PALEOTECTONICS OF SOUTHERN CALIFORNIA 48-0 MA BASED ON FINITE-ELEMENT RECONSTRUCTION

Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains and rotations, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate and stress direction at 6 m.y. intervals. The input data derive from a thorough literature search from 49° to 20° N. On average, specific fault offsets, cross-section reconstructions and paleomagnetically determined rotations of the model slightly underfit target data. This systematic underfitting is compensated at regional scale by continuum strain. Piercing-point pairs restored along the San Andreas-Gulf of California plate boundary include the Chocolate Mountains anticlinorium at 6 Ma, the Pinnacles/Neenach Volcanics at 22 Ma, and the Jolla Vieja and Poway conglomerates adjacent their Sonoran source at 48-42 Ma. Offset of 70 km along the San Gabriel-Canton fault system continued south along the Cristianitos fault and faults hidden beneath the younger Los Angeles basin. Reversal of extension on the Oceanside detachment fault places San Nicolas Island adjacent to Rosarito at 18 Ma. Since 18 Ma, the western Transverse Ranges rotated 70° clockwise; sinistral faults in this province originated with NNE trends. First contact between the Pacific and North America plates at 28 Ma was associated with NW-SE extension in the Plush Ranch-Vasquez-Diligencia basins. The 48 Ma map was modified in order to better match paleomagnetic and geologic data: 1. the Tehachapi block was rotated counterclockwise 44 degrees; 2. adjoining Mesozoic plutonic rocks of the Salinian block were also rotated counterclockwise and translated eastward; 3. the Chocolate Mountains anticlinorium was rotated counterclockwise and the San Gabriel Mountains block (southeastern Salinia) was translated eastward in order to align offset depositional, structural and basement rocks along the anticlinorium. Primary achievements include: 1. reversal of dextral slip on the San Andreas fault system; 2. reversal of clockwise rotations of the Transverse Ranges; 3. closure of the Inner Borderland; 4. closure of the Gulf of California; and 5. reversal of crustal extension of the Basin and Range. Our 48 Ma paleogeologic map is a robust palinspastic reconstruction for southwestern North America near the end of the Laramide orogeny.