Rooted detachment faults and detachments beneath rootless slide blocks exhibit many similar structural characteristics. However, while rooted detachments are thought to penetrate into the midcrust and to accommodate significant crustal extension, rootless detachments break to the surface downdip and are not directly involved in such extension. Distinguishing between these two mechanically different kinds of structure is central to the assessment of extension magnitude. Here we examine deformation along the Mormon Peak detachment, a feature that has been cited as an example of both a rooted and a rootless structure. Located in the Mormon Mountains of southeastern Nevada, this detachment has been interpreted as one of three low-angle normal faults of regional scale that together are thought to have accommodated more than 50 km of Basin and Range extension. For the most part, however, the Mormon Peak detachment is expressed as a series of isolated exposures where Paleozoic rocks are in brittle fault contact with nonmylonitized underlying rocks. Individual blocks contain high-angle normal faults that terminate downward at their respective detachment surfaces, yielding a geometry common to both modes of emplacement. In order to test between these competing interpretations, we studied deformational characteristics close to the detachment surface, reasoning that a seismogenic fault ought to differ fundamentally from a surficial slide block, particularly if the slide block was emplaced in a single event rather than by protracted or episodic creep. An examination of the contact mapped as the Mormon Peak detachment reveals that the character of deformation is indistinguishable from that of known gravity-driven slide blocks and is fundamentally different from that associated with seismically cycled faults. Moreover, the orientation of kinematic indicators observed at detachment surfaces is consistently close to the downdip direction, which in many places diverges strongly from the expected direction of movement in the rooted detachment model. We conclude that outcrops of the inferred upper plate of the Mormon Peak detachment represent an assemblage of individual rootless gravity-driven slide blocks and not the erosional remnants of a formerly contiguous extensional allochthon. If similar misidentifications have been made elsewhere in the Basin and Range Province, total Cenozoic extension may have been significantly overestimated. Implications for the interpretation of extensional geology in general are far-reaching.
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