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GSA release 02-40: Sept. GSA BULLETIN media highlights

Geological Society of America

Boulder, Colo.--The September issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest is new understanding of the early geologic history of Antarctica.

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 at for copies of articles and for additional information or assistance.

Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica.
Paul M. Myrow, Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA; et al. Pages 1070-1088.
Keywords: Archaeocyatha, Cambrian, Neoproterozoic, reef, Ross orogeny, Transantarctic Mountains.

Detailed mapping in the remote central Transantarctic Mountains has provided the basis for reconstructing much of the early history of the continent. The mountain range presently separates East and West Antarctica, but during the late Precambrian to early Paleozoic it was a continental margin undergoing major transformation from a passive to active tectonic setting. The mountain building event, the Ross Orogeny, has been poorly understood, in part because of complex deformation of the rocks. We have unraveled the nature and timing of this early history by combining structural, sedimentological, stratigraphic, and paleontological data. A key to deciphering this history was the discovery of a transitional contact between large reefs built by the extinct organisms archeocyathis and overlying coarse conglomeratic strata. The acheocyathan reefs had up to 40 m of relief above the seafloor. We found evidence for rapid drowning of these reefs and their subsequent coating by a thick layer of phosphate. Shortly thereafter, the basin rapidly filled and a thick wedge of coarse continental (fluvial and alluvial fan) deposits built outwards from the new mountain range. The timing of this event was provided by a new discovery of trilobite fossils in strata that record the early filling of the marine basin subsequent to the drowning of the archeocyathid reefs. This margin of East Antarctica was transformed from a passive continental margin (e.g., present-day eastern North America) to a convergent margin during the Cambrian. Although this transformation took place over a long time interval, from ~540-480 Ma, the onset of active faulting, uplift, and erosion in the central Transantarctic Mountain is now constrained to have occurred in the late Cambrian (515-510 Ma).

Arcuate thrust systems in sandbox experiments: A comparison to the external arcs of the Western Alps.
W.H. Lickorish, Geologisches Institut, Eidgenössische Technische Hochschule Zentrum, CH-8092, Zürich, Switzerland; et al. Pages 1089-1107.
Keywords: arcuate structures, collision, kinematics, mechanical stratigraphy, sand analogue experiments, Western Alps.

The arcuate form of the western Alps has long fascinated geologists. How can such arcuate structural systems develop in external zones during collision? A compilation of published and new kinematic data shows that, instead of a truely arcuate distribution of tectonic transport directions, there are in fact two distinct directions around the western alpine arc. The principal transport was towards the NW while a secondary system developed toward the SW. Variation in amount and age of shortening around the arc are critical in distinguishing these two systems. Sandbox experiments carried out at Rennes University examine arcuate structural systems that developed ahead of a rigid indentor that followed a straight, oblique rotational or curved trajectory as it was pushed into layered sand. Comparison of these experiments with alpine arc data indicates that the alpine indentor (Apulia or Adria) followed a slightly diagonal path with respect to the European plate margin from the Eocene to the early Miocene and curved anticlockwise by 10-15° in the mid-Miocene. On a more local scale, the importance of foreland mechanical stratigraphy, specifically the distribution and thickness of a weak décollement horizon, in the development of arcuate systems is also examined in a series of experiments and compared favourably with the second order Jura arc.

Numerical modeling of fluvial strath-terrace formation in response to oscillating climate.
Gregory S. Hancock, Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA; and Robert S. Anderson, Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA. Pages1131-1142.
Keywords: fluvial features, landscape evolution, modeling, river terraces, sediment supply.

In the article, Numerical modeling of fluvial strath terrace formation in response to oscillating climate, by Hancock and Anderson, the authors present a modeling study that suggests the formation of a particular kind of river terrace, known as a strath terrace, is more complicated than previously recognized. Strath terraces are essentially abandoned river valley floors, and sequences of such terraces are widespread on rivers of western North America. Their formation has typically been subscribed to river response to oscillation between glacial and interglacial periods. Straths are cut and widening in response to high sediment loads during glacial periods, and then abandoned to form terraces during interglacial periods. The modeling work presented here suggests that this general idea is reasonable, but the timing and duration of strath formation may be much different than previously recognized. The results suggest that using fluvial terraces to determine the timing of climatic events and river erosion rates may not be entirely straightforward.

Weathering profiles, mass-balance analysis, and rates of solute loss: Linkages between weathering and erosion in a small, steep catchment.
Suzanne Prestrud Anderson, Center for Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064-1077, USA; et al. Pages 1143-1158.
Keywords: chemical erosion, denudation, physical weathering, soil dynamics, uplift, weathering.

Weathering describes the suite of physical and chemical processes that break down solid rock to produce soil. Evidence from the chemical loads of large rivers suggests that chemical weathering rates are higher in areas subject to active physical erosion processes. The linkage between physical and chemical weathering processes is not well understood. In this study we were able to analyze separately the chemical weathering taking place within the soil and that within the bedrock in a steep, headwater basin. Although both soil and bedrock contribute equally to the dissolved load produced from the basin, the rate of chemical weathering is found to be higher in the soil than in the bedrock. More rapid chemical weathering rates in the soil probably occur because of the organic acids present in the forest soil, and because high erosion rates in the basin continually feed new material into the soil "reactor". The weathering rates determined from this basin are greater than those measured in sites with much older soils, which suggests that turnover of material in the weathering zone is important. The fact that chemical weathering also takes place within the bedrock shows that a landscape without a soil mantle will also undergo chemical weathering, although at a lower rate.

Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 42°S and 46°S: An appraisal based on fission-track results from the transpressional intra-arc Liquiñe-Ofqui fault zone
Stuart N. Thomson, Institut für Geologie, Mineralogie, und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany. Pages 1159-1173.
Keywords: Cenozoic, denudation, fission-track dating, landscape evolution, Patagonian Andes, transpression.

The Patagonian Andes of southern Chile are a unique, but still poorly studied geological environment. They are situated above the only present-day world-wide example of an active oceanic spreading center being subducted below a major continent, and contain one of the largest single structures of the Andean Cordillera of South America, the approximately 1000 km long Liquiñe-Ofqui fault. In this article the mineral dating technique fission-track analysis has been applied to investigate for the first time the erosion history and development of the mountains along the southern part of the Liquiñe-Ofqui fault. The data reveal that this fault was the focus of enhanced, but localized rock uplift and erosion in the last 16 million years, and in particular between 7 and 2 million years ago. Several other previously disregarded lesser faults have been found to have been major active geologic features that acted to areally limit enhanced rock uplift and erosion at this time. Most of the main faults of this region were previously thought to be caused by rock masses sliding horizontally past each other (strike-slip faults). However, the discovery in this study of rock uplift requires that horizontal compressive forces also acted perpendicular to the faults resulting in local upwards squeezing (transpression) of rocks. Near uniform landscape across regions that have experienced different amounts and rates of rock uplift supports ideas that propose a certain threshold elevation, above which glacial erosion is so efficient that it can counteract any rock uplift, regardless of its rate or total amount, hence maintaining a near steady-state topography. This study thus emphasizes that transpression combined with fast acting erosion (in this case glaciation) can act as an effective mechanism to quickly bring rocks from great depth to the Earth's surface. The fastest rates of erosion in response to rock uplift occur at the same time as the initial stages of subduction of the oceanic spreading center at the southern termination of the Liquiñe-Ofqui fault, implying a causal link between these two processes.


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