JURASSIC COLLISIONAL TECTONICS IN THE NORTHWESTERN U.S. CORDILLERA, AND PAIRED GROWTH OF THE LUNING-FENCEMAKER THRUST BELT AND UTAH-IDAHO TROUGH

New data emerging from NW Nevada and eastern Oregon prompt us to reconsider the regional driving forces, thrust-belt dynamics, and basinal response to Jurassic construction of the Luning-Fencemaker thrust belt (LFTB) in NW Nevada. Based on compelling correlations, and evidence for ~400 km dextral offset along the Early Cretaceous MSNI fault, the Blue Mountains province (BMP) of eastern Oregon restores to the latitude of NW Nevada during Jurassic time (Wyld and Wright, 2001, 2005). Recent synthesis of the BMP provides evidence for growth of a large marine collisional basin (Izee basin) during the Jurassic (Dorsey and LaMaskin, submitted). Synchronous events in NW Nevada include deformation and metamorphism in the Black Rock arc terrane and LFTB. Farther east, there was simultaneous widespread subsidence in the Utah-Idaho Trough (UIT), interpreted as a flexural response to thrust loading in the west (Bjerrum and Dorsey, 1995). While supported by stratigraphic data and basin geometry, the flexural model is debated because the reconstructed eastern edge of the LFTB is located ~100-200 km west of its predicted position. However, new illite-crystallinity data show that rocks in the LFTB were tectonically buried ~7-14 km deep (Wyld et al., 2003), whereas the flexural model used a thrust belt only ~5-6 km thick. Moreover, areas of Jurassic shortening east of the LFTB are not generally considered part of the thrust load for the UIT, but probably should be. New analysis shows that use of a thicker and wider thrust load can likely resolve problems with the 1995 flexural model. Based on the above input, we propose that protracted collision between amalgamated oceanic terranes of the BMP and the magmatic arc-backarc region in Nevada (prior to their later separation along the MSNI fault) was the major driving force that caused Jurassic growth of the LFTB and flanking sedimentary basins (UIT in the east and Izee basin in the west). According to this model, during most of Jurassic time the NW U.S. Cordillera did not experience normal subduction of oceanic crust along an integrated Andean-type margin, but instead was the site of a long-lived collisional regime. This geometry resembles that of present-day Papua New Guinea, except that prior to collision northern Australia was a passive continental margin, whereas Nevada in the Triassic was a Japan-type magmatic arc and back-arc basin.