UNUSUAL ‘GRADED’ FAULT ROCKS FROM THE SNAKE RANGE DETACHMENT, NV –EVIDENCE BOTH FOR SEISMIC SLIP ON LOW-ANGLE NORMAL FAULTS AND PALEODIP AT THE TIME OF SLIP

Since the 1850’s, field and laboratory description of fault rocks have almost uniformly referred to the random fabric of fault breccias. We describe an outcrop of the Snake Range detachment in East-central Nevada with clearly graded fault breccia, ranging from cobble-sized clasts to silt-sized carbonate and quartz grains. Well-sorted, silt-sized porous breccia-derived material is locally injected into tensile fractures in the hanging wall for a distance of several meters, reminiscent of a natural ‘sand-frac’. Field relations indicate these graded fault zone materials formed within the fault zone at a late stage in the detachment’s history.

The presence of sub-horizontally-graded, fluid-settled, porous fault zone materials within the Snake Range detachment has important implications for the behaviour of low-angle normal fault zones. 1.) Late in the detachment’s history, the detachment was at a shallow-enough crustal level (<~2 km) for open cavities to locally form inside the fault core and remain open. 2.) Breccia material was unconsolidated and un-cemented at times of fluidization 3) Fluid moving within the fault core at cm/sec to m/sec entrained unconsolidated breccia and transported it in a fluvial manner within the fault core. 4) Transient overpressure above the minimum confining stress caused injection of the unconsolidated, already graded, breccia material into the hanging wall in a manner similar to sand injectites.

The transient overpressure causing hydrofracture and injection of breccia-derived material into the hanging wall virtually requires fault slip at seismic velocities. The 5-7° ESE of the bedding of the breccia inside the detachment, which currently dips 12º to the ESE, demonstrates that the detachment became inactive at a dip of 5-7°. These results indicate that a low-angle normal fault was capable of slip at seismic velocities at dips <10 degrees and without the presence of low-friction fault zone materials.