LATE QUATERNARY EVOLUTION OF THE EAST-CENTRAL SAN GABRIEL MOUNTAINS FRONTAL FAULT SYSTEM RECORDED BY UNROOFING SEQUENCE

Alluvial deposits and debris flows preserved between modern-day San Dimas and San Antonio drainages record progressive uplift and exposure of distinct crystalline bedrock units that accompanied southward migration of the east-central San Gabriel Mountains frontal fault system during Late Quaternary time. Our mapping, initiated after wildfires of 2003, focused on Quaternary gravels presently stranded on the hanging wall of the active, north-dipping Sierra Madre thrust. These gravels are spatially restricted to the Potato Mountain Block, which is separated from precipitous Sunset Ridge by the north-dipping Evey Canyon thrust. Two major units are recognized from stratigraphic field relationships and consistent provenance signals preserved in pebbles and cobbles. The older unit is subdivided into three geographic domains: eastern and western fluvial terraces associated with San Antonio and San Dimas drainages, and a central expanse of alluvial fans exposed over the intervening area. The terraces preserve basal nonconformities elevated 10-50 m above the modern stream channels. Clasts are derived predominantly from Sunset Ridge, with a minor proportion eroded from more northerly, upstream bedrock sources that include Pelona schist and Proterozoic augen gneiss. The older alluvial fans, composed almost entirely of Sunset Ridge clasts, are elevated as much as 80 m above base level. The younger unit consists of red debris flows derived principally from iron-rich Potato Mountain bedrock sources. It overlies a paleosol developed at the top of the older unit. Observed provenance relationships are consistent with progressive transfer of slip from the Evey Canyon thrust to the Sierra Madre thrust. Early displacement on the Evey thrust (recorded by clasts derived from Sunset Ridge) was succeeded by activation of the Sierra Madre thrust (and the emergence and erosion of Potato Mountain).

Results are pending from samples of both units collected for cosmogenic dating. These ages will help constrain the Late Quaternary uplift rate related to slip on the Sierra Madre thrust and the sinistral-reverse San Antonio-San Jose Fault system that intersects the frontal fault. Ongoing gravity and seismic refraction surveys seek to precisely locate the traces of these active faults that are obscured by Holocene alluvial fan deposits.