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Seismic zones, river deltas, landslides, fossil reptiles, and more new Geology science

Boulder, Colo., USA - Geology posted ahead of print 14 Feb.-2 Mar. is a dynamic collection of papers covering modeling studies of the U.S. New Madrid Seismic Zone; landslide prediction through examination of the Slumgullion landslide, Colorado; investigation of a potential nuclear waste repository site in Finland; understanding river delta formation and long-term evolution with insights from the Mekong River, Vietnam; and an explanation of how drought drove forest decline and dune building in eastern upper Michigan, USA.

Highlights are provided below.

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Kinematics of the New Madrid seismic zone, central United States, based on stepover models
Thomas L. Pratt, U.S. Geological Survey, School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195, USA. Published online 14 Feb. 2011; doi: 10.1130/G32624.1.

Seismicity in the New Madrid seismic zone (NMSZ) of the central United States is generally attributed to a stepover structure in which the Reelfoot thrust fault transfers slip between parallel strike-slip faults. However, parts of the seismic zone do not fit this simple model. Comparison of the NMSZ with an analog sandbox model explains all of the arms of seismicity as only part of the extensive pattern of faults that characterizes stepover structures. Computer models show that the stepover structure may form because of differences in the trends of lower crustal shearing and inherited upper crustal faults. Thomas L. Pratt of the U.S. Geological Survey concludes that the models predict that the modern seismicity occurs only on a subset of the faults in the New Madrid stepover structure, that only the southern part of the stepover structure ruptured in the infamous 1811-1812 earthquakes, and that the stepover formed because the trends of older faults are not the same as the current direction of shearing.

Gas domes in soft cohesive sediments
Mark A. Barry et al., Department of Oceanography, Dalhousie University, 1355 Oxford Street, PO Box 15000, Halifax, B3H 4R2 Nova Scotia, Canada. Published online 14 Feb. 2012; doi: 10.1130/G32686.1.

The accumulation of gaseous methane can generate seabed domes that can in turn lead to seafloor instability, act as precursors to pockmark formation, and potentially pose a threat to seafloor drilling. The generation of these domes may also increase in frequency due to future melting of gas hydrates within the seabed, leading to increased methane release to the atmosphere. Despite their geological and practical significance, sediment gas-dome formation remains poorly understood. Mark A. Barry of Dalhousie University and colleagues conducted small-scale laboratory doming experiments with marine sediments to gain a quantitative understanding seabed doming. The domes created were well described by elastic mechanics, from which it is possible to predict the gas pressure required to create natural domes. Their results suggest that observed seabed domes require surprisingly small over-pressures to form.

Regional moisture balance control of landslide motion: Implications for landslide forecasting in a changing climate
Jeffrey A. Coe, U.S. Geological Survey, Denver Federal Center, MS 966, Denver, Colorado 80225, USA. Published online 14 Feb. 2012; doi: 10.1130/G32897.1.

The Slumgullion landslide is a thick, complex landslide in the San Juan Mountains of Colorado, USA, that has been moving for at least 300 years. Movement of landslides like Slumgullion is difficult to forecast because they incorporate a variety of materials, have many parts, and respond to long-term groundwater conditions. For Slumgullion, Jeffrey A. Coe of the U.S. Geological Survey forecasts that increased air temperatures and evapotranspiration from global warming will cause the landslide to slow in the 21st century, and that the upper part of the landslide will stop moving in the middle of the century. To make this forecast, Coe measured the movement of 18 points on the landslide for 12 years (1998-2010) and linked the observed movement to moisture balance index values from the same time period. Moisture balance index values were a measure of precipitation minus potential evapotranspiration. High moisture index values corresponded to faster landslide movement and low values corresponded to slower landslide movement. Coe used the link between movement and moisture balance, in combination with predictions of decreasing future moisture balance values, to forecast decreasing landslide movement. The moisture balance technique may be useful for forecasting the movement of thick landslides in other parts of the world.

Stress-controlled fluid flow in fractures at the site of a potential nuclear waste repository, Finland
Jussi Mattila and Eveliina Tammisto, Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland. Published online 14 Feb. 2012; doi: 10.1130/G32832.1.

Jussi Mattila and Eveliina Tammisto of the Geological Survey of Finland investigate the relationships between present-day stresses and bedrock fractures acting as conduits for groundwater at Olkiluoto, Finland. This area is designated as the potential repository for highly active nuclear waste, expected to become operational in 2020. Based on the analysis of a fracture database consisting of 38,703 fracture observations, combined with detailed stress measurements, Mattila and Tammisto conclude that fractures with distinct signs of groundwater flow display patterns that can be attributed to the effect of the present-day stress state and especially to the direction of minimum compression. In addition, based on their analysis, the highest groundwater flows are also associated with fractures having minimum compression perpendicular to the fracture planes. The results of the study are considered to yield important background information for the safety assessment of underground nuclear waste repositories.

The advent of hard-part structural support among the Ediacara biota: Ediacaran harbinger of a Cambrian mode of body construction
Erica C. Clites et al., Glen Canyon National Recreation Area, P.O. Box 1507, Page, Arizona 86040, USA. Published online 28 Feb. 2012; doi: 10.1130/G32828.1.

A team of paleontologists lead by Erica C. Clites of the Glen Canyon National Recreation Area has discovered the oldest animal with a skeleton. Called Coronacollina acula, the organism is between 560 million and 550 million years old, which places it in the Ediacaran period, before the explosion of life and diversification of organisms that took place on Earth in the Cambrian. Coronacollina acula is visualized in the fossils as a depression measuring a few millimeters to two centimeters deep. Notably, it is constructed in the same way that Cambrian sponges were constructed. The fate of the earliest Ediacaran animals has been a subject of debate, with many suggesting that they all went extinct just before the Cambrian. This discovery shows that they did not. Coronacollina acula lived on the sea floor. It is shaped like a thimble to which at least four 30-40-centimeter-long needle-like "spicules" were attached, and most likely held itself up by these spicules. Clites and colleagues believe it ingested food in the same manner a sponge and that it was incapable of locomotion. The finding provides insight into the evolution of life -- particularly, early life -- on the planet, why animals go extinct, and how organisms respond to environmental changes. The discovery also can help scientists recognize life elsewhere in the universe.

Incipient melt segregation as preserved in subaqueous pyroclasts
C. Ian Schipper et al., Institut des Sciences de la Terre (ISTO), Centre National de la Recherche Scientifi que (CNRS), Universit d'Orlans, 1a rue de la Frollerie, Orlans Cedex 2, 45071, France. Published online 28 Feb. 2012; doi: 10.1130/G32582.1.

Magma crystallization yields both the crystals themselves and a chemically evolved residual melt. Melt segregation is the process by which this residual melt may physically separate from the growing network of crystals with which it coevolved. Melt segregation occurs over many time and length scales and can explain many geologic phenomena, such as the generation of large volumes of crystal-free silicic melt and segregation structures within lava flows. C. Ian Schipper of the Universite d'Orleans, France, and colleagues present melt segregation structures of an embryonic type in subaqueous pyroclasts. These structures, termed intravesicular extrusions, are balloon-like protrusions of crystal-free basaltic glass that penetrate the walls of preformed vesicles locked in a crystal rich groundmass. Their analysis shows that extrusions represent the movement of residual melt that was enriched in fast-diffusing H2O, but not yet evolved in slower-diffusing major elements, allowing Schipper and colleagues to put time scales on the onset of melt segregation within the framework that these subaqueous pyroclasts are among the fastest-quenched natural volcanic rocks known. Their results shed light on the earliest stages of melt segregation that are normally overprinted in slower cooling magmas and indicate that segregation begins over time scales of seconds rather than the days to centuries required of phenemonologically similar processes in lava flows or magma chambers.

Origin and evolution of interdistributary delta plains; insights from Mekong River delta
Toru Tamura et al., Geological Survey of Japan, AIST (National Institute of Advanced Industrial Science and Technology), Central 7, 1-1-1 Higashi, Tsukub, Ibaraki 305-8567, Japan; and Sheffield Centre for International Drylands Research, University of Sheffield, Winter Street, Sheffield S10 2TN, UK. Published online 28 Feb. 2012; doi: 10.1130/G32717.1.

River deltas have a range of geometry resulting from the interplay of coastal processes, river discharge, and sediment supply. Almost 25% of the world's population lives on deltaic lowlands, so prediction of delta growth is critical. However, knowledge of processes responsible for delta geometries is not well established, even though such knowledge is critical for risk management of land use and settlements on deltas. The Mekong River delta of southern Vietnam is one of the largest deltas in the world and offers a unique opportunity for understanding the sedimentary evolution of a river delta that is formed by a combination of tidal and wave processes. Toru Tamura of the Geological Survey of Japan and colleagues constrained the three-dimensional sedimentary evolution of the delta plains by determining ages of landforms that record past shoreline position and subsurface delta sediment. Several delta plains of the Mekong delta did not initiate simultaneously, but propagated laterally since 3500 years ago. Delta plains formed as a result of successive emergence of bars that bifurcated the river mouth. This study by Tamura and colleagues, based on the Mekong delta, demonstrates for the first time that bar emergence is a key process in the long-term evolution of deltaic lowlands.

A new technique for identifying rock avalanchesourced sediment in moraines and some paleoclimatic implications
Natalya V. Reznichenko et al., Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Published online 28 Feb. 2012; doi: 10.1130/G32684.1.

Moraines are widely used as indicators of past climate changes, and their chronologies are crucial for reconstructing global climate-change patterns. However, glaciers do not build moraines only in response to climate disturbances. Where a large rock avalanche covers enough of the ablation zone of a glacier, the reduced ablation can cause the glacier to advance independently of climate and redeposit the rock avalanche material as a moraine before retreating again. Such moraines should clearly not be used in glacial paleoclimatic databases. Hitherto there was no reliable technique to identify rock avalanche material in moraines. Natalya V. Reznichenko of the University of Canterbury and colleagues describe a new method to distinguish rock avalanche sediment from glacially produced material based on the effects of the contrasting stress, strain, and fracture conditions in a rock avalanche and beneath a glacier. As a result of these conditions, rock avalanche sediments contain large quantities of ultra-fine fragments that form agglomerates, but neither the ultrafine particles nor the agglomerates are produced by subglacial rock comminution. Reznichenko and colleagues note that the presence of these agglomerates in a moraine shows that the moraine contains rock avalanche material, and thus may be the result of a non-climatic advance. They also tested this technique on some dated moraines in the Southern Alps, New Zealand.

Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust
Thorsten J. Nagel et al., Steinmann-Institute, Universitt Bonn, 53115 Bonn, Germany. Published online 28 Feb. 2012; doi: 10.1130/G32729.1.

Around four billion years ago, the first continents emerged through partial melting of hydrated basaltic rocks. The tectonic setting of this event is a matter of intense debate and is connected to the problem of how plate tectonics worked on the early Earth. The trace element composition of the oldest well-preserved continental fragments found in western Greenland has widely been interpreted to have originated from melting of basaltic crust at great depths in a subduction zone. Thorsten J. Nagel of Universitat Bonn, Germany, and colleagues combine thermodynamic calculations of partial melting and subsequent trace element modeling to simulate the formation of continental crust in different tectonic environments. They find the best coincidence between observed and calculated trace element compositions in model runs assuming melting of early Archean metamorphic basalts from western Greenland at much shallower depths than expected for melting in a subduction zone. They also find that these local basalts show similarities to modern arc volcanics rather than to mid-ocean ridge basalts, which would be expected to enter a subduction zone. The results presented here by Nagel and colleagues challenge the traditional view of continental crust formation via melting of normal oceanic crust in a down-going slab and support scenarios of melting within tectonically thickened, hot crust.

Drought drove forest decline and dune building in eastern upper Michigan, USA, as the upper Great Lakes became closed basins
Walter L. Loope et al., U.S. Geological Survey, Great Lakes Science Center, Munising, Michigan 49862, USA. Published online 28 Feb. 2012; doi: 10.1130/G32937.1.

The presence of thousands of (now forested) sand dunes spread across 12,000 square kilometers of eastern upper Michigan, more than 500 km east of the present forest-prairie ecotone, sounds an alarm calling for the long-accepted, pollen-based models of vegetation history in the eastern mid-continent to be reviewed. Walter L. Loope of the U.S. Geological Survey and colleagues recently used 57 optically stimulated luminescence (OSL) ages on quartz sand from stabilized dunes in eastern upper Michigan to document dune building about eight to ten thousand years ago. Noting that pollen-based models suggest that closed canopy forest covered the region throughout the last 11,500 years, Loope and colleagues show that their data require that a sharp, sustained decline in forest cover occurred between about eight to ten thousand years ago. Their signal of forest decline and dune building is roughly coincident with drought-driven hydrologic closure of the upper Great Lakes, determined by other methods. Forest openings (a minimal requirement for dune development) are rarely detected in pollen sums from eastern upper Michigan because faint signatures of non-arboreal pollen are largely obscured by abundant and highly mobile pine pollen. OSL dating of eolian sediment deposition provides a direct, spatially explicit archive of greatly diminished forest cover during the driest episode of the past 11,500 years.

Dynamic deviation of fluid pressure from hydrostatic pressure in turbidity currents
Joris T. Eggenhuisen, Dept. of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Netherlands; and William D. McCaffrey. Posted online 2 Mar. 2012; doi: 10.1130/G32627.1.

Large sediment avalanches transport gravel, sand, and mud from coastal areas into the world's deep seas and oceans. These events are difficult to predict, are destructive, and take place in the most inhospitable part of our planet (the deep marine), so we still know very little about them. In the past 40 years, researchers have assumed these flows are somewhat similar to rivers, but Joris T. Eggenhuisen Utrecht University and William D. McCaffrey of the University of Leeds present experiments that show that a critical assumption about the pressure in river flow cannot be applied to submarine currents. This changes our view of how sediment avalanches build submarine channels and slopes, and necessitates a revision of widespread modeling approaches.

How tough is tuff in the event of fire?
M.J. Heap et al., Laboratoire de Gophysique Exprimentale, Institut de Physique de Globe de Strasbourg (UMR 7516 Centre National de la Recherche Scientifi que, Universit de Strasbourg/Ecole et Observatoire des Sciences de la Terre), 5 rue Ren Descartes, 67084 Strasbourg cedex, France. Posted online 2 Mar. 2012; doi: 10.1130/G32940.1.

Tuff has been extensively used as a building material in volcanically and tectonically active areas over many centuries, despite its inherent low strength. A common and unfortunate secondary hazard accompanying both major volcanic eruptions and tectonic earthquakes is the initiation of catastrophic fires. Here, M.J. Heap of the Institut de Physique de Globe de Strasbourg and colleagues report new experimental results on the influence of high temperatures on the strength of three tuffs that are commonly used for building in the Neapolitan region of Italy. Their results show that a reduction in strength was only observed for one tuff; the other two were unaffected by high temperatures. The cause of this strength discrepancy was found to be a product of the initial mineralogical composition, or, more specifically, the presence of thermally unstable zeolites within the initial rock matrix. The implications of these data are that, in the event of fire, only the stability of buildings or structures built from tuff containing thermally unstable zeolites will suffer. Unfortunately, this includes the most widespread dimension stone in Neapolitan architecture. Heap and colleagues recommend that this information be considered during fire hazard mitigation in the Neapolitan area and that other tuffs used in construction worldwide should be tested in a similar way to assess their fire resistance.

Applying Benford's Law to volcanology
A. Geyer and J. Mart, Institut de Cincies de la Terra Jaume Almera, Consejo Superior de Investigaciones Cientfi cas, c/Lluis Sol i Sabaris s/n, 08028 Barcelona, Spain. Posted online 2 Mar. 2012; doi: 10.1130/G32787.1.

Benford's Law predicts that the distribution of first digits of real world observations is not uniform, but instead the lower digits (1, 2, and 3) occur more frequently than the higher ones (, 8, 9). It has been shown that the use of Benford's Law may help as a validity check on databases and that the first digit rule may provide new ways to detect anomalous signals in data sets. In fact, nonconformity to Benford's Law could be an indicator of (a) incompleteness, (b) excessive data round-off, or (c) data errors, inconsistencies, or anomalies. This law has long been known but has received little attention the earth sciences. In this work, A. Geyer and J. Mart of the Institut de Ciencias de la Terra Jaume Almera, Spain, first test the conformity of three volcanology-related data sets to Benford's Law: age and area of collapse calderas and the duration in days of the volcanic eruptions that occurred between 1900 and 2009 according to the Smithsonian's Global Volcanism Program catalogue. They also consider the relevance and potential utility of using Benford's Law to assess the integrity and authenticity of the presented volcanological data. The present analysis shows that excessive data round-off, data errors or anomalies may be detected when comparing the data with Benford's Law expected frequencies.

Climate change and the formation of nickel laterite deposits
Robert L. Thorne et al., School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK, and CSIRO Earth Science and Resource Engineering, Australian Resources and Research Centre, Kensington, Western Australia 6151, Australia. Posted online 2 Mar. 2012; doi: 10.1130/G32549.1.

Robert Thorne of the University of Southampton and colleagues report on a study detailing the climatic conditions necessary to form nickel laterite deposits, which account for 40% of global nickel production. Combining these conditions with climatic records from Turkey and Albania has shown that the climate was most conducive to nickel laterite deposit formation in these countries between the Late Cretaceous and Middle Eocene (70 to 45 million years ago). Nickel laterites form in climates with relatively high temperatures and rainfall, which are necessary to facilitate the intense weathering of nickel-bearing ultramafic rocks exposed at the surface. The weathering of these rocks forms laterite deposits containing relatively high concentrations of nickel. The method detailed in this study could be applied to other locations worldwide to help identify new regions for mineral exploration.

Volcano- and climate-driven changes in atmospheric dust sources and fluxes since the Late Glacial in Central Europe
Gal Le Roux et al., AGEs, Department of Geology, Lige University B18, Sart-Tilman, Alle du 6 Aot, B-4000 Lige, Belgium, and EcoLab, UMR5245 CNRSUniversit de Toulouse, campus ENSAT avenue de l'Agrobiople, 31326 Castanet-Tolosan, France. Posted online 2 Mar. 2012; doi: 10.1130/G32586.1.

Atmospheric dust is an important part of the global climate system and also plays an important role in the marine and terrestrial biogeochemical cycles of major and trace nutrient elements. A peat bog record of atmospheric deposition in Switzerland shows considerable variation in dust deposition during the past 15,000 years, with abrupt changes in fluxes. Using geochemical and isotopic tracers, Gael Le Roux of Liege University, Belgium, and colleagues show that it is possible to clearly distinguish between volcanic inputs and those driven by climate change, such as the long-term aridification of the Sahara and regional erosion due to forest clearing and soil cultivation activities. Their results indicate that a major dust event in North Africa and Europe preceded a known northern hemisphere cooling event: the cold event 8,200 years ago by 200 years. This dust event may have played an active role in the climate cooling following the 8,200-year-old event. Isotopic fingerprinting evidence also record a relatively slow change in the dust regime over Europe from 7,000 years ago to 5,000 years ago due to expansion of the Sahara. These findings show that high-resolution peat records can provide remarkably sensitive indicators of dust load and sources and can support the need to better identify the impact of dust loading and the links with direct and indirect climate feedbacks.

No gap in the Middle Permian record of terrestrial vertebrates
Michael J. Benton, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Posted online 2 Mar. 2012; doi: 10.1130/G32669.1.

New work on fossil reptiles from Russia shows a more continuous evolutionary record than had been assumed. A key concern about the fossil record is that it is incomplete. Major gaps -- times when no fossils were preserved -- can hide the detail of certain episodes in the history of life. During the Permian period 300-250 million years ago, the basis of modern terrestrial ecosystem was established. Worldwide climates became warmer and drier, and reptiles rose in importance. The first plant-eating reptiles appeared, and Late Permian ecosystems were broadly comparable to modern ones. Until recently, however, the Permian record of reptiles was said to be incomplete, with a gap of up to 5 million years. This was because paleontologists had to look at rock successions from different continents, and it seemed there was a major time hiatus between the well-known Lower Permian successions of North America and the Middle and Upper Permian successions of South Africa and Russia. New dating evidence shows that the Russian rock record overlaps the North American record, and the gap is closed. We can study the story of change in terrestrial ecosystems through the Permian without a major lack of knowledge.

Rapid erosion beneath the Greenland ice sheet
T. Cowton et al., School of Geosciences, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK. Published online 2 Mar. 2012; doi: 10.1130/G32687.1.

Erosion by ice sheets has played a key role in sculpting landscapes across large areas of Europe and North America. The rate at which these landscapes were cut is difficult to measure, but the same process can be observed through studying the modern Greenland ice sheet. Vast volumes of meltwater drain through cracks in the ice and flow along the ice sheet bed, entraining debris produced by the force of the ice on the bedrock. T. Cowton of the University of Edinburgh and colleagues measured the volume of this debris washed out from beneath a 600-square-kilometer region of the ice sheet over a two year period. This debris equated to the removal of a 5 mm layer of bedrock per year from this region, which is between 10-100 times greater than previous estimates of the rate of erosion by ice sheets. Cowton and colleagues attribute this rapid erosion to the flushing effect of the meltwater, removing debris and exposing fresh bedrock to the destructive force of the ice. The implications of this are that ice sheets are capable of changing the landscape much more rapidly than previously thought, and that this process is likely to be most effective during periods of climate warming when melting is at its greatest.

Rapid reef island formation and stability over an emerging reef flat: Bewick Cay, northern Great Barrier Reef, Australia
P.S. Kench et al., School of Environment, University of Auckland, Private Bag 92019, Auckland, New Zealand. Published online 2 Mar. 2012, doi: 10.1130/G32816.1.

The study presents new and significant scientific findings on the conditions necessary for the formation of reef islands on the western boundary of the Pacific Ocean during the Holocene. P.S. Kench of the University of Auckland and colleagues present a model that resolves reef island formation in relation to both reef platform substrate development and mid-Holocene sea level change for Bewick Island, northern Great Barrier Reef (GBR), based on morphostratigraphic analysis and radiocarbon dated island sediments and reef corals. On Bewick, elevated and fossil corals record reef flat development at higher sea level (1.5 m) by 6,500 years before present. Island building began on a partially emergent reef flat 5,000𔃂,000 years before present, when sea level was 0.5 m above present. As sea level fell to its present level, the reef platform closed the process window and the island core stabilized. Results present the first unequivocal evidence of island building directly over a reef flat comprising micro-atolls, and the first detailed model of island formation from the GBR. The model demonstrates that the interplay of sea level and reef surface elevation can vary between sites but their convergence is critical for island initiation. Future trajectories of island change will vary dependent on the unique relative sea level and substrate depth conditions that govern island formation.

What makes a volcano tick -- A first explanation of deep multiple seismic sources in ascending magma
Mark E. Thomas and Jurgen Neuberg, Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS12 9JT, UK. Posted online 2 Mar. 2012; doi: 10.1130/G32868.1.

At many volcanoes, low-frequency earthquakes have often been associated with the state of a volcanic system and have been employed for eruption prediction. Several models attempt to explain the generation of such earthquakes but fail to describe their clustering in tight spatial swarms and their highly repetitive nature. Mark E. Thomas and Jurgen Neuberg of the University of Leeds present a new model that not only explains the generation of a single event, but also accounts for the swarm behavior and cyclic activity. By considering magma rupture as a source mechanism of seismic events, they demonstrate that a change in conduit geometry is a likely cause for their generation. Their model matches the observed spatial and temporal behavior of low-frequency seismicity and contributes to the understanding necessary to provide estimates of magma ascent rates. This enables Thomas and Neuberg to provide an improved estimation of magma flow rates at depth which can now be linked directly to observed seismograms, and may lead to the development of reliable seismic forecasting tools for volcanic eruptions.

Constant cosmogenic nuclide concentrations in sand supplied from the Nile River over the past 2.5 m.y.
Michael Davis et al., " Institute of Earth Sciences, Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel. Posted online 2 Mar. 2012; doi: 10.1130/G32574.1.

Quartz sand in the eastern Mediterranean coastal plain is supplied through an extended transport system, which includes the Nile River, east Mediterranean longshore currents, and inland eolian transport. While the concentrations of cosmogenic nuclides and their ratio in modern sand deposited along the coast of the eastern Mediterranean reflect the combined effect of exposure and burial during transport, the concentrations of these nuclides in buried sands are the result of decay of this initial dosing.

Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust
John Adam et al., GEMOC, Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia. Posted online 2 Mar. 2012; doi: 10.1130/G32623.1.

The origin of Earth's earliest continental crust is a topic that has created much debate. In this study, John Adam of Macquarie University and colleagues perform high temperature and pressure melting experiments on potential continental crust source rocks from the 4.28 Ga Nuvvuagittuq Complex, Quebec. This fold belt contains the oldest known rocks discovered on the planet so far. Their results show that the chemical characteristics of the experimental melts are like early continental crust. This suggests that they may have been inherited from source rocks that were themselves the product of an earlier episode of crustal recycling. Adam and colleagues suggest that the processes are similar to modern plate tectonics.

Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading
Andrew M. McCaig and Michelle Harris, Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Posted online 2 Mar. 2012; doi: 10.1130/G32789.1.

According to Andrew M. McCaig and Michelle Harris of the University of Leeds, rates of seafloor spreading in the Pacific Ocean are fast (~10 cm/yr), such that the ocean crust has an extremely consistent layered structure. In particular, note McCaig and Harris, there is a transition from coarse grained, slowly cooled gabbros upward to fast-cooled sheeted dikes at about 1.5 km below the seafloor. This transition is controlled at the spreading ridge crest by a balance between rapid hydrothermal cooling by circulating seawater above and by slow cooling of molten magma below. Faulting plays only a minor role in this mode of spreading. In the Atlantic, rates of seafloor spreading are much slower and the ocean crust is much more heterogeneous. A new "detachment mode" of seafloor spreading has recently been discovered in which extension is controlled by movement on long-lived, convex-up detachment faults, and the degree of magmatic extension is more limited. These detachment faults raise mantle rocks and slowly cooled gabbros to be exposed on the seafloor. McCaig and Harris present evidence from the Integrated Ocean Drilling Program (IODP) in the Atlantic that a dike-gabbro transition can be located within a detachment fault and exhumed to the seafloor. Movement on the fault increases permeability and localizes hydrothermal circulation in the fault zone, and this prevents the gabbroic magma from rising to high levels. These dike-gabbro transitions are therefore controlled by a balance between hydrothermal circulation and magmatism, just as in the Pacific, but lead to very different crustal structures.


Contact: Christa Stratton
Geological Society of America

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