EXPERIMENTALLY PRODUCED DEFORMATION BANDS IN ETCHEGOIN SANDSTONE: IMPLICATIONS FOR INFERRING STRESS DIRECTIONS IN CENTRAL CALIFORNIA

The Pliocene Etchegoin Formation of central California hosts deformation bands that are exposed in outcrop in several structures northeast of the San Andreas fault. These tabular strain localization features occur in poorly cemented sandstones as both individual bands and zones of closely spaced bands, both of which accommodate shear displacement where observable. Measurements of deformation band orientations over a broad field area between Parkfield, CA (1-3 km from the San Andreas fault) and Kettleman Hills (32 km) indicate that deformation bands in the Etchegoin formation primarily develop in two sets of steeply dipping conjugate sets. It is possible that stress directions can be inferred by bisecting the acute angle between this set of conjugate sets.

We conducted triaxial deformation experiments on cylindrical cores of Etchegoin sandstone that do not host deformation bands to better understand how deformation bands form in response to applied stress and strain. During the experiments, confining and pore fluid pressures were held constant and simulate conditions at approximately 1 km depth. A piston applied an axial load and advanced at a constant axial strain rate on the order of 10^-7 to 10^-6 s^-1. Samples were deformed up to 10% axial strain and exhibited overall strain hardening. The suite of experiments was conducted with the axial stress parallel and perpendicular to bedding and at axial strains between 2 and 10% to examine how these variables affect deformation band formation.

Deformed samples either contained deformation bands or showed no signs of strain localization. Where possible, we measured the angle between the applied maximum compressive stress and the deformation bands. Thin sections from cores, impregnated with blue epoxy, allow us to make direct microstructural and petrophysical comparisons between experimentally formed and natural deformation bands in the Etchegoin formation.