Crustal Rotation and Fault Slip in the Continental Transform Zone in Southern California

Abstract

Computations from global plate circuits show that since the end of Oligocene time (24 Ma) the Pacific plate has moved northwestward relative to southern California 1072 ± 122 km (J. Stock and P. Molnar, 1988). During part of this period displacement across the San Andreas fault was 315 ± 15 km (5% error?). Paleomagnetic data infer that the Transverse Ranges crustal blocks rotated clockwise since 16 Ma, about 86 degrees west of the fault and 41 degrees on the east. These rotations were simultaneous with NW- SE dextral shear distributed on faults in non-rotating crust north and south of the Transverse Ranges. This shear is computed to be 265 ± 38 km west of the San Andreas and 110 ± 11 km east of it. The sum of the distributed shear and San Andreas slip is 690 ± 42 km.This sum is 382 ± 129 km less than the displacement between the plates. The difference is significant, and the missing displacement should be looked for in the eastern Mojave Desert (east of the Granite Mountain fault) and west of the western Transverse Ranges (west of the Hosgri fault). Because the timing of the rotations and slip on the San Andreas are also known, the partitioning of displacements across southern California can be described as a function of time. During Early Miocene time 45 km of dextral slip occurred on the northern San Andreas fault. An area of extended crust in the Mojave Desert also rotated 50 degrees clockwise during this time. During Middle Miocene time most of the shear occurred north and south of the western Transverse Ranges as they rotated almost 60 degrees. During this same interval the Tehachapi Mountains rotated clockwise about 41 degrees and the northern San Andreas fault may have slipped 30 km. During Late Miocene time the rotation rate decreased in the west. The northern San Andreas fault slipped about 55 km which may have been transferred to the San Gabriel fault and faults in the southern California borderland. Since Late Miocene time the western Transverse Ranges rotated 30 to 40 degrees and the San Andreas slipped 185 km north of the Transverse Ranges and 240 km in southern California. The eastern Transverse Ranges and portions of the central Mojave Desert rotated 41 degrees clockwise, presumably at the same time.