ALLOCHTHONOUS PALEOZOIC-JURASSIC TERRANES OF NW NEVADA AND SE OREGON DISPLACED DEXTRALLY FROM THE SOUTHWEST CORDILLERA IN THE EARLY CRETACEOUS: WHERE IS THEIR HOMELAND?

NW Nevada and SE Oregon contain three unique Paleozoic-Jurassic terranes - the Fox Range terrane (FRT), Granite Range terrane (GRT), and Pueblo Mountains terrane (PMT). These terranes bear no resemblance in stratigraphy, structure, or magmatic history to established terranes farther east (Black Rock and Jungo terranes). Stratigraphic and detrital zircon data indicate the western terranes were displaced dextrally from the SW U.S. Cordillera (ca. 400-450 km) in the Early Cretaceous, along a tectonic boundary connecting the Mojave-Snow Lake fault to the Salmon River-Idaho suture (Wyld and Wright, 2001, 2007). Exact homeland of the terranes is not clear, however, and we hope that presentation of terrane data at this Arizona conference might yield answers. PMT contains a thick (≥4 km) sequence of andesitic to rhyolitic volcanic rocks, intruded by quartz diorite plutons. Sedimentary strata are absent. U-Pb zircon ages indicate most magmatism was 182-178 Ma. Volcanic rocks include lava and pyroclastic deposits (pumice and ash flows, block and ash flows, surge deposits, welded ash flows). Stratigraphy and geochemistry indicate that PMT is a proximal part of a continental stratovolcano. GRT consists of Early Permian shallow marine carbonate, felsic to andesitic pyroclastic tuffs, and andesitic lava, overlain unconformably by Early Jurassic cpx-plag lava and related clastics, and intruded by a 196 Ma monzodiorite pluton (ages based on fossils and U-Pb zircon analyses). Along the unconformity is a unit of polymict conglomerate containing quartz arenite clasts, whose U-Pb zircon detrital signature is similar to that of the Aztec Sandstone. FRT consists of a polydeformed and structurally inverted sequence of: 1) ≥1300 m of shallow marine black argillite and quartz arenite, with minor limestone, 2) ≥800 m of quartz-mica schist and quartzite, and 3) small klippe of cordierite-quartz-mica gneiss, all of which are no older than Late Triassic (based on U-Pb detrital zircon analyses). No volcanic strata are present. Detrital zircon signatures are similar to those of Colorado plateau Jurassic erg deposits, particularly the Aztec and Navajo sandstone. Each terrane (PMT, GRT, FRT) is distinct; they appear to be fragments from different parts of the SW U.S. Cordillera caught up along the Early Cretaceous dextral fault zone.