Abstract

The SanGabriel Basin is a Pliocene–Pleistocene Transverse Ranges structure bounded by the San Gabriel Mountains on the north, the San Jose and Puente Hills on the east, and the Repetto and Montebello Hills on the west. A basement boundary between gneissoid rocks of San Gabriel Mountains aspect and Peninsular Ranges rocks strikes east across the basin and connects with the San Jose fault. This boundary may represent the eastward continuation of the Canton fault, a middle Miocene precursor of the San Gabriel fault system with up to 30 km of right slip. Canton fault displacement is largely canceled by ∼22.5 km of left slip on a precursor to the Raymond fault such that an Oligocene dacite in the San Jose Hills and clasts derived from it show little evidence of large horizontal displacement between the San Gabriel Mountains and Puente Hills. Clockwise rotation in the middle Miocene probably influenced the distribution of lateral slip. The fill in the northeast-trending Pliocene–Pleistocene San Gabriel Basin is a basal shallow-marine sequence overlain by the nonmarine Duarte Conglomerate, in contrast to the deep-water Fernando Formation of the Los Angeles Basin. The paleoslope trends south in the direction of the Repetto and Puente Hills, evidence that uplift of these hills postdated deposition. The triangular San Gabriel Basin is currently a stable block bounded by the right-slip, northwest-striking East Montebello fault on the west and the left-slip(?), northeast-striking Walnut Creek fault on the east. Farther east, the San Jose and Puente Hills are underlain by an east-trending active fold belt, including the San Jose reverse fault and anticline, the Walnut anticline, and the Puente Hills anticline adjacent to the right-lateral Whittier fault. The East Montebello fault is bounded on the west by the Elysian Park anticlinorium, a fold belt in downtown Los Angeles. The left-slip Holocene Raymond fault joins the Sierra Madre reverse fault at its segment boundary marked by the Clamshell-Sawpit fault. The triangular stable block between east-trending fold belts is bounded on the south by the Puente Hills blind thrust and on the north by the Sierra Madre fault, documenting north-south Quaternary convergence. East-west extension is also indicated, although not confirmed by geodetic measurements.

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