UNUSUAL ‘GRADED' FAULT ROCKS FROM THE SNAKE RANGE DETACHMENT, NV – IMPLICATIONS FOR FLUID INVOLVEMENT IN LOW-ANGLE NORMAL FAULT PROCESSES AND THE ANGLE OF SLIP OF CORE COMPLEX DETACHMENTS

Since the middle of the 19^th century, field and laboratory description of fault breccias have almost uniformly referred to the random fabric of rocks found both in cohesive and incohesive fault breccias. The existence of clastic sediments forming in caves has, however, been documented for many years. The outcrop we describe is analogous to a cave fill sediment, but formed entirely within a low-angle normal fault zone (the Snake Range detachment, NV) in the presence of sufficient fluid to permit clast settling as a function of size. We present field and microstructural descriptions of two distinct graded units found within an outcrop of the North Snake Range detachment. 1.) A graded, carbonate-cemented gravel or breccia with clear upward-fining from cobble to coarse sand-sized sub-angular to sub-rounded clasts. 2.) A 1 m long lens of coarse- to fine-grained moderately well-sorted, horizontally-bedded carbonate sand, locally enclosing larger clasts. Field relations indicate these clastic sediments formed within the fault zone at a late stage in the detachment’s history and shortly after slip ceased respectively. The clear grading of the sediment indicates both the gravel and later sand were deposited in open space filled with fluid. CL studies document a single late cement for both units, distinct from the complex cementation history of other fault breccia rocks from the detachment.

The presence of sub-horizontally-graded, fluid-settled, intra-fault-zone sediments 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 (<~2-3 km) for open cavities to locally form inside the fault core and remain open. 2.) These cavities inside the fault core were filled with enough fluid for clasts to settle out in a fluid medium, but insufficent fluid pressure to hydrofracture wallrocks. The evidence of transport of sand-sized grains in a fluid indicates that locally, fluids in LANF detachments were capable of moving at velocities on the order of cm/sec. 3.) The horizontal bedding of the sediments forming inside the detachment, which currently dips 12º to the ESE, demonstrates that the detachment became inactive at its current dip.