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19 Sept. 2012
GSA Release No. 12-68
Kea Giles
Managing Editor,
GSA Communications
+1-303-357-1057
Earth

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Geosphere Adds to Four Themed Issues, Plus More New Science

Geosphere articles posted online 19 September 2012

Boulder, CO, USA – New Geosphere papers in themed issues include a study showing, for the first time, that a major fault runs under central Reno, Nevada, USA ("Origin and Evolution of the Sierra Nevada and Walker Lane"). Detailed field mapping adds new information to the study of "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen," and isochron cosmic burial data adds to the understanding of "CRevolution 2: Origin and Evolution of the Colorado River System II."

Abstracts for these and other Geosphere papers are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of Geosphere articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geosphere in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

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Post-2.6 Ma tectonic and topographic evolution of the northeastern Sierra Nevada: The record in the Reno and Verdi basins
Patricia H. Cashman et al., Dept. of Geological Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES764.1.

The landscape along the east side of the Sierra Nevada near Reno, Nevada, USA, is extremely young and dynamic. This paper by Patricia Cashman and colleagues describes the geologic evolution of the eastern margin of the Sierra Nevada near Reno area over the past 2.6 million years in unprecedented detail, based on new surface and subsurface data. Sedimentary rocks exposed west of Reno record an abrupt change in topography about 2.6 million years ago. Rapid erosion created a canyon that was then filled by granitic boulder conglomerate and sandstone derived in part from the interior of the Sierra Nevada to the west. These rocks are now tilted east; the amount of tilt decreases upward, documenting ongoing faulting and tilting during deposition. Gravity and borehole data from the Reno basin show the north-striking, west-side-down normal fault that bounds the tilted section -- under central Reno. Many small faults throughout the sedimentary section were active at the same time as tilting; together, they accommodate east-west extension. These results are significant because they show that the modern topography evolved more recently than was previously known, and that a major fault underlies the middle of the Reno basin, rather than forming the eastern or western edge as suggested by previous interpretations.

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Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes
Terry L. Pavlis, Dept. of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00753.1.

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. Terry Pavlis and colleagues present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region.

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New incision rates along the Colorado River system based on cosmogenic burial dating of terraces: Implications for regional controls on Quaternary incision
Andrew L. Darling et al., Arizona State University, School of Earth and Space Exploration, Interdisciplinary Science and Technology Building 4, Room 795, Tempe, Arizona 85287-1404, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00724.1.

New cosmogenic burial and published dates of Colorado and Green river terraces are used to infer variable incision rates along the rivers in the past 10 million years. A knickpoint at Lees Ferry separates the lower and upper Colorado River basins. Through studying isochron cosmic burial data, Andrew L. Darling and colleagues find that, near Bullfrog Basin, Utah (upstream of Lees Ferry), the average incision rate of 126 meters per million years above the knickpoint is three times older than a cosmogenic surface age on the same terrace, suggesting that surface dates inferred by exposure dating may be minimum ages. Incision rates below Lees Ferry are faster, at about 170 meters to 230 meters per million years Ma, suggesting upstream knickpoint migration over the past several million years. They conclude that higher incision rates, gradient, and discharge along the upper Colorado River relative to the Green River are consistent with differential rock uplift of the Colorado Rockies relative to the Colorado Plateau.

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Estimating two-dimensional static stabilities and geomorphic settings of precariously balanced rocks from unconstrained digital photographs
David E. Haddad et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00788.1.

Earthquakes that rupture Earth's topography can cause significant loss of human life and destruction to anthropogenic structures, such as schools, hospitals, and office buildings. Fortunately, these large earthquakes leave telltale marks about their sizes and distributions over geologic time. For example, large earthquakes can displace stream channels, river terraces, fluvial ridges, and even topple fragile geologic features. The purpose of this paper by David Haddad and colleagues is to demonstrate how laser scanning, also known as LiDAR, can be applied in paleoseismic research to measure the magnitude and extent of past large earthquakes. The paper presents three case studies that include millimeter-scale imaging of sedimentary deposits and faults in paleoseismic trenches, measuring decadal-scale erosion rates of fault scarps, and high-resolution imaging of precariously balanced rocks. All case studies show that laser scanning is an effective analytical tool for documenting past earthquakes and has the potential for being an integral component of the paleoseismic toolbox.

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Formation of pluton roofs, floors, and walls by crack opening at Split Mountain, Sierra Nevada, California
John M. Bartley et al., Dept. of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111, USA. Posted online 19 Sept. 2012; doi: GES00722.1.

Igneous plutons are the direct geologic record of the plumbing system by which magma ascends through the crust. Better understanding of the processes by which magma is intruded to form plutons should translate into better understanding of related phenomena such as precursors to volcanic eruptions -- thereby improving our ability to predict eruptions -- and formation of ore deposits of metals such as gold, copper, silver, lead, zinc, and molybdenum which are byproducts of intrusion of magma into the Earth's crust. The Split Mountain area of the Sierra Nevada provides extraordinary exposures of several plutons and of their relationships to the wall rocks in which they grew. Field observations by John M. Bartley and colleagues indicate that the plutons grew in place as cracks progressively opened in the wall rocks to admit ascending magma. Crack orientations, and therefore the resulting shapes of the plutons, varied with time from horizontal to steep and were influenced by the structure of the wall rocks. The observations are inconsistent with the more traditional view that plutons like those at Split Mountain were produced by the ascent of large bodies of magma by a combination of diapirism, in which wall rocks accommodate the ascent of a large magma body by flowing downward around it, and stoping, in which wall-rock fragments break off of the roof of the magma body and sink through it.

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Uplift prior to continental breakup :Indication for removal of mantle lithosphere?
Raphael Esedo et al., Dept. of Earth and Atmospheric Sciences, University of Houston, 312 Science & Research Building 1, Houston, Texas 77204, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00748.1.

Uplift or reduced subsidence prior to continental breakup is a key component of the rift-drift transition. This uplift causes lateral variations in the lithospheric potential energy, which can increase intraplate deviatoric tension, thereby facilitating continental rupture. There is a growing body of evidence that pre-breakup uplift is a global phenomenon characteristic of magmatic and magma-poor rifted margins. Evidence is provided by the subaerial extrusion of lava interpreted from drill logs, stratigraphic records, the presence of breakup unconformities, and the spatial extent of uplift associated with Afar (the Ethiopian-Somali plateau), which may be at the stage of rupture. Raphael Esedo and colleagues show that dynamic uplift resulting from passive upwelling asthenosphere below the rift is limited (~200 m). These results underpin their proposed model for continental breakup, wherein removal of mantle lithosphere by either detachment or formation of gravitational instabilities is a characteristic process. Observations of depth-dependent thinning and geochemical data support this model.

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Reconstructing the Upper Permian sedimentary facies distribution of a tight gas field in Central Europe on the basis of a modern analog field study in the Panamint Valley, western U.S.
Anna Alexandra Vackiner et al., Geological Institute, Energy and Mineral Resources Group, RWTH Aachen University, Wullnerstasse 2, 52062 Aachen, Germany. Posted online 19 Sept. 2012; doi: 10.1130/GES00726.1.

Anna Vackiner and colleagues investigate the influence of tectonic movements on reservoir rock deposition in a natural gas field in NW Germany. In Germany, the depositional environment of the reservoir rocks can only be reconstructed from core material and geophysical subsurface data. In order to improve the understanding of the reservoir rock distribution and quality, Vackiner and colleagues conducted a field campaign in the Panamint Valley, which exhibits a modern surface analog to the Upper Permian in the German subsurface.

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Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA
Facundo Fuentes et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00773.1.

This paper by Facundo Fuentes and colleagues focuses on the geometry and the temporal deformation progression of the fold-thrust belt in northwest Montana. This region contains significant structural features that have no apparent counterparts in other parts of the Cordilleran thrust belt. Of particular interest is the Lewis thrust system, an immense thrust system that has tectonically transported a slab of rock greater than seven kilometers in thickness more than 100 km toward the foreland region. In contrast to the other major Precambrian carrying thrust faults in the Cordillera, which formed in the interior part of the thrust belt in Idaho and Utah during the early Cretaceous, the Lewis is a frontal thrust system that sustained motions as late as late Paleocene-early Eocene. An almost complete record of sedimentation in the foreland basin system, local igneous rocks and cross-cutting relationships provide control on the major deformation events in the thrust belt. Excellent existing geologic maps and subsurface data were used to construct a ~150 km balanced cross-section that represents the geometry of the thrust belt along this segment of the Cordillera.

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Paleomagnetism of the Crocker Formation, northwest Borneo: Implications for late Cenozoic tectonics
Andrew B. Cullen et al., Shell International Exploration and Production Company, 100 Hoekstade, Rijswijk, Netherlands; and Chesapeake Energy Corporation, 6100 N. Western Avenue, Oklahoma City, Oklahoma 73118, USA. Posted online 19 Sept. 2012; doi: 10.1130/GES00750.1.

Tectonic models for Borneo's Cenozoic evolution differ in several aspects, particularly in the extent to which they include paleomagnetic data suggestive of strong counterclockwise rotation between 30 and 10 million years ago. Key areas are undersampled. Andrew B. Cullen and colleagues present the results of a paleomagnetic study of Eocene to Early Miocene sandstones from northwest Sabah, principally from the Crocker Formation.

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Radar scattering in an alpine glacier: Evidence of seasonal development of temperate ice beneath ogives
John McBride et al., Dept. of Geological Sciences, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA. Posted online 19 Sept. 2012; doi:10.1130/GES00804.1.

New ice-penetrating radar data provide an unprecedented view of scattering of electromagnetic energy in glacial ice that correlates to the development of glacial ogives (transverse topographic, wave-like surface features that form below icefalls on some alpine glaciers). A team of faculty and Brigham Young University students, working over three field seasons, has collected and analyzed radar data over the Zwillingsgletscher (part of the Gorner glacier system) in the Swiss Alps in order to map the distribution of internal scattering. They propose that periodic variations in scattering are caused by seasonal fluctuations in water infiltration to crevasses during the summer and subsequent freezing of that water during winter. This seasonal infilling and freezing of water alters the distribution of temperature, creating zones of temperate ice (meaning ice and water phases coexist) that follow seasonal ogive development. The presence of such water-rich temperate zones would have rheological implications for the dynamics of glacial flow and englacial water drainage. Future work should include drilling and borehole thermometry in order to penetrate the scattering layer and test these hypotheses for the origin of the scattering and its relation to glacier dynamics.

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