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News Release October 11, 2001
GSA Release No. 01-42
Contact: Ann Cairns
Director–Communications and Marketing
acairns@geosociety.org, (303) 357-1056
FOR
IMMEDIATE
RELEASE

October Media Highlights:
The Geological Society of America Bulletin

BOULDER, Colo. The Geological Society of America has released the October issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

Stratigraphic architecture, magnetostratigraphy, and incised-valley systems of the Pliocene-Pleistocene collisional marine foreland basin of Taiwan
Wen-Shan Chen et al. Department of Geology, National Taiwan University, 245 Choushan Road, Taipei, Taiwan, ROC. Pages 1249-1271
The Pliocene Pleistocene foreland basin of Taiwan is the result of the oblique collision of the Luzon volcanic arc with the southeastern margin of China. Analysis of the 3 to 5-km-thick section of foreland basin deposits using stratigraphic analysis, magnetostratigraphy, and seismic profiles allows us to understand the stratigraphic development of the Taiwan foreland basin. The Pliocene Pleistocene strata of the foreland basin of Taiwan record ~2.3 million years of deposition based on our magnetostratigraphy. Sediment accumulation rate was on the order of ~950 meters per million years during the earlier stages of basin development. During the later stages of basin development, sediment accumulation rate increased to ~1900 meters per million years. These rates represent some of the highest sedimentation rates known on Earth.

Resolving complexities associated with the timing of macroscopic folds in multiple deformed terrains: The Spring Hill synform, Vermont
Ken A. Hickey and T.H. Bell. School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia. Pages 1282-1298
The collision of tectonic plates causes the Earth's crust to compress and mountains to form. During this process, rock strata in the crust undergo repeated periods of shortening and are deformed into large folds. Timing the development of these large folds is difficult, especially for rocks that have undergone a succession of overprinting deformations. The Spring Hill synform in southeast Vermont is an example of such a fold. Understanding the origin and timing of this structure is crucial to unraveling the history of mountain building that produced the Appalachian mountain range in Vermont. We have reassessed the origin of this fold using a combination of detailed structural mapping and the analysis of mineral textures preserved at the microscopic scale. We believe the fold formed during a deformation path that involved a succession of overprinting, a near-orthogonal period of shortening that produced a series of new subvertical and subhorizontal alignments of metamorphic minerals called foliations. Orthogonal overprinting of foliations have recently been observed in rocks from other mountain chains and may represent a fundamental product in the accommodation of subhorizontal crustal collision. The Spring Hill synform was previously thought to have formed as a subhorizontal fold associated with large-scale lateral extrusion of rock (thrusting and nappe formation) and development of the main subhorizontal foliation preserved in the rocks. However, we demonstrate that the Spring Hill synform developed as a large subvertical fold prior to the development of the metamorphic foliations now preserved in the rocks. We suggest that the synform formed at a much earlier stage in the development of the Appalachian mountain range in Vermont and was subsequently modified and rotated to its present geometry by the long history of west-east shortening that dominated the later stages of this phase of tectonism. Our results suggest that large-scale folds commonly develop early in the deformation history of crustal collision and are simply reused and reoriented without being completely overprinted at the same scale.

Petrogenesis of the contact-metamorphic rocks beneath the Stillwater Complex, Montana
Theodore C. Labotka, Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, and Randy L. Kath, Department of Geosciences, State University of West Georgia, Carrollton, Georgia 30118. Pages 1312-1323
The rocks beneath the Stillwater Complex in southwest Montana consist of Archaean (more than 2.5 billion years old) metasedimentary rocks, primarily biotite-garnet schist. The rocks were regionally metamorphosed before the intrusion of the gabbroic magmas of the Stillwater Complex, when rocks close to the complex were contact-metamorphosed. Minerals in the metamorphic rocks include biotite, cordierite, garnet, gedrite, staurolite, and quartz. In the zone of contact metamorphism, cummingtonite and hypersthene appear. Both regional and contact metamorphism occurred under low-pressure conditions, with estimates from mineral equilibria of less than 2 to 3 kilobars. The contact-metamorphic association of cordierite + cummingtonite indicates pressures less than 2 kilobars. The temperature of metamorphism ranged from less than 500 °C in the regional metamorphic rocks to 815 °C at the contact. The pressure at the contact is consistent with shallow emplacement of the Stillwater Complex.

Detrital zircon provenance of Mesoproterozoic to Cambrian arenites in the western United States and northwestern Mexico
John H. Stewart et al. U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025. Pages 1343-1356
The source of Proterozoic (upper Precambrian, about 550 million years ago) and Cambrian arenites of the western United States and northwestern Mexico is investigated by U-Pb dating of individual detrital zircons. The arenites had multiple sources including Precambrian crystalline basement rocks from a variety of regions in western North America, from Proterozoic silicic volcanic fields in western North America, and possibly from sources that were rifted from North America and are now far distant from North America.

To view abstracts for the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, go to www.gsajournals.org. To obtain a complimentary copy of any BULLETIN article, contact Ann Cairns.

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