Deformation Above an Extensional Lateral Ramp, Crater Flat, Nevada

Tectonic models of Yucca Mountain (YM), Nevada, site of the USA's proposed high-level nuclear waste repository, are based on understanding the nature of deformation affecting the site. Using mapped fault dips (50° in the north to 70° in the south) from the Bare Mountain fault (BMF, 15 to 25 km [9.3 to 15.5 mi] west of YM) and the surface geology of Crater Flat (CF, immediately west of YM) and YM itself, we constructed cross-sections across northern and southern CF. The sections indicate that the BMF has a listric shape detaching at a depth of about 6 km (3.7 mi) under northern CF and 12 km (7.5 mi) under southern CF. We present the results of physical analog modeling that tests the hypothesis that the BMF has a lateral ramp linking a shallow detachment under northern CF with a deeper detachment under southern CF. We constructed a footwall with two contrasting profiles based on cross sections generated using a constant thickness algorithm. The northern profile had a horizontal detachment at half the depth of the southern profile and the two fault surfaces were linked by a steep, S-dipping lateral ramp. The hanging wall of the model consisted of a plastic sheet conforming to the footwall shape, overlain by dry sand. To prevent frictional edge effects, the sand pack was large and laterally unconstrained. Undyed and acrylic-dyed sand simulated mechanically passive layering. Dry sand was chosen because of its ability to represent the brittle crust, and the ease with which it can be sectioned. The entire hanging wall moved in concert, constant velocity displacement was driven by a stepper motor attached via a rigid, strike-parallel assembly to the plastic sheet. After 2.8 cm (1.1 in) of horizontal displacement, synkinematic growth layers of sand were added to the model. After 5 cm (2 in) of horizontal displacement the model was stopped and sand added to stabilize the model prior to saturation and sectioning. Hanging wall deformation in the model was dominated by the formation of a half-graben, accomplished by development of normal faults and a crestal collapse graben within the hanging wall. Variation in extension across the lateral ramp was accommodated by en echelon, left-stepping antithetic faults, and by obliquely oriented small-displacement faults. We interpret development of normal faults within the hanging wall to be the response to outer arc extension imposed by bending of the hanging wall as it was displaced over the listric master fault. Fault-accommodated deformation in our model strongly resembles the character of CF and YM: (i) southward widening and deepening of CF basin; and (ii) extension transferred laterally by a left-stepping system of NS trending, antithetic normal faults (Windy Wash, Fatigue Wash, Solitario Canyon faults), and by NW-SE trending faults (Sever Wash, Pagany Wash, Drill Hole Wash faults). The BMF can be successfully modeled as two detachment depths linked by a lateral ramp. This abstract was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) and its contractors for the Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-02-012. The activities reported here were performed on behalf of the NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste Repository Safety. This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.