NEW MAPPING IN THE EAST RANGE, NEVADA: IMPLICATIONS FOR THE WINNEMUCCA AND FENCEMAKER DEFORMATION BELTS

Mesozoic shortening in north-central NV is attributed to two Jurassic deformation belts, the east-vergent Luning-Fencemaker fold-thrust belt (LFTB) and the west-vergent Winnemucca belt (WDB). Documenting the structural history and timing of deformation in these belts is important for regional models of Mesozoic orogenesis. We present the results of new mapping in the East Range (ER), a key area for both the LFTB and WDB, and the type locality for the "Willow Creek thrust", a defining structural feature of the WDB. Our mapping indicates that one strand of this "thrust" (Rockhill Canyon) is actually a high-angle fault of probable Cenozoic age. The other strand separates carbonate-dominated units in the area surrounding Lee Pk. from the Ordovician Valmy Fm. Known as the Lee Pk. window, these units were originally thought to be Triassic and later interpreted as Ordovician. Our mapping indicates that they are less extensive than previously thought and that the Ordovician fossils actually came from the Valmy Fm.: the window units thus could be part of the Triassic Star Peak Group, as originally mapped. The fault encircling the window units is a low-angle, brittle structure that is cut by a Jurassic pluton and dips generally outward away from the pluton. It may have formed by doming and extension above the pluton during emplacement. Based on these results, the concept of a Jurassic "Willow Creek thrust" in the ER is misleading. We also found no evidence for west-vergent folding, the other purported hallmark of the WDB. Therefore, it does not appear that any west-vergent Jurassic deformation belt exists in the ER. The concept of the WDB, which is recognized in only two other nearby ranges, may need to be re-evaluated. In contrast to the WDB, we did find intriguing new evidence for east-vergent deformation in the form of a previously undocumented major ductile shear zone along the west side of the central ER. The shear zone is localized within the Mississippian Inskip Fm. Mylonites within this shear zone dip to the west and display top-to-E shear sense indicators. The shear zone is cut by a Jurassic stock and lies on strike with the Fencemaker thrust: we conclude that it is an exposed ductile portion of this thrust. U-Pb zircon dating of pre- and post-tectonic intrusions is in progress and will help constrain the age of structures in the ER.