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Poster #092, San Andreas Fault System (SAFS)

The Black Belt Mylonite Zone and the Role of Tectonic Inheritance in Brittle Faulting in the San Gabriel Block

Sarah N. Sengpiehl, Joshua Schwartz, Elena A. Miranda, Keith A. Klepeis, & Gabriela Mora-Klepeis
Poster Image: 

Poster Presentation

2021 SCEC Annual Meeting, Poster #092, SCEC Contribution #11598 VIEW PDF
The Black Belt mylonite zone is a major mid-crustal ductile shear zone in the San Gabriel Mountains and its east-west strike parallels the strike of the Quaternary Cucamonga Thrust Fault. Here, we examine the role of pre-existing ductile tectonic fabrics in controlling brittle fault geometry during Quaternary faulting. We focus on the southern boundary of the Black Belt mylonite zone in Cucamonga Canyon where previous workers have proposed that ductile fabrics were reactivated by a brittle fault system that parallels the strike of the Cucamonga Thrust Fault.

We find that the southern boundary of the Black Belt mylonite zone preserves evidence for an overprinted magmatic contac...
t between high-temperature (>800-600ºC), lower-crustal granulitic gneisses and mid-crustal tonalites. Both the granulites and the tonalites share similar deformational and metamorphic histories, including early development of garnet granulite-facies gneissic foliations that are overprinted by amphibolite-facies mylonites and subsequent brittle faults. Gneissic foliations in the granulites are defined by recrystallized plagioclase and the alignment of biotite, pyroxene and amphibole, and strike NE-SW and dip steeply (70-90º) to the SE. These granulite-facies gneisses are overprinted by amphibolite-facies that strike NW-SE and dip moderately (40-65º) to the north. The mylonitic fabrics are defined by the alignment of biotite, plagioclase, amphibole and titanite. The mylonites also exhibit monoclinic kinematic deformation geometry, and display a dextral, strike-slip sense of shear (i.e., top to SE). Undeformed to weakly deformed tonalites outside of the Black Belt mylonite zone yield weighted-average zircon 206Pb/238U ages ranging from 89 to 86 Ma. Metamorphic titanites aligned with mylonitic fabrics in the Black Belt mylonite zone give an age of 81 Ma, which documents the timing of amphibolite-facies mylonitic fabric development. Brittle faults overprint all other high-temperature fabrics and they generally strike N-S and dip moderately to steeply to the west, similar to high-temperature granulite fabrics but distinct from the E-W-striking Cucamonga Fault to the south. Our observations highlight the generation of multiple high-temperature fabrics in the Late Cretaceous and their role in acting as stress guides for brittle fault development.