A Reappraisal of Paleogeography and Early Slip on the San Andreas Fault Based Upon Detrital Zircon Geochronology of Paleogene Forearc Strata in Central California
Sharman, Glenn R.; Graham, Stephan A.; Grove, Marty; Hourigan, Jeremy
Detrital zircon provenance analysis of Paleogene forearc strata in central California has yielded important new insights into the middle Cenozoic paleogeography and tectonics of central California including a reappraisal of the early slip history of the San Andreas fault. Our detrital zircon data indicate that a major contrast in Eocene provenance along the California margin was situated at the latitude of Bakersfield, California. Forearc strata that crop out for 100’s km to the north are characterized by abundant mid- to Late Cretaceous zircon and lesser amounts of Paleogene, Jurassic, and pre-Mesozoic grains. This signature represents the provenance signature of the “Main Sierran batholith”. Conversely, Eocene forearc strata south of Bakersfield, including those now exposed in the San Emigdio Mountains, and strata presently west of the San Andreas fault in northern Salinia, are characterized by subequal populations of Cretaceous and Jurassic zircon, generally small amounts of Permian-Triassic zircon, and few pre-300 Ma grains. We interpret this signature to represent a “southeastern Sierran and northwestern Mojave provenance”.
Our results contradict the prevailing view that northern Salinia: (1) extended significantly northward into the southern San Joaquin basin in Eocene time and (2) was a significant sediment source to local, “continental borderland” basins. These concepts were fundamental to piercing points previously established along the San Andreas fault. Our results alternatively indicate that displaced portions of the hypothesized middle Eocene Butano-Point of Rocks submarine fan were never contiguous across the San Andreas fault, thereby removing a key Paleogene tie used to constrain slip on the San Andreas fault in central California. Recognizing that Eocene provenance characteristics exhibited by northern Salinian strata link it with the southeastern Sierra Nevada and northwestern Mojave implies that northern Salinia was located at least 75-50 km farther south in Eocene time than previously recognized. This reconstruction eliminates the need for Cretaceous batholith exposures in Salinia to extend north of the southern tail of the Sierra Nevada during Eocene time.
In summary, 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 America 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 Salinia similar to the deformation that occurs today along the strike-slip Sumatra Fault System.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013