Strain partitioning in highly oblique rift settings: Inferences from the southwestern margin of the Gulf of California (Baja California Sur, Mexico)

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 dipâEUR 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.