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102nd Annual Meeting of the Cordilleran Section, GSA, 81st Annual Meeting of the Pacific Section, AAPG, and the Western Regional Meeting of the Alaska Section, SPE (8–10 May 2006)

Paper No. 3
Presentation Time: 8:00 AM-11:30 AM

QUATERNARY ALLUVIAL FAN DEPOSITION AND ACTIVE FAULTING ALONG THE SAN GABRIEL MOUNTAIN FRONT, SAN DIMAS AND MARSHALL CANYONS, LOS ANGELES COUNTY, CALIFORNIA


ELLIS, Robert A., RUOTOLO, Allison M. and MARSHALL, Jeffrey S., Geological Sciences Dept, Cal Poly University, 3801 W. Temple Ave, Pomona, CA 91768, amruotolo@csupomona.edu

Active uplift along southward-verging frontal thrust faults of the Cucamonga-Sierra Madre fault system has generated abrupt topographic relief between the San Gabriel Mountains and the northern margin of the Los Angeles basin. Pleistocene alluvial fan remnants are uplifted between active strands of the frontal thrust system. The abrupt mountain front and uplifted fan deposits are also offset along a series of steep NE-to-ENE trending left-lateral oblique-slip faults (e.g., San Dimas Canyon and San Antonio Canyon fault systems).

A 37 m thick stratigraphic section within uplifted Pleistocene deposits between San Dimas Canyon and Marshall Canyon reveals interbedded fluvial gravels and debris flow deposits with occasional sandy horizons. Clasts consist primarily of granite and gneiss of the San Gabriel basement terrain, with a minor component of andesite and rhyolite of the Miocene Glendora volcanics. The deposit is capped by a >2 m thick deep-red clay-rich soil, and clasts exhibit substantial decomposition with distinct weathering rinds.

Hanging-wall uplift along the mountain front has diverted local drainages, isolating the Pleistocene fan remnants from their paleo-source canyons within the interior mountain block. The uplifted Pleistocene alluvium is now being incised and reworked by modern drainage networks, resulting in a younger phase of active fan deposition into the adjacent low-lying basin to the south.

Abrupt triangular facets along a steep NE-trending segment of the mountain front suggest active slip along the San Dimas Canyon fault. The Pleistocene alluvial fan surface exhibits apparent vertical displacement across this fault (west side up). Rapid tectonic uplift along the San Gabriel Mountain front and displacement of Quaternary alluvial fan deposits is the result of regional transpression within the Transverse Ranges restraining bend of the San Andreas Fault Zone.

Session No. 6--Booth# 7

T14. GSA: GSA Undergraduate Research (Posters)
Monday, 8 May 2006: 8:00 AM-11:30 AM
Geological Society of America Abstracts with Programs. Vol. 38, No. 5, p.8
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