SAND SPRINGS LITHOTECTONIC ASSEMBLAGE, WEST-CENTRAL NEVADA: POSSIBLE LINK BETWEEN SIERRA NEVADA AND LUNING-FENCEMAKER THRUST BELT

The Sand Springs lithotectonic assemblage (SSA) of the Mesozoic marine province in west-central Nevada consists of greenschist to lower amphibolite facies metamorphic tectonites that form part of the hinterland of the late Mesozoic Luning-Fencemaker thrust belt (LFT). Isolated exposures over an area of 9400 km² are composed of Late Triassic to Middle Jurassic clastic and carbonate rocks consisting of early to middle Norian laminated limestone and limestone breccia, late Norian black shale and interbedded quartzose sandstone and limestone, and Sinemurian volcanogenic sandstone and shale overlain by quartz arenite and chert-pebble conglomerate. SSA rocks are deformed in three generations of superposed structures. The first structures are close to isoclinal folds, penetrative foliations, faults, and intersection, mineral, and stretching lineations formed at greenschist to lower amphibolite facies conditions. Second and third structures are non-metamorphic and include pervasive NE-trending, open to close folds and coeval SE-vergent thrust faults, and NW-trending, gentle to open folds, respectively. A Cretaceous granitoid pluton and associated dikes and sills, dated by K-Ar as 79.6 ± 2 to 76 ± 2 Ma, are post-kinematic and crosscut all three generations of structures. Locally, a Jurassic pluton, dated by K-Ar on biotite and hornblende, respectively, as 158 ± 4 to 155 ± 7 Ma, truncates penetrative fabric of first-generation structures and apparently predates second-generation structures. SSA stratigraphy and structures link and differentiate the LFT and the eastern Sierra Nevada (SN). SSA stratigraphy is related to that of the eastern SN and different from other rocks in the LFT. Greenschist to lower amphibolite facies metamorphism and first-phase structures are widespread in the SN, where they are cross-cut by a 169 to 165 Ma pluton dated by U-Pb, and ostensibly correlated with first-generation structures in the SSA of the LFT. SSA second-phase structures are found throughout the LFT but are absent in the SN, and the two assemblages were separated by the northerly-trending, transpressional Pine Nut fault system. NW-trending folds of older metamorphic structures in the eastern SN are truncated by a 107 Ma pluton and formed together with third-phase folds of the SSA during regional shortening.