Cenozoic Evolution of the Central Sierra Nevada (California) and the Birth of the New Plate Boundary
spe438-12 page 1 The Geological Society of America Special Paper 438 2008 The ancestral Cascades arc: Cenozoic evolution of the central Sierra Nevada (California) and the birth of the new plate boundary Cathy J. Busby Jeanette C. Hagan Department of Earth Science, University of California, Santa Barbara, California 93106, USA Keith Putirka Department Earth and Environmental Sciences, California State University, Fresno, California 93710, USA Christopher J. Pluhar Department Earth Science, University of California, Santa Cruz, California 95064-1077, USA Phillip Gans Department of Earth Science, University of California, Santa Barbara, California 93106, USA David L. Wagner California Geological Survey, 801 K Street, Sacramento, California 95814-3500, USA Dylan Rood Steve B. DeOreo Department of Earth Science, University of California, Santa Barbara, California 93106, USA Ian Skilling Department of Geology and Planetary Sciences, University of Pittsburgh, 200 SRCC, Pittsburgh, Pennsylvania 15260, USA ABSTRACT We integrate new stratigraphic, structural, geochemical, geochronological, and magnetostratigraphic data on Cenozoic volcanic rocks in the central Sierra Nevada to arrive at closely inter-related new models for: (1) the paleogeography of the ancestral Cascades arc, (2) the stratigraphic record of uplift events in the Sierra Nevada, (3) the tectonic controls on volcanic styles and compositions in the arc, and (4) the birth of a new plate margin. Previous workers have assumed that the ancestral Cascades arc consisted of stratovolcanoes, similar to the modern Cascades arc, but we suggest that the arc was composed largely of numerous, very small centers, where magmas frequently leaked up strands of the Sierran frontal fault zone. These small centers erupted to produce andesite lava domes that collapsed to produce block-and-ash fl ows, which were reworked into paleocanyons as volcanic debris fl ows and streamfl ow deposits.
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