ANOMALOUS() EARLY JURASSIC DEFORMATION IN THE WESTERN US CORDILLERA

New data from the Black Rock Desert of northwest Nevada indicate that Paleozoic and early Mesozoic magmatic are rocks in this area were affe cted by regional shortening deformation and metamorphism in the Early Jurassic (ongoing at similar to 201 Ma). This is similar to 30-50 m.y. prior to the time at which significant Jurassic shortening deformatio n is thought to have begun in the western U.S. Cordillera and is furth er anomalous because the Early Jurassic time frame in most other are a ssemblages of the Cordillera is characterized by accumulation of strat a and neutral to extensional tectonism. These conflicting relations su pport the following conclusions. (1) The Black Rock Desert is probably separated from other are assemblages to the west and south (in the Kl amath Mountains and Sierra Nevada) by a younger strike-slip fault boun dary that juxtaposes are elements with different Early Jurassic struct ural histories. (2) Tectonic conditions varied along the Early Jurassi c plate margin such that shortening occurred to the north but not to t he south, an aspect of the Mesozoic evolution of the Cordillera that h as not been previously appreciated. At present, regional Early Jurassi c deformation is documented only in the Black Rock Desert, but it may have impacted a much larger area in this part of the Cordillera, based on relations in the Blue Mountain province of Oregon and the Luning-F encemaker fold-and-thrust belt of central Nevada.