MID MIOCENE RECONSTRUCTION OF THE EAST-CENTRAL SAN GABRIEL MOUNTAINS BASEMENT STRATIGRAPHY

Geological mapping of the east-central San Gabriel Mountains basement reveals a range-scale stratigraphy dismembered into diamond-shaped blocks by Late Cenozoic strike-slip and reverse faults. These faults record response of the San Gabriel Mountains block to translations and rotations that accompanied development of the transform plate boundary in southwestern California. Sequential restoration of conjugate strike-slip faults is accomplished in map view by matching distinct Proterozoic and Mesozoic crystalline sheets. This procedure removes 22 km Late Miocene dextral displacement on the northern San Gabriel-Icehouse Canyon-Scotland-Lytle Creek fault system and minor Pliocene-Quaternary sinistral slip on the Stoddard Canyon, San Antonio Canyon, San Dimas Canyon-Weber, Burro Canyon, and Pine Mountain faults. One model considers 10-15 km Late Miocene dextral displacement on the Sawpit-Clamshell fault, feeding into the eastern part of the north San Gabriel fault. Quaternary shortening associated with the Cucamonga-Sierra Madre thrust is not restored. The reconstruction provides an integrated mid-Miocene paleogeography of the San Gabriel Mountains block containing several provocative features: (1) a consistent layered configuration of plutonic and metamorphic rock units composing both plates of the Paleocene Vincent thrust system, (2) distinction between map-scale folds that deform the Vincent thrust and smaller scale Proterozoic folds, (3) reassembly of a Late Oligocene granite pluton and a mid-Miocene mafic-intermediate dike swarm into narrow belts that mark zones of pre-San Andreas thermal activity, and (4) residual mismatches marking north-northeast striking Early Miocene faults whose southerly projections may have played important roles in development of the adjacent Los Angeles basin. The basis for reconstruction is correlation of distinct Proterozoic and Mesozoic metamorphic rocks recognized at similar structural levels. Important map units include Pelona schist, Vincent thrust mylonite, and Paleoproterozoic banded gneisses and augen gneisses. Especially conspicuous marker units are the Triassic Mt. Lowe intrusive complex and two sheets of Late Cretaceous quartz diorite-granodiorite mapped between Mt. Wilson and Mt. San Antonio-Ontario Ridge. The along-strike continuity of these crystalline rock units should prove useful in establishing additional ties across the San Andreas fault in the Chocolate Mountains and Sonora, Mexico.