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News Release May 7, 2002
GSA Release No. 02-30
Contact: Christa Stratton
+1-303-357-1093
FOR
IMMEDIATE
RELEASE

May Media Highlights:
The Geological Society of America Bulletin

Boulder, Colo. - The May issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest is a study of late Quaternary glaciation in Costa Rica which offers evidence that the American tropics experienced much colder Ice Age temperatures than previously believed. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

Tectonic history of a Jurassic backarc basin sequence (the Gran Cañon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits
Salvatore Critelli, Dipartimento di Scienze della Terra, Università degli Studi della Calabria, 87036 Arcavacata di Rende (CS), Italy, et al. Pages 515-527.
Detailed petrofacies analysis of volcaniclastic sediments in a preserved example of ancient backarc basin fill, the Jurassic Gran Cañon Formation (Cedros Island, Baja California, Mexico), is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc basin settings.
The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows
A.J.L. Harris, Hawai'i Institute of Geophysics and Planetology/School of Ocean and Earth Science and Technology (HIGP/SOEST), University of Hawai'i, 2525 Correa Road, Honolulu, Hawai'i 96822, USA, et al. Pages 533-546.
Thick, slow-moving block lava flows typically occur at volcanoes that erupt dacitic lava. Such lava flows can be extremely hazardous, where collapse of the block lava flow front can generate devastating pyroclastic flows. These have the potential to kill thousands of people; one such flow may have killed as many as 5,000 people in 1929 at Santiaguito volcano in Guatemala. Using data collected at this volcano during January 2000 we show that such block lava flows are extremely well insulated by thick, cool surface crusts. This results in extremely low rates of core cooling, so that the flow can remain fluid for many years. This means that, although a block lava flow may advance extremely slowly (just meters per day), it has the potential to eventually extend a great distance. As a result, a block lava flow front can slowly extend towards and into vulnerable communities, thus moving the source of the lethal pyroclastic flows closer to populated areas.
Late Quaternary glaciation of Costa Rica
Matthew S. Lachniet and Geoffrey O. Seltzer, Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA. Pages 547-558.
Lachniet and Seltzer describe evidence for Ice Age glaciation of the Costa Rican high mountains. This glaciation is evidence that the American tropics experienced much colder Ice Age temperatures than previously believed, with mean annual temperatures in the highlands as much as 9 degrees Celsius (18 degrees Fahrenheit) lower than modern temperatures. They base their findings on the mapping of glacial deposits and landforms and ancient glacier reconstructions, and estimation of how much colder temperatures had to be to produce the glaciers. The results are especially surprising when standing on the highest glaciated peak, Cerro Chirripo (3,819 meters[12,526 feet] above sea level), where in one view, both glacial landforms and the torpid Pacific beach, lined with palm trees and lush vegetation, can be seen.
Revised ages for tuffs of the Yellowstone Plateau volcanic field: Assignment of the Huckleberry Ridge Tuff to a new geomagnetic polarity event
Marvin A. Lanphere, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, et al. Pages 559-568.
The precision of ages for the three major ash-flow tuffs of the Yellowstone Plateau volcanic field has been improved by sing the 40-argon/39-argon dating method on sanidine feldspar. The ages for the ash-flow tuffs are: Huckleberry ridge Tuff = 2.059 ± 0.004 million years, Mesa Falls Tuff = 1.285 ± 0.004 million years, and Lava Creek Tuff = 0.639 ± 0.002 million years. The Huckleberry Ridge Tuff has a transitional magnetic direction and has been assigned to a new geomagnetic polarity event.
Timing of multiple late Quaternary glaciations in the Hunza Valley, Karakoram Mountains, northern Pakistan: Defined by cosmogenic radionuclide dating of moraines
Lewis A. Owen, Department of Earth Sciences, University of California, Riverside, California 92521-0423, USA, et al. Pages 593-604.
The Karakoram Mountains are the most glaciated area outside the polar regions and contain some of Earth's highest mountains, including K2 (the second highest mountain in the world). Using cosmogenic radionuclide dating, the timing of multiple past glaciations in this region has been determined for the first time. The results suggest a strong link between glacial fluctuations and precipitation changes related to oscillations in the South Asian monsoonal system combined with cooling that is broadly associated with high frequency climatic change in the North Atlantic. The initial results provide data and a framework to help predict how glaciers may respond to climate change in the future, particular in the light of the possibility of human-induced global warming. The link between glaciation and the South Asian monsoonal system hints at the possibility that glaciers may initially grow as the world warms because the South Asian summer monsoon will penetrate farther north into the Himalayas and Karakoram Mountains, and the increased snowfall at high altitudes will feed glaciers and allow them to advance.
Late Cenozoic paleoenvironment in southern Victoria Land, Antarctica, based on a polar glaciolacustrine deposit in western Victoria Valley
Meredith A. Kelly, Institute for Quaternary and Climate Studies, University of Maine, Orono, Maine 04469, USA, et al. Pages 605-618.
The origin of a widespread drift sheet in western Victoria Valley, Antarctica, is investigated and discussed, in particular as to its implications for the stability of the East Antarctic Ice Sheet during the late Cenozoic Era. This drift sheet was previously interpreted as a set of recessional moraines deposited during an expansion of Victoria Upper Glacier. However, recent examinations show that the geomorphology, sedimentology, and weathering of the deposits are different from typical polar moraines. An alternative hypothesis, presented by the authors, is that the deposits were formed by a lake-ice conveyer system. Such a system of sediment transport and deposition occurs only in cold polar environments, which support perennially ice-covered, proglacial lakes. The interpretation of this drift sheet as a lake-ice conveyer deposit supports the hypothesis of East Antarctic Ice Sheet stability during the deposition of the drift sheet.
Tightening the Belt: Paleomagnetic-stratigraphic constraints on deposition, correlation, and deformation of the Middle Proterozoic (ca. 1.4 Ga) Belt-Purcell Supergroup, United States and Canada
D.P. Elston, 6300 Country Club Drive, Flagstaff, Arizona 86004, USA, et al. Pages 619-638.
The 1.4 billion-year-old Belt-Purcell Supergroup, lying mostly in Montana, Idaho, and British Columbia, is an excellent laboratory for studying Precambrian Earth processes. The paleomagnetic record of its red sediments and volcanics are near ideal, allowing one to consider their depositional setting, the correlation of units around the basin, and the deformation of the basin over its subsequent history. This paper summarizes about 20 years of paleomagnetic study of this basin. We reconcile a long-standing problem concerning the position of North America through the Middle Proterozoic and provide important constraints on what continent rifted away from North America to form the Pacific Ocean. We show that Earth processes (plate tectonic and geomagnetic reversal rates) in the Middle Proterozoic are similar to those seen during the Cretaceous Period (140 to 65 million years ago) in relatively recent times. As well, we see how the formation of the North American Cordillera broke this region west of the Rockies into blocks which rotate against one another.

To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.

To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns.
The Geological Society of America: http://www.geosociety.org/

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