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GSA Bulletin--August media highlights

Geological Society of America

Boulder, Colo.--The August issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest are: a study that suggests that the rate of ridge production of new oceanic lithosphere has remained essentially steady over the last 180 million years, thereby challenging current models of sea level change and variations in oceanic and atmospheric chemistry; and research concluding that the rangeland practice of converting native shrublands to non-native grasslands results in a precipitous decrease in root strength that destabilizes hillslopes and leads to increased rates of landsliding. 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.

Rate of plate creation and destruction: 180 million years ago to present.
David B. Rowley, Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA. Pages 927-933.

One of the primary measures of plate tectonics is the history of production of new oceanic lithosphere. A direct estimate of the rate of plate creation can be derived from the area-age distribution of the modern oceanic lithosphere. Inversion of the most recent area-age data from a Müller study in 1997 suggests that the rate of production has not varied over the past 180 million years from a mean rate of 3.4 square kilometers per year. Reconstruction of the cumulative area of subducted lithosphere over the past 90 million years is in excellent agreement with a fixed rate of ridge production. The conclusion that the rate of ridge production has not varied significantly contrasts markedly with most existing estimates in which the rate is modeled as decreasing by 50% or more since about 100 million years ago and has important implications for models of sea level, and pCO2, among other phenomena that have been linked to variations in global rates of seafloor spreading.

Linked thrust and strike-slip faulting during Late Cretaceous terrane accretion in the San Juan thrust system, Northwest Cascades orogen, Washington.
Steffen G. Bergh, Department of Geology, University of Tromsø, N-9037 Tromsø, Norway. Pages 934-949.

This article presents a detailed structural investigation of Cretaceous fault zones of the San Juan Islands, forming part of the western Cascade foothills of Washington. In the Cretaceous time period (~95-80 million years ago) various crustal plates collided obliquely with the western edge of the North American continent. The resulting compressions and lateral motions of plates along the continental edge caused the rocks to become highly folded and faulted. Common fault processes operating during this collision, that is, processes that are also acting along collisional margins today, are demonstrated and discussed.

The kinematic history of the central Andean fold-thrust belt, Bolivia: Implications for building a high plateau.
Nadine McQuarrie, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 950-963.

The Andean mountain chain borders the one of the largest orogenic plateaus in the world. While the Tibetan plateau is thought to be the result of continental collision between the Indian and Asian continents, the Andean plateau is associated with convergence between oceanic and continental lithosphere. The processes involved in building a high-elevation plateau are still hotly debated, with many of the arguments centered around the relative importance of crustal underthrusting, magmatic additions, thermal effects and distributed shortening. The goal of this paper is to link the growth of the central Andean plateau to sequential deformation within the Andean fold-thrust belt by presenting regional balanced cross sections based on field traverses across the entire Andean fold-thrust belt. Balanced cross sections are vertical slices though the Earth that show an interpretive profile of the contractional deformation that built the plateau. The faults and the folds shown in the cross section can be sequentially undone to allow for sequential restoration, tracking the evolution of the fold-thrust belt with time. They also provide a means for determining realistic estimates of shortening that can account for the total thickness of the Andean plateau from the eastern edge of the volcanic arc to the foreland. The cross sections and kinematic evolution models presented in this paper suggest that the Andean plateau may be a manifestation of unique features in the fold-thrust belt, such as 15-km-thick basement thrust sheets. This implies that the growth history of the plateau cannot be understood outside the context of the kinematic history of the fold-thrust belt.

The Kingston terrane, southern New Brunswick, Canada: Evidence for an Early Silurian volcanic arc.
Sandra M. Barr et al., Department of Geology, Acadia University, Wolfville, Nova Scotia B0P 1X0, Canada. Pages 964-982.

This paper puts southern New Brunswick, Canada, in a new light in terms of its geological history. Instead of a single passive continental block during the building of the Appalachian Mountain belt, about 440 million years ago, it consisted of at least two separate blocks that were colliding, forming violently erupting volcanoes and contributing to the mountain-building process. This process may explain, in part, similar mountain-building processes that occurred in coastal Maine where less evidence for the colliding blocks themselves has been preserved.

Landslides on coastal sage-scrub and grassland hillslopes in a severe El Niño winter: The effects of vegetation conversion on sediment delivery.
Emmanuel J. Gabet, Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA, and Thomas Dunne, Donald Bren School of Environmental Science and Management and Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106, USA. Pages 983-990.

The conversion of vegetation from native communities to introduced species is a common practice in rangelands. The replacement of shrublands by grasslands on hillslopes, primarily to increase pasturage for cattle, often results in increased rates of landsliding. In this contribution, the authors investigate over 30 shallow landslides that occurred near Santa Barbara, California, during the 1997-1998 El Niño to understand the role of root reinforcement in landslide mechanics. They conclude that a sudden decrease in root strength will destabilize hillslopes that were previously stable.

Drainage reversals in Mono Basin during the late Pliocene and Pleistocene.
Marith C. Reheis et al., U.S. Geological Survey, M.S. 980, Federal Center, Denver, Colorado 80225, USA. Pages 991-1006.

Pleistocene Mono Lake (Lake Russell) last overflowed to the south into the Owens River drainage. Prior to the tectonic opening of this outlet to the south, Lake Russell overflowed northward into the East Walker River on at least several occasions between 2 and 1 million years ago. This drainage reversal permitted southward migration of Lahontan fish into Owens Valley as late as 700,000 years ago. Although cutthroat trout and whitefish do not live this far south along the Sierra Nevada today, ancient deposits of Owens Lake contain their fossils as well as those of other fish that were widely distributed throughout the region before creation of the present rugged topography.

Initiation of magmatism during the Cambrian-Ordovician Ross orogeny in southern Victoria Land, Antarctica.
Andrew Allibone, School of Earth Sciences, James Cook University of North Queensland, Townsville, Q4811, Australia, and Richard Wysoczanski, Research School of Earth Sciences, Australian National University, Canberra, ACT0200, Australia and Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, USA. Pages 1007-1018.

The Ross orogeny spans the full width of the Antarctic continent and represents part of the eroded root of an ancient volcanic belt similar to the modern "ring of fire" that encircles the Pacific Ocean. A series of samples of granite have been dated to determine when volcanism in the Ross orogeny initiated. By dating these rocks, geologists can gain a clearer understanding of the length of time over which volcanic belts like the Pacific "rim of fire" were active in the past and how the continents were arranged while volcanism was occurring.

Sedimentologic-magnetic record of western Pangean climate in upper Paleozoic loessite (lower Cutler beds, Utah).
G.S. (Lynn) Soreghan, et al., School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. Pages 1019-1035.

Wind-blown silt deposited in the western equatorial region of the Pangean supercontinent (present-day southeastern Utah) during "icehouse" conditions 300 million years ago exhibits sedimentologic and rock magnetic evidence for both short term (tens of thousands of years) and long-term (hundreds of thousands of years) climate change. The silt deposit contains numerous fossil soil horizons that show kicks in magnetic susceptibility. This magnetic signal appears attributable to genesis of magnetic minerals in soils that formed during climatic optimums in a manner apparently analogous to processes that have produced such magnetic signatures in young (less than 2.5-million-year-old) loess of, for example, the Chinese Loess Plateau. The loess-soil couplets documented in this study appear to record relatively short-term glacial-interglacial climate fluctuations at equatorial latitudes of western Pangea. Over the hundred-thousand-year scale, the silt section exhibits a change from wind-blown to river-transported sediments that, together with changes in soil type, appears to record a long-term climate change from drier to wetter conditions driven by evolution of mega-monsoonal circulation in the western Pangean supercontinent.


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