SOLUM, JOHN G., University of Michigan, Dept. of Geological Sciences, MI

ABSTRACT: Changes in Clay Mineralogy and Fabric Development Across Selected Fault Zones

Changes in clay mineralogy and the development of clay fabrics across fault zones provide information about the conditions that existed along faults when they were active, which has implications for fault mechanics. Clay gouge is a common component of fault cores in most tectonic settings, thus insight into faulting gained from this study may reflect conditions that are common to many types of faults worldwide.

Changes in clay mineralogy, specifically the transformation of smectite to illite and changes in the polytypes of illite, are quantified using software modeling of X-ray diffraction spectra or equations that involve ratios of polytype-specific peaks to peaks that are shared by all polytypes.

The development of clay fabrics is evaluated using high-resolution X-ray texture goniometry. The development of a preferred orientation is significant because it contributes to anisotropy of fluid flow in fault zones (with enhanced flow parallel to the fault) and because a well-developed fabric parallel to the fault may result in a lowered coefficient of friction perhaps accounting for the weak behavior of some faults.

Sampled faults include the Moab and Hurricane normal faults as well as unnamed small displacement normal faults and the Parowan Gap thrust fault in Utah, a normal fault in Nevada, and the right lateral Punchbowl fault in southern California.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid