GSA Bulletin highlights: New research posted March 2-23, 2011

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Widespread basement erosion during the late Paleocene-early Eocene in the Laramide Rocky Mountains inferred from 87Sr/86Sr ratios of freshwater bivalve fossils
Majie Fan et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 2 Mar. 2011; doi: 10.1130/B30219.1.

The regional-scale pattern of erosional exhumation of the Laramide uplifts is crucial to understanding the deformation processes of the Laramide Rocky Mountains. Majie Fan of the University of Arizona and colleagues first explore the relationship of modern river water 87Sr/86Sr ratios and bedrock lithology in the region. They further present 87Sr/86Sr ratios of Late Cretaceous-early Cenozoic river water in six basins reconstructed from 87Sr/86Sr ratios of freshwater bivalve fossils to examine the regional pattern of Precambrian basement erosion. Fan and colleagues found that basement erosion was widespread during the late Paleocene-early Eocene in the Laramide Rocky Mountains, and basement was eroded earlier in the drainage of the Powder River Basin in northeastern Wyoming than the areas adjacent to the Sevier thrust belt in western Wyoming.

Anomalous clastic wedge development during the Sevier-Laramide transition, North American Cordilleran foreland basin, USA
Jennifer Aschoff, Dept. of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois St., Golden, Colorado 80403, USA; and Ron Steel. Published online 2 Mar. 2011; doi: 10.1130/B30248.1.

Jennifer Aschoff of the Colorado School of Mines and Ron Steel of The University of Texas at Austin provide evidence that basement-involved deformation of Earth's crust began as early as ~77 million years ago in central Utah, and that the transition from thin-skinned to basement-involved deformation facilitated the rapid and extensive progradation of clastic detritus shed from the Sevier highlands. Aschoff and Steel define three Campanian-aged, alluvial-to-marine sediment wedges that traversed 200-400 km from the highlands in West-Central Utah to Colorado. Two of these wedges of sediment (Wedges A and C) are thicker successions with rising-trajectory shoreline stacking patterns that reflect relatively slow progradation of narrow, wave-dominated shorelines. In contrast, Wedge B consists of thinner successions with a flat-to-falling, shoreline stacking pattern that suggests rapid progradation of embayed, mixed-energy (wave- and tide-influenced) shorelines. The anomalously extensive Wedge B is unique in the Utah-Colorado segment of the Cordilleran foreland basin because of its long extent, rapid progradation rate, abundant tide-influenced strata and nested architecture. Stratigraphic relationships indicate development of Wedge B coeval with both Sevier- and Laramide-style deformation in Utah. Aschoff and Steel attribute the development of the unique, Campanian-aged sediment wedge (Wedge B) to the flexural response of Earth's crust during the transition from, or overlap of, one deformation style to another.

Mechanics of V-shaped conjugate strike-slip faults and the corresponding continuum mode of continental deformation
An Yin, Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA; and Michael H. Taylor. Published online 2 Mar. 2011; doi: 10.1130/B30159.1.

A fundamental goal of earth scientists is to relate observed faults in Earth's crust to the forces that created them. An Yin of the University of California at Los Angeles and Michael H. Taylor of the University of Kansas provide a new theoretical work to address this issue. In particular, Yin and Taylor used field examples, experimental simulations, and theoretical calculations to relate the commonly observed V-shaped fault pairs in nature to the forces and mechanical properties of the crust that hosts the faults. This new theory has important applications to the studies of planetary geology such as Mars and Venus. That is, scientists can now infer the mechanical conditions and mechanical properties of their crust by analyzing the fault geometry shown on satellite images without directly touching the rocks. This study was supported in part by a grant from the U.S. National Science Foundation.

A scenario for late Neogene Andean shortening transfer in the Camisea Subandean zone (Peru, 12°S): Implications for growth of the northern Andean Plateau
Nicolas Espurt et al., Centre Europeen de Recherche et d'Enseignement des Geosciences de l'Environnement, UMR 6635, Aix-Marseille Universite, Europole de l'Arbois, BP80, F-13545 Aix-en-Provence cedex 04, France. Published online 23 Mar. 2011; doi: 10.1130/B30165.1.

Nicolas Espurt of the European Centre for Research and Education in Environmental Geosciences and colleagues deal with the geological structures and kinematics of the Camisea basin. This basin belongs to the eastern Andean foothills of Peru (central Peruvian Subandean zone), adjacent to the northern Andean Plateau. The construction of a retrodeformable cross section at the scale of the basin combined with exhumation ages (apatite fission track and vitrinite reflectance) on the main Pongo de Mainique thrust present a step-by-step evolution of the shortening through time. The results suggest an initiation of the deformation into the central Peruvian Subandean zone approximately 14 million years ago. North-eastward thrusting arrived in the Camisea basin approximately 6 million years ago. This was followed by the development of the geological structures of the basin from 6 million years ago to present-day. Finally, the results suggest that the Late Neogene growth of the northern Andean Plateau mostly resulted from a continuous crustal shortening accommodated within the central Peruvian Subandean zone.

Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)
David S. Jones et al., Dept. of Geology, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA. Published online 23 Mar. 2011; doi: 10.1130/B30323.1.

The first mass extinction in the history of skeletal life occurred at the end of the Ordovician Period, approximately 444 million years ago. Climate change has been implicated as a potential cause of the extinction -- large ice sheets are known to have existed at the time. Geologists have also recognized substantial disturbances to the global carbon cycle associated with end-Ordovician events. David S. Jones of Amherst College and colleagues present a detailed geochemical record of carbon stable isotopes from sedimentary rocks on Anticosti Island (Quebec, Canada) in order to develop high-resolution correlations of the upper Ordovician rocks exposed there. Based on these correlations, they develop an interpretation of the timing of end-Ordovician sea-level fluctuations, which they argue are caused by the growth of ice sheets on polar landmasses. This sea-level model provides a means to establish the relative timing between glaciation, extinction, and carbon cycle changes. The work lends support to the hypothesis that changing weathering regimes during glaciation led to anomalies in the carbon isotope records.

###Keywords: Laramide Rocky Mountains, Sevier-Laramide transition, Cordilleran foreland basin, fault geometry, Camisea basin, central Peruvian Subandean zone, Anticosti Island, Quebec, Canada

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