Wyld, Sandra J., Wright, James E., Nutaitis, James, Department of Geology, University of Georgia, Athens GA 30602
Time-space patterns of Jurassic shortening in the northwest Great Basin are an important constraint on models of Mesozoic tectonism and orogenesis in the U.S. Cordillera; however, conflicting data on structural and timing relationships have led to very different regional interpretations. Critical to this debate is the timing of deformation in NW Nevada, particularly the Luning-Fencemaker fold thrust belt (LFTB). In one popular model, shortening began at ~160 Ma in the LFTB, driving late Jurassic basin subsidence in Utah, and then propogated eastward through the end of the Cretaceous, forming the Sevier fold thrust belt. Other models consider Jurassic and Cretaceous orogenic events to be separate and unrelated, and argue for older Jurassic flexural basin development in Utah. We here summarize key timing constraints and structural relations for NW Nevada, based on recent and ongoing studies. Terranes involved are, from west to east: the Black Rock terrane (BRT), a Paleozoic to early Mesozoic oceanic arc succession; the basinal terrane, an early Mesozoic back-arc marine basin; and the shelf terrane, the shallow marine counterpart to the basinal terrane. Jurassic shortening initiated in the BRT at 201-199 Ma (U-Pb ages of syn-tectonic plutons; 40Ar/39Ar ages from syntectonic hornblende). Deformation propogated to the east between 193-165 Ma as the BRT was thrust over the back-arc basin, and the basinal terrane was internally shortened by SE-vergent, tight folds, reverse faults, and pervasive foliation. These structures represent the main, D1, phase of shortening in the LFTB. Timing constraints are based on 40Ar/39Ar dating of phyllites, U-Pb ages of post-tectonic plutons, and stratigraphic relations indicating syntectonic sedimentation. LFTB shortening culminated with a spatially restricted phase (D2) of SE-vergent folding associated with thrust emplacement of deformed basinal terrane rocks over the shelf terrane along the Fencemaker thrust. D2 structures cut a 165 Ma pluton (U-Pb) and are reportedly older than a pluton with a K-Ar cooling age of ca 153 Ma: we are currently working to confirm this relation and obtain a crystallization age for the pluton. Jurassic shortening, metamorphism and plutonism in NW Nevada was followed by exhumation and erosion of 7-14 km of overburden, prior to deposition of coarse alluvial strata of the King Lear Formation (KLF) in the late Neocomian (125-123 Ma; U-Pb). Depth of erosional unroofing across this major unconformity is based on a quantitative analysis of metamorphic grade in exhumed basinal terrane rocks (including illite crystallinity values). There is little other evidence of Cretaceous tectonism affecting the BRT or basinal terrane except in the thermal aureoles of plutons dated at 108-98 Ma (U-Pb). These data indicate that shortening in NW Nevada is primarily of Early to Middle Jurassic age, and that this deformation was separated from any Cretaceous shortening by a prolonged period of exhumation and erosion. These results pose serious problems for models invoking a continuum between Jurassic and Cretaceous (Sevier belt) orogenesis, including formation of Late Jurassic flexural basins in Utah. They are more consistent with the opposing models noted above.