TIMING AND KINEMATICS OF DEFORMATION IN THE NORTHERN BEAR MOUNTAINS FAULT ZONE, SIERRA NEVADA FOOTHILLS, CALIFORNIA

The N-S trending Bear Mountains Fault Zone (BMFZ) in the Sierra Nevada foothills is a 300 km long by ~10 km wide mélange zone with local high strain zones. It separates two lithotectonic belts and has been variably interpreted as a suture zone, an intra-arc reverse fault, and a transverse "mega-shear" zone. Previous work also suggests the timing of ductile deformation along the BMFZ may vary significantly along strike, from prior to ~160 Ma in the north to as young as ~123 Ma in the south.

This study examines the timing and deformation history of the northern BMFZ near Auburn, where excellent exposure occurs along the American River. This area is dominated by a zone of foliated greenschist-facies serpentine matrix mélange up to 11 km wide. Blocks include gabbro, pillow basalt and breccia, massive metabasite, chert, and limestone, with lesser volcaniclastic rocks and clastic sediment. One higher-grade block consists of strongly foliated hornblende amphibolite. Metadiorite plutons that intrude the mélange have been strongly deformed into wide zones of uniform greenschist that previous workers mapped as metavolcanic rocks.

Anastamosing foliation dips steeply east and contains a weak to moderately-developed down-dip lineation. Rootless isoclinal folding at the cm-scale is ubiquitous. A 4 km-wide high strain zone has deformed the BMFZ up to its border with an intact, less-deformed volcanic and plutonic sequence previously dated to ~162 Ma. Kinematic indicators from the high strain zone show that flattening strain is dominant. Both reverse and normal motion are also indicated, likely due to large-scale folding. Of note, no conclusive evidence of transverse motion is present.

New isotopic ages bracket the timing of ductile deformation in this area to the Early and Middle Jurassic. Hornblende from the amphibolite block has a ⁴⁰Ar/³⁹Ar age of 195.9±2.4 Ma, and a weakly-deformed pluton intruding the high strain zone has a SHRIMP U/Pb zircon age of 160.9±1.3 Ma. Dikes and sills with ⁴⁰Ar/³⁹Ar muscovite and biotite ages of ~124 Ma are undeformed. Based on these results and structural interpretations, we infer that the northern BMFZ is a middle Jurassic or older feature that incorporates syndeformational plutonism and high strain, rather than an upper Jurassic-lower Cretaceous feature as seen in the south.