SEGMENTATION AND KINEMATICS OF PLATE-MARGIN FAULT SYSTEMS IN THE SOUTHERN GULF OF CALIFORNIA

In the southern Gulf of California, integrated transtensional plate motion has been accommodated on three mechanically and/or spatially distinct fault systems that have reactivated preexisting crustal weaknesses. Initial rifting (c.a. 12.5 Ma) occurred across continental dextral-normal faults on either side of Baja California isolating it as a microplate. (1) West of Baja California, oblique dextral-normal faults and major east-down normal faults have reactivated and reversed the sense of subduction across the accretionary wedge. Single faults in this domain commonly extend 100’s of km and have anomalously large length:displacement ratios due to the reactivation of the structural grain of the accretionary complex. (2) East of Baja California, a major system of predominantly east-down normal faults and oblique dextral-normal faults extends 300 km from Loreto to Los Cabos. This system follows the core facies of the Miocene volcanic arc that may have thermally weakened the crust and most faults strike NNW subparallel to the regional foliation of Cretaceous batholith. Single faults generally do not extend more than 100 km and the rifted margin can be subdivided into three main segments. Much of the early dextral shearing in this domain was accommodated by vertical axis rotation of tectonic blocks like the Sierra Trinidad and Isla Carmen. The mainland rifted margin is also strongly thinned, but the style of faulting is more difficult to characterize because of greater sedimentation. (3) Although all three fault systems are still active, most of the modern plate motion occurs across an en echelon array of transform faults and oceanic spreading centers that exists along the axis of the gulf. Well developed oceanic ridges like the East Pacific and Alarcon Rises in the south contrast markedly with the series deep (3-4 kmbsl), magmatically-starved, fault-bounded depressions like the Farallon and Carmen basins immediately north. The northern spreading centers commonly overlap each other and seismicity becomes more broadly distributed indicating the plate margin is still being dynamically organized into a system of transforms and spreading centers. The largest oceanic spreading centers in the southern gulf (Alarcon and Carmen) nucleated in accommodation zones between the main segments of the Baja rifted margin.