A Re-Evaluation of the Million Hills Wash Fault, South Virgin Mountains, Nevada

The Million Hills Wash fault in the South Virgin Mountains, southern Nevada is a gently dipping normal fault that places Cambrian through Mississippian strata atop Paleoproterozoic gneiss, and is reported to have slipped at an inclination as low as 11° based on the downward termination of more steeply inclined faults in its hanging wall (Brady et al., 2000, GSAB, v. 112, p. 1375-1397). A re-evaluation of critical field relations at the south end of Azure Ridge, including new mapping and new measurements of fault orientations and cut-off angles, reveals the following: 1) The MHWF is a map-scale composite of several faults with a range of attitudes. 2) Hanging-wall faults with either normal or reverse separation (present-day co-ordinates) are typically characterized by stratigraphic offsets of no more than a few meters. 3) A combination of small separation, incomplete exposure, modest obliquity with the MHWF and the evident complexity of the latter makes it difficult to establish that these steeper faults terminate downwards. 4) The MHWF is demonstrably offset in some cases. A mechanically plausible interpretation for these observations and for published geological mapping in the South Virgin Mountains is as follows. Normal faults developed with steep to moderate dips. The most prominent, ridge-bounding structures were not necessarily active over precisely the same spans of time (cf. simple domino model). The MHWF was tilted passively to a very low dip and offset by mostly minor more favorably inclined faults after it became locked. Synchronous slip on high- and low-angle normal faults is not required. Nor is there evidence for extreme listric curvature, with normal faults becoming subhorizontal within gneiss at depths of less than a few kilometers. The MHWF in any case provides no support for such an interpretation. These conclusions are pertinent to the construction of regional geological cross-sections.