HR: 15:10h
AN: GP12A-07    [PDF]
TI: Comparing magnetic and clast fabric in gouge and breccia from the Black Mountain detachments, Death Valley, CA: Implications for the micro-mechanics and kinematics of shallow crustal shear zones.
AU: * Hayman, N W
EM: nickh@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington Box 351310, JHN 063, Seattle, WA 98195 United States
AU: Housen, B
EM: bernieh@cc.wwu.edu
AF: Geology Department, 516 High St. Western Washington University, Bellingham, WA 98225-9080 United States
AU: Cladouhos, T T
EM: tcladouhos@webpe.com
AF: Department of Earth and Space Sciences, University of Washington Box 351310, JHN 063, Seattle, WA 98195 United States
AU: Livi, K J
EM: klivi@jhu.edu
AF: Department of Earth and Planetary Sciences, Johns Hopkins University 3400 N. Charles St., Baltimore, MD 21218 United States
AB: The Black Mountain detachments, Death Valley, CA, place Pliocene-Quaternary sediment against Miocene and older crystalline rocks. The detachments comprise sharp slip surfaces and centimeter-to-meter scale shear zones. The shear zones contain gouge and breccia that exhibit well-developed mesoscopic foliation but no evidence for deformation from crystal plasticity or penetrative pressure solution. Measurements of the anisotropy of magnetic susceptibility (AMS), partial anhysterestic remanent magnetization (pARM), and shape preferred orientation (SPO) of greater than 50 micron grains define fabric consistent with the extension direction of the faults inferred from geologic and geodetic data. Many lines of evidence including low-Temperature MS experiments, pARM, transmission and scanning electron microscopy, and optical petrography demonstrate that the magnetic carriers within the gouge and breccia are dominantly nanometer-to-micrometer scale grains that grew within the shear zones prior to the most recent deformation. In contrast, SPO was measured for populations of grains that were inherited from the wall-rock. SPO was measured on thin sections cut parallel to three orthogonal planes-of-view. Because it is not known if the SPO vectors are eigenvectors a priori the resulting SPO vectors were compared in relative length and orientation with the eigenvectors of the AMS ellipsoid. Flattened and some elongated AMS and SPO ellipsoids characterize the fabrics. The long axes of these ellipsoids are shallowly inclined to the shear plane, but in some cases are inclined out of the plane of inferred shear. The similarity of fabric defined by clasts (SPO) and matrix (AMS) is inconsistent with a plastically yielding matrix or a dispersive mode of polyphase flow. Although mechanically the gouge and breccia were/are frictional materials that deform via a complex granular flow, we find that the orientation of the SPO and AMS are best described using kinematic models rooted in Jeffery's theories for fluids. Because the magnetic carriers demonstrably grew after the larger SPO-defining clasts began rotating, we propose that the SPO records more finite strain than the AMS. This can explain most of the discrepancies between the SPO and AMS orientations.
UR: http://www.ess.washington.edu/~cowan/
DE: 1518 Magnetic fabrics and anisotropy
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8030 Microstructures
DE: 8094 Instruments and techniques
DE: 8100 TECTONOPHYSICS
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting