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News Release December 20, 2002
GSA Release No. 02-58
Contact: Christa Stratton
+1-303-357-1093
FOR
IMMEDIATE
RELEASE

January Media Highlights:
The Geological Society of America Bulletin

Boulder, Colo.-The January issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest is new work demonstrating the link between mass extinction and environmental change at the end of the Ordovician Period, when as many as 86% of species became extinct. 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.

Please note that GSA will be closed from 12:00 p.m. MST on Monday, December 23, until Thursday, December 26. We will also be closed from 12:00 p.m. MST on Tuesday, December 31 until Thursday, January 2.

Because of holiday closings and vacation schedules, requests for articles may be directed as follows:
Monday a.m.Dec. 23 contact Ann Cairns at acairns@geosociety.org
Thursday and FridayDec. 26 & 27 contact Joan Manly at jmanly@geosociety.org
MondayDec. 30 contact Margo Good at mgood@geosociety.org
Tuesday a.m.Dec. 31 contact Ann Cairns at acairns@geosociety.org
ThursdayJan. 2, and ongoing contact Ann Cairns at acairns@geosociety.org

East-west extension and Miocene environmental change in the southern Tibetan plateau: Thakkhola graben, central Nepal
Carmala N. Garzione, Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA; et al. Pages 3-20.
Keywords: extension, Nepal, Thakkhola graben, Tibetan plateau.
The Tibetan Plateau stands at an average elevation of ~5000 m. It is generally thought that the latest stage of uplift of the Tibetan plateau led to gravitation collapse, forming the E-W extensional basins that we see across the plateau today. Previous estimates on the timing of uplift of the Tibetan plateau have focused on an 8 million year age for uplift, based on several indicators of environmental change at that time and age constraints of 8 to 5 million years ago for another extensional basin in Tibet. Garzione et al. present the first detailed study of the sedimentology of one of these grabens, the Thakkhola graben in north-central Nepal. Sedimentological data such as rock types and thicknesses, measurements of flow directions in ancient river systems, and the source rocks and locations from which sediment was derived are used to decipher the tectonic evolution of the Thakkhola graben as well as the paleoclimate of this region. The implications of these data are discussed in light of previous work by Garzione et al. and other researchers, which suggests an age of >11 million years for uplift of the Tibetan Plateau.
The Tula uplift, northwestern China: Evidence for regional tectonism of the northern Tibetan Plateau during late Mesozoic-early Cenozoic time
Delores M. Robinson, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA; et al. Pages 35-47.
Keywords: Asian tectonics, China, syntectonic processes, Tibetan Plateau.
Geologic mapping combined with laboratory studies in the Tula uplift of western China provides insights into the tectonic evolution of the northern edge of the Tibetan Plateau. The tectonic history from 150 to 20 million years ago (Ma) is preserved in the Tula uplift, which includes sedimentary strata now exposed in a large fold, pre-150 Ma metamorphic basement, and plutons that have an age of 74 m.y. Petrographic analyses of 150-20 Ma sandstones show that rocks in the area were derived from lithologically diverse source terranes consisting of sedimentary, metasedimentary, and igneous rocks. These relations imply that uplift in the Tula area began in Late Jurassic time (ca. 150 Ma) and that uplift of the Tibetan Plateau's northern edge may have been initiated long before the early Tertiary (ca. 55 Ma) India-Asia collision. Continued tectonic activity in the Tula area is recorded by intrusion of ca. 74 Ma granitoid bodies, shortening from 75 to 20 Ma, uplift of pre-150 Ma basement rock, the composition of the 140 to 20 Ma sandstones, and folding of all the basin strata into a regional north-vergent fold. The northern range-bounding thrust fault of the Tula uplift has been recently active, suggesting that uplift and thickening continues in the northern Tibetan Plateau.
Early to middle Tertiary foreland basin development and the history of Andean crustal shortening in Bolivia
Peter G. DeCelles, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and Brian K. Horton, Department of Earth and Space Sciences, University of California, Los Angeles, California 90095 USA. Pages 58-77.
Keywords: Andes, foreland basins, fold-thrust belts, Tertiary, sedimentology, sedimentary petrology.
This article addresses the geological history of development of the Andes Mountains in Bolivia. The Andes are made of a large volcanic arc and a fold-thrust belt. The fold-thrust belt has developed by horizontal shortening of Earth's crust as the South American and Nazca plates converge upon each other. Two of the biggest remaining questions in Andean tectonics are the timing of initial horizontal shortening in the fold-thrust belt, and whether this horizontal shortening is enough to explain the present thickness of the crust in the Andes. Our analysis of sediments that were shed off of the rising Andes between ca. 65 million years ago (Ma) and ca. 20 Ma suggests that the fold-thrust belt began to develop by at least 55 Ma. The Andes have formed in part by the off-scraping of sedimentary rocks from the Brazilian crust as it underthrusts westward beneath the growing mountain range. Our data also suggest that a slab of Brazilian crust approximately 1,000 km long (measured in an east-west direction) has been buried beneath the fold-thrust belt. Crowding of this much crust beneath the mountain range can explain the present thickness of crust beneath the Andes.
High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation
P.J. Brenchley, Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 7GP, UK,et al. Pages 89-104.
Keywords: stable isotopes, Ordovician, Hirnantian, extinction, glaciation, Baltic region.
The mass extinction at the end of the Ordovician Period (about 430 million years ago) was one of the most dramatic in Earth's history: as many as 86% of species on Earth are thought to have become extinct. New work to be published in the GSA bulletin (Brenchley et al.) demonstrates the link between extinction and environmental change. The carbon and oxygen isotopic "fingerprints" preserved in sediments and fossils from the period reflect global changes in the world's oceans as Earth's climate suddenly deteriorated — and then equally suddenly warmed. The same pattern of change is reflected in sediments from the Baltic States to Nevada. The isotopic fingerprints enable the authors to start to dissect the extinction event and reveal distinct stages in the patterns of extinction and evolution. Although the changes were very rapid, they were not instantaneous. Some species were apparently more resistant to environmental change than others: some became extinct during the cooling others during the warming episode.
Genesis of flood basalts and Basin and Range volcanic rocks from Steens Mountain to the Malheur River Gorge, Oregon
Victor E. Camp, Department of Geological Sciences, San Diego State University, San Diego, California 92181, USA, et al. Pages 105-128.
Keywords: Basin and Range province, flood basalt, Oregon, mantle plume, Steens basalt, Columbia River basalt.
Two regionally important volcanic successions lie in eastern Oregon between Steens Mountain and the Malheur River Gorge: (1) Miocene flood basalts (~16.6-15.3 million years old) — a voluminous sequence of lavas that may be related to melting of a mantle plume currently underlying Yellowstone National Park; and (2) Basin-and-Range volcanic rocks (<15.3 million years old) — a sporadic but widespread sequence of lava flows and fragmented volcanic rocks associated with continental extension of the western United States. Chemical and stratigraphic variations demonstrate that the main phase of flood-basalt volcanism generated ~230,500 km3 of basalt over an interval of ~1.3 million years. This rapid accumulation of lava appears to be contemporaneous with a markedly consistent northward propagation of regional uplift, basalt regression, and vent migration across the breadth of eastern Oregon and into southeastern Washington. The northward advancement of volcanism and uplift may represent the surface expression of a spreading mantle plume head deflected against the ancient continental margin of North America.

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.

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