ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.