GSA Bulletin: Alaska, Russia, Tibet, the Mississippi River, and the Great Green River Basin

(Press-News.org) Boulder, CO, USA - New GSA BULLETIN science published online 24 Feb. includes work on the Chugach Metamorphic Complex of southern Alaska; news and data from the first non-Russian science team to make a helicopter over-flight of Shiveluch volcano in Kamchatka, Russia, after its large 2005 eruption; and a study by a team from the Lamont-Doherty Earth Observatory that proposes a new calibration model for the Eocene segment of the Geomagnetic Polarity Time Scale (GPTS).

Large-scale, short-lived metamorphism, deformation, and magmatism in the Chugach metamorphic complex, southern Alaska: A SHRIMP U-Pb study of zircons
D. Gasser et al., Department of Earth Science, University of Graz, Universitatsplatz 2, A-8010 Graz, Austria; doi: 10.1130/B30507.1.

The Chugach Metamorphic Complex of southern Alaska developed within sediments of the southern Alaskan accretionary prism and consists of high-grade metamorphic rocks that underwent partial melting. U-Pb dating of small zircon overgrowths from these migmatites by D. Gasser of Austria's University of Graz and colleagues reveals that peak metamorphic conditions were reached during a very short time interval at 55-52 million years ago on an ~200-km-long section. Together with maximum depositional ages for the sediments in which the complex developed, this leaves only about five million years for burial to a more than 20 km depth and heating to up to 700 degrees Celsius of these sediments. This is relatively short compared to migmatitic complexes developed in continental collision zones, and reflects the short time scales and active tectonic environment in an ocean-continent subduction zone environment.

Surface textures and dynamics of the 2005 lava dome at Shiveluch Volcano, Kamchatka
Michael S. Ramsey et al., Dept. of Geology and Planetary Science, University of Pittsburgh, 4107 O'Hara Street, Pittsburgh, PA 15260, USA; doi: 10.1130/B30580.1.

In August 2005, Michael S. Ramsey of the University of Pittsburgh and colleagues from the Alaska Volcano Observatory became the first non-Russian science team to make a helicopter over-flight of Shiveluch volcano. Soon after the large 2005 eruption of Shiveluch Volcano in Kamchatka, Russia, Ramsey and colleagues were granted access to the volcano. The team collected visible and thermal infrared images of the active dome. The data reveal an actively deforming dome with a central crease structure and complex thermal patterns on the surface. The presence of a these two features are both unexpected discoveries that reveal how the lava dome growth is occurring. The data were later compared to spaceborne thermal data in order to estimate the lava extrusion rate. The results demonstrate a straight forward approach of how to link ground, air, and spaceborne thermal image data, which could easily be applied to other active volcanoes around the world.

Rapid and widespread response of the Lower Mississippi River to eustatic forcing during the last glacial-interglacial cycle
Z.X. Shen et al., Dept. of Earth and Environmental Sciences, Tulane University, 6823 St. Charles Ave., New Orleans, Louisiana 70118-5698, USA; doi: 10.1130/B30449.1.

Z.X. Shen of Tulane University and colleagues surveyed 2450 m of sedimentary pile (sample spacing of 2.7 m) in addition to a previous magnetostratigraphic section of 840 m overlying the sampled sequence. Stability tests indicate that magnetization was blocked during early diagenetic stages. Magnetite is the main magnetic carrier, with variable iron sulphide content and occasionally hematite. A global correlation, based on a magnetostratigraphic composite section derived from this work (constrained by biostratigraphic record), allows Shen and colleagues to propose a complete chronostratigraphic frame for the Ainsa Basin infill of between 55 and 45 million years. Inferred accumulation rates are relative low, corresponding to a marine shelf, until roughly 43 million years ago, and progressively higher onward, related to overall continentalization of the basin.

Pulsed Miocene range growth in northeastern Tibet: Insights from Xunhua basin magnetostratigraphy and provenance
R.O. Lease et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA; doi: 10.1130/B30524.1.

Tibetan Plateau basins are sensitive recorders of the tectonic and climatic processes that operate during continental collision. Despite burgeoning knowledge of sedimentary basin evolution in northeastern Tibet during the onset of India-Asia collision 50 million years ago as well as over the past 12 million years, surprisingly little is known about the intervening period. To better understand the spatiotemporal patterns of deformation, erosion, and environmental change during this period of time, R.O. Lease of the University of California at Santa Barbara and colleagues present new magnetostratigraphy from the Xunhua basin, Tibet, that spans from 30 to 9 million years ago. An integrated analysis of sedimentology, subsidence, and detrital zircon provenance from this magnetostratigraphic section reveals the sequential, pulsed erosion of multiple mountain ranges in northeastern Tibet. Initial growth of the north-northeast-vergent Laji Shan mountain range in the early Miocene and subsequent growth of the east-vergent Jishi Shan range about 10 million years later supports interpretations of a middle Miocene kinematic reorganization in northeastern Tibet.

Fine-tuning the calibration of the Early to Middle Eocene Geomagnetic Polarity Time Scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins
Kaori Tsukui and William C. Clyde, Division of Geochemistry, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA, and Richard Gilder Graduate School at the American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA; doi: 10.1130/B30545.1.

The Geomagnetic Polarity Time Scale (GPTS) is an important component of the Geologic Time Scale, and it enables integration of stratigraphic data into a coherent temporal framework. The GPTS is built based on a sequence of geomagnetic polarity reversals calibrated by radio-isotopic dating of volcanic ash beds within a terrestrial sequence. However, calibration of the reversals often relies on interpolation and extrapolation because the datable ash beds are few and far in between, limiting the temporal precision of the calibration. In this study, Kaori Tsukui and Willilam Clyde of the Lamont-Doherty Earth Observatory determined a paleomagnetic polarity of a series of Eocene ash beds from the Greater Green River Basin and its surrounding basins in Wyoming, Utah, and Colorado. The polarity data were combined with published radioisotopic data of the same ash beds to evaluate the published calibration models, including that of the current GPTS. Tsukui and Clyde propose a new calibration model for the Eocene segment of the GPTS that reconciles all the available and relevant stratigraphic data from the Eocene strata in the study area.

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