Abstract

The Specter Range quadrangle, Nevada, lies across the projected trend of the Las Vegas Valley Shear Zone, a right-lateral fault zone with more than 25 miles of displacement. At the north end of the quadrangle a narrow strip of Paleozoic rocks is stratigraphically continuous and nearly continuous in outcrop across the projected trend of the shear zone, but shows no evidence of displacement by a large strike-slip fault. The Las Vegas Valley Shear Zone is interpreted as a deep crustal structure (Longwell, 1960), and in the Specter Range quadrangle most of the displacement on the shear zone at depth is taken up by bending in the overlying sedimentary rocks rather than by fracturing.

The Las Vegas Valley Shear Zone should be regarded as a separate structure from the Walker Lane in the Cedar Mountain area. The westward extension of the Las Vegas Valley Shear Zone appears to be south of the western part of the Walker Lane.

The most characteristic structural features of the Specter Range quadrangle are the high-angle faults, many of which are topographically expressed. Folding is relatively unimportant and local. Three sets of high-angle faults are recognized on the basis of similar strike: (1) a northwest set, (2) a north-south set, and (3) a northeast set. The north-south set is interpreted as a regional set of normal faults whereas the northwest and northeast sets are interpreted as right-lateral oblique-slip and left-lateral oblique-slip faults, respectively.

Evidence indicates that the Johnnie thrust mapped by Nolan (1929) is not present in the Specter Range quadrangle. The contact between the Johnnie Formation and the overlying Stirling Quartzite is conformable in some places and not marked by a thrust. The “tectonic” contact visible at some localities is regarded as bedding-plane slip between competent and incompetent rock types.

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