Lithosphere posts new research in California, Nevada and the Tibetan Plateau

(Press-News.org) Boulder, Colo., USA – New Lithosphere research about Earth's crust and upper mantle presents what may be the best-documented ancient sedimentary record of subduction initiation along a continental margin in the El Paso Mountains region of California; an integrated approach to understanding the Karakoram Fault Zone, Tibet; and back-and-forth exchanges between field-based observations and lab analyses and modeling that lead researchers to 40-year-old interpretation of the geologic history of the Walker Lane belt, Nevada.

Stratigraphic record of subduction initiation in the Permian metasedimentary succession of the El Paso Mountains, California
Jean L. Rains et al., California State University Northridge, Northridge, CA 91330-8266, USA. Posted online 3 April 2012; doi: 10.1130/L165.1.

Subduction leads to volcanism and mountain building, but the beginnings of this process are poorly understood. Sedimentary/volcanic rock layers deposited from ca. 280 to ca. 260 million years ago in the El Paso Mountains region of California may be the best-documented ancient sedimentary record of subduction initiation along a continental margin. Results from this study by Jean L. Rains and colleagues are consistent with computer models of this fundamental tectonic process that is inherent to the creation of convergent plate margins.

Exhumation history of the Karakoram Fault Zone mylonites: New constraints from microstructures, fluid inclusions and Ar40/Ar39 analyses
Barun K. Mukherjee et al., Wadia Institute of Himalayan Geology, Petrology & Geochemistry Group, 33 GMS Road, Dehra Dun, Uttrakhand 248 001, India. Posted online 3 April 2012; doi: 10.1130/L163.1.

The Karakoram Fault Zone (KFZ) marks the western margin of the Tibetan plateau. This fault zone accommodates present-day movement of the Tibetan plateau in response to India-Eurasia collision. Barun K. Mukherjee and colleagues take an integrated approach combining microstructures, fluid inclusions, and geochronology to interpret the evolution and exhumation of KFZ rocks. Their data set infers progressive mylonitization of KFZ rocks triggered by low-temperature deformation at the last stage of exhumation, when two phases of fluids coexisted and were widespread.

Timing, magnitude, and style of Miocene deformation, west-central Walker Lane belt, Nevada R. Ernest Anderson et al., U.S. Geological Survey, P.O. Box 347, Kernville, California 93238, USA. Posted online
3 April 2012; doi: 10.1130/L174.1.

Progress in the earth sciences often relies on back-and-forth exchanges between field-based observations and lab analyses and modeling. This paper by R. Ernest Anderson of the U.S. Geological Survey and colleagues is a good example. It presents a broad spectrum of newly recognized structures (mainly folds) and some new timing data that aid in choosing models best suited to explain how Earth's crust deformed in an area near Hawthorne in Lyon and Mineral Counties, Nevada, USA, that has become important to geoscientists' "world view" of interrelations between deformation at the margin of the North America plate and in its interior. The new field data reveal a much greater role for flexure than rigid tilting during deformation. Integration of flexure leads the authors away from extreme amounts of rates of deformation inherent in published two-dimensional rigid models toward a less extreme three-dimensional model capable of embracing much of the complexity that emerged from the new field study as well as much of the published analytical data. Interestingly, the new model also leads Anderson and colleagues back to a 40-year-old interpretation of the geologic history based on geologic fieldwork, an interpretation that was abandoned as analytical data were integrated into modeling that did not include flexure.

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