THE FOX RANGE, NORTHWEST NEVADA: DISPLACED FRAGMENT ALONG AN EARLY CRETACEOUS DEXTRAL STRIKE-SLIP FAULT?

Recent studies indicate that a major strike-slip boundary (MSNI fault) of Early Cretaceous age extends from southeast CA through NV into western ID, and accommodated 400-500 km dextral offset (e.g., Wyld and Wright 2001, 2005). The Fox Range of northwest Nevada (north of Pyramid Lake) lies along an understudied section of this fault system. To the northeast is the Paleozoic to Mesozoic Black Rock arc terrane. To the east is the Late Triassic back-arc basinal terrane, which is deformed by the SE-vergent Jurassic Luning-Fencemaker fold-thrust belt (LFTB). Our detailed mapping of the Fox Range indicates that it is unrelated to these terranes and was not deformed in the LFTB. Three pre-Late Cretaceous units are present: 1) a graphitic argillite unit, composed of thin to thick-bedded black argillite and siltstone, with minor interbeds of impure quartzite and carbonate, overlain by 2) a quartzite unit, consisting of massive beds of clean quartzite, with less common interbeds of pure carbonate and metapelite, locally capped by 3) a quartz-feldspar-biotite paragneiss, occurring enigmatically only in small patches at high elevations. It is not yet clear whether the gneiss represents a distinct unit or a higher grade version of the quartzite unit. All units are deformed by three phases of deformation. Bedding is deformed by tight to isoclinal D1 folds that verge to the west, accompanied by an axial planar foliation (S1). D2 folds of S1 are generally open and refolded locally by gentle D3 folds. Syntectonic metamorphism increases in grade upsection from the argillite unit into the gneiss unit. Either these units were overthrust by a now-eroded hot thrust sheet, or the gneiss unit patches are relicts of this thrust sheet. The argillite and quartzite units are reminiscent lithologically of the Paleozoic miogeocline, but detrital zircon ages from a quartzite sample indicate deposition after 212 Ma, and zircon signatures are identical to those of Jurassic erg deposits of the Colorado plateau and their re-deposited equivalents in the southwest Cordillera. Reconstruction of 400 km offset on the MSNI fault places the Fox Range between the latitudes of the Inyo Mountains and Mono Lake. Ongoing work seeks to determine whether Fox Range geological equivalents can be identified in that area.