Abstract

Lamprophyre dikes and evidence for flows were recently found in Upper Devonian rocks of the Roberts Mountains allochthon (RMA) in the Tuscarora Mountains of north-central Nevada. Elsewhere in the allochthon, Upper Devonian greenstones commonly form flows and pyroclastic rocks. Famennian bedded barite deposits of the RMA are syndiagenetic, locally fossiliferous, and interpreted as sedimentary-exhalative in origin. Small scale block faulting accompanied deposition of barite. These features indicate significant extensional or strike-slip faulting before or during the early phases of the Antler orogeny in Nevada. The depositional basement of the RMA was probably both attenuated continental crust and oceanic crust on the margin of North America.

Extensional or transcurrent tectonism also affected other regions of the North American western margin during the mid-Paleozoic, but without Antler compression. Three possible hypothesis may explain Late Devonian basinal extension in Nevada: (A) incipient (back-arc?) rifting, or (B) strike-slip faulting, or (C) shallow-level normal faulting along the westward-subducting continental plate during the early Antler orogeny. Hypothesis) does not explain the production of lamprophyres and greenstones but is compatible with some Famennian to Kinderhookian barite and phosphatic rocks. Taken together, the evidence favors a model of back-arc thrusting or ‘incipient’ subduction for the Antler orogeny.