USING DEFORMATION BANDS TO CHARACTERIZE DEFORMATION AT KETTLEMAN HILLS, CALIFORNIA: IMPLICATIONS FOR OFF-FAULT DEFORMATION IN THE SAN ANDREAS FAULT BORDERLANDS

The Kettleman Hills anticline is one of several folds cored by blind thrust faults along the edge of the San Joaquin Valley northeast of the San Andreas fault in central California. The main phase of fold growth occurred in the past 2-2.5 m.y. making it one of the youngest structures in the region. The fold is well expressed in the topography, with its hinge slightly oblique to the local trace of the San Andreas fault, and includes three, right-stepping, en echelon segments known as North, Middle, and South Domes. Our study is focused on North Dome, which has excellent exposures of Pliocene-age rocks.

We collected structural data throughout North Dome including the orientation of deformation bands. These features are tabular structures unique to porous granular material, which often act as a precursor to faults. At North Dome, deformation bands are most commonly developed in poorly cemented, coarse blue sandstones of the Upper Etchegoin and Lower San Joaquin Formations. In general, four main orientations are observed: two share a ~340˚ strike, dipping either NE or SW by 60-80˚, and two more share a ~020˚ strike, dipping either NW or SE by 60-80˚. In other words, the deformation bands form a pair of conjugate sets striking NNW and NNE. Field evidence supports normal motion along most of the deformation bands with displacements of ~1 cm per band.

Unfolding North Dome causes increased scatter in the deformation band orientations, suggesting that the structures did not form prior to folding. There are no spatial variations in their orientations along or across the fold, suggesting that the deformation bands record regional rather than local fold-related deformation. Thus, these structures may be used as an indicator of recent strains in this area. Their geometry is consistent with models of faults developed in three-dimensional strain fields suggesting that the minimum horizontal stretching direction (or maximum compression direction) is approximately NS. This direction is quite different from that inferred from the fold shape alone or from normal faults developed along the fold hinge. This study demonstrates the value of using small-scale structures, such as deformation bands, for characterizing off-fault deformation in the San Andreas fault borderlands.