ALBERT

Description: Interpretation of new multichannel seismic reflection profiles indicates that the Palomares margin formed by crustal-scale extension and coeval magmatic accretion during middle to late Miocene opening of the Algero-Balearic basin. The margin formed at the transition between thinned continental crust intruded by arc volcanism and back-arc oceanic crust. Deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between ~N50°E striking reverse faults and associated folds like the Sierra Cabrera and Abubacer anticlines, and N10-20°E sinistral strike-slip faults like Palomares and Terreros faults. Parametric sub-bottom profiles and multibeam bathymetry offshore, structural analysis, available GPS geodetic displacement data and earthquake focal mechanisms jointly indicate that tectonic inversion of the Palomares margin is currently active. The Palomares margin shows a structural pattern comparable to the north Maghrebian margins where Africa-Eurasia plate convergence is accommodated by NE-SW reverse faults, NNW-SSE sinistral faults and WNW-ESE dextral ones. Contractive structures at this margin contribute to the general inversion of the Western Mediterranean since ~7 Ma ago, coeval to inversion at the Algerian margin. Shortening at the Alboran ridge and Al-Idrisi faults occurred later, since 5 Ma, indicating a westward propagation of the compressional inversion of the Western Mediterranean.

Description: A P-wave seismic velocity model has been obtained for the Central Iberian Zone, the largest continental fragment of the Iberian Variscan Belt. The spatially dense, high-resolution, wide-angle seismic reflection experiment ALCUDIA-WA, was acquired in 2012 across central Iberia, aiming to constrain the lithospheric structure and resolve the physical properties of the crust and upper mantle. The seismic transect, ~310 km long crossed the Central Iberian Zone from its suture with the Ossa-Morena Zone to the southern limit of the Central System mountain range. The energy generated by 5 shots, was recorded by ~900 seismic stations. High amplitude phases were identified in every shot gather for the upper crust (Pg and PiP) and Moho (PmP and Pn). In the upper crust, the P-wave velocities increase beneath the Cenozoic Tajo Basin. The base of the upper crust varies from ~13 km to ~20 km between the southernmost Central Iberian Zone and the Tajo Basin. Lower crustal velocities are more homogeneous. From SW-NE, the travel-time of PmP arrivals varies from ~10.5 s to ~11.8 s, indicating lateral variations in the P-wave velocity and the crustal thickness, reflecting an increase towards the north related with alpine tectonics and the isostatic response of the crust to the orogenic load. The results suggest that the high velocities of the upper crust near the Central System might correspond to igneous rocks and/or high grade metamorphis rocks. The contrasting lithologies and the increase in the Moho depth to the north evidence differences in the Variscan evolution.

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

Description: 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.

Description: 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.

Description: The Rheic Ocean suture resulted from pre-Carboniferous oceanic subduction followed by Late Devonian-Carboniferous Variscan collision. In SW Iberia, this suture has been classically located along the boundary between the Ossa-Morena and South Portuguese Zones based on the presence of three units: (i) A conspicuous metamafic unit (Beja-Acebuches) that crops out along this boundary and has been interpreted as a pre-Carboniferous Rheic Ocean ophiolite; (ii) a low-grade metasedimentary unit with minor MORB-like metabasalts (Pulo do Lobo unit), thought to represent a Rheic Ocean subduction-related accretionary prism; and (iii) the allochthonous Cubito-Moura unit that contains high-pressure and ophiolitic-like rocks. We report new structural and geochronological data that allow us to reinterpret the origin and internal structure of the Beja-Acebuches and the Pulo do Lobo units. Thus, both the Beja-Acebuches protoliths and the Pulo do Lobo metabasalts would have been formed in the context of an intracollisional extensional stage that interrupted the Variscan collision at early Carboniferous time, after the Rheic Ocean consumption and the first continental collision. Later on, collision was resumed in an oblique left-lateral regime that gave way to coeval frontal (folds and thrusts) and lateral (shear zones and strike-slip faults) structures, with variable pressure-temperature conditions and space distribution along time. As a consequence of the superposition of transtension and complex transpression, the Rheic suture in SW Iberia has an obscure nearly cryptic appearance.

Description: The El Salvador Fault Zone (ESFZ) is an active, c. 150 km long and 20 km wide, segmented, dextral strike-slip fault zone within the Central American Volcanic Arc striking N100° E. Although several studies have investigated the surface expression of the ESFZ, little is known about its structure at depth and its kinematic evolution. Structural field data and mapping suggest a phase of extension, at some stage during the evolution of the ESFZ. This phase would explain dip-slip movements on structures that are currently associated with the active, dominantly strike-slip and that do not fit with the current tectonic regime. Field observations suggest trenchward migration of the arc. Such an extension and trenchward migration of the volcanic arc could be related to slab roll-back of the Cocos Plate beneath the Chortis Block during the Miocene/Pliocene. We carried out 4D analogue model experiments to test whether an early phase of extension is required to form the present-day fault pattern in the ESFZ. Our experiments suggest that a two-phase tectonic evolution best explains the ESFZ: an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures. This extensional phase is followed by a strike-slip dominated regime, which results in inter-segment areas with local transtension and segments with almost pure strike-slip motion. The results of our experiments combined with field data along the Central American Volcanic Arc indicate that the slab roll-back intensity beneath the Chortís Block is greater in Nicaragua and decreases westward to Guatemala.

Description: Abstract We present a 2D p‐wave velocity model and a coincident multichannel seismic reflection profile characterizing the structure of the southern Costa Rica margin and incoming Cocos Ridge. The seismic profiles image the ocean and overriding plates from the trench across the entire offshore margin, including the structures involved in the 2002 Osa earthquake. The overriding plate consists of three domains: Domain I displays thin‐skinned deformation of an imbricate thrust system composed of fractured rocks. Domain II shows ~15 km‐long landward‐dipping reflection packages and active deformation of the shelf sediment. Domain III is little fractured and appears to be dominated by elastic deformation, overlain by ~2 km‐thick landward‐dipping strata. The velocity structure supports the argument that the bulk of the margin is highly consolidated rock. Thick‐skinned tectonics probably causes the uplift of Domains II and III. The oceanic plate shows crustal thickness variations from ~14 km at the trench (Cocos Ridge) to 6‐7 km beneath the shelf. We combine (1) interplate geometry and fracturing degree, (2) tectonic stresses and brittle strain, and (3) earthquake locations, to investigate relationships between structure and earthquake generation. The 2002 Osa sequence nucleated at the leading flank of subducting seamounts in the area of highest tectonic overpressure. Both estimated rock fracturing and modelled brittle strain steadily increase from the leading flank of the subducting seamounts to their top, reflecting the progressive damage caused by the seamount. Therefore, the seismicity and structural‐mechanical evolution of the upper plate reflect the downward propagation of the leading edge of seamounts.

Description: Abstract This study aims to analyze the modalities of strain accommodation within a highly oblique rift, taking the Gulf of California as a prototype. Rifting in the Gulf of California is accomplished by intra‐Gulf strike‐slip (transform) faults, and mostly dip‐slip displacement on the rift‐margin faults. We have collected fault‐slip data and samples for radiometric dating at selected sites in southeastern Baja California, which is host to the southwestern margin of the rift. We have identified three styles of faulting, particularly (1) WSW‐dipping normal faults, (2) E‐ENE‐dipping normal faults, and (3) steep NNE‐NE‐trending left‐lateral faults. The E‐ENE‐dipping normal faults define the western margin of the Gulf of California rift and are most likely coeval (Late Miocene to Recent) with both the ~NNE‐NE‐trending left‐lateral faults and some of the WSW‐dipping faults. Fault‐slip data have often been collected on potentially active Gulf of California rift‐margin faults, which invariably display dominant dip‐slip kinematics (generally with minor dextral component). Distribution of extension directions determined from stress inversion of brittle fault kinematic data indicates a peak of 080°‐090°, which is strikingly similar to the orientations of T axes from earthquake focal mechanisms of both rift‐margin normal/faults and intra‐Gulf strike‐slip faults. These findings suggest that this stretching may have been occurring throughout the protracted rift history. Furthermore, highly oblique rifts do not show across‐rift variations in the orientation of local extension, which is instead typical of continental rifts with lower obliquity.