-temperature relationship suggests a higher than normal geotherm, likely associated with regional magmatic activity. As a case study we examined the timing of uplift of the Guadalupe Mountains previously attributed to the compressional Laramide orogeny (ca. 90 to 50 Ma), later extensional tectonics associated with Basin and Range (ca. 36 to 28 Ma) or the opening of the Rio Grande Rift (ca. 20 Ma to present). We show that most of the spar origin is coeval with the ignimbrite flare-up between 36 – 28 Ma. Our results constrain the initiation of Guadalupe Mountains block uplift, relative to the surrounding terrain, to between 27-16 Ma and reconstruct the evolution of a low-lying regional landscape prior to block uplift from 185 to 28 Ma, in support of models that attribute regional surface uplift to extensional tectonics and associated volcanism.

Beschreibung: Unraveling the temperature distribution and composition of Earth's crust is key for understanding its origin, evolution and mechanical behavior. Models of compressional (V P ) and shear wave (V S ) velocity are obtained from seismological studies and can be interpreted in terms of temperature and composition, using relationship defined through laboratory experiments. These empirical evidences often do not properly account for the effects driven by temperature, pressure, water content and phase change of minerals. In this study, we use thermodynamic modeling to properly investigate the role of these variables in affecting seismic properties, as a tool to guide (joint-) inversion and interpretation of geophysical data. We find that mineralogical phase transitions can be more seismically relevant than a change in chemical composition. In particular, the α-β quartz transition would cause a jump in acoustic impedance and V P /V S ratio 〉8%, occurring in the 15-25 km depth range, depending on the thermal gradient. Moreover, in the case of a cold lower crust, the consumption of plagioclase in favor of high-velocity minerals might represent another relevant seismic discontinuity. Different chemical compositions proposed for the Italian crust would be seismically indistinguishable, since they give overlapping seismic properties. Values of V S 〈3.6 km s -1 would imply a strong contribution of sediments and/or partial melt. The V S /density ratio shows a narrow variability, suggesting that densities at depth can be directly derived in first approximation from V S .

Beschreibung: Global distribution of the centroid of magnetic layer, Zo , was estimated from spectral analysis of aeromagnetic and marine magnetic anomalies using recently released new global digital magnetic anomaly data, WDMAM version 2.0. Zo distribution provides a comprehensive view of regional-scale trends of lithospheric thickness; Zo are generally shallower in oceanic-ridges, and deeper older continents, which is consistent with previous works. Zo is closely related to reasonable average thermal regime and its correspondence with tectonic regime indicates that Zo is useful to delineate regional crustal thermal structure. It is expected that Zo combined with multidisciplinary data should help to infer geophysical and geological information in less explored regions.

Beschreibung: A new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large-scale D2 extensional shear zone (i.e. Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite-rich rocks. The lower layer consists of high-grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large-scale D3 folds (e.g. Marofa synform), later deflected and refolded by a D4 strike-slip shear zone (i.e. Juzbado-Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of post-kinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

Beschreibung: An M w 6.2 earthquake occurred in Central Italy on August 24, 2016. The objective of this study was to reveal the imprint of rupture directivity using the strong motion recordings. The strong motion stations were separated into two groups: southeast (SE) and northwest (NW). The effects of rupture directivity on the peak ground acceleration (PGA), peak ground velocity (PGV), and pseudo spectral acceleration (PSA) were investigated. The observed values of these parameters were compared with predicted values derived from ground motion prediction equations. The results showed the residuals between the observed and predicted PGAs, PGVs, and PSAs at periods of T 〈 1 s were correlated significantly with azimuth angle and generally larger in the NW sector, reflecting that the observed PGAs, PGVs, and short-period PSAs in the NW sector were generally larger than observed in the SE sector. These phenomena are accordant with the theoretical law that the rupture directivity causes higher amplitudes in the forward direction compared with the backward direction. Finally, selected source rupture parameters were inverted using PGAs and PGVs. This revealed the rupture was predominantly unilateral rupture, the major rupture was likely at an azimuth of ~360°, and the length of the major rupture was proportional to 70%–100% of the total ruptured fault, confirming that rupture directivity caused the differences in the ground motions observed in the SE and NW sectors.

Beschreibung: ABSTRACT The thermal evolution of distal domains along rifted margins is at present poorly constrained. In this study, we show that a thermal pulse, most likely triggered by lithospheric thinning and asthenospheric rise, is recorded at upper crustal levels and may also influence the diagenetic processes in the overlying sediments, thus representing a critical aspect for the evaluation of hydrocarbon systems. The thermal history of a distal sector of the Alpine Tethys rifted margin preserved in the Ligurian Alps (Case Tuberto-Calizzano unit) is investigated with thermochronological methods and petrologic observations. The studied unit is composed of a polymetamorphic basement and a sedimentary cover, providing a complete section through the pre-, syn- and post-rift system. Zircon fission-track analyses on basement rocks samples suggest that temperatures exceeding ~240±25 °C were reached before ~150-160 Ma (Upper Jurassic) at few kilometres depth. Neo-formation of green biotite, stable at temperatures of ~350 to 450°C, was syn-kinematic with this event. The tectonic setting of the studied unit suggests that the heating-cooling cycle took place during the formation of the distal rifted margin and terminated during Late Jurassic (150-160 Ma). Major crustal and lithospheric thinning likely promoted high geothermal gradients (~60-90°C/km) and triggered the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly. Our results suggest that rifting can generate thermal perturbations at relatively high temperatures (between ~240 and 450°C) at less than 3 km depth in the distal domains during major crustal thinning preceding breakup and onset of seafloor spreading.

Beschreibung: A new kinematic reconstruction that incorporates estimates of post-20 Ma shortening and extension in the Apennines, Alps, Dinarides and Sicily Channel Rift Zone (SCRZ) reveals that the Adriatic microplate (Adria) rotated counter-clockwise as it subducted beneath the European Plate to the west and to the east, while indenting the Alps to the north. Minimum and maximum amounts of rotation are derived by using, respectively, estimates of crustal extension along the SCRZ (minimum of 30 km) combined with crustal shortening in the eastern Alps (minimum of 115 km), and a maximum amount (140 km) of convergence between Adria and Moesia across the southern Dinarides and Carpatho-Balkan orogens. When combined with Neogene convergence in the western Alps, the best fit of available structural data constrains Adria to have moved 113 km to the NW (azimuth 325°) while rotating 5 ± 3° counter-clockwise relative to Europe since 20 Ma. Amounts of plate convergence predicted by our new model exceed Neogene shortening estimates of several tens of kilometers in both the Apennines and Dinarides. We attribute this difference to crust-mantle decoupling (delamination) during roll-back in the Apennines and to distributed deformation related to the northward motion of the Dacia Unit between the southern Dinarides and Europe (Moesia). Neogene motion of Adria resulted from a combination of Africa pushing from the south, the Adriatic-Hellenides slab pulling to the northeast and crustal wedging in the western Alps, which acted as a pivot and stopped further northwestward motion of Adria relative to Europe.

Beschreibung: The metasupracrustal units within the north-central Chelan block of the North Cascades Range, Washington are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have: (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (ca. 134–79 Ma). Sediment was likely underthrust into the active arc by ca. 74–65 Ma, as soon as 7 m.y. after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (ca. 79–60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.

Beschreibung: The Turkana Depression of northern Kenya, lies at the intersection of the NW-SE trending late Mesozoic-early Paleogene South Sudan and Anza rifts and the N-S trending late Paleogene-Recent East African Rift System (EARS). A low-temperature thermochronology study in the Lapur Range reveals a complex tectonothermal evolution related to multiple periods of regional and local tectonism. Zircon (U-Th)/He data from Precambrian basement record rapid Early Cretaceous denudational cooling. Coeval subsidence in the adjacent Anza and South Sudan rifts suggests that the northern Turkana region acted as a basement high separating the grabens, as well as an axial source of sediment. Between ~95-90 Ma, a period of reheating commenced with burial of Lapur basement beneath ~500 m of Turonian-Eocene Lapur Sandstone and ~1.5-3.5 km of latest Eocene-early Miocene Turkana Volcanics. Apatite fission track (AFT) and apatite (U-Th-Sm)/He data record a transition to rapid denudational cooling in the mid Miocene (~14 Ma) in response to EARS-related extension in the northern Turkana Basin. Thermal history models indicate the Lapur Range experienced ~90-100 °C of mid-Miocene to Plio-Pleistocene cooling, yielding the first Neogene AFT ages reported from Kenya related to EARS exhumation. We attribute the larger magnitude of cooling in the Lapur Range compared to other regions of the EARS to the attenuated crustal thickness and elevated heat flow of the Turkana Depression, crustal properties inherited from earlier Cretaceous-Paleogene rifting. The resulting low effective elastic thickness of the Turkana lithosphere allowed for increased isostatic footwall uplift in response to EARS extension.

Beschreibung: Recent geophysical imaging indicates that the Hayward Fault hard-links to the Rodgers Creek Fault at 5 m depth within the San Pablo Bay, CA, suggesting that earthquakes may be able to rupture continuously through the fault network. To investigate fault propagation, interaction and linkage in segmented fault networks, including those within the San Pablo Bay, we simulate the development of two idealized, underlapping faults within an extensional step over at seismogenic depths using work optimization. We test the sensitivity of fault growth to strength anisotropy, material heterogeneities and initial fault geometry. The optimal faults propagate toward each other until linking with the other fault at its tip, and form a single hard-linked transverse fault. These faults propagate with relatively high propagation power, or rate of efficiency gain. Less efficient faults form wider basins, and develop with reduced propagation power. Models with initial fault geometries that more closely match the shallowly imaged Hayward and Rodgers Creek faults suggest that the faults link at seismogenic depths if a mapped segment of the Rodgers Creek that extends into the San Pablo Bay is currently inactive. Predictions of average slip rate, slip per earthquake and earthquake magnitude from these models closely match paleoseismic estimates. The hard linkage of the Hayward and Rodgers Creek faults imaged in the near-surface, and predicted by these models, increases local seismic hazard by increasing the upper limit of through-going earthquakes to M 7.6.

Beschreibung: We welcome the comments of van Hinsbergen et al. [2017] on the recent efforts of Barnett-Moore et al. [2016] . Specifically, van Hinsbergen et al. [2017] raise concerns about two of the major conclusions made by Barnett-Moore et al. [2016] . Firstly, Barnett-Moore et al. [2016] choose to negate the Cretaceous Iberian paleomagnetic database as a viable plate kinematic constraint on the plate motions of Mesozoic Iberia. This conclusion, criticized by van Hinsbergen et al. [2017], was based on citing the previous efforts of Neres et al. [2012; 2013] , which exposed several shortcomings, elaborated on below, within this dataset. Secondly, van Hinsbergen et al. [2017] criticize Barnett-Moore et al. [2016] for dismissing mantle tomographic interpretations in support of a preserved Cretaceous Pyrenean ‘subducted slab’ beneath northern Africa. Below, we have addressed each of these major criticisms from van Hinsbergen et al. [2017] in a two-section layout, similar to their comment above.

Beschreibung: No abstract is available for this article.

Beschreibung: ABSTRACT Low-angle detachment fault systems are important elements of oblique-divergent plate boundaries, yet the role detachment faulting plays in development of such boundaries is poorly understood. The west Salton detachment fault (WSDF) is a major low-angle normal fault that formed coeval with localization of the Pacific-North America plate boundary in the northern Salton Trough, CA. Apatite U-Th/He thermochronometry (AHe; n=29 samples) and thermal history modeling of samples from the Santa Rosa Mountains (SRM) reveal that initial exhumation along the WSDF began at ca. 8 Ma, exhuming footwall material from depths of 〉2 to 3 km. An uplifted fossil (Miocene) helium partial retention zone is present in the eastern SRM, while a deeper crustal section has been exhumed along the Pleistocene high-angle Santa Rosa fault (SFR) to much higher elevations in the southwest SRM. Detachment-related vertical exhumation rates in the SRM were ~0.15 – 0.36 km/Myr, with maximum fault slip rates of ~1.2 – 3.0 km/Myr. Miocene AHe isochrons across the SRM are consistent with northeast crustal tilting of the SRM block, and suggest the post-WSDF vertical exhumation rate along the SRF was ~1.3 km/Myr. The timing of extension initiation in the Salton Trough suggests that clockwise rotation of relative plate motions that began at 8 Ma is associated with initiation of the southern San Andreas system. Pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low- to high-angle faulting, and indicates local fault geometry may at times exert a fundamental control on rock uplift rates along strike-slip-fault systems.

Beschreibung: In the High Andes of Central Chile, above the flat-slab segment, analysis of more than 1,000 fault-slip data from Miocene outcrops provides evidence for a change of the regional tectonic regime from compressional to strike-slip. This shift in tectonic regime occurred during the waning stages of arc volcanism between 14 and 11 Ma, as a result of the shallowing of the Nazca plate, in conjunction with the migration of deformation to the Precordillera. During the early to middle Miocene, a compressive regime with horizontal σ 1 axis (N86°E) was responsible for reverse slip along NNE to N-striking faults. During the late Miocene, a shift to strike-slip tectonics took place due to an increase in the absolute magnitude of the vertical stress component as the crust thickened and the gravitational potential energy increase. We argue that instead of the previously accepted highly compressional setting in the arc region during the slab flattening, the change to a strike-slip regime was the main control on mineralization. Mineralization was controlled by the promotion of fluid expulsion from the magma chambers along active, sub-vertical strike-slip fault systems with a high slip tendency, and focusing of fluids in localized areas undergoing extension. Under this strike-slip regime, the El Indio, Tambo and La Despensa fault systems formed as dextral strike-slip systems. The tips and jog-sites along these faults experienced local extensional stress fields, forming the El Indio and Tambo mineral districts.

Beschreibung: The Tumblagooda Sandstone in Western Australia documents Early Paleozoic rifting in East Gondwana and provides the earliest evidence for terrestrial activity of multicellular animals on Earth. We constrain the provenance of this sequence using 737 concordant U–Pb ages of detrital zircons from two stratigraphic wells in the northern Perth Basin and Southern Carnarvon Basin, and outcrops of the type sections near Kalbarri. Detrital zircon age signatures are linked to infrared spectral data and stratigraphic logs. These ages span 3312-466 Ma, including major Precambrian age peaks at 1079-1544 Ma, 1695-2403 Ma, and 2640-2879 Ma, consistent with igneous sources in the West Australian Craton. Significant Early Paleozoic age peaks at 499-541 Ma suggest a North Indian Orogen source. The maximum depositional age is constrained by the youngest detrital zircon, which yields an age of 466±8 Ma. Our age constraints imply that terrestrial activity of multicellular animals on Earth may not be older than Middle Ordovician (Darriwilian). Rifting resulted in the exposure of the Yilgarn Craton and the segmentation of the Pinjarra Orogen. The Northampton Complex segment of the Pinjarra Orogen constituted a basement high that separated sub-basins during the onset of rifting. Discordant Archean zircons provide a consistent record of radiogenic Pb loss at ca. 470 Ma, which we interpret as being related to the denudation of the Yilgarn Craton. The Pb loss event suggests that intracratonic rifting in the Pinjarra Orogen was initiated in the Middle Ordovician, after the Kuunga Orogeny completed the final amalgamation of Gondwana.

Beschreibung: The cause of intraplate seismic zones persists as an important scientific and societal question. Most intraplate earthquakes are concentrated in specific seismic zones along or adjacent to large-scale basement structures (e.g., rifts or sutures at ancient plate boundaries) within continental crust. The major intraplate seismic zones are limited to specific segments and are not distributed along the lengths of the ancient structures. We present a new hypothesis that major intraplate seismic zones are restricted to places where concentrated crustal deformation (CCD) is overprinted on large-scale basement structures. Examples where CCD affects the stability of specific parts of large-scale structures in response to present-day stress conditions include the most active seismic zones in central and eastern North America: Charlevoix, Eastern Tennessee, and New Madrid. Our hypothesis has important implications for the assessment of seismic hazards.

Beschreibung: The Eger Rift is an active element of the European Cenozoic Rift System associated with intense Cenozoic intraplate alkaline volcanism and system of sedimentary basins. The intracontinental Cheb Basin at its western part displays geodynamic activity with fluid emanations, persistent seismicity, Cenozoic volcanism, and neotectonic crustal movements at the intersections of major intraplate faults. In this paper, we study detailed geometry of the crust/mantle boundary and its possible origin in the western Eger Rift. We review existing seismic and seismological studies, provide new interpretation of the reflection profile 9HR and supplement it by new results from local seismicity. We identify significant lateral variations of the high-velocity lower crust and relate them to the distribution and chemical status of mantle-derived fluids and to xenolith studies from corresponding depths. New interpretation based on combined seismic and isotope study point to a local-scale magmatic emplacement at the base of the continental crust within a new rift environment. This concept of magmatic underplating is supported by detecting two types of the lower crust: a high-velocity lower crust with pronounced reflectivity and a high velocity reflection-free lower crust. The character of the underplated material enables to differentiate timing and tectonic setting of two episodes with different times of origin of underplating events. The lower crust with high reflectivity evidences magmatic underplating west of the Eger Rift of the Late Variscan age. The reflection-free lower crust together with a strong reflector at its top at depths of ~28-30 km forms a magma body indicating magmatic underplating of the Late Cenozoic (Middle and Upper Miocene) to recent. Spatial and temporal relations to recent geodynamic processes suggest active magmatic underplating in the intracontinental setting.

Beschreibung: The kinematics of deformation in the overriding plate of convergent margins may vary across timescales ranging from a single seismic cycle to many millions of years. In Northeast Japan, a network of active faults has accommodated contraction across the arc since the Pliocene, but several faults located along the inner forearc experienced extensional aftershocks following the 2011 Tohoku-oki earthquake, opposite that predicted from the geologic record. This observation suggests that forearc faults may be favorable for stress triggering and slip inversion, but the geometry and deformation history of these fault systems are poorly constrained. Here we document the Neogene kinematics and subsurface geometry of three prominent forearc faults in Tohoku, Japan. Geologic mapping and dating of growth strata provide evidence for a 5.6-2.2 Ma initiation of Plio-Quaternary contraction along the Oritsume, Noheji and Futaba faults, and an earlier phase of Miocene extension from 25 to 15 Ma along the Oritsume and Futaba faults associated with the opening of the Sea of Japan. Kinematic modeling indicates these faults have listric geometries, with ramps that dip ~40-65° W and sole into subhorizontal detachments at 6-10 km depth. These fault systems can experience both normal and thrust sense slip if they are mechanically weak relative to the surrounding crust. We suggest that the inversion history of Northeast Japan primed the forearc with a network of weak faults mechanically and geometrically favorable for slip inversion over geologic timescales, and in response to secular variations in stress state associated with the megathrust seismic cycle.

Beschreibung: In their recent paper, Barnett-Moore et al. [2016] reflect on current models of Iberian plate motion in the Jurassic and Cretaceous as well as ongoing debates on the reliability of the various types of kinematic data that form independent constraints on Iberia's motion relative to Eurasia. They question the validity of various marine geophysical, seismic, tomographic, geological, and paleomagnetic datasets from the Bay of Biscay, Central Atlantic Ocean and Iberia for kinematic reconstruction of Iberia and conclude that neither models invoking Aptian-Albian transtension, nor compression, are consistent with currently available data. An important element in their analysis is that they discard the large paleomagnetic dataset from the Jurassic and Cretaceous from Iberia based on perceived limitations of that dataset. In addition, they argue that seismic tomographic images exclude a scenario of subduction in the Aptian-Albian in the Pyrenees, and based on this “ question the validity of current plate reconstructions, their constraints, and geodynamic scenarios, which are in support of this scenario [e.g., Vissers et al., 2016]” . We welcome the discussion raised by Barnett-Moore et al. [2016] on the reliability and usefulness of paleomagnetic data as independent constraint for Iberia's plate motion in the Mesozoic. Taking these paleomagnetic data at face value, Vissers et al. [2016] recently showed that these are consistent with a ~40° counterclockwise rotation of Iberia in the Aptian, requiring up to 500 km of Aptian convergence across the Pyrenees, i.e. through subduction. In this comment, we aim to critically assess whether and how the concerns on the quality of paleomagnetic data raised by Barnett-Moore et al. [2016] may allow for an alternative explanation, particularly one with a Mesozoic rotation of Iberia that is small enough so as to not requiring subduction. We also re-assess whether seismic tomographic images indeed refute subduction scenarios, using 8 S-wave and P-wave tomographic models including those used in Barnett et al . [2016].