Ductility in fault gouge from a normal fault system, Death Valley, California: A mechanism for fault-zone strengthening and relevance to paleoseismicity

Abstract

Fault gouge along the Badwater Turtleback fault system displays abundant evidence for mesoscopic ductility, as foliation, isolated and/or rotated inclusions, and isoclinal folds. Fault-gouge thicknesses also vary dramatically. At the outcrop scale, gouge thicknesses can range from 0 to >1 m, and at the 100 m scale, they can range from 0 to >5 m. Because these variations are closely associated with other ductility-related features, they probably also formed because of ductile flow. Ductile flow appears to locally strengthen the fault zone by severe attenuation of the gouge and by transport of inclusions. Where gouge pinches out to zero thickness, hanging-wall rock rests directly on footwall rock, a condition that produces an overall stronger fault zone as well as stick-slip behavior in laboratory tests. Transport of inclusions within the gouge zone may create local inclusion “jams” which may also have a strengthening effect. The presence of these stronger portions of the fault zone may give indirect evidence for ancient seismicity on the fault.