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Is the Vincent fault in southern California the Laramide subduction zone megathrust?

Haoran Xia, & John P. Platt

Published August 15, 2018, SCEC Contribution #10847

The Vincent fault in the San Gabriel Moun-tains, 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 ther-mal evolution of the Vincent fault and the associated mylonite zone requires a reevalua-tion 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 recrystal-lized 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 be-tween 46.9 and 71.4 MPa. The microstruc-tures show that part of the lower mylonite zone was overprinted by the upper mylonite zone and that the mylonites were formed dur-ing retrogressive metamorphism and exhu-mation. 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 analy-sis 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.

Xia, H., & Platt, J. P. (2018). Is the Vincent fault in southern California the Laramide subduction zone megathrust?. GSA Bulletin, 131(1-2), 120-136. doi: 10.1130/B31870.1.