DEFORMATION BANDS IN THE ETCHEGOIN FORMATION AT PARKFIELD: IMPLICATIONS FOR STRESSES ON THE SAN ANDREAS FAULT IN CENTRAL CALIFORNIA
Deformation bands within the syncline are typically either individual bands or zones of multiple, closely spaced bands. We interpret both types as small faults, where each band accommodates mm- to cm-displacements. Deformation bands are more numerous on the fold limb closest to the San Andreas fault suggesting that they record seismic events. At this location, we observe five distinct orientations. One set strikes WNW, parallel to bedding, and dips 45-65˚ N, slightly shallower than bedding. This set likely parallels foresets in the sandstone and is always cut by the other four sets of deformation bands, which are themselves mutually cross-cutting. The four sets dip 70-80˚ and form two conjugate pairs striking ~040˚ and ~350˚. Absolute motion is difficult to determine, but apparent offsets in cross-section are always normal. In plan-view, the 350˚-striking set has dextral offsets while the 040˚-striking set has sinistral offsets. One fault surface in a deformation band zone has oblique-slip lineations suggesting that offset on many bands could be a combination of normal and strike-slip motion.
We believe that the four sets of deformation bands are syn- or post-folding features because unfolding the syncline causes more scatter to their conjugate geometries. Thus, these structures may be used to infer recent stresses. We estimate the orientation of the maximum principal stress direction (σ1) by bisecting the acute angle between the conjugate sets. Thus, σ1 is subhorizontal trending towards 015˚, approximately 55˚ from the local strike of the San Andreas fault. This orientation matches other studies including shallow observations in the SAFOD pilot hole and calculations from earthquake focal mechanisms. Our field observations from Parkfield have implications for the weak fault model for the San Andreas fault.