CLAST ANALYSIS OF QUATERNARY GRAVELS ALONG THE EAST-CENTRAL SAN GABRIEL MOUNTAIN FRONT: IMPLICATIONS FOR EVOLUTION OF THE SIERRA MADRE-EVEY CANYON THRUST SYSTEM

The Quaternary Sierra Madre-Evey Canyon thrusts are part of an active fault system that transects the east-central San Gabriel Mountains range front. Fault segments situated directly north of densely populated communities of Claremont and La Verne are poorly understood. Our efforts focused on the southward extending Upper Quaternary gravels associated with uplift and erosion of hanging walls of the thrusts. Our aim in studying this sedimentary system was to understand the regional geologic setting of an important seismic hazard and develop a model for fault block uplift.

We mapped and sampled portions of two basement block uplifts and collected pebble samples of associated gravels for clast analysis. The Sunset Ridge Block (SRB) forms the hanging wall of the north-dipping Evey thrust. This block is composed of a layered sequence of distinctive plutonic and metamorphic rock units, including Cretaceous tonalite, Precambrian quartzofeldspathic gneiss, Jurassic diorite and granite, and Triassic “dalmationite.” Situated farther south in the footwall of the Evey fault and the hanging wall of the active Sierra Madre thrust, the Potato Mountain Block (PMB) is composed mainly of Precambrian quartzite, biotite gneiss / phyllite and Cretaceous leucogranite. Stratigraphic relationships constrain two generations of gravel deposits. Clast analyses of 8 gravel samples (60 to 200 pebbles / sample) and field inspection of boulders from 2 sites yielded systematic provenance ties to the two basement blocks and a consistent time progression: older gravels were derived from the SRB while younger gravels were shed from PMB bedrock sources. These data indicate the two fault blocks were uplifted sequentially such that early displacement on the Evey thrust (coincident with uplift of SRB) was succeeded by activation of the Sierra Madre thrust (and uplift of PMB). Thus the two gravel units record southward transfer of slip from Evey Canyon to the present-day mountain front.

Our study has increased understanding of the local geologic history and will eventually serve as an important tool for city planners and emergency personnel in reducing seismic risk. Samples from both Quaternary units have been collected for cosmogenic dating; results are pending. This information will provide constraints on the uplift rate of the Quaternary gravels.