GEOLOGIC MAPPING, GRAVITY, AND PALEONTOLOGICAL STUDIES IN THE PINNACLES NATIONAL PARK REGION, CENTRAL CALIFORNIA, REVEAL ~14 TO ~6 MA TRANSTENSIONAL HISTORY OF THE SAN ANDREAS FAULT SYSTEM

Middle and late Miocene strata occupying a down-dropped fault block west of the present San Andreas fault and east of Pinnacles volcanics record releasing bend extension on an early set of San Andreas system faults, accumulation of coarse clastics derived from volcanic and granitic rocks to the west, and subsequent folding, erosional truncation, and sedimentary overlap as the releasing bend was abandoned. These conclusions arise from new geologic studies in and around Pinnacles National Park undertaken in cooperation with the National Park Service. Reconnaissance mapping and new gravity-data collection extend the study area to the south and east. Siliceous shale from two units yielded age-diagnostic Miocene diatoms. The new data reveal the following geologic history:

~14-8 Ma, the San Andreas fault system occupied a releasing step that included faults west of the present San Andreas. A tectonic escarpment was formed and uplifted volcanic and granitic sources west of the escarpment contributed coarse detritus that was transported generally eastward and deposited as arkosic sandstone and boulder conglomerate, with angular to subrounded clasts in places larger than 1 m. Transport and deposition of coarse sediment was episodic and mostly by sediment gravity flows in marine environments, perhaps on parts of one or more fan-deltas. Interbedded with the coarse-grained strata are diatomaceous mudstone layers representing periods of quiet-water deposition.

Between ~8-6 Ma, the accumulated strata were folded, eroded, and overlain in angular unconformity by unsorted shallow marine and nonmarine deposits of sand, gravel, and cobbles, again sourced from uplifted granitic and volcanic rocks to the west. We take these changes to signify that activity on the releasing step was tapering off, resulting in one or more periods of compression and erosion and lower energy depositional environments.

Gravity data reveals that the east-facing escarpment is at least ~60 km long, suggesting at least that much offset through the releasing bend between ~14-6 Ma.

After ~6 Ma, the transtensional faults were abandoned and overlapped by shallow marine deposits. The sedimentary rocks of the transtensional depocenter have been offset northward by slip on San Andreas and other faults to the east (~165 km since ~5.5 Ma based on studies elsewhere).