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Anomalous(?) Early Jurassic deformation in the western U.S. Cordillera

¹ Department of Geology and Geophysics, MS-126, Rice University, 6100 Main Street, Houston, Texas 77005

New data from the Black Rock Desert of northwest Nevada indicate that Paleozoic and early Mesozoic magmatic arc rocks in this area were affected by regional shortening deformation and metamorphism in the Early Jurassic (ongoing at 201 Ma). This is 30–50 m.y. prior to the time at which significant Jurassic shortening deformation is thought to have begun in the western U.S. Cordillera and is further anomalous because the Early Jurassic time frame in most other arc assemblages of the Cordillera is characterized by accumulation of strata and neutral to extensional tectonism. These conflicting relations support the following conclusions. (1) The Black Rock Desert is probably separated from other arc assemblages to the west and south (in the Klamath Mountains and Sierra Nevada) by a younger strike-slip fault boundary that juxtaposes arc elements with different Early Jurassic structural histories. (2) Tectonic conditions varied along the Early Jurassic plate margin such that shortening occurred to the north but not to the south, an aspect of the Mesozoic evolution of the Cordillera that has not been previously appreciated. At present, regional Early Jurassic 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-Fencemaker fold-and-thrust belt of central Nevada.

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