Abstract: Miocene Palinspastic Restoration of Southwestern California

FRITSCHE, A. EUGENE, California State University, Northridge, CA 91330-8266

Previous models that attempted to restore southwestern California to its pre-Miocene configuration fell short of the goal because three constraints of the geologic data base were not adhered to in constructing the model. First, published paleomagnetic data from the western Transverse Ranges indicate that the amount of clockwise rotation increases from ~37 degrees in the San Gabriel Mountain block to ~130 degrees in the Point Conception area. This means that prior to rotation, the western Transverse Ranges, which are now relatively straight, were bent at an ~90 degree angle. Second, rocks that are presently on opposite sides of the San Gabriel fault need to be backrotated along the fault and must not lose contact with the fault surface. Third, the Mesozoic rocks in the Santa Monica Mountains must be restored to a position immediately north of their truncated counterparts at the northern end of the Santa Ana Mountains. These constraints are incorporated into a new model by (1) back-rotating the San Gabriel block ~37 degree in a counterclockwise direction; (2) closing the gap created by Miocene transrotational extension by back-sliding the Santa Monica Mountains rocks along the curved San Gabriel fault in a counterclockwise, rotational, left-slip direction across the eastern Los Angeles basin until the Mesozoic rocks in the Santa Monica and Santa Ana Mountains are juxtaposed; and (3) bending the rocks in the westernmost Transverse Ranges, west of the Santa Monica Mountains, in an additional ~90 degree counterclockwise back-rotation until they come into contact with the southern California coast. When these three steps are completed, the Eocene forearc basin deposits of the San Diego and western Transverse Ranges areas are restored into straight alignment with the Eocene forearc basin deposits to the north in the San Rafael and Sierra Madre areas, as would be expected with forearc basin geometry. This new model shows that the San Gabriel fault was not created initially as an early transform fault of the San Andreas system, but was initiated as a curved fault surface by the rotation process.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California