RELATIONS BETWEEN CRUSTAL BLOCKS AND EVOLUTION OF THE SAN ANDREAS FAULT SYSTEM

Viewed as a passive crustal response to thermal and gravitational forces that drive plate tectonics, the structural complexity of the San Andreas Fault (SAF) system is not simply the result of an oceanic transform fault slicing across the western margin of the North American continent (NA), but rather of a more complicated transform interaction between the Pacific and NA plates beginning ~30 Ma. From this viewpoint, the holistic relation between the role of crustal blocks and the evolving spatial and temporal patterns of faults within and bounding them can help us understand the SAF system.

Balanced reconstruction of basement rocks and overlying strata along the SAF system reveals the pre-late Cz paleogeology of southern CA (Powell, 1993). Palinspastically restored assemblages define three major basement blocks: the Peninsular Ranges and Sierra Nevada batholiths (PRB & SNB) and an intervening block formed by reassembly of crystalline rocks of the Mojave Desert-Transverse Ranges-Salinian proto-block (MTSB) (Powell, 1981).

As transform-margin deformation evolved, the PRB and SNB acted as rigid blocks between deforming crust to the E and W (Great Basin/Walker Lane E of SNB and Coast Ranges/SAF W of SNB; Sonoran Basin and Range/Gulf of CA E of PRB and continental borderland W of PRB). In contrast, faults were distributed through the MTSB, which was deformed sequentially by zones of transtensional dextral shear, by sets of right- and left-lateral faults, and ultimately by contractional folding and thrusting. As do the SNB and PRB, the MTSB consists of Mz batholithic rocks--but in contrast to those blocks, the MTSB also contains widespread exposures of Proterozoic crust and is underplated by Mz oceanic crust and metamorphosed Mz-Cz rocks in exposures of Pelona-Orocopia-Rand Schist exhumed beneath low-angle faults.

The evolving strain pattern suggests that the rigid PRB and SNB rotated clockwise within right-lateral couples, whereas the more pervasively deforming crust of the MTSB, bounded to the N by the evolving Garlock Fault and to the S by faults along the S flank of the Transverse Ranges, responded as a more readily deformable “soft” domain in a sinistral couple between the two rigid blocks. As plate-margin deformation progressed the PRB and SNB indenters converged on one another while the MTSB “escaped” from between the two.