DETACHMENT FAULTING IN THE NORTHERN BRISTOL MOUNTAINS, CALIFORNIA

The crust of the Mojave Desert has been dramatically rearranged by at least three distinct episodes of low-angle faulting since the Cretaceous, resulting in a relatively thin upper plate of felsic plutonic rocks underlain by subduction-channel schist. However, the faults responsible for this mess have only been mapped in a handful of areas. Recent study, including a January 2019 Cal Poly Pomona field class, has revealed the presence of a previously unrecognized detachment fault in the northern Bristol Mountains, east of Broadwell Mesa. This area within the Kelso Dunes Wilderness was geologically mapped on foot at a scale of 1:6000, with the aid of satellite imagery. The detachment fault, which generally dips shallowly to the west, separates a footwall of relatively homogeneous K-spar-phyric granodiorite from a more heterogeneous complex of intermediate, amphibolite-facies meta-plutonic rocks in the hanging wall. Along with a poorly-exposed meter-scale gouge layer, the shear zone is marked by an anastomosing mylonite layer, which varies from just a few meters to over 100 m thick, with lineations that generally trend WSW. Observed sense-of-shear indicators, most notably S-C fabrics, consistently indicate top-to-the-west motion. Fault corrugations run parallel to the mylonitic lineation, with a wavelength of 300-400 m and an amplitude of 20-40 m. In turn, these structures are affected by gentle to open folding along NNW axes and are cut by steep, WNW-trending dip-slip faults. Because both the footwall and hanging wall of this detachment contain high-grade rocks, it is likely that both underlie an as-of-yet unmapped master detachment. The age of detachment faulting is unknown, but may be early Neogene, based on the occurrence of calcite-ankerite-siderite veins similar to mineralization associated with the also structurally similar early Neogene Central Mojave Metamorphic Core Complex, ~100 km to the west. Future mapping, geochronology, and petrology may clarify this fault's significance to Mojave tectonics.