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.
In the deep seated gravity-driven deformation systems of the Gulf of Mexico contemporaneous extension and contraction of the overburden is favored by mechanical decoupling from the basement along thick salt sequences (up to 4 km). The updip extension is located inland, on the continental shelf of northeast Mexico, and is characterized by extensional listric faults and roll-overs; the downdip shortening zone is located at the deep waters and is characterized by a fold and thrust belt detached above the salt layer. Two physical experiments are used to discuss some aspects of these gravity-driven systems. The experimental setup includes a motor-driven experimental table, an inclined brittle basement (1°), a silicone layer simulating the salt sequences, and sand layers simulating the pre-kinematic Jurassic-Cretaceous strata before Laramide shortening. Deformation resulted in further tilting of the basement (3° to 4°). After the onset of deformation, thin sand layers were added at regular time intervals simulating the syntectonic sedimentation. The experiments reproduced the geometry of the deformation at the frontal ramp characterized by a seaward vergent thrust and its associated deformed region (the Perdido fold belt). The fold and thrust belt localization was favored by the change in basement inclination (a built-in slope change). Key elements interpreted in one available section of the area were reproduced in the model: a) the presence of an antithethic roll-over in the extensional zone and, b) the basinward vergence of folds and thrusts observed in the downdip shortening zone in the mexican Perdido fold belt.
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