VERTICAL- AND INCLINED-AXIS ROTATIONS IN EXTENSIONAL SETTINGS

Although vertical-axis rotations are more commonly associated with thrusting or strike-slip faulting, they can also occur in response to extension. One example is in Crater Flat basin, Nevada, an area that encompasses the site for the proposed high-level nuclear waste repository at Yucca Mountain. We interpret the crustal deformation of Crater Flat basin and adjacent areas as differential outer-arc extension of the hanging wall above the listric Bare Mountain fault. Differential extension of the hanging wall results from a north-to-south increase in dip of the listric Bare Mountain fault. Because of this change in dip of the Bare Mountain fault, equal amounts of east-west extension across the fault require more fault-parallel slip and a greater amount of hanging wall extension where the fault dip is steepest. The differential extension and vertical-axis rotations predicted by this model are consistent with field observations of hanging wall structures and vertical and inclined axis rotations recorded in a large set of paleomagnetic data. Based on this model, we demonstrate that most of the post-15-Ma extension of Crater Flat basin occurred between 12 and 11 Ma. Rapid extension of the Crater Flat basin is constrained to this interval because of differences in vertical axis rotation recorded by volcanic rocks older than about 12 Ma compared to un-rotated volcanic rocks and other strata younger than about 11 Ma. Cumulative throw across the Bare Mountain fault between about 12 Ma and 11 Ma ranges from 1–2 km [3300–7000 ft], suggesting a fault-slip rate of 1–2 mm/yr [0.04–0.08 in/yr]. After about 11 Ma, this slip rate likely reduced to present rates of about 0.06 mm/yr [0.002 cm/yr]. This composite geologic record of Crater Flat basin provides a fundamental component in the evaluation of the volcanic and seismic hazards at Yucca Mountain.

[This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the U.S. Nuclear Regulatory Commission]