AN OVERLOOKED WIDE ZONE OF HIGH STRAIN IN THE 'AUTOCHTHON' OF THE EARLY-MIDDLE JURASSIC FENCEMAKER DEFORMATION BELT, NORTHWEST NEVADA: IMPLICATIONS FOR CORDILLERAN OROGENESIS, PALEOGEOGRAPHY, AND ORE DEPOSITS

Traditional views of the Jurassic Fencemaker fold-thrust belt (FFTB; north part of Luning-Fencemaker fold-thrust belt) in NW Nevada are that: it primarily affected deep marine Triassic strata (basinal terrane) deposited offshore of the Triassic continental shelf (shelf terrane); most shortening occured in a D1 event (isoclinal folds, thrusts, foliation); culmination of deformation occured when basinal terrane strata ramped partway over the shelf along the Fencemaker thrust (FT); the autochthonous shelf terrane was little affected by FFTB deformation; and the FFTB eastern limit is marked by the FT. New work near the inferred FT boundary in the northern East (nER) and southern Humboldt (sHR) Ranges contradict elements of this traditional view. In the nER, we find that rocks previously assigned to the basinal terrane are actually shelf terrane units, the FT previously shown to cut this area does not exist, and shelf strata are strongly deformed (polyphase folds, foliation, thrusts). In the sHR, previously shown to contain basinal and shelf strata separated by the FT, we have found only Triassic shallow marine strata that are complexly deformed (polyphase folds, foliation, thrusts, shear zones), with no evidence of a terrane-bounding FT. Farther north in the HR, shelf strata also show widespread deformation (thrusts, folds, foliation). Age of deformation in the region is constrained by 164-162 Ma U-Pb zircon ages from syn- and post-tectonic plutons. Collectively, these data require a revised view of the FFTB, with multidisciplinary implications. (1) The western shelf terrane was affected by significant shortening during FFTB development, likely more than is found in the basinal terrane. (2) Eastern limit of FFTB deformation is within the shelf terrane, >6 km farther east than previously inferred. (2) FT does not mark the eastern limit of FFTB deformation, and it is likely just one of many FFTB thrusts. (4) A Middle Jurassic and wider FFTB that involved the shelf has implications for correctly identifying an FFTB foreland basin, and for identifying sediment eroded from the orogen. (6) Finally, the belt of strongly deformed shelf terrane strata is host to multiple precious metal deposits - improved understanding of this previously unappreciated deformation zone will be critical to future exploration