Deformation bands in the Etchegoin Formation sandstone: Triaxial deformation experiments and implications for principal stress directions in central California

I present structural measurements of deformation bands in the Miocene-Pliocene Etchegoin Formation of central California collected at several sites spanning four large-scale folds northeast of Parkfield, CA. From these data, I infer the direction of maximum horizontal compressive stress (SHmax) by bisecting the acute angle between sets of conjugate sets of deformation bands—a common pattern across the field area. These SHmax measurements, at 60°-70° to the strike of the San Andreas fault, are at odds with values reported by the World Stress Map, which generally form high angles (\u3e80°) with the strike of the San Andreas fault. Because of this discrepancy and the potential for deformation bands to serve as an indicator of regional stress patterns in central California, I use triaxial deformation experiments on samples of Etchegoin Formation sandstone to better understand how deformation bands form in response to applied stresses. I investigate the impact of varying bedding direction and the amount of axial strain on experimentally deformed samples. At the low effective confining pressures (5-20 MPa) used in this study, no localized features are observed in the deformed samples for any amount of applied axial strain (2-10%). Analysis of thin sections made from deformed cores confirm that deformation is distributed throughout the samples in grain fracturing and pore space collapse. Although the results of these experiments are not instructive for inferring stress directions from natural deformation bands, they are a step towards understanding how the Etchegoin Formation deforms and provide some direction for future triaxial experiments