The Vincent fault in the San Gabriel Mountains, southern California, which forms the contact between the Proterozoic gneisses and Mesozoic magmatic arc in the upper plate and the subduction-associated Pelona Schist in the lower plate, has long been regarded as the Laramide subduction zone megathrust. Our investigation of the structural and thermal evolution of the Vincent fault and the associated mylonite zone requires a reevaluation of its tectonic significance. The base of the upper plate and the uppermost section of the lower plate are involved in a mylonite zone up to 1 km thick. Two mylonitic subzones are recognized within the upper-plate mylonites. The lower mylonite zone records pressure-temperature conditions of 0.46−0.52 GPa and 362−409 °C. Estimated shear stresses for the lower mylonite zone based on the recrystallized quartz grain size lie between 17.0 and 29.5 MPa. The upper mylonite zone records pressure-temperature conditions of 0.19−0.22 GPa and 323−341 °C, and shear stresses between 46.9 and 71.4 MPa. The microstructures show that part of the lower mylonite zone was overprinted by the upper mylonite zone and that the mylonites were formed during retrogressive metamorphism and exhumation. Sense of shear indicators preserved in the mylonitized Pelona Schist and the mylonitic gneisses consistently show that the upper plate moved southeastward relative to the lower plate. Zircon fission-track analysis suggests that the upper plate of the Vincent fault is not cut by any major faults, except for late Miocene to recent strike-slip faults of the San Andreas system. These observations do not support the idea that the Vincent fault is the Laramide subduction zone megathrust; instead, we conclude that it is a normal fault that contributed to the exhumation of the Pelona Schist.

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