2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 8
Presentation Time: 1:30 PM-5:30 PM


HAYMAN, Nicholas W., Department of Earth and Space Sciences, Univ of Washington, Box 351310 or JHN 063, Seattle, WA 98195, nickh@u.washington.edu

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.