Computer-Assisted Geometric and Kinematic Analysis of Subsurface Faulting in the Vicinity of Yucca Mountain, Nevada, Using Balanced Geologic Cross Sections

by Stephen R. Young, Southwest Research Inst, San Antonio, United States,
Gerry L. Stirewalt, Southwest Research Inst, San Antonio, United States,
Robert A. Ratliff, Southwest Research Inst, San Antonio, United States,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1991


Computer-assisted geological cross section balancing methods are used in the geometric and kinematic analysis of subsurface structures in the vicinity of Yucca Mountain, Nevada, including underlying listric normal fault and detachment fault geometries and their relationships in a linked fault system. Dips of hanging wall fault blocks are directly related to the shapes of underlying curved normal and detachment faults. Arrays of small faults that are pervasively distributed through the hanging wall blocks are interpreted to be the bulk deformation mechanism of the block. As the hanging wall slides over the underlying fault surface, the fault block deforms to accommodate itself to the shape of the fault. Distributed slip on the small fault array is assumed to accomplish this shape change. The deformation is modeled as general simple shear, such that changes in shape of the model fault blocks take place by distributed slip on uniformly oriented slip surfaces that are evenly spaced through the area of the block, replicating the behavior of a deck of cards. Computed listric normal fault trajectories and surface geologic data constrain the range of compatible depths to potential detachment fault zones below Yucca Mountain to between about 3.5 and 6 kilometers. This is at least 1 to 2 kilometers deeper than the contact between the base of the Tertiary volcanics and the Paleozoic section. Preliminary balanced versions of sections A-A' and B-B' of Scott and Bonk show that assumption of a vertical simple shear deformation mechanism for fault block distortion will produce geologically reasonable subsurface models of faulting.

Subject Headings: Radioactive wastes | Geology | Subsurface environment | Kinematics | Geometrics | Walls | Soil deformation | Nevada | United States

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