Pre-Luning-Fencemaker Metamorphism and Deformation in the Northern Sand Springs Range, Nevada

A 1:8,000-scale geologic map of the northern Sand Springs Range (SSR) shows a pre-Luning Fencemaker deformation and metamorphic event. The Sand Springs Range is located within the Sierran Arc and the Luning Fencemaker Fold and Thrust Belt (LFTB). LFTB folds and faults deform D1 metamorphic tectonites that are cross cut by Basin and Range extensional strike-slip faults. Page (1964), Wilden and Speed (1974), and Satterfield (2005) previously mapped this area. Oldow (1983) and Wyld (2002) describe contrasting regional sequences of deformation phases. The northern SSR contains four metamorphic tectonite map units: foliated marble, andalusite schist, quartz-rich schist, and Triassic meta-quartz porphyry. Cretaceous granitoid plutons and sills and Tertiary rhyolite sills intrude tectonites. Tertiary basalt and interbedded ash flow tuff overlie tectonites. The SSR contains three phases of deformation, a syn-metamorphic event (D1) followed by two non-metamorphic folding and thrusting phases characteristic of the LFTB. D1 occurred after Triassic quartz porphyry intruded and before Cretaceous granitoids. D1 map-scale and outcrop-scale folds are typically tight to isoclinal, strike NE, and dip steeply to the SE. A map scale D1 antiform located in the SW portion of the map area folds quartz-rich schist and andalusite schist map units. S1 axial-planar foliations strike NE and dip moderately to steeply NW and SE. Metamorphic minerals present in D1 are garnet, biotite, and andalusite indicating lower amphibolite facies. Mapping is significant because pre-LFTB amphibolite facies metamorphism and deformation has not been described elsewhere in the LFTB and because D1 metamorphism and foliations have been attributed to forceful pluton emplacement. Work on this project was funded by a 2015 SW AAPG research grant.