Anisotropic Magnetic Susceptibility of Fault Rocks From Death Valley, CA: Comparison With Shape Preferred Orientation, and Implications for Kinematic Models of Brittle Foliation Development.

Anisotropic Magnetic Susceptibility (AMS) was measured for 11 samples of gouge and breccia from detachment faults that bound the Black Mts, Death Valley, CA. We reason that the AMS ellipsoid roughly describes the bulk deformational fabric within the <2-10μm matrix because the larger clasts are dominantly of quartzo-feldspathic mineralogy and, where present, any fabric within the clasts pre-dates fault-related deformation. Furthermore, there is a weak correlation between bulk-rock Fe₂O₃ and mean susceptibility; changes in Fe₂O₃ are thought to be partly due to mineral reactions within the matrix. Low temperature measurements are underway to further determine the source of the AMS. There is a distribution of prolate, oblate, and intermediate AMS ellipsoids, with a bias towards prolate ellipsoids. The declination of the maximum principal susceptibility k_max is somewhat dispersed with k_max of the prolate ellipsoids either normal or 40-60° to X, the dip and transport direction of the detachment; Y is the strike, and Z is normal to the X-Y plane. kmax of the more oblate ellipsoids is either within 15° of parallel to X or within 15° of parallel to Y. AMS ellipsoids with k_max parallel to Y likely track the intersection lineation of a composite foliation, or some component of vorticity and constriction, whereas those with k_max parallel to X developed in response to stretching or flattenting.In the cases where k_max is oblique to X the AMS developed under general shear. AMS was measured for the same samples used by Cladouhos (1999, Journal of Structural Geology, v.19, pp. 419-448) to measure the shape preferred orientation (SPO) of the larger clasts suspended within the matrix. Though the fault rocks have a mesoscopic foliation, the presence of a SPO is not intuitive as the foliation is cryptic at the scale of a thin section. The anisotropy of the magnetic susceptibility is also not predicted a priori as the foliation is poorly defined in the matrix at the scale of a thin section including in electron images. Cladouhos (1999) proposed that the SPO developed during a kinematic flow wherein clasts with axial ratios (R) >1.4 freely rotated (e.g. Jeffrey's-model behavior) within a general flow and upon reaching a critical orientation became insensitive to continuing strain. The inclination of the minimum principal susceptibility of the AMS ellipsoids is, for more than half of the samples, normal to the SPO and thus the fabric that is tracked by the SPO is also present in the matrix. This scale independence is described by the March model wherein the inclination of the foliation is proportional to the finite shear strain. In this model, the inclined foliation in the fault rocks developed with shear strains ca 4, and much of the >3.5 km of slip on the detachment accrued in gouges with a shear-zone parallel foliation or on localized slip surfaces. However, the AMS ellipsoids are not simply tracking one foliation within the X-Z plane and there is other evidence that the fault rocks developed through a protracted history of distributed deformation. A kinematic model involving more complex general flow thus may be more appropriate for describing the kinematic evolution of the fault zone.