A reappraisal of the early slip history of the San Andreas fault, central California, USA

The modern San Andreas fault system (California, United States) is widely considered to have formed in response to the initiation of Pacific-North American plate interaction ca. 27 Ma. Although there is general consensus on the magnitude and timing of Neogene displacement along the San Andreas system, its Paleogene history remains unresolved. In particular, similar to 100 km of right-lateral offset between mid-Cretaceous plutonic rocks of the northern Salinian block and the western edge of Sierra Nevada basement remains unaccounted for after restoration of Neogene displacement along strike-slip faults of the San Andreas system. Our detrital zircon data invalidate a key Paleogene piercing point by demonstrating that displaced portions of the hypothesized Middle Eocene Butano-Point of Rocks submarine fan were never contiguous across the San Andreas fault. We instead show that the Eocene provenance characteristics exhibited by northern Salinian strata closely match those of the southern Sierra Nevada and northwestern Mojave Desert. This implies that the northern Salinian block was located at least 75-50 km farther south in Eocene time than previously recognized. Our data require (1) pre-23 Ma dextral slip along the San Andreas fault in central California, and/or (2) slip along a predecessor fault that formed prior to Pacific-North American plate interaction. This previously undocumented slip may indicate that significant Pacific-North American plate interaction propagated from the plate margin into the continental interior much earlier than conventionally believed. Alternatively, late Paleogene slip could predate the development of the modern plate boundary and represent inboard dextral strike-slip displacement along the eastern margin of the Salinian block, similar to the deformation that occurs today along the strike-slip Sumatra fault system.