Mesozoic Fabric Inheritance in the Ductile Roots of the Quaternary Cucamonga Fault

Francine A. Robles, Omid A. Arzili, Joshua Schwartz, Elena A. Miranda, Keith A. Klepeis, & Gabriela Mora-Klepeis

Published August 15, 2021, SCEC Contribution #11460, 2021 SCEC Annual Meeting Poster #118

The geologic framework of Southern California is superimposed upon blocks of basement rocks containing inherited ductile fabrics, including Late Cretaceous mylonite, which function as stress guides for strain localization along major, recently active faults. Here, we study pre-existing fabrics in crystalline basement rocks exposed within the San Gabriel lithotectonic block with the goal of constraining the geometry and rheology of the ductile roots of an ancient fault system that was subsequently captured by a major Quaternary fault (the Cucamonga Fault) in the SAF system. We focus our study on the southeast corner of the San Gabriel block, near the intersection of the Cucamonga Fault and the San Jacinto Fault, where a thick section of lower- to middle-crustal shear zone fabrics have been exhumed and exposed. These rocks preserve the deepest section of crustal shear zone rocks in Southern California and are likely analogous to the deep crustal fabrics inferred from geophysical studies.

We find that the Quaternary Cucamonga Thrust Fault (dip = 35-25º) overprinted a sequence of high-temperature (>800-600ºC), granulite- to amphibolite-facies mylonites. These mylonites overprint garnet- and pyroxene-bearing gneisses, marbles, and amphibolites that were metamorphosed at ~30 km depth. Near the Cucamonga Fault, mylonites strike ~E-W, and dip moderately (40-50º) to the north where they are overprinted by brittle faults showing subhorizontal slickenlines, indicating a dominantly sinistral sense of shear with a minor component of reverse motion (i.e., top-to-the WSW). New U-Pb zircon geochronology of the host rocks indicates that magmatism occurred in the Middle Jurassic to Early Cretaceous (169-119 Ma) and host rocks were metamorphosed at 85.1 +/- 1.5 Ma. A date on a syn-mylonitic dike indicates that high-temperature deformation occurred at 84.4 +/- 1.3 Ma, in association with high-temperature metamorphism. Our results document that the geometry of deep-crustal mylonitic fabrics exerted a strong influence on the location and orientation of slip planes within the Cucamonga fault.

Key Words
ductile roots, ancient fault system, geochronology, U-Pb zircon geochronology

Citation
Robles, F. A., Arzili, O. A., Schwartz, J., Miranda, E. A., Klepeis, K. A., & Mora-Klepeis, G. (2021, 08). Mesozoic Fabric Inheritance in the Ductile Roots of the Quaternary Cucamonga Fault. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)