Abstract

The Muddy Mountain overthrust, exposed in the Buffington window, southeastern Nevada, consists of a Paleozoic carbonate sheet thrust over Mesozoic Aztec Sandstone, with a molasse filling topographic lows. Evidence suggests that the thrust sheet moved across an erosional surface and that the molasse may have been a forethrust debris. A sharp contact with gouge marks the fault surface. The base of the overthrust sheet is a tectonic breccia containing injections of gouge that are rooted at the contact. Thrust-related changes in the underlying rocks related to proximity of the thrust plane include (1) increase in abundance of microfractures and decrease in grain size due to cataclastic deformation; (2) increase in intensity of macrofracturing parallel and at a low angle to the contact; (3) increase in degree of induration; (4) loss of well-defined bedding planes and color contrast within the Aztec Sandstone; and (5) slabs of dolomite sheared from the upper plate. Laboratory mechanical tests in conjunction with field observations suggest that the shear strength of the undeformed Aztec Sandstone was lower than the frictional strength of the sandstone sliding on quartz gouge. Therefore, cataclastic deformation within a 10- to 100-m-thick zone accompanied the initial advance of the thrust sheet. Following induration, which strengthened the cataclastic sandstone, slip was localized at the thrust contact. During this later stage the high permeability of the fractured upper plate and the Aztec Sandstone suggests that fluid communication with the surface at the leading edge of the thrust was rapid and, therefore, the advance of the thrust could not have been aided by high pore pressure.

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