TECTONIC GEOMORPHOLOGY AND CLAST PROVENANCE OF UPLIFTED ALLUVIAL FAN GRAVELS, SAN GABRIEL MOUNTAIN FOOTHILLS, EASTERN LOS ANGELES COUNTY, CALIFORNIA

Uplifted remnants of a late Pleistocene alluvial fan lie along the San Gabriel Mountain foothills between San Dimas and Marshall canyons, eastern Los Angeles County. These deposits of “older alluvium” have been elevated by slip along a system of range-bounding faults, including southward-verging thrusts of the Sierra Madre-Cucamonga fault system, and steep NE-to-ENE trending oblique sinistral faults such as the San Dimas Canyon and Evey Canyon faults. Rapid uplift along these structures produces abrupt topographic relief between the mountain front and the adjacent alluvial lowlands of the San Gabriel Valley. Field mapping, topographic surveying, and stratigraphic analyses provide insights into the tectonic geomorphology and provenance of faulted alluvial fan gravels along the mountain front.

Topographic profiles surveyed across a 3 km² area of mapped paleo-fan deposits reveal apparent offsets across bounding faults. Deposit thicknesses of 5 to >30 m are constrained by mapped basement-gravel contacts and well logs. A road-cut along San Dimas Wash exposes a 31 m thick stratigraphic section comprised of semi-stratified debris flow deposits with intermittent fluvial gravel and sand horizons. This section is capped by a ~2 m thick deep-red clay-rich soil, indicative of a late Pleistocene age. Clast counts were conducted in one-meter square grids at six sites spaced at ~5 m vertical intervals. Clast lithologies consist mostly of locally derived Cretaceous and Precambrian crystalline basement rocks, with a minor component of Miocene Glendora volcanics. Quartzite and meta-sedimentary gneiss appear in the middle to upper part of the stratigraphic section. These distinctive lithologies outcrop >6 km to the east, suggesting that the “older alluvium” at this site may be a distal section of a dismembered paleo-alluvial fan originating from San Antonio Canyon. This interpretation is consistent with the anomalous SW slope of the fan surface toward San Dimas Wash. Hanging-wall uplift along the San Gabriel foothills may have isolated the fan remnants from source areas within the interior mountain block. Uplift and displacement of paleo-alluvial fan deposits is the result of regional transpression within the Transverse Ranges restraining bend of the San Andreas Fault Zone.