LATE CENOZOIC TECTONIC EVOLUTION OF SOUTHWESTERN CALIFORNIA

We synthesize available geologic and geophysical data from southern California and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern Coast Ranges, western Transverse Ranges, and borderland regions of California since 30 Ma. Premises include specified relative plate motions for times prior to 10.5 Ma and after 3 Ma, a mid-Tertiary bight in the continental margin of southern California, midcrustal detachment faults under much of California and adjoining regions, rotation of the western Transverse Ranges about an eastern pivot, and specified fault displacement histories. Prior to 18 Ma, about 90% of the tangential component of Pacific-North America relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope. From 18 Ma to 5.5 Ma, dextral slip was accommodated predominantly on the offshore system but also on a second, inboard system that included the San Andreas fault. Fault-bounded crustal blocks between the offshore and inboard fault systems underwent translation, rotation, and extension from 18 Ma to 11.5 Ma but not from 11.5 Ma to 5.5 Ma. Since 5.5 Ma, dextral slip has been accommodated predominantly on the inboard fault system; dextral slip ceased on the offshore fault system north of the Transverse Ranges but continues on faults west of or within Baja California. The San Gregorio-San Simeon-Hosgri fault system has accommodated less than 5 km of the late Cenozoic dextral slip. The distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction, and, to a lesser degree, changes in magnitude of Pacific-North America relative motion. Based on our reconstruction, the tangential component of Pacific-North America relative motion was approximately 90mm/yr for the period 10.5 Ma to 5.5 Ma and approximately 60mm/yr for the period 5.5 Ma to 3 Ma.