Abstract: Quaternary Uplift of the Eastern San Gabriel Mountains: a Case for Crystalline Basement Rock Folding

KENNEY, MILES, Earth Consultants International, San Diego, CA

The eastern San Gabriel Mountains (ESGM) represent a composite range that has resulted from pre-Quaternary extensional tectonics, motion on the range-bounding Cucamonga (CF) and Sierra Madre (SMF) fault zones, and local secondary faulting and crystalline basement folding associated with the interactions of the San Andreas (SAF), San Jacinto (SJF) and CF fault zones. Motion on the CF and SMF thrust faults have caused the ESGM to uplift as a block. Local crystalline basement folding and secondary faulting also occur in the rocks near the intersection of the SJF and SAF from localized compression associated with the interactions of the CF, SJF and SAF in the ESGM. Secondary deformation is required to accommodate geometric problems not resolved by motion on the CF, SJF and SAF alone. The deformed basement exhibits two northwest trending folds; a doubly plunging antiform located south of the SAF within the high peaks region of the ESGM, and a northwestward plunging monocline north of the SAF. The high points of the two folds, adjacent across the SAF, have migrated together from the western San Bernardino Mountains during offset on the SAF. The antiform develops as material moves through the restraining bend at the junction of the SJF with the SAF. The monocline results from a northwest migrating subsurface restraining bend in the SAF produced as the Cucamonga thrust channels the Perris Block beneath the ESGM block and collides with the SAF at a depth of 8-10 km. The monocline develops like a snow-plow as the rocks of the Mojave Block move up and over the north-dipping subsurface restraining bend in the SAF as it migrates toward the northwest.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California