Structure and petrology of gouge and breccia bearing shallow crustal shear zones of detachment faults in Death Valley, California

The Black Mountain (Death Valley, CA) low angle normal, detachment faults place Pliocene-Quaternary sediment against crystalline rocks. High angle normal faults extend the sedimentary section and define earthquake scarps on the valley floor. The high angle faults do not offset and are coupled with the detachments. The wedge-shaped hanging wall is critically stable when the effective friction of the detachment is weakened by roughly 50% of typical earth materials. The weakening is partly because of gouge and breccia within the detachment shear zones. The fault rocks exhibit well-developed mesoscopic foliation but do not exhibit evidence for deformation from high-temperature deformation mechanisms. Measurements of the anisotropy of magnetic susceptibility (AMS) and shape-preferred orientation (SPO) of >50-micron grains define fabric with orientations consistent with the transport and extension directions of the faults. Magnetic experiments and microscopy demonstrate that the magnetic carriers within the gouge and breccia are dominantly nanometer-to-micrometer grains that grew within the shear zones prior to the most recent deformation. The kinematic development of SPO and AMS is best treated as owing to the rotation of passive markers within a passively yielding matrix. However, the similarity of fabric defined by clasts (SPO) and matrix (AMS) is more consistent with deformation via cataclastic and granular flow, each producing distinctive microstructures. Accompanying the development of the fabric was the growth of new minerals within pore spaces and the dissemination of new minerals along reaction fronts. The youngest phases are dominantly oxides and some orthoclase and dolomite, whereas earlier phases are phyllosilicates, including polytypes of illite and chlorite with interstratified swelling clays. The mineralogy can be separated into three different assemblages that grew along a retrograde reaction pathway. The reactions occurred between meteoric water and the footwall, over the last 6Ma, and from >120°C to near surface temperatures. Fault rock development does not appear to have been accompanied by elevated fluid pressure, or large water-rock ratios. Therefore, the weakening of the detachment and its propensity for seismic slip may be/have been dependent on the development of the new minerals and the change in deformation mechanism from cataclastic to granular flow.