THE NEOGENE THROUGH QUATERNARY DEFORMATION HISTORY OF THE SOUTH BRISTOL MOUNTAINS, CA

The South Bristol Mountains lie near the eastern border of the Mojave block and eastern California shear zone. The rock units composing the range are dissected by at least two generations of faults: early Miocene normal faults associated with early Miocene volcanism and sedimentation, and high angle northwest striking faults of the South Bristol Mountains fault (SBMF), including fault splays that cut Quaternary units. Total offset on the SBMF is estimated to be at least 6 km based on correlation of plutonic rocks across the fault zone. The range is composed of remnants of Proterozoic plutons, metamorphosed Paleozoic passive margin rocks, Jurassic hypabyssal and plutonic rocks, and Neogene volcanic and sedimentary sequences. Our mapping shows that the basal nonconformity between Mesozoic basement and Miocene volcanic and sedimentary rocks is overlain in places by buff to dark orange, arkosic, massive, non-volcanic conglomeratic sandstone that locally grades into saprolite, and elsewhere by a fluvial to lacustrine sequence of rhythmic sandstone beds with increasing volcanic content. The onset of Late Oligocene to Early Miocene volcanism and sedimentation in the South Bristol mountains region is indicated by tuffaceous sediments in the lacustrine beds underlying a newly dated ash with a U-Pb zircon age of 23.8 ± 0.4 Ma (1 sigma). Rare camelid hoofprints are found in the lowest fluvial sequence, and fossilized plant remains in limestone are found within the volcanic sequences. The lake bed sequence and subsequent volcanic and sedimentary sequences have been dismembered by Neogene and Quaternary faulting. Detailed mapping of the Neogene sedimentary and volcanic rocks should allow us to determine the extent of Early Neogene extension and basin formation, and to produce a more detailed total slip budget for the Quaternary strike slip faulting within the South Bristol Mountains.