GRADED FAULT ROCKS AND A HYDROFRACTURED FAULT ZONE AT THE SNAKE RANGE DETACHMENT, NV, USA: SLIP ON A LOW-ANGLE NORMAL FAULT AT SEISMIC VELOCITIES

Field evidence for either a) slip on a low-angle normal fault at low dip or b) slip at seismic velocities, with the exception of pseudotachylityes, remains rare. An outcrop of the Snake Range detachment in East-central Nevada contains visibly graded upward-fining fault breccia, ranging from cobble-sized clasts to silt-sized sub-angular carbonate grains. The well-sorted, sub-angular carbonate silt is injected into hydrofractures in the hanging wall for several meters, reminiscent of sand injectites, and into upper-plate normal faults. Field relations indicate these fault zone silt injectites formed within the fault zone at a late stage in the detachment’s history, and coevally with random-fabric late, porous fault breccias.

Sub-horizontally-graded, fluid-settled, porous fault zone materials together with injections of well sorted but sub-angular silty fault zone material have implications for the late behaviour of the detachment. 1.) Late in the detachment’s history, the detachment was at a depth <2 km, with wall rocks strong enough to permit open cavities inside the fault core to form and remain open. 2.) Fault breccia was unconsolidated and un-cemented. 3) Fluid moving within the fault core at cm/sec to m/sec entrained unconsolidated breccia and transported it in a fluvial manner within open spaces in the fault core. 4) Rapid fluid movement induced transient overpressure above the minimum confining stress causing injection of the unconsolidated, already graded, breccia material into fractures in the hanging wall, similar to sand injectites. 5) Intraclasts of already-graded material are found in both typical and graded breccias, indicating breccia grading was a repeated process.

Hydrofracture and injection of graded breccia-derived material into the hanging wall requires pressures at or above the local fracture gradient. As the hanging wall is brittle carbonate and jointed, slip at seismic velocities is virtually required to cause a transient overpressure. 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°, without the presence of low-friction fault zone materials.