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9 August 2011
GSA Release No. 11-51
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093

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August 2011 Geosphere Highlights

Boulder, CO, USA - The August 2011 GEOSPHERE announces additions to several themed issues: Exploring The Deep Sea and Beyond; Origin and Evolution of The Sierra Nevada and Walker Lane; Geodynamics and Consequences of Lithospheric Removal in The Sierra Nevada, California. The August issue also features several articles not associated with a specific theme.

Highlights are provided below. Representatives of the media may obtain complimentary copies of GEOSPHERE article by contacting Christa Stratton at the address above. Review abstracts for these articles at http://geosphere.gsapubs.org/. 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 assistance.

Keywords: Gulf of Alaska, submarine canyons, Sierra Nevada, Mojave Desert, Gulf of Mexico, New Mexico, Chugach metamorphic complex, Himachal Himalaya

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

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EXPLORING THE DEEP SEA AND BEYOND THEMED ISSUE
Tectonic and climatic influence on the evolution of the Surveyor Fan and Channel system, Gulf of Alaska

Robert S. Reece et al., Jackson School of Geosciences, The University of Texas at Austin, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road, Austin, Texas 78758-4445, USA; doi: 10.1130/GS654.1.

The Surveyor Fan and Channel system in the deep-water Gulf of Alaska is a unique sedimentary system that connects the St. Elias Range on land to the Aleutian Trench at its end. Because of this relationship, the Surveyor system has physical characteristics not typical of other submarine sedimentary systems. Additionally, the St. Elias Mountains are so close to the coast that the majority of sediment from erosion caused by glaciers and tectonic uplift moves into the ocean and into the Surveyor system. This has allowed us to study sedimentary layers and channels in the ocean floor to determine how changes in climate and tectonics on land have led to changes in erosion and shaping of the continental margin.

Measuring currents in submarine canyons: Technological and scientific progress in the past 30 years
J.P. Xu, U.S. Geological Survey, 345 Middlefield Road, MS-999, Menlo Park, California 94025, USA; doi: 10.1130/GS640.1.

The development and application of acoustic and optical technologies and of accurate positioning systems in the past 30 years have opened new frontiers in the submarine canyon research communities. This paper reviews several key advancements in both technology and science in the field of “currents in submarine canyons” since the publication of the similarly titled book by Francis Shepard in 1979. Precise placements of high-resolution, high-frequency instruments have not only allowed researchers to collect new data that are essential for advancing and generalizing theories governing the canyon currents, but have also revealed new natural phenomena that challenge the understandings of theorists and experimenters alike in their predictions of submarine canyon flow fields. Turbidity currents are found to occur frequently in submarine canyons such as Monterey Canyon. A concerted experiment with multiple monitoring stations along the canyon axis and on nearby shelves is required to characterize the storm-trigger mechanism for turbidity currents.

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ORIGIN AND EVOLUTION OF THE SIERRA NEVADA AND WALKER LANE THEMED ISSUE
Birth of the Sierra Nevada magmatic arc: Early Mesozoic plutonism and volcanism in the east-central Sierra Nevada of California

A.P. Barth et al., Dept. of Earth Sciences, Indiana University-Purdue University, Indianapolis, 723 West Michigan Street, SL118, Indianapolis, Indiana 46202, USA; doi: 10.1130/GS661.1.

The granitic core of the Sierra Nevada dominates the landscape of California, and understanding its construction is a key to understanding the assembly of northern and central California. Along the high Sierra crest east of Yosemite National Park, granitic and volcanic rocks record the birth and early growth stages of the granitic Sierra Nevada. New uranium-lead ages outline a long-lived magma system active in earliest Mesozoic time (about 232 to 218 million years ago). This magma system formed widespread bodies of granitic rock, and also fed explosive volcanic eruptions that blanketed the Triassic landscape with quartz-bearing volcanic ash. Because both granitic and volcanic rocks are well preserved, this part of the Sierra Nevada provides a baseline for understanding the magmatic processes that led to further rapid growth of California later in Mesozoic time.

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GEODYNAMICS AND CONSEQUENCES OF LITHOSPHERIC REMOVAL IN THE SIERRA NEVADA, CALIFORNIA THEMED ISSUE

Structure of the Sierra Nevada from receiver functions and implications for lithospheric foundering
Andrew M. Frassetto et al., Dept. of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark; doi: 10.1130/GS570.1.

This recent study conducted by Andrew M. Frassetto of the University of Copenhagen and colleagues across the Sierra Nevada mountains uses energy from distant earthquakes to map geologic layering from the subsurface to tens of kilometers deep. Mountain ranges are generally supported by deep roots of low-density crust, but this work demonstrates that the highest part of the Sierra Nevada, running from the Mojave Desert to Lake Tahoe, is supported by abnormally shallow and buoyant mantle. In contrast, the adjacent western foothills have considerably thicker crust paired with low average elevations. Their results, combined with several independent observations, support the idea that unusually dense continental crust resides beneath the westernmost Sierra Nevada and keeps the elevation near sea-level. This material has lingered here for about 85 million years and small, abundant, and unusually deep earthquakes suggest that it may finally be detaching from the base of the continent. This material is absent beneath the eastern Sierra Nevada despite a common geologic history, and its probable recent removal has likely contributed to the present high elevations and volcanic activity seen across this part of California.

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CURRENT ISSUE ARTICLES

History of Cenozoic North American drainage basin evolution, sediment yield, and accumulation in the Gulf of Mexico basin
William E. Galloway et al., Institute for Geophysics, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758-4445, USA; doi: 10.1130/GS647.1.

Using a series of detailed maps, this paper presents the locations of drainage basins and major river systems of continental North America that have emptied into the Gulf of Mexico during the past 65 million years. Locations of the rivers and their evolution has been mapped using their preserved record in intracontinental basins, known physiography of the continent interior, and the sedimentary record of the rivers in the preserved delta systems of the northern Gulf of Mexico continental margin.

Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation
James V. Jones III et al., Dept. of Earth Sciences, University of Arkansas, Little Rock, Arkansas 72204, USA; doi: 10.1130/GS614.1.

New radiometric ages of detrital minerals from metamorphosed sedimentary rocks in northern New Mexico provide a better understanding of sedimentation patterns, sediment sources, and the relationship between regional sedimentation and mountain-building events during the evolution of the North American continent ca. 1.7 to 1.4 billion years ago. In particular, this work identifies a new ca. 1.4 billion-year-old deposit called the Marqueñas Formation, which was previously correlated with ca. 1.7 billion-year-old rocks in the region. However, on the basis of our new age information, extreme deformation and metamorphism of the Marqueñas Formation must have occurred ca. 1.4 billion years ago and provides some of the best evidence yet of a major mountain-building event in New Mexico and surrounding regions at this time.

Deformation and structure in the Chugach metamorphic complex, southern Alaska: Crustal architecture of a transpressional system from a down plunge section
Mitchell R. Scharman et al., Dept. of Geological Science, University of Texas, El Paso, Texas 79968, USA; doi: 10.1130/GS646.1.

Geoscientists have long pondered how large strike-slip faults like the San Andreas fault pass downward through the deep crust and mantle lithosphere. This study examines an unusual area in southern Alaska where erosion has exhumed an oblique crustal section across an approximately 50 million-year-old strike-slip structure. In this strike-slip system, we can trace structures from shallow crustal faults, through middle crustal shear zones, and into lower crustal zones of distributed flow. Our observations support the long-held view that upper crustal faults continue downward into the middle crust as discrete shear zones rather than dispersed deformation. At greater depths, however, this study documents a major structural transition at the textural change from schist to gneiss. This transition may represent a mid-crustal detachment horizon between distributed flow below and block motion along shear zones and faults above. However, we propose that an alternative explanation of variable fold amplification with structural level could explain some of our observations, and more work is needed to clarify the nature of mid-crustal transitions observed in this system.

Cenozoic tectonic history of the Himachal Himalaya (northwestern India) and its constraints on the formation mechanism of the Himalayan orogen
A. Alexander G. Webb et al., Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA; and Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, USA; doi: 10.1130/GS627.1.

Himalayan mountains contain a diverse rock record along the front of the India-Asia collision, Earth's best developed ongoing continent-continent collision. Therefore, these mountains represent a premier natural laboratory for discoveries of Earth's plate tectonic system. This study reports new observations of bedrock geology from a key segment of the Himalaya, the region of the Indian state Himachal Pradesh. This area preserves important relationships between two faults, the Main Central thrust and South Tibet detachment, which together played dominant yet controversial roles in the tectonic evolution of the Himalayan Mountains. New field mapping and analytical results (thermobarometry, geochronology, geochemistry, and thermochronology) extending across the fault zones are presented and used to test models for the Himalayan tectonic development. The results document that the Main Central thrust and South Tibet detachment merge in the up-dip direction along a branch line that is locally buried. This indicates that during the period of activity along these structures, the Himalayan Mountains evolved via tectonic wedging, and the crystalline core of the range was emplaced at depth.

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