THE SOUTHERN CALIFORNIA BATHOLITH FLARE-UP: HIGH-TEMPERATURE ARC PROCESSES DURING FLAT SLAB COLLISION?

We use U-Pb zircon and titanite SIMS and LA-SF-ICPMS geo- and thermochronology to investigate the relationship between arc magmatism, metamorphism and deformation in the lower to middle crust of the Southern California batholith and northern Peninsular Ranges batholith during the initiation of flat-slab collision. Seven zircon dates from the Waterman batholith, Mt. Josephine Intrusion, and Mt. Wilson pluton in the San Gabriel terrane and one date from the San Sevaine tonalite in the San Antonio terrane document a period of wide-spread tonalitic to granitic magmatism in the Southern California batholith from 86 to 72 Ma. This period of arc magmatism was synchronous with metamorphism in the lower crust, where metamorphic zircons in the Cucamonga terrane record garnet-granulite facies metamorphism from 86 to 84 Ma at 8–9 kbars and 775–800°C. Migmatitic tonalites with metamorphic zircon rims in the San Antonio terrane also record continued high-temperature metamorphism and local partial melting in the middle crust until 75.5 ± 5.3 Ma. These data demonstrate that the Southern California batholith remained hot and magmatically active through ca. 75 Ma.

Arc magmatism and metamorphism in the Southern California batholith were synchronous with deformation in a series of shear zones that record arc-parallel motion and contraction. In the San Gabriel Mountains, sinistral shearing and contraction are recorded in the Black Belt mylonite zone, a 20-km long, 600-meter-wide amphibolite-facies shear zone that was active from 86 to 74 Ma. The Borrego Springs and Santa Rosa shear zones in the northern Peninsular Ranges record slightly older deformational fabrics associated with ductile, top-to-the west thrust sense of shear. Titanites aligned with deformational fabrics in these rocks give a lower-intercept date of 86.3 ± 2.4 Ma in the Borrego Springs shear zone, and 88.5 ± 1.7 Ma in the Santa Rosa shear zone. In both the Borrego and Santa Rosa shear zones, recrystallized titanites give Zr-in-titanite temperatures of 670-680°C. Collectively, these data show that high-temperature arc processes in the Southern California batholith and eastern Peninsular Ranges batholith continued for 10-15 myr after proposed flattening of the subducting slab and collision of the Shatsky rise with Southern California starting at ca. 88 Ma.