Abstract

The Roberts thrust is a major thrust in Nevada on which Ordovician to Devonian siliceous facies rocks were carried more than 80 km eastward over contemporaneous carbonate facies. For more than two decades, a mid-Paleozoic age for this structure has been widely accepted. The bases for dating the thrust are (1) the assumption that the Roberts thrust formed at the time of the Antler orogeny—a reliably dated mid-Paleozoic event, and (2) the concept of an overlapping assemblage consisting of sedimentary strata deposited on both the allochthon and autochthon. The first assumption is unproved, and the second relies on another assumption: that the overlap assemblage was not cut and telescoped by the Roberts thrust. In the years since a mid-Paleozoic age for the Roberts thrust became widely accepted, much new information on the stratigraphy and facies relations of Mississippian rocks in northern Nevada has accumulated. One interpretation of these new data suggests that in the Pinon Range, where a mid-Paleozoic age for the thrust was most convincingly displayed, the overlap assemblage actually may have been cut by a major thrust that juxtaposed contrasting facies of Mississippian rocks. If so, the principal evidence used to date the Roberts thrust is compromised, and the time has come for an agonizing reappraisal of all evidence bearing on the question. A mid-Paleozoic age can no longer be taken for granted, and a post-Paleozoic age cannot be ruled out.

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