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Find Your Science at GSA
15 July 2010
GSA Release No. 10-34
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
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Geology and GSA Today Highlights

August 2010

Boulder, CO, USA - Digging dinosaurs, burrowing worms, weakened rocks, immobilized uranium, slowstanding sea levels, isolated nunataks, colliding sediments, retreating ice sheets, westerly winds, electrically conductive lithosphere, dynamic debris flows, reactivated landslides, rock incompetence, recrystallized quartz, and biased sedimentation are some of the topics presented in the August issue of GEOLOGY. August GSA TODAY science presents new data and digital elevation models acquired during the February 2000 Shuttle Radar Topography Mission aboard the Space Shuttle Endeavour.

Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. GSA TODAY articles are open access. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.

Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.

geology

Prehistoric earthquakes on the Caribbean-South American plate boundary, Central Range fault, Trinidad
Carol S. Prentice et al., U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA. Pages 675-678.

Carol Prentice of the U.S. Geological Survey and colleagues describe the first geologic investigation of the Central Range fault that shows this is an active strike-slip fault capable of producing large earthquakes on the island of Trinidad. The Central Range fault is a major part of the Caribbean-South American plate boundary. Prentice et al. show that the most recent earthquake on the Central Range fault occurred between 2710 and 550 years before the present, indicating that the Central Range fault is a major seismic hazard in Trinidad.

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Investigation of the strength contrast at the Moho: A case study from the Oman Ophiolite
Janelle M. Homburg et al., Dept. of Earth and Environmental Sciences, Columbia University, Lamont Doherty Earth Observatory, Palisades, New York 10964, USA. Pages 679-682.

The strength contrast at the Moho (lower crust to upper mantle transition) controls such processes as the depth of earthquakes, interactions between the crust and the mantle, and the deformation and motion of the tectonic plates. While previous work has concluded that the lower crust is weaker than the mantle at the Moho, recent work, based on the location of deep earthquakes in Tibet and other areas around the world, has suggested the lower crust may be stronger than the mantle. In this paper, scientists from the Lamont Doherty Earth Observatory and Brown University show compelling geological evidence that crustal rocks are significantly weaker than mantle rocks at Moho conditions, using outcrop scale and microstructural observations. Their observations are unique because they studied lower crustal rocks and upper mantle rocks that deformed together at Moho conditions. Their results are in good agreement with predictions based on extrapolation from laboratory observations. Homburg et al. conclude that the lower crust is weaker than the mantle wherever the crustal composition is similar to the rocks they studied. This study was funded in part by the U.S. National Science Foundation.

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Potassium and uranium in the upper mantle controlled by Archean oceanic crust recycling
Sune G. Nielsen, Dept. of Earth Science, University of Oxford, Parks Road, OX1 3PR Oxford, UK. Pages 683-686.

Processes that occurred in the early stages of Earth's history are difficult to study because few clues remain that bear evidence of those times. Geoscientist Sune Nielsen of the UK’s University of Oxford shows that the ratio observed today between the elements potassium (K) and uranium (U) in Earth's mantle was controlled by the environmental conditions at the surface during the first 2000 million years of Earth's history. Presently, the mantle displays K/U far higher than the bulk Earth value, but no modern-day mechanism can account for this disparity. The atmosphere contained little or no oxygen in the early Earth and this would have rendered uranium immobile during weathering, as it is only soluble in the presence of oxygen. Conversely, potassium was delivered amply to the oceans because its solubility does not depend on surface oxygenation. Consequently, the ocean crust would have inherited a high K/U through hydrothermal alteration and sedimentation. This material was then mixed back into the mantle through subduction at convergent plate margins, and thus slowly forced K/U toward the high value observed today. This process was halted when the atmosphere became oxygenated about 2400 million years ago, and thus K/U in the mantle records a snapshot of early Earth weathering.

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Quantum magmatism: Magmatic compositional gaps generated by melt-crystal dynamics
Josef Dufek and Olivier Bachmann, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA. Pages 687-690.

For more than a century, geologists have observed gaps in the chemical composition of many erupted magmas from worldwide volcanic centers. The clustering of these compositions, rather than a continuous distribution of compositions in the eruptive record, influences the mechanical and thermal properties of the crust, and holds clues about how our planet differentiates. Geoscientists Dufek and Bachmann of the Georgia Institute of Technology have explored the potential of magma dynamics to generate compositional gaps using analytical calculations and numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs. They show that gaps are inherent to crystal fractionation for all compositions, as crystal-liquid separation takes place most efficiently within a crystallinity window around 50-70 vol% crystals. The probability of melt extraction from a crystal residue in a cooling magma chamber is highest in this crystallinity window due to (1) enhanced melt segregation in absence of chamber-wide convection, (2) buffering by latent heat of crystallization, and (3) diminished chamber-wall thermal gradients. This mechanical control of igneous distillation is likely to have played a dominant role in the formation of the compositionally layered Earth's crust by allowing multiple and overlapping intrusive episodes of relatively discrete or quantized composition that become more silicic upward. This study was funded in part by the U.S. National Science Foundation.

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Mount Etna-Iblean volcanism caused by rollback-induced upper mantle upwelling around the Ionian slab edge: An alternative to the plume model
W.P. Schellart, School of Geosciences, Monash University, Melbourne, VIC 3800, Australia. Pages 691-694.

Mount Etna is the largest volcano in Europe, and its origin has long been a mystery. Most volcanism on Earth occurs at plate boundaries, in places where tectonic plates move apart (e.g., Iceland) and in places where tectonic plates come together with one plate diving (subducting) below the other plate into the mantle (e.g., Pacific "ring of fire"). Some volcanism (named intraplate volcanism) occurs far from plate boundaries, and its origin is more controversial. The chemistry of the volcanic rocks from Mount Etna and the nearby Iblean volcanics in Sicily indicate that they are intraplate volcanics. Interestingly, the volcanoes are located close to, but are laterally offset from, the Calabrian subduction zone plate boundary, where the African plate sinks below the European plate. This suggests that the volcanics are somehow related to the Calabrian subduction zone. New modeling of subduction and mantle flow by W.P. Schellart of Monash University confirms this, showing that backward sinking of the African plate at the Calabrian subduction zone induced flow around the southern edge of the subducted plate and upward below Sicily. The upward flow induced decompression melting of upper mantle material and these melts extruded at the surface in Sicily, forming Mount Etna and the Iblean volcanics.

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Drowned coastal deposits with associated archaeological remains from a sea-level “slowstand”: Northwestern Gulf of Maine, USA
Joseph T. Kelley et al., Dept. of Earth Sciences, University of Maine, Orono, Maine 04469-5790, USA. Pages 695-698.

The rise of sea level since the end of the last Ice Age has drowned the continental shelves of the world and, along with them, innumerable archeological sites. In formerly glaciated areas like the Gulf of Maine, sea-level change has involved more than a simple rise in water level. Owing to the weight of the ice, the landscape was initially depressed and flooded as the ice receded. Following melting of the ice, the land rose back up and exposed extensive areas of the Gulf of Maine. After the land stabilized, sea level continued to rise, but at an uneven rate. Between 11,500 and 7,500 years ago, sea level barely rose off the Maine coast, a time this team from the University of Maine and the University of New Hampshire have termed the "slowstand." During this time, glacial deposits were eroded and formed beaches that sheltered lakes and estuaries. Off Bass Harbor, Maine, scallop draggers recovered stone tools from this time period on the shores of what their bathymetric imagery, seismic reflection profiles, and cores suggest was a beach associated with freshwater wetlands and tidal flats. The time of the slowstand and the associated depth range, 15-25 m, represent the most probable settings from which submerged archeological remains will be recovered in the Gulf of Maine.

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Predatory digging behavior by dinosaurs
Edward L. Simpson et al., Department of Physical Sciences, Kutztown University of Pennsylvania, 424 Boehm, Kutztown, Pennsylvania 19530, USA. Pages 699-702.

This paper by Edward Simpson of Kutztown University of Pennsylvania and colleagues provides evidence for a dinosaur hunting strategy for securing fossorial mammals. The predatory behavior of dinosaurs has commonly been identified through specific adaptations, jaws, teeth and post-cranial elements, taphonomic associations, and trace fossil evidence, including bite marks, gut contents, coprolites, and trackways. Minimal direct evidence exists in the rock record of dinosaurs and mammals behaving as predators and prey, respectively. However, a newly discovered Late Cretaceous trace fossil association of digging traces of maniraptoran theropod dinosaurs and mammalian den complexes indicates a predator-prey relationship. Three distinct associated trace fossils occur within a floodplain siltstone-mudstone bed of the Upper Cretaceous Wahweap Formation in southern Utah, United States. Simpson et al. show that one trace records digging by a maniraptoran theropod dinosaur, possibly a dromeosaurid or troodontid. The other two are interpreted as mammalian den complexes. The fact that these traces are so close together suggests that dinosaurs used excavation techniques to prey on mammals.

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Pleistocene dynamics of the interior East Antarctic ice sheet
Kat Lilly et al., Dept. of Geology, University of Otago, PO Box 56, Dunedin, New Zealand. Pages 703-706.

Kat Lilly of the University of Otago and colleagues from New Zealand, Australia, and the UK measure how long nunataks (islands within a glacier or ice sheet) have been exposed above the ice in the remote interior of the East Antarctic ice sheet. Their results provide an understanding of the long-term stability of the ice sheet over the past 4 million years. The study turns up extremely old exposure ages, indicating both that erosion rates in the area are very low and that this part of the East Antarctic ice sheet has been remarkably stable over the past few million years. Modeling by Lilly et al. also shows that the ice-sheet surface elevation has oscillated with the global ice age cycles but has also very slowly lowered at a rate of about 50 meters per million years.

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Source-side shear wave splitting and upper mantle flow in the Chile Ridge subduction region
R.M. Russo et al., Dept. of Geological Sciences, P.O. Box 112120, 241 Williamson Hall, University of Florida, Gainesville, Florida 32611, USA. Pages 707-710.

Subduction of active spreading ridges is a geodynamic oddity: if ridges occur at mantle upwellings, then they should not coincide with regions of downwelling or subduction. Ray Russo of the University of Florida and colleagues show that, in the region where the Chile Ridge subducts beneath South America (46.5°S), the upper mantle flows through a gap - a slab window - formed between the trailing edge of the subducting Nazca Plate and the leading edge of the subducting Antarctic Plate. The upper mantle flows horizontally southward, parallel to the Nazca trench, north of the slab window, but turns eastward into the window where the gap between the Nazca and Antarctic slabs exist. This upper mantle flow has been implicated in the unusual pattern of Late Cenozoic tectonics and volcanism in overriding South America. This study was funded in part by the U.S. National Science Foundation.

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Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary
Jean Vannier et al., UMR 5125 PEPS, Universite de Lyon, Universite Lyon 1, Bat. Geode, 2 rue Raphael Dubois, F-69622 Villeurbanne Cedex, France. Pages 711-714.

The Precambrian-Cambrian transition (about 550 million years ago) represents a key period in the evolution of life on Earth, during which the direct ancestors of most present-day animal phyla appear in the fossil record. These anatomical novelties generated an unprecedented behavioral complexity that allowed animals to colonize new horizons of the marine ecospace. However, major uncertainties remain concerning the actors, the chronology, and the amplitude of this colonization. Jean Vannier and a team of geoscientists from France, Germany, and Poland take an innovative approach that combines fossil data and laboratory experiments with recent worms from Sweden that reveals that priapulid worms were of particular importance in the early colonization of marine sediment. Priapulid worms are known to be one of the earliest pioneer burrowers of the sea floor. Their horizontal burrow systems are preserved in the Cambrian rocks of all continents. These worms were carnivorous, preying upon a variety of small animals living at the water-sediment interface. This is attested by exquisitely well-preserved gut contents from the Burgess Shale Lagerstatte. Priapulids survived all major extinctions and are still living in present-day environments, showing a remarkable ability to tolerate very low oxygen levels.

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Equivalence of abrupt grain-size transitions in alluvial rivers and eolian sand seas: A hypothesis
Douglas J. Jerolmack and Theodore A. Brzinski III, Dept. of Earth and Environmental Science, University of Pennsylvania, 240 S. 33rd Street, Philadelphia, Pennsylvania 19104-6316, USA. Pages 719-722.

Rivers and deserts are not so different, according to Douglas Jerolmack and Theodore Brzinski of the University of Pennsylvania. Rivers often undergo a rapid transition from gravel to sand substrate, with a notable absence of intermediate grain sizes. Desert sand seas show a similar pattern, in which dune fields composed of sand are fringed by deposits of silt-sized particles called loess. Jerolmack and Brzinski hypothesize that the collision of sediments during transport produces a distinct population of smaller particles by chipping and rounding of corners in both systems. They calculate the collision dynamics of gravel in rivers and sand in deserts, and show that they are equivalent. Abrasion occurs for the fraction of grains that travel in "saltation," essentially bouncing along the sediment surface. Chips broken off by abrasion are small enough to be suspended in the current and moved long distances. As grains wear down from saltation, their collision energy decreases until abrasion becomes ineffective. The lack of intermediate grain sizes - between the smallest abraded particles and their products - generates an abrupt transition when sediment is sorted by water or wind.

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Large-scale glaciation and deglaciation of Antarctica during the Late Eocene
Shanan E. Peters et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 723-726.

A coastal, incised river valley complex in the Western Desert of Egypt documents the timing and magnitude of an ~40-m eustatic sea-level fall and rise that was driven by the transient growth and retreat of a large Antarctic ice sheet prior to substantial global cooling at the Eocene-Oligocene boundary. The sea-level fall and rise is coincident in time with an oscillation in atmospheric CO2 concentrations of about 750 parts per million. Because many of the carbon emission scenarios for the coming century predict that atmospheric CO2 will cross this same 750 parts per million threshold, results from this study by Shanan Peters and colleagues from the University of Wisconsin the University of Michigan raise the possibility that global climate could transition to a state not unlike the late Eocene, when a large, permanent Antarctic ice sheet was not sustainable. This study was funded in part by the U.S. National Science Foundation.

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Covariability of the Southern Westerlies and atmospheric CO2 during the Holocene
P.I. Moreno et al., Dept. of Ecological Sciences and Institute of Ecology and Biodiversity, University of Chile, Santiago, Chile. Pages 727-730.

Ice core records of atmospheric CO2 variations feature a steady deglacial rise that reached a maximum value about 11 thousand years ago, a conspicuous reversal between 11 and 8 thousand years ago, and a steady multi-millennial increase since then. Although several attempts to explain these aspects have been proposed, no single mechanism has been able to account for the timing and structure of natural atmospheric CO2 changes during the Holocene. Recent studies have proposed that changes in the latitudinal position and strength of the Southern Hemisphere Westerly Winds (SWW) can greatly influence large-scale ocean circulation and degassing of the deep ocean via changes in wind-driven upwelling in the Southern Ocean (SO); however, very few paleoclimate records allow testing of this hypothesis. Based on terrestrial ecosystem proxies from western Patagonia, P.I. Moreno of the University of Chile and colleagues reconstruct variations in the intensity of zonal flow over the past 14,000 years. These variations correspond to the timing and structure of atmospheric CO2 changes, and are consistent with the modeled magnitude of CO2 changes induced by varying strengths of the SWW. The close match between data and models supports the view that the SWW-SO system underpins multi-millennial CO2 variations during the current interglacial and, possibly, during glacial cycles.

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Deep deformation pattern from electrical anisotropy in an arched orogen (Betic Cordillera, western Mediterranean)
Ana Ruiz-Constan et al., Departamento de Geodinamica, Universidad de Granada, Granada, Spain. Pages 731-734.

Our knowledge of lithospheric mantle deformation is poor, due to the lack of accurate deep geophysical techniques and the scarcity of peridotite outcrops. The magnetotelluric (MT) method allows for characterization of the electrical conductivity of the lithosphere, proving a sensitive tool for determining its structure. The Betic Cordillera makes up the western end of the Alpine orogen in Europe and was formed in a general setting of Eurasian-African plate convergence. Ana Ruiz-Constan and colleagues from Spain, Mexico, and Portugal present MT data acquired in the Betic Cordillera and its foreland that provides the first evidence of electrical anisotropy in the upper mantle of the Mediterranean region. These data reveal preferred structure orientation related to olivine elongation in the mantle and an increase in the amount of deformation toward the axis of the plate boundary. At deep levels, all the sites show a common north-south geoelectrical strike, which may represent a low-intensity deformation, possibly related to “frozen” pre-alpine plate tectonics. For lower crust levels, a north-south constant strike at the Betic Cordillera sites contrasts with the east-west strike in the Iberian Massif. The integration of MT data with previous seismic studies allows discussion of the recent evolution of the western Mediterranean, which is a matter of great controversy among earth-science researchers.

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Evolution of a natural debris flow: In situ measurements of flow dynamics, video imagery, and terrestrial laser scanning
Scott W. McCoy et al., CIRES and Dept. of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA. Pages 735-738.

Debris flows are rapidly moving mixtures of soil, rock, and water that pose immediate hazards to down-slope communities and infrastructure. This team from CIRES, the USGS, and East Carolina University uses a novel combination of in situ measurements of debris-flow dynamics, video imagery, and high-resolution laser scanning to capture a natural debris-flow event at Chalk Cliffs in central Colorado (United States). The monitored debris flow started from clear-water flow and rapidly entrained sediment to form multiple, hazardous boulder-rich fronts. These new data confirm the importance of pore-fluid pressure and the non-uniform distribution of flow resistance in controlling hazardous aspects of debris flows such as travel distance, flow depth, and peak discharge. The researchers emphasize the fact that dynamic flow properties, such as pore-fluid pressure, are of equal or greater importance than channel topographic properties in determining where a particular debris flow will stop. Though this fact makes hazard assessments more difficult, incorporation of these additional controls on flow behavior should increase the accuracy of predictive models. This study was funded in part by the U.S. National Science Foundation.

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Platy layer silicate minerals for controlling residual strength in landslide soils of different origins and geology
Shinya Nakamura et al., University of the Ryukyus, Senbaru 1, Nishihara-cho, Okinawa 903-0213, Japan. Pages 743-746.

"Residual" strength is the final strength that develops on particle-reoriented surfaces of soil and rock after large displacements. Therefore, the residual strength is extremely important for understanding the stability for reactivated landslides. Shinya Nakamura of the University of the Ryukyus and colleagues describe the relationship between the residual strength and clay mineralogy of landslide soils to clarify the role of lamellar to platy clay minerals in developing the residual condition of a slip surface and to provide an estimate of the residual strength from the mineralogical soil composition. They examine the residual strengths and mineralogical compositions of soils collected from different landslides and showed that the total content of smectite, vermiculite, chlorite, and mica in the soil particle size fraction smaller than 425 micrometers is a controlling factor of the residual strength. These minerals are 2:1-type layer silicate minerals, which are lamellar to platy in shape and apt to undergo particle reorientation after large-displacement shear, leading to a decrease in shear strength of the residual state. Nakamura et al. believe that the total content of smectite, vermiculite, chlorite, and mica in the particle size fraction smaller than 425 micrometers is a suitable mineralogical parameter for estimating the magnitude of the residual strength. It is interesting that minute particles such as clay minerals can be one of the determining factors for the occurrence of landslides.

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Paraná flood basalts: Rapid extrusion hypothesis confirmed by new 40Ar/39Ar results
David S. Thiede and Paulo M. Vasconcelos, School of Earth Sciences, The University of Queensland, St Lucia, Queensland 4072, Australia. Pages 747-750.

The relationship between continental flood basalts (CFB) and mass extinctions has been a subject of vigorous debate. Geochronology for four of the largest CFB provinces show both rapid extrusion rates and temporal overlaps between volcanism and major mass extinction events, leading to suggestions of a causal relationship between CFB volcanism and mass extinctions. However, the comparably sized Paraná CFB in South America appears to be an exception, as no major extinction event overlaps the proposed time of volcanism. The age and duration of Paraná volcanism has been in dispute due to conflicting sets of 40Ar/39Ar geochronology data. One set of results indicates a short, rapid eruption similar to other CFBs linked to mass extinction events, and another set of results supports a longer, slower eruption sequence that would be consistent with a lack of associated extinction event. David Thiede and Paulo Vasconcelos of the University of Queensland resolve the age and duration controversy of the Paraná CFB by reanalyzing the exact samples that previously gave the oldest and youngest ages in the protracted eruption results. Their new ages indicate a short eruption event. A rapid extrusion for the Paraná CFB and the lack of a major co-temporal extinction event challenge proposed direct links between CFB volcanism and mass extinctions.

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Mélange rheology and seismic style
Ake Fagereng and Richard H. Sibson, Dept. of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand. Pages 751-754.

Deformation of Earth's crust is commonly accommodated by shear displacement on faults. This deformation occurs in a range of styles, including fast earthquake ruptures and slow aseismic slip. There is, however, much uncertainty in understanding which factors determine whether a fault will slip seismically or aseismically. Simple fault models treat faults as discrete, planar interfaces. Ake Fagereng and Richard Sibson of the University of Otago review observations in fault zones at a range of depths, in light of observations in faults exposed on Earth's surface, and suggest that some faults are rather thick layers of deformed rocks of mixed composition. If this is true, then faults may comprise strong, competent rock fragments mixed with weaker, incompetent material. The relatively incompetent rocks likely accommodate deformation by aseismic flow, while competent rocks break in faster, seismic events. It follows that the composition of a fault zone, and the relative proportions of competent and incompetent rocks, must be considered critical controls on the seismic behavior and earthquake potential of a fault.

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A new perspective on paleopiezometry: Dynamically recrystallized grain size distributions indicate mechanism changes
Michael Stipp et al., Marine Geodynamics, IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany. Pages 759-762.

Deformation of Earth's crust and mantle, ranging from seismic rupture on discrete faults to plastic flow processes in wide shear zones, is controlled by tectonic stresses. Direct stress measurements are possible in boreholes down to shallow depths in the crust; indirect measurements at greater depths can only be carried out using microstructural indicators in exhumed rocks. The most reliable and widely used stress indicator (piezometer) is the dynamically recrystallized grain size resulting from plastic flow within the deeper crust and upper mantle. Although measured in many studies on deformed rocks and also metals and ceramics, global analyses on the frequency distribution of recrystallized grain sizes are completely lacking. Michael Stipp of Germany’s IFM-GEOMAR and colleagues present the first systematic investigation of the recrystallized grain-size distribution for the mineral quartz. The distribution is strikingly discontinuous, indicating the operation of three distinct recrystallization mechanisms, which breaks new ground for theoretical conceptions of recrystallization and its stress dependence. The findings are not only relevant for quartz but for any rock-forming mineral (e.g., olivine, feldspar, calcite), for water ice, and for any crystalline solid used in the fabrication of ceramics, metals, semiconductors, or other materials.

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Shallow-marine records of pyroclastic surges and fallouts over water in Jeju Island, Korea, and their stratigraphic implications
Y.K. Sohn and S-H. Yoon, Dept. of Earth and Environmental Sciences, Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Republic of Korea. Pages 763-766.

One of fundamental questions in stratigraphy and sedimentary geology is how faithfully the geological processes are preserved in sedimentary records. On the basis of their study of a shallow-marine volcaniclastic deposit in Jeju Island, Korea, Y.K. Sohn and S-H. Yoon of Gyeongsang National University show that there can be extreme biases in Earth's stratigraphic records. The Jeju Island deposit accumulated very rapidly during a brief volcanic eruption, about a million times faster than the adjacent sedimentary strata. Because of the unusually high sedimentation rate, the volcaniclastic deposit could record the “usual” fair-weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits. This finding highlights the biases in Earth’s stratigraphic records and tells us that volcanic deposits, commonly regarded as the products of catastrophic events, can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot record.

GSA Today Science Article

Structural analysis of three extensional detachment faults with data from the 2000 Space-Shuttle Radar Topography Mission
Jon E. Spencer, Arizona Geological Survey, 416 W. Congress St. #100, Tucson, Arizona, 85701, USA

The Shuttle Radar Topography Mission (SRTM), flown in February of 2000 aboard the Space Shuttle Endeavour, acquired new data used to produce a new digital elevation model (DEM) of almost all land areas between 60 degrees N and 56 degrees S latitude. The new DEM provides very high quality topographic data for 80% of Earth's land, much of which was not well surveyed before the SRTM. The mission has given us extraordinary images associated with some recent faults, revealing domal, planar, and grooved surfaces. Some of these surfaces form remarkably smooth topography over tens to hundreds of square kilometers in regions otherwise deeply incised by rapid erosion. Here, images of three spectacular examples (from New Guinea, the Himalayas, and Indonesia) are presented, and some structural, tectonic, and geomorphic processes associated with these exhumed faults are discussed. While data from the mission have been a bonanza for geomorphologists studying extensional tectonic regimes, aspects of the behavior and morphology of the faulting, as related in particular to the geothermal gradient, remain areas of significant research opportunities.

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