AGE OF MYLONITIZATION IN THE MILL CANYON WINDOW AND CRETACEOUS-PALEOCENE REGIONAL DEFORMATION ALONG THE VINCENT THRUST IN THE SAN GABRIEL MOUNTAINS, SOUTHERN CALIFORNIA

Zircon ²⁰⁶Pb/²³⁸U and mica ⁴⁰Ar/³⁹Ar ages of granitic rocks and mylonites in the central and western San Gabriel Mountains illuminate deep arc processes during low-angle subduction. Mylonites of the Mill Canyon window structurally underlie an upper-plate continental arc terrane that includes ~1.7 Ga gneisses, a 1.19 Ga anorthosite – syenite intrusive complex and its associated granulite aureole, and Mesozoic arc rocks including the Triassic Mount Lowe Intrusion and Cretaceous Josephine Mountain and Waterman Mountain intrusive suites. Zircon age distributions in the youngest upper-plate arc rocks are complex, but the youngest populations yield estimated times of assembly of these intrusive suites at ca. 82-76 Ma. The ages of these plutonic rocks are assumed to establish a maximum age for regional displacement along the underlying Mill Canyon mylonite zone. Amphibolite, granodioritic and granitic mylonites comprise the core of the Mill Canyon window, with foliation dipping northeast to south-southwest that contains a gently northwest- to southeast-plunging lineation. May and Walker (1989) interpreted shear fabrics in the mylonites to indicate top-west deformation. A granitic mylonite yielded a zircon age of 73 ± 2 Ma, and a second mylonite at a similar structural depth yielded a muscovite Ar/Ar age of 61.6 ± 1.7 Ma and a biotite age of 57.6 ± 1.9 Ma. These mylonite mica cooling ages are significantly younger than the nearest upper-plate biotite cooling age of 69.1 ± 1.8 Ma from an undeformed granite. These new data indicate the upper-plate basement complex and the Mill Canyon mylonites were juxtaposed by top-west shear between about 73 and 62 Ma. This age assignment suggests that the Mill Canyon mylonites are correlative with the Vincent thrust mylonites in the eastern San Gabriel Mountains (Jacobson et al., 1996). This correlation implies that the exposed basement complex of the central and western San Gabriel Mountains is a thin allochthonous sheet above the Pelona Schist, exposed in structural highs in the East Fork area and Sierra Pelona (Ehlig, 1981). As observed at Sierra Pelona, the age discontinuity between the upper plate and mylonites implies this structure could be a portion of the shallow-dipping subduction thrust that underplated the arc crust with schist, or a slightly younger exhumation structure.