Sense of Displacement along Foothills Fault System: New Evidence from the Melones Fault Zone, Western Sierra Nevada, California

Abstract

Sense of displacement along the Foothills fault system, western Sierra Nevada, California, has been interpreted in terms of reverse, thrust, and strike-slip faulting. Within the Melones fault zone (part of the Foothills fault system) near Downieville, synoptic fault-plane intersections, in cataclastically deformed metasedimentary rocks and serpentinite, plunge steeply northwest, as do axes of minor flexure folds in metasedimentary rocks. Fold axes in serpentinite plunge steeply to the northeast and southeast; slickenside orientations are varied, but plunge chiefly southward at steep to moderate angles. Cleavage in slate on either side of the fault zone strikes approximately north and dips steeply eastward. These relationships imply a complicated chronology of events characterized by possible early emplacement of serpentinite and a late phase of strike-slip displacement. I conclude that present fault dips near Downieville probably approximate initial dips, that the pattern of faulting suggests a conjugate relationship best explained in terms of strike-slip displacement, that the latter interpretation is supported by attitudes of minor-fold axes, but not by those of slickensides, and that such an interpretation does not preclude earlier oblique-slip or dip-slip displacements.