PALINSPASTIC RECONSTRUCTIONS OF SOUTHERN CALIFORNIA AND THE NORTHERN GULF OF CALIFORNIA: IMPLICATIONS FOR MOJAVE AND TRANSVERSE RANGES TECTONIC HISTORIES

Map-view reconstructions of the southern California-northern Gulf of California region from Pinnacles to the midrift islands at 2, 4, 6.5, and 8-12 Ma are in progress. They incorporate available geologic, neotectonic, geodetic, thermochronological, and paleomagnetic constraints, including new data from the Gulf and spreading rates from Alarcón basin. These assumptions and methods are used: (1) Western mainland Mexico, southwestern Arizona, the Baja California peninsula south of the Agua Blanca fault, the western Mojave, the Sierra Nevada-Great Valley, and the west side of the central San Andreas are held rigid. (2) Extension and contraction are minimized across major strike-slip faults: central and Salton Sea segments of the San Andreas, San Jacinto, Elsinore, Cerro Prieto, Garlock, and Ballenas transforms.

Slip discrepancies of ~100 km among various earlier reconstructions of the southern San Andreas are reconciled by along-strike displacement gradients and the conclusion that ~40-60 km of slip occurred on the Mojave segment between ~6.5 and 8-12 Ma. An unacceptible gap opens along the Mojave-Coachella segments of the San Andreas, so one or more of the conditions in (1) or (2) is unrealistic. Comparing the 8-12 Ma reconstruction with those of the Sierra Nevada relative to the Colorado Plateau by Wernicke and Snow (W&S) (1998) show significant mismatch (~100-180 km). Preliminary analysis suggests that the mismatch may be reduced if slip on the Mojave segment began ~8 Ma rather than ~12 Ma, if the western Mojave was not rigid during earlier time slices, and (or) if W&S restore the Sierra Nevada block too far south. Their reconstruction and independent considerations imply Mio-Pliocene clockwise rotation of that block. Coupled with western Mojave deformation, this reduces or eliminates the unacceptable gap, eases space problems at the Garlock-San Andreas junction, and implies that clockwise rotation of the central San Andreas (instead of anticlockwise rotation of the Mojave segment) formed the big bend.