LARAMIDE THRUSTING IN SE CALIFORNIA: NEW AGE CONSTRAINTS FROM LATE SYN-KINEMATIC PEGMATITES AND ACCESSORY MINERAL PETROCHRONOLOGY

The Cargo Muchacho Mountains of SE California comprise predominantly Jurassic arc meta-volcaniclastic and -plutonic rocks, which display a ductile foliation of variable intensity. Early workers thought this foliation was related to the nearby Laramide-age Chocolate Mountains Thrust due to their general parallelism, whereas others considered it Jurassic as the younger Jurassic granites appear undeformed. The age of this deformation is key to deciphering the tectonic evolution of this complex region.

We present new data that confirms a Late Cretaceous Laramide deformation age in the Cargo Muchacho Mountains. This deformation entailed N/NW-directed ductile-to-brittle thrusting under conditions that cooled from ca. 500-700°C to below 280°C, at a maximum rate of 24°C/My over a 10 My period. Earlier, high-T deformation is recorded by all units. The Jurassic granites display intense recrystallization in thin section, indicating that deformation post-dated their emplacement. Rare feldspar sigma clasts indicate top-to-N/NW kinematics, and quartz microstructures reflect rapid grain-boundary migration, suggesting deformation temperatures of ca. 500-700°C. Titanite in these granites yield dates of ca. 143-64 Ma, consistent with the timing of granite crystallization and with Late Cretaceous-early Paleocene ductile deformation, as recorded by the strong foliation and deformed pegmatites. These pegmatites form conjugate sets that cross-cut the high-T ductile foliation; pegmatites at low angles to the foliation are intensely boudinaged, whereas those at high angles are folded. This reflects a strain ellipsoid consistent with N-directed thrusting, in line with kinematics recorded in the sheared granite. Pegmatite intrusion is dated at 67-63 Ma by zircon U-Pb analysis; their moderate deformation (folding and boudinage, but no transposition or recrystallization) suggests emplacement during the waning stages of ductile shearing.

Finally, apatite U-Pb ages of ca. 54-47 Ma may indicate cooling through a closure T of ca. 450-500°C, likely reflecting erosion related to crustal thickening during Laramide-age thrusting. Some pegmatites are cut by foliation-parallel brittle faults with a top-N thrust sense, which must have formed after cooling below ca. 280°C, and suggest continued lower-T thrusting.