Abstract

Four of the mountain ranges near Death Valley, California, display exhumed, low-angle normal faults on their flanks, features originally referred to as turtleback structures. These fault surfaces are smooth, planar to curviplanar, and defined by faceted spurs and coincident interfluve crests. The lowest parts of these faults are overlain in tectonic contact by poorly indurated fanglomerate, and locally by Quaternary volcanic rocks. Steep fault scarps in alluvium are present at the foot of each turtleback-type flank.

Although these may be examples of active low-angle normal faults, it is difficult to establish the dip histories of these faults. The problem with constraining the dip histories is one of missing and concealed information, because the exposed hanging-wall rocks are some of the youngest material cut and displaced by the (currently) shallow-dipping faults. Another important relationship along these range flanks is the intersection between the low-angle faults and steep, scarp-forming faults that cut alluvium at the range fronts. Although this relationship remains unconstrained in most of the ranges, a shallow-dipping fault is cut by one of the steep faults in Panamint Valley, suggesting that, in one area at least, the low-angle faults are not active.

The low-angle faults, and not the steep neotectonic faults, are probably responsible for most of the opening of the present valleys, because each low-angle fault intersects the valley floor at the range front. None of the low-angle faults are perched high in the ranges, as would be the case if the steep faults had uplifted them significantly. Therefore, even if there has been a geologically recent transition from low-angle normal faulting to steep normal (with or without strike slip) faulting, the modern topography still reflects the former regime. Strictly speaking, the Death Valley region may not be an example of active low-angle normal faults and supradetachment basins.

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