DILEK, YILDIRIM, Department of Geology and Geography, Vassar College, Poughkeepsie, NY, and ELDRIDGE M. MOORES, Department of Geology, University of California, Davis, CA

ABSTRACT: The Across-Strike Anatomy of the Cordilleran Orogen at 40 degrees N Latitude and Implications for the Mesozoic Paleogeography of the Western United States

A geologic traverse across the northern Sierra Nevada foothills (California) and western Nevada along the 40 degrees N latitude shows the existence of, from west to east, a Jurassic ensimatic arc terrane and its oceanic basement, late Paleozoic-early Mesozoic subduction zone complexes, late Paleozoic-Jurassic continental margin arc, Jurassic-Cretaceous Sierra Nevada batholith, Triassic-Jurassic volcanic arc terrane, and an early Mesozoic marine province consisting of a Triassic-Jurassic basinal succession and a Triassic shelf sequence. The Jurassic ensimatic arc terrane in the west is composed of Lower to Upper Jurassic volcanic, volcaniclastic, hypabyssal, and plutonic rocks forming a polygenetic intra-oceanic arc sequence. It was constructed on and across a late Paleozoic-early Meso oic disrupted oceanic basement comprising multiply deformed heterogeneous oceanic crust and overlying marine rocks. Subduction zone complexes to the east include an accretionary prism and trench-slope deposits of the Calaveras complex and the blueschist-bearing Feather River peridotite, and mark the approximate trace of the early Mesozoic subduction zone dipping beneath the continent. East of the subduction zone complexes is the composite magmatic arc of the western U.S. Cordillera that contains late Paleozoic and Jurassic arc sequences deposited over a continental basement. The Jurassic continental arc sequence includes submarine to subaerial Lower to Middle Jurassic volcanic-volcaniclastic rocks underplated by coeval plutonic rocks, and displays structural and stratigraphic evidence fo the persistence of an extensional tectonic regime during its late-stage evolution. However, the presence of east-directed thrust faults within and east of this arc indicates period(s) of crustal shortening around the Middle Jurassic. The volcanic arc terrane east of the Sierra Nevada batholith consists of Triassic to Middle Jurassic volcanic, volcanogenic, and sedimentary rocks commonly associated at depth with a comagmatic suite of plutonic and intrusive rocks. The Jurassic Humboldt igneous complex in west-central Nevada constitutes part of this volcanic arc terrane and tectonically rests on an early Mesozoic marine province. Stratigraphic and structural studies and correlations within the continental Jurassic arc terrane in the Cordilleran system suggest its evolution in an orogen-par llel extensional graben depression that underwent rifting and strike-slip faulting synchronous with plutonism and rapid deposition. The volcanic arc terrane and the basinal succession, composed of Triassic to Jurassic clastic and carbonate rocks, tectonically overlie the Triassic carbonate shelf along the east-vergent Luning-Fencemaker thrust fault.

The existence of a major Early Cretaceous dextral fault along the magmatic axis of the orogen (Sierra Nevada batholith) has been recently documented by several workers on the basis of regional studies and correlations of lithologic units and structures. Reconstruction of the orogen and removal of up to 400 km of dextral translation along the inferred fault system provide a pre-Early Cretaceous paleogeography in which the marine province and the volcanic arc terrane of western Nevada align with and become the northward continuation of the rifting and subsiding continental arc in the Sierra Nevada foothills. The ensimatic Jurassic are terrane outboard of this continental arc represents part of a continent-fringing arc system that collapsed into the continental margin in the Middle Juras ic, causing extensive contractional deformation in the western U.S. Cordillera.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.