Strain hardening and strain softening in fault zones from foreland thrusts

The movement of thrust sheets is accomplished, in part, by deforming the rocks adjacent to thrust faults. In foreland fold-and-thrust belts, emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet. Ductile deformation of rocks in the layer occurs by displacing fault-bounded blocks relative to one another in a kind of mesoscopic “grain boundary” sliding. Analyses of data from three southern Appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening, then by strain softening, and it is accommodated by intra-block fractures, small faults, and veins. The density of accommodation features increases near thrusts, and where these features become sufficiently numerous, a layer of cataclastic rocks forms along the thrust. We describe foliated cataclasites from two thrusts, the microstructures of which indicate that the cataclasites exhibited extreme ductility, interrupted occasionally by brittle processes. Deformation of the cataclasites was accompanied by strain softening. Strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites. During the formation of the cataclasite, rocks in the sheet are impelled to cross a strain energy “hump,” after which they deform at either higher rates or lower stresses.