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Find Your Science at GSA
29 September 2008
GSA Release No. 08-54
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093

Geosphere

October Media Highlights

Boulder, CO, USA – The October GEOSPHERE, published by the Geological Society of America, is now online. This issue delves into Earth's crust to examine the uplift of the Southern Rocky Mountains, Himalayan kinematics, fault geometry in the central Mississippi valley’s New Madrid seismic zone, the magmatic mosaic of the Northern Sierra Nevada Mountains, volcanic processes that endanger areas surrounding the Nevado de Toluca near Mexico City, and trace metal concentrations in ancient marine sediments.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Christa Stratton for additional information or other assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .

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Epeirogeny in the Southern Rocky Mountains region: Evidence and origin
Gordon Eaton, Texas A&M University, 9505 Northpointe Boulevard, Suite 1002, Spring, Texas 77379, USA

The number of peaks in the Southern Rocky Mountains higher 10,000 feet in elevation greatly exceeds those in other parts of North America, including Alaska, British Columbia, and California. This is because the Southern Rockies were uplifted after their initial formation by the development of a large regional swell of Earth's crust and mantle beneath them. The swelling was caused by a flow of heat energy from deeper levels in the earth that made the crust and mantle buoyant, thereby lifting the mountains. This lifting continues today as determined from repeated surveys of elevations along U.S. Highway 50 from Las Animas (out on the Great Plains) to Salida (in the mountains) of Colorado. This paper draws upon, integrates, and interprets many lines of evidence that uniquely define the mechanism of mountain uplift.

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Forward modeling the kinematic sequence of the central Himalayan thrust belt, western Nepal
Delores M. Robinson, Dept. of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487-0338, USA. NSF funding received.

One method of describing the geology of remote regions is to visit the region, make observations and measurements, collect data, and then make maps of a particular region. The Himalayan Mountains are so vast and remote that geologists who make maps of the region are the first to do so. It is difficult to determine whether these maps are correct without visiting the same locations. However, hypotheses regarding how the mountains were built can be checked by producing models. The models at least eliminate what was not possible and shows what was possible. In this paper, cross sections -- vertical slices through the crust -- showing the stratigraphy of the rock units and the deformation of those units in far western Nepal are checked with computer modeling. Models are constructed and built in a computer program that moves in time steps sequentially from 25 million years ago to the present. The models confirm that cross sections that proposed approximately 75% shortening from the India-Asia collision are plausible and thus, the mapping in the region is also viable. In addition, these models provide an unprecedented detailed step-by-step view of how the Himalayan Mountains evolved into the highest mountain range in the world.

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New Madrid seismic zone fault geometry
Ryan Csontos, Ground Water Institute, University of Memphis, Memphis, Tennessee 38152, USA; and Roy Van Arsdale. NSF funding received.

The New Madrid seismic zone (NMSZ) of the central Mississippi River valley has been interpreted to be a right-lateral strike-slip fault zone with a left step-over restraining bend (Reelfoot reverse fault). This model is overly simplistic, as New Madrid seismicity continues 30 km southeast of the step-over. In this study, we have analyzed a recent dataset of more accurately located earthquake hypocenters to better map fault geometry in the New Madrid seismic zone. Most of these earthquakes appear to lie along five discrete fault planes: the Axial, Reelfoot South, Reelfoot North, Risco, and New Madrid North faults. Regional mapping of the top of the Precambrian crystalline basement indicates the basement has undergone differential displacement along each of the five faults. Seismic reflection profiles reveal reverse displacement on top of the Paleozoic and younger strata across the Reelfoot fault; however, at deeper levels, the Reelfoot North and Reelfoot South faults appear to be basement normal faults. The Reelfoot North and Reelfoot South faults differ in strike, dip, depth, and displacement, and only the Reelfoot North fault has a surface scarp (monocline). Thus, the Reelfoot fault is actually composed of two left-stepping restraining bends and two inverted faults that together extend across the entire width of the Reelfoot rift. The NMSZ faulting is more complex than previously modeled, and these new details will better help us understand the region’s mechanics and continued seismic threat.

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Geology, geochronology, and geochemistry of the Miocene–Pliocene Ancestral Cascades arc, northern Sierra Nevada, California and Nevada: The roles of the upper mantle, subducting slab, and the Sierra Nevada lithosphere
Brian Cousens et al., Dept. of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S5B6, Canada

The past 20 million years (Late Tertiary Period) has been a period of extensive volcanic activity and significant tectonic change in the western United States, producing a mosaic of magma types erupted in a variety of shifting tectonic settings. Much of the Sierra Nevada of northern California and adjacent parts of western Nevada are blanketed by volcanic rocks of this age, of which relatively little is known because few published studies of these rocks exist. Remarkably, this poorly understood fragment of the western U.S. magmatic mosaic lies in a tectonic setting that is transitional between subduction-related continental arc and Basin and Range-related extension, raising the possibility that magmatism may be controlled by processes distinct to both subduction and extensional regimes. Utilizing geochronological and geochemical data, we show that late Tertiary volcanism in the Sierra Nevada is due to subduction, but many of the geochemical attributes of the lavas are similar to those arising from purely extension-related volcanism in southeastern California. We thus propose that both arc volcanism in the Sierra and extension-related volcanism in southeastern California tap old lithospheric mantle as a magma source. The major role played by the lithospheric mantle in Late Tertiary arc magmatism contrasts strongly with its apparent minor role in modern continental arc volcanism in the Cascade Range of western North America.

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Quaternary sector collapses of Nevado de Toluca volcano (Mexico) governed by regional tectonics and volcanic evolution
G. Norini, et al., Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Campus Juriquilla-UNAM, Queretaro, Qro. 76230, Mexico

Nevado de Toluca is a Quaternary volcano located in an active continental volcanic arc in Mexico. Although the volcano is dormant, a renewal of activity could affect more than 25 million inhabitants, including Mexico City. Among the main volcanic processes that endanger the surrounding areas, gravitational collapses of the volcano flanks represent a significant source of hazard. In the past 50 thousand years, Nevado de Toluca suffered at least three gravitational sector collapses on its flanks. Field and remote sensed data supported by scaled analogue models revealed that these catastrophic events were strongly correlated to the presence in the area of the active Tenango Fault System. The analysis presented in this paper can improve hazard mitigation on the basis of better knowledge of growth and collapse mechanism of the volcano in response to basement tectonics. The numerous examples of composite volcanoes with similar structural-volcanological characteristics of Nevado de Toluca imply that the model results can also act as a guide to study other volcanoes in continental and island volcanic arcs.

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Trace-metal covariation as a guide to water-mass conditions in ancient anoxic marine environments
Thomas J. Algeo, Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA; and J. Barry Maynard. NSF funding received.

The concentrations of trace metals such as molybdenum (Mo), uranium (U), and vanadium (V) in ancient marine sediments have long been used to make inferences concerning the degree to which the overlying waters were oxic or anoxic. Recent work has shown that trace-metal concentrations in sediments are dependent not only on redox (reducing-oxidizing) conditions but also on the concentration of trace metals in the watermass itself. In bodies of water in which the deep part of the water column is partly isolated from the open ocean, the concentration of trace metals in the watermass can be reduced over a period of time to low levels through uptake of trace metals by the sediment without a compensatory resupply. Such changes in watermass concentrations can occur at different rates for different trace metals, leading to ratios of trace metals in the sediment that differ strongly from those expected for a fully open-marine environment. Analysis of the concentration ratios of trace metals in ancient marine sediments can thus yield information about the degree of isolation of the watermass as well as its history of chemical evolution. The application of this technique is demonstrated in the present paper through a comparative analysis of the chemistry of modern anoxic marine environments, such as the Black Sea, with 300 to 360 million-year-old organic-rich shales from the Devonian of the Appalachian Basin (Ohio-Kentucky) and the Carboniferous of the Midcontinent region (Kansas).

To review the abstracts for these articles, go to www.gsajournals.org.
Click here to view the complete table of contents for the current issue of GEOSPHERE.

www.geosociety.org

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