Geology and tectonic evolution of the Bear Mountains Fault Zone, Foothills Terrane, central Sierra Nevada, California

Abstract

The Late Jurassic‐Early Cretaceous Bear Mountains fault zone (BMFZ) is the westernmost strand of the Foothills fault system in the Western Metamorphic belt of the Sierra Nevada. The tectonic significance of this 300‐km‐long fault zone has been downplayed in the past, but we contend that it is a major discontinuity within the Foothills terrane. The BMFZ is an ˜ 5‐km‐wide shear zone consisting of slate‐metagraywacke‐matrix melange that experienced intense polyphase deformation and metamorphism. Small‐scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone. A host of lithologically diverse tectonic blocks are enclosed in the BMFZ. Blocks of sandstone, metavolcanic rocks, and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the Foothills terrane, whereas other block types are exotic. Exotic blocks include ultramafites that locally contain pods of gabbro, garnet amphibolite, greenschist, sedimentary breccia, and volcaniclastic rocks. The breccia is composed primarily of clasts of amphibolite and minor garnet‐bearing impure quartzite (metachert?); it probably accumulated at the base of fault scarps. The exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the Foothills terrane, and they record a deformation prior to incorporation in the slate‐metagraywacke matrix. Amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the Tuolumne ophiolite east of the BMFZ. The presence of exotic blocks implies large‐scale mixing and large displacements in the BMFZ, as do differences in stratigraphy across the fault zone. Metamorphic grade does not vary significantly across the BMFZ, however, suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose. There is no direct evidence for major strike slip along the BMFZ as has been proposed by others; any such slip must have occurred before the reverse‐slip related structures of the zone. We thus interpret the BMFZ as an intra‐arc reverse fault of moderate to large displacement, with possible earlier strike slip, that incorporates ophiolitic basement showing an older, complicated history.