CANTON FAULT: MIDDLE MIOCENE PRECURSOR TO THE SAN GABRIEL FAULT, SOUTHERN CALIFORNIA

The San Gabriel fault (SGF) began in late Miocene, 11.5 ± 2.1 Ma, based on the distribution of Violin Breccia and the offset of a lower Mohnian turbidite fan at least 35 km from a source in the San Gabriel Mountains. Offset of San Gabriel basement across both strands of the fault farther east is 27-35 km. In contrast, the 16-20-Ma basal Caliente Formation in Lockwood Valley contains anorthosite-bearing conglomerate offset 60-75 km from a source in the San Gabriel Mountains. The discrepancy is resolved by partitioning displacement between the SGF and the middle Miocene Canton fault (CF), which diverges from the SGF west of the Ridge Basin and passes west of a granitic basement ridge in the east Ventura Basin, where it is overlain unconformably by the lower Mohnian fan. Marine and nonmarine Paleogene strata in upper Piru Creek might be offset 31 ± 5 km from similar rocks in the subsurface in the east Ventura Basin. The CF separates heterogeneous basement in the Verdugo Mountains, northern San Gabriel Valley, and San Jose Hills from granitic rocks in the southern San Fernando Valley, Whittier Narrows, and Puente Hills and Santa Monica Slate in the central San Gabriel Valley. However, the continuity of 27-28-Ma Mountain Meadows Dacite between the San Gabriel Mountains and Puente Hills precludes a 30-km right-slip fault in the subsurface. This paradox is resolved by 22.5 km left-lateral offset of the contact between Peninsular Ranges batholith and Santa Monica Formation on the proto-Raymond fault in middle Miocene, thereby annihilating most of the 30-km right slip on the CF. The CF is coeval with precursors to the San Andreas fault: the San Francisquito-Fenner-Clemens Well fault and the Russell-Chimeneas-San Juan fault in the Cuyama Basin. Strike slip is accompanied by rifting, volcanism, and much greater clockwise rotation west of the CF than east.