EVOLUTION OF THE SOUTHERN SAN JOAQUIN BASIN AND MIDTERTIARY TRANSITIONAL TECTONICS, CENTRAL CALIFORNIA

A Cenozoic tectonic and sedimentary history is proposed for the Southern San Joaquin Basin (SSJB) and Tehachapi Mountains that evolved adjacent to the plate margin off central California Seismic reflection, borehole, field. biostratigraphic, and paleomagnetic data are integrated into geologic and fault structure maps, cross sections, and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence, timing, and distribution of complex, tectonically linked events. The largely buried structures and strata in the SSJB preserve an unusually complete record of the mid-Tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the Pliocene. Significant structural relief, existing across both extensional and contractile features, is preserved in the subsurface and an active fold-thrust belt propagates basinward along the margin of the U-shaped Tejon embayment. The Cenozoic evolution of the SSJB reflects the regional deformation of central California as different tectonic events followed each other along the adjacent North American plate margin. Five Oligocene-Miocene basin phases are identified in the SSJB: (1) late Oligocene/early Miocene extensional subsidence, with high- and low-angle normal faulting, accompanied by volcanism and deposition of coarse syntectonic conglomerates, (2) middle Miocene uplift; (3) later mid-Miocene transtensional subsidence to lower bathyal depths; (4) alternating subsidence and uplift until the late Miocene; and (5) flexural subsidence due to Pliocene to Recent contraction. Reconstructions of mid-Tertiary California place the southern San Joaquin/Tehachapi extensional terrane as a paleotectonic block located between the Western Mojave terrane (then to the east) and the Western California terrane (then to the south and west). Regional extension occurred during a long transition period between convergent and transform boundaries along the North American plate margin. Significant slip along the San Andreas transform postdates this extensional event. Its origin apparently coincided with a regional middle Miocene uplift event, indicating that the San Andreas fault is younger than previously supposed. The Tertiary SSJB has subsided due to extension/transtension, crustal tilting, and thrust-related loading. The Maricopa Subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the Sierra Nevada and locally has subsided beyond 12 km. In contrast, the relatively stable Tejon embayment, underlain by Sierran crystalline crust, achieved its maximum subsidence in the Miocene. Since Pliocene time, the SSJB has filled and continued to deepen after the basin was tectonically shut off from the Pacific Ocean.