Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 10-6
Presentation Time: 3:25 PM


MILLER, Elizabeth, Department of Geological Sciences, Stanford University, Stanford, CA 94305 and RAFTREY, Mark E., Stanford Law School, Stanford, 559 Nathan Abbott Way, Stanford, CA 94305

Metamorphic core complex detachment faults are controversial structures that have spawned multiple ideas and hypotheses for their origin and kinematic histories. To better understand these faults, we focused on better charting the Miocene slip history of the BCF (Wright and Troxel, 1993). The footwall of the BCF is exposed for 30 km in the slip direction with separation of once contiguous units almost that distance.

At high levels, the fault juxtaposes Cenozoic strata in a klippe against Late Precambrian Sterling Quartzite with inferred offset of ~4-5 km. Cenozoic strata dip steeply, truncated beneath by the flat BCF. The Lee’s Camp anticline in the footwall is interpreted as a drag fold; it rotates units into parallelism with the fault. Bedding (30-80°) to fault angle (flat) in the klippe suggest 60° rotation of fault and hanging wall strata. Slip initiated after 12.7 Ma (MDA based on DZ), compatible with previous thermochronology suggesting ~10-6 Ma slip.

A set of parallel NW dip normal faults in lower plate Sterling tilt units to the SE and shingle the Sterling so that it now underlies 22 km of the lower plate in the slip direction. In addition to map-scale faults, quartzites are riddled with mm to cm offset normal faults that effect bulk ductile strain at the map scale. Motion indicators show N66W slip. These faults post-date the klippe-bounding fault. Their offsets increase to the NW and we infer them to interact at depth with a lower ductile (?) fault or a ductile-brittle transition zone (DBTZ). The lower plate of these faults emerges near Chloride City where the mapped low-angle Eastern Shear Zone and Chloride Cliff Detachment occur beneath the BCF. Ductile and mylonitic deformation are increasingly developed NW towards Monarch Canyon (stretching lineations W-NW) where the BCF fault gently dives beneath the northern Grapevine Mountains (fault corrugations N60W).

These key relationships suggest a rapid, kinematically coordinated but multi-stage origin for the BCF. Multiple detachment levels are inferred; with vertical rise of the DBTZ and T’s upwards through the crust by > than several km’s due to syn-extensional flow or magmatism at depth, required to explain the significant amounts of rotation and sub-horizontal stretching leading to the present geometry of the fault.