A TEXTURAL PROGRESSION OF BRITTLE FAULT ROCKS ALONG THE STRIKES OF THE DEATH VALLEY DETACHMENTS

Low angle normal, or detachment, faults floor sedimentary wedges exposed along the Black Mountains, Death Valley, CA. An analysis of high angle normal faults that cut the hanging-wall sediment, but are linked with the detachment, predicts that the fault rocks develop with a slip rate of <1 mm/yr under a vertical s₁, with an effective friction (µ) of 0.36, and associated seismicity. There is a south-to-north, along detachment-strike, textural progression (I-V) of the fault rocks within the detachments that correlates with increasing age of hanging-wall sediment: (I) South of (below) exposures of the detachment, the crystalline basement is ‘damaged’ with local cataclasis and alteration. (II) Hematitic stains and abrasion-type striae mark the base of the youngest (Late Pleistocene) sediment. (III) There is no distinct slip plane below sediments that contain 0.67 Ma ash, but rather a >1m wide disrupted zone, with elongate cobbles of footwall blocks supported by a fine-grained, orange and white matrix. (IV) The fault zone narrows to <1m beneath sediment containing 1.2–0.77 Ma tephra, where the fault rocks are stratified with a clay gouge at the top, granular gouges in the center, and a foliated breccia at the base, with only minor brittle deformation in the footwall. Roughly 30% of the gouge is <2µm with a tri-modal particle size distribution (PSD). Syn-tectonic authigenic illite and smectite mixtures, authigenic orthoclase, and disseminated Mn-oxides comprise the clay gouge. (V) The <1m detachment beneath the oldest hanging-wall sediments with Pliocene (3.1-6 Ma) tephra contains scaly clay gouge notable for a complex set of interstratified illite, chlorite, and smectite. Roughly 18% of the gouge is <2µm and PSD spectra are bi-modal. A <1mm polished and striated slip plane cuts the central portion of Zone V and separates the top of Zone IV from the hanging wall. However, there are areas in Zone V where hanging-wall faulting and associated textural irregularities in the gouge offset the slip plane. Some of the characteristics of Zone V could arise from dilation imposed from hanging-wall faulting down-dip. Otherwise, the fault rock textural progression is inferred to roughly track increasing net slip and the cumulative effects of alternating intervals of weakening and unstable slip, and strengthening and stable creep.