September 19, 2012
30 New Articles Added By GEOLOGY
New Geology articles posted online ahead of print 4—18 September 2012
Highlights are provided below. Geology articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary Geology articles by contacting Kea Giles at the address above.Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geology in articles published. Contact Kea Giles for additional information or assistance.
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Large normal-sense displacement on the South Tibetan fault system in the eastern Himalaya
F.J. Cooper et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Posted online 4 Sept. 2012; 10.1130/G33318.1.
The Himalayan mountain range is Earth's most dramatic example of continent-continent collision. As expected, the majority of faults within the range are contractional, stacking up thrust sheets as the topographic front develops. However, the range also contains a family of extensional faults, the South Tibetan fault system (STFS), that does not fit typical models of contractional mountain building. The importance of this extensional deformation in the development of the Himalayas is unclear because geoscientists do not know exactly for how long the STFS has been active or how much displacement it has accommodated. In this study, F.J. Cooper and colleagues integrate ASTER remote sensing data and field observations to map the areal extent of the extensional STFS in northwestern Bhutan and estimate a minimum displacement across it of approx. 65 km. This measurement is comparable to displacement estimates on the major thrust faults of the eastern Himalaya, and suggests that normal faulting has played a fundamental role, perhaps as important as thrust faulting, in the evolution of the mountain range. These results are consistent with the idea that the STFS and the Main Central thrust system to the south collectively sustained Miocene southward extrusion of the metamorphic core of the Himalaya.
Glaciation of North America in the James Bay Lowland, Canada, 3.5 Ma
Cunhai Gao et al., Ontario Geological Survey, Sudbury, Ontario P3E 6B5, Canada. Posted online 4 Sept. 2012; doi: 10.1130/G33092.1.
Glaciers expanded in the Northern Hemisphere three to four million years ago (Ma) during the middle Pliocene. However, given a climate much warmer than today during this geological episode, a key question remains unanswered: Did glaciers extend into the midlatitude lowlands or were they restricted to the circum-arctic continents and mountains like today? Knowing whether extensive glaciations occurred during the middle Pliocene is critical for a better understanding of Earth's climate history and the cause of amplified ice growth and decay beginning in the late Pliocene (3 to 2.6 million years ago). A record obtained recently from a buried, deep bedrock trench in the James Bay Lowland, Canada (52°49.5'N, 83°52.5'W) indicates the deposition of till by an early ice sheet, and hence continental glaciation, in the midlatitude lowlands about 3.5 million years ago (3.6 to 3.4 million years) on the basis of paleomagnetism measurements and a pollen-derived biostratigraphy. After the glaciation, rapid warming permitted warmth-loving trees now exotic to the area to grow, including oak, hickory, sweetgum, and cypress. Furthermore, pollen data from the overlying lake sediments indicate alternating Carolinian deciduous and boreal evergreen forests under a climate that oscillated and cooled gradually during a prolonged postglacial period from 3.5 to 3.0 million years ago.
Forecasting catastrophic stratovolcano collapse: A model based on Mount Taranaki, New Zealand
Anke V. Zernack et al., INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand. Posted online 4 Sept. 2012; doi: 10.1130/G33277.1.
The life cycles of andesite stratovolcanoes are characterized by episodic edifice growth and regular large-scale collapse. This behavior has dominated the past 130,000 years of volcanic activity at Mount Taranaki, where 14 catastrophic edifice failures have produced debris avalanche deposits of up to 7.5 cubic kilometers. The timing and magnitudes of these failures are uncorrelated with Last Glacial climate fluctuations and gradual changes in magma composition. Furthermore, similar debris avalanche deposit characteristics suggest similar proto-edifice structures, collapse trigger mechanisms, and runout path characteristics. Since internal and external conditions can be considered constant over long periods of time, large-scale collapses appear to be governed primarily by the magma-supply rate. Based on this conclusion, Anke Zernack and colleagues developed a simple steady-state volume-frequency model that allows the definition of natural edifice stability limits and forecasts of both the maximum likely event scenarios and decadal or millennial likelihoods for the most hazardous process known at stratovolcanoes. In the Mount Taranaki case, the maximum potential size of a collapse at present is estimated to be 7.9 cubic kilometers, while the maximum interval before the next collapse is less than 16.2 thousand years. The current annual collapse probability is around 0.00018 with the most likely collapse being a small one (less than two cubic kilometers).
Provenance and tectonic evolution of Ganderia: Constraints on the evolution of the Iapetus and Rheic oceans
Cees R. van Staal et al., Geological Survey of Canada, 625 Robson Street, Vancouver, British Columbia V6B 5J3, Canada. Posted online 4 Sept. 2012; doi: 10.1130/G33302.1.
The provenance of the microcontinents Ganderia and Avalonia in the northern Appalachians, the time of their rift-drift, and the tectonic mechanisms responsible for their Early Paleozoic transfer from West-Gondwana to ancestral North America (Laurentia) provide estimates for the width, timing, and rates of opening and closing of the Early Paleozoic Iapetus and Rheic oceans. Opening of the Iapetus Ocean was estimated to have taken place at a rate between 2 and 5 cm per year, whereas the Rheic Ocean opened at a rate of about 9 cm per year in a back-arc position, following the rifting and drifting of Ganderia and Avalonia away from Gondwana towards Laurentia. As a result, the active, leading edge of Ganderia accreted to Laurentia about 455 million years ago, closing the main tract of the Iapetus Ocean. The drift of the trailing edge of Ganderia and Avalonia had slowed down to approx. 5 cm per year after 470 million years ago due to opening (approx. 4 cm per year) of a backarc basin, which may have achieved a width of 800 km. Closure of the latter back-arc basin led to accretion of the Gander margin to Laurentia by about 430 million years ago. Opening and closing of the Iapetus and Rheic oceans was largely driven by slab pull and slab rollback.
Allophane detection on Mars with Thermal Emission Spectrometer data and implications for regional-scale chemical weathering processes
E.B. Rampe et al., NASA Johnson Space Center, Houston, Texas 77058, USA; and Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831, USA. Posted online 4 Sept. 2012; doi: 10.1130/G33215.1.
E.B. Rampe and colleagues model regional thermal-infrared data from the martian surface with poorly crystalline aluminosilicate weathering products, allophane, and high-SiO2 gel, in the spectral library. Models suggest that allophane and gel are present in several low-albedo martian surfaces. The presence of allophane indicates that these surfaces experienced low-temperature chemical weathering at low water-to-rock ratios and neutral to mildly acidic pH. Previous models of chemical weathering on Mars have suggested that acidic alteration has occurred on a global scale for the past 3.5 billion years. However, the presence of allophane indicates that mildly acidic to neutral chemical weathering has been an important regional-scale aqueous process on Mars.
Tracking the relict bases of marine methane hydrates using their intersections with stratigraphic reflections
Richard J. Davies et al., Centre for Research into Earth Energy Systems (CeREES), Dept. of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK. Posted online 4 Sept. 2012; doi: 10.1130/G33297.1.
Methane hydrates trapped in sediment in oceans can melt due to changes in pressure and temperature, but the hard evidence for this in our oceans is scarce. Being able to track episodes of methane hydrate melting is very important because they could be caused by previous episodes of global warming and in turn release greenhouse gases into the atmosphere. Understanding the link between methane hydrate melting and global warming has significant scientific and societal importance. Richard Davies and colleagues show that seismic reflection data can essentially take a photo of the past positions of the base of the methane hydrate -- therefore recording episodes of methane hydrate melting. In the dataset we present the bases of the ancient hydrates have left a regular pattern of semi-circular concentric rings in the sediment -- reminiscent of tree rings -- but these are on a giant scale extending for 20 km along the African ocean margin. Their discovery means geoscientists may be able to test whether there are interdependencies between methane hydrate melting and climate change.
Climatic backdrop to the terminal Pleistocene extinction of North American mammals
Victor J. Polyak et al., Earth & Planetary Sciences, University of New Mexico, 200 Yale Boulevard, Northrop Hall, Albuquerque, New Mexico 87131, USA. Posted online 4 Sept. 2012; doi: 10.1130/G33226.1.
The cause of extinction of the large mammals that once roamed across the North American continent during the Pleistocene is part of a long, ongoing debate. Victor Polyak and colleagues report on evidence for a terminal Pleistocene drought in the southwestern United States from the study of Fort Stanton Cave stalagmites and Carlsbad Cavern shelfstone, New Mexico, USA, starting just before 14,500 years ago and extending to the Younger Dryas around 12,900 years ago. Uranium and carbon isotope values (delta-234U and delta-13C) from 14,500 years ago to the top of the stalagmite 11,400 years ago rose abruptly and significantly, supporting the drying conditions that halted stalagmite growth. Shelfstone that rims (now dry) pool basins in the Big Room of Carlsbad Cavern represent periods when the basins were full to the brim. All shelfstone samples analyzed are older than 13,500 years ago, also supporting drying conditions at the terminal Pleistocene. The high-resolution record obtained by Polyak and colleagues indicates that thousands of years of dry climate must have taken a toll on the large mammals in southwestern and other regions of North America. They suggest that this drought was extensive, and in combination with the prior intense glacial period, was a major factor in the extinction.
Sulfur isotope evidence for a Paleoarchean subseafloor biosphere, Barberton, South Africa
N. McLoughlin et al., Dept. of Earth Science and Centre of Excellence in Geobiology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway. Posted online 4 Sept. 2012; doi: 10.1130/G33313.1.
The Archean sub-seafloor has been proposed as an environment for the emergence of life with titanite microtextures in pillow lavas of the Barberton Greenstone Belt South Africa, argued to be the earliest traces of microbial micro-tunneling. In this paper, N. McLoughlin and colleagues test the geochemical evidence associated with these approx. 3.45 billion-year-old candidate traces of life. Previously reported carbon linings in the titanite microtextures could not be found, despite the high-sensitivity of the nano-scale ion microprobe (NanoSIMS) mapping technique. This lack of organic linings in both the type- and fresh drill core samples raises questions about the biogenicity of the microtextures and their formation by Archean endolithic microbes. However, independent geochemical evidence comes from sulfide grains discovered in these samples that exhibit pronounced sulfur isotope fractionations and points to sulfate reducing microbes inhabiting the Archean sub-seafloor. Thus, McLoughlin and colleagues show that in-situ sulfur isotope analysis provides an alternative approach to investigating the possibility of an Archean subseafloor biosphere that is independent of the titanite microtextures.
A subseafloor carbonate factory across the Triassic-Jurassic transition
Sarah E. Greene et al., Dept. of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA. Posted online 4 Sept. 2012; doi: 10.1130/G33205.1.
The end-Triassic mass extinction is associated with a paucity of preserved carbonate -- a possible consequence of ocean acidification. This study by Sarah Greene and colleagues examines three Triassic-Jurassic marine sections which contain unique early diagenetic carbonate, i.e., carbonate that grew within the sediment soon after deposition. Diagenetic features are "secondary" and therefore commonly ignored because they obscure the primary sedimentary record and are presumed to contain no useful information about the overlying ocean environment. The team's examination of these unusual carbonate features reveals that they grew just below the sediment-water interface (likely under acidic conditions) nearly concomitant with primary sediment deposition. Thus, carbonate burial in the shallow sub-seafloor, a previously unheralded pathway in the carbon cycle, is a carbonate sink of unknown size and may be a predictable consequence of ocean acidification, whereby carbonate precipitation first returns within the sediment before recovering in the water column.
A 1600 yr seasonally resolved record of decadal-scale flood variability from the Austrian Pre-Alps
Tina Swierczynski et al., GFZ German Research Centre for Geosciences, Section 5.2, Climate Dynamics and Landscape Evolution, Telegrafenberg, D-14473 Potsdam, Germany. Posted online 4 Sept. 2012; doi: 10.1130/G33493.1.
A flood time series reaching 1,600 years back into the past, for the first time at seasonal resolution, is reported from Lake Mondsee, an annually laminated sediment record in Austria. A novel methodological approach based on microscopic sediment analyses allowed Tina Swierczynski and colleagues to separate spring/summer floods from local mass movements in the sediment record. The flood time series exhibits a pronounced decadal-scale variability with a series of six 30-50-year periods of high flood activity evidencing a non-stationary behavior of flood frequencies. The observed flood episodes culminated around the middle of the first millennium A.D. and the first half of the last millennium. Flood frequencies during the last century remained moderate. No simple relationship between temperature and extreme hydro-meteorological events is seen, because flood frequency was lowest during both the warmest period of the Medieval Climate Anomaly and the coldest decades of the Little Ice Age, whereas most floods occurred during transitional phases of climatic cooling. This is likely a consequence of changes in atmospheric circulation patterns and their regional effects in the northeast European Alps. These results imply that anticipation of future flood frequencies under global warming remains a complex challenge necessitating consideration of potential changes in large-scale atmospheric circulation.
Long-term growth of the Himalaya inferred from interseismic InSAR measurement
RaphaÃ«l Grandin et al., Ecole Normale SupÃ©rieure, UMR 8538, F-75231 Paris, France. Posted online 18 Sept. 2012; doi: 10.1130/G33154.1.
The Himalayan range is a major tectonic plate boundary that slowly accumulates strain and occasionally breaks during large earthquakes that release immense amount of elastic energy. Yet, the mountain range continues to grow, suggesting that a fraction of the tectonic deformation accumulates irreversibly in the long term. Using satellite-borne radar data, Raphael Grandin and colleagues measure the strain accumulating across the Himalaya in a period devoid of major earthquake. They show that contemporary uplift in Central Nepal accumulates to the south of the region of long-term uplift. Their observations bring support to a model of mountain growth involving a flux of crustal material across the thrust plate interface. The underlying Indian plate abandons sheets of rocks below the Himalayan domain as it gets underthrusted, thus sustaining the long-term growth of the mountain range.
Unraveling the complexity of deep gas accumulations with three-dimensional multimodal CARS microscopy
Robert C. Burruss et al., U.S. Geological Survey, Reston, Virginia 20192, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33321.1.
Coherent Anti-Stokes Raman Scattering (CARS) microscopy is an important method for label-free, molecule-specific imaging in the life sciences. CARS is a nonlinear optical analog of Raman spectroscopy but is more than 10,000 times more sensitive. Robert C. Burruss and colleagues present the first application of CARS microscopy to the geosciences. This non-invasive method is not only molecule-specific (e.g., methane, carbon dioxide, water, higher hydrocarbons, etc.) but also allows for rapid recording of vibrational Raman spectra (showing pressure effects, hydrate formation, etc.), even in highly fluorescing samples such as crude oil inclusions. The simultaneous recording of second harmonic (revealing crystallographic features) and two-photon fluorescence (revealing aromatic hydrocarbons) images allows -- for the first time -- their correlation with molecule-specific imaging. As an illustration, Burruss and colleagues apply CARS microscopy to the study of fluid inclusions in deeply buried sedimentary basins. This allowed them to identify separate gas sources, timing of generation, migration, and mixing, as well as potential contributions from the deep crust or upper mantle. There are significant implications for other broad areas of geosciences including mineralogy, paleobiology, and geochemistry. Burruss and colleagues believe that these broadly applicable imaging methods will potentially transform the characterization of geological materials.
Archean mantle fluids preserved in fibrous diamonds from Wawa, Superior craton
Evan M. Smith et al., Dept. of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada. Posted online 18 Sept. 2012; doi: 10.1130/G33231.1.
Evan M. Smith and colleagues studied fluid inclusions in 2.7-billion-year-old fibrous diamonds. Fibrous diamond is an impure variety of diamond with a high content of micron-sized fluid inclusions. This variety is the only type of diamond that routinely carries fluid inclusions, and is therefore of great importance for understanding mantle fluids. However, most fibrous diamonds were formed relatively recently in earth history, probably less than 500 million years ago. Thus, the fluids in fibrous diamonds studied to date are also relatively young. This is the reason the Archean age of the samples presented in this study is significant. Smith and colleagues´ findings show that the Archean diamond fluids are potassium-, sodium-, and chlorine-rich and bear a strong resemblance to fluids in younger diamonds. The resemblance suggests a uniformitarian view that the mantle processes that created the Archean fibrous diamonds were the same as those that have operated more recently. Strontium isotopes strengthen this view and point toward a convecting mantle or kimberlite-like source for the fluid. Additionally, trace element measurements show that the chlorine-rich nature of the fluid makes it capable of accumulating europium and creating mantle "europium anomalies" that may be mistaken as a signature from shallower, crustal rocks.
Tracking halogens through the subduction cycle
Mark A. Kendrick et al., School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. Posted online 18 Sept. 2012; doi: 10.1130/G33265.1.
This study by Mark Kendrick and colleagues tracks the movement of volatile elements like iodine and chlorine, during the subduction of tectonic plates beneath arc volcanoes, such as those that form the circum-Pacific "Ring of Fire." The fate of iodine during subduction is of special interest because it is an essential element for life that exists in very few geologic materials. Iodine dissolved in seawater is incorporated into organic matter and the mineral serpentine that forms close to the seafloor. It was originally assumed that volatile elements, like iodine, are efficiently removed from subducting plates and returned to Earth's surface through volcanism, and that Earth's oceans originally formed by volcanic outgassing of the mantle. In contrast, new data presented by Kendrick and colleagues demonstrate that a limited proportion of seawater-derived volatiles bypass arc volcanoes and are transferred to the deeper mantle during plate subduction. The presence of seawater-derived iodine in the mantle supports the recent view that modern seawater has a complex origin and may have been extensively cycled through Earth's mantle by the forces of plate tectonics over much of Earth's 4.5 billion year history.
Nile Delta´s sinking past: Quantifiable links with Holocene compaction and climate-driven changes in sediment supply?
Nick Marriner et al., CNRS, CEREGE UMR 7330, EuropÃ´le de l´Arbois, BP 80, 13545 Aix-en-Provence cedex 04, France. Posted online 18 Sept. 2012; doi: 10.1130/G33209.1.
The Nile Delta hosts about two thirds of Egypt's population and 60% of the country's food production. Global warming and human impacts have sharpened focus on the delta's potential resilience to present and future changes. Nick Marriner and colleagues use data from more than 100 cores to quantitatively reevaluate the drivers of Nile Delta surface dynamics and sediment mass balance during the Holocene. Their reconstruction demonstrates that over the past 8,000 years, two important contributors to changes in Nile Delta mass balance have been sediment compaction and orbitally forced changes in sediment supply. Between 8,000 and 4,000 years ago, spatially averaged sedimentation rates were greater than subsidence, meaning that delta aggradation was the dominant geomorphological process at the regional scale. By contrast, during the past 4,000 years, a sharp climate-driven fall in Nile sediment supply, coupled with the human-induced drainage of deltaic wetlands, has rendered the delta more sensitive to degradation by sea-level rise and extreme flood events. Although the current predicament of the Nile delta -- starved of significant sediment input since the completion of the Aswan High Dam -- is without precedent for the Holocene, these new mass balance data suggest that the Nile delta has a particularly long history of degradation by climate-driven changes in sediment delivery and human impacts.
Tectonic erosion in a Pacific-type orogen: Detrital zircon response to Cretaceous tectonics in Japan
Kazumasa Aoki et al., Dept. of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan. Posted online 18 Sept. 2012; doi: 10.1130/G33414.1.
In order to constrain the Cretaceous putative tectonic erosion in Japan, we conducted high-resolution U-Pb dating of detrital zircons of the Cretaceous sandstones and metasandstones from southwestern Japan by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These results suggest that the tectonic erosion likely occurred twice in the Cretaceous subduction-related margin around Japan.
Lake sediments record cycles of sediment flux driven by large earthquakes on the Alpine fault, New Zealand
Jamie D. Howarth et al., Dept. of Geography, University of Otago, P.O. Box 56, Dunedin, New Zealand. Posted online 18 Sept. 2012; doi: 10.1130/G33486.1.
In mountain environments, large earthquakes are important drivers of erosion because they trigger extensive landsliding, which can be hazardous for society and infrastructure. However, the magnitude and duration of enhanced landsliding triggered by large earthquakes is not well understood because the long recurrence times between events generally preclude any direct observation of landscape responses. Jamie Howarth and colleagues present a new approach for addressing this issue, using lake sediments to reconstruct the duration and magnitude of the landscape response to earthquakes on the Alpine fault, New Zealand. Their data show that an elevated sediment flux from catchments adjacent to the Alpine fault persists for approximately five decades after each of the last four large earthquakes. These post-seismic landscape responses were responsible for 27% of the erosion in the lake catchment over the past 1,100 years. Howarth and colleagues conclude that Alpine fault earthquakes are one of the most important drivers of erosion in the Southern Alps and that the response to these events represents considerable delayed hazard that persists long after an earthquake.
Reconstructing Greenland ice sheet runoff using coralline algae
Nicholas A. Kamenos et al., School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK. Posted online 18 Sept. 2012; doi: 10.1130/G33405.1.
The Greenland Ice Sheet (GrIS) contains the largest store of fresh water in the northern hemisphere, equivalent to about seven meters of sea level rise. Currently available GrIS melting records are about 50 years long and reveal that melting has increased since the records began. However, to understand if this recent melting, and subsequent runoff into the sea, is part of a natural see-saw melting pattern or if it represents a sudden increase in melting rates, records spanning longer than 50 years are needed. At present, these do not exist. Nicholas Kamenos and colleagues have developed a technique that allows them to reconstruct historic melting rates of the GrIS by determining past changes in the salinity of Greenland´s seas: the more the GrIS melts, the less salty the seas become. They demonstrate this technique by reconstructing melt from part of the GrIS since 1939 and observe that since the mid-1980s, there has been a pronounced increase in melting and runoff, possibly in response to atmospheric warming. This technique will enable the production of longer melt records to understand how recent melting rates compare to historic melting rates and what is driving the melting patterns.
U-Pb zircon crystallization age of the Muslim Bagh ophiolite: Enigmatic remains of an extensive pre-Himalayan arc
M. Ishaq Kakar et al., Centre of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan. Posted online 18 Sept. 2012; doi: 10.1130/G33270.1.
India wasn't always an Asian superpower, when the dinosaurs roamed the planet, an ocean separated India from Asia. Very little of this ocean remains, but on the Pakistan-Afghanistan border, the largest piece of this ocean (named Tethys) is preserved. New ages from rocks from crystallized magmas in this piece of Tethys date, for the first time, the age of this ocean to 80 million years old. The chemistry of these rocks also shows that they formed as India traveled north and the ocean began to be subducted.
The interface-scale mechanism of reaction-induced fracturing during serpentinization
Oliver PlÃ¼mper et al., Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, Blindern, N-0136 Oslo, Norway. Posted online 18 Sept. 2012; doi: 10.1130/G33390.1.
The hydration of mantle olivine to form serpentine (serpentinization) is arguably the most important metamorphic hydration reaction on Earth as it has fundamental repercussions on geophysical and geochemical processes. As serpentinization relies on the addition of water fluid pathways through an originally impermeable rock need to be created. Although these pathways can be created by tectonic stresses there are ample indications that the reaction itself can fracture the rock. Until now, however, the mechanism linking processes at the reaction interface with fracture propagation that allows km-scale areas of the Earth's uppermost mantle to be serpentinized has not been fully understood. By combining evidence from the microstructural characteristics of olivine serpentinization with theoretical considerations, Oliver Plumper and colleagues have identified a microstructurally consistent, self-propagating fracturing mechanism. They suggest that the chemical reaction of olivine to serpentine is powerful enough to fracture an original impermeable rock.
Flemish Cap-Goban Spur conjugate margins: New evidence of asymmetry
Joanna Gerlings et al., Dept. of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada. Posted online 18 Sept. 2012; doi: 10.1130/G33263.1.
Joanna Gerlings and colleagues present the results of deep seismic surveys across the Flemish Cap-Goban Spur conjugate margin pair, which they use to infer rifting style and breakup. Profiles on both margins extend into oceanic crust, hence making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust. Their results indicate that asymmetric structures are formed during all stages of rifting, breakup, and complex transition to oceanic spreading. This result contrasts with a previous interpretation of the conjugate margin pair´s rifting style, which was in support of symmetric rifting. The differing nature of the two ocean-continent transition zones is particularly striking. For Flemish Cap, the results show a wide region of highly thinned continental crust. In contrast, results for the Goban Spur transition zone indicate a primary composition of exhumed serpentinized mantle.
Revised conceptual model for maar-diatremes: Subsurface processes, energetics, and eruptive products
Greg A. Valentine and James D.L. White, Dept. of Geology, 411 Cooke Hall, University at Buffalo, Buffalo, New York 14260, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33411.1.
Maars are volcanic craters cut into the landscape, caused by highly energetic, explosive interactions between magma and groundwater or surface water. The craters are surrounded by low profile rings of volcanic ejecta deposits and underlain by funnel-shaped bodies of debris known as diatremes. An existing, widely accepted model for maar-diatremes assumes that magma-water explosions begin at shallow levels and move downward during the volcano's lifetime, resulting in ejection of material from progressively deeper levels in the subsurface. Greg A. Valentine and James D.L. White propose a new model in which explosions occur at a range of depths throughout an eruption. Only shallow explosions eject material, while deeper explosions do not erupt but cause mixing of deep and shallow materials within the diatreme.
Widespread loess-like deposit in the Martian northern lowlands identifies Middle Amazonian climate change
James A. Skinner Jr. et al., Astrogeology Science Center, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33513.1.
Identifying packages of rock and sediment in the Martian northern lowland that clearly illustrate the type and timing of geologic events through time is tenuous, despite their covering nearly one-third of the planet´s surface. By mapping and measuring topographically subtle landforms, we identify a previously unrecognized sedimentary unit in the Martian lowlands that post-dates more widely recognized planar rocks and sediments by more than 2 billion years. The new unit, named the Middle Amazonian lowland unit (mAl), forms scattered and low-relief patches, is defined by subtle marginal scarps, has mean average thickness of 32 meters, and occurs in the northern mid- to high-latitudes of Mars. Craters located atop topographic promontories and nested arcuate ridges surround unit mAl, indicating current outcrops are vestiges of a more extensive deposit. James A. Skinner Jr. and colleagues propose the unit accumulated at the same time, and at the expense of, the erosion of layered ice and sediment that makes up the north polar plateau. Ice volume estimates correspond to global water volume thicknesses of 14-91 cm and 72-460 cm for modern and past unit mAl extents, respectively. This work connects ancient sedimentary processes to climate change scenarios that occurred before those implied by current orbital models.
Linking orography, climate, and exhumation across the central Andes
Jason B. Barnes et al., Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33229.1.
Reconstructing the co-evolution of mountain uplift and orographic rainfall patterns is important for understanding the dynamic interactions between Earth's surface and climate. This new study by Jason Barnes and colleagues illustrates how this is becoming possible using a novel combination of techniques, including climate modeling and isotope data that reconstructs a landscapes' history of deformation and erosion. Barnes and colleagues suggests that central Andes topography reached threshold elevations (greater than 75% modern) necessary to trigger the onset of precipitation patterns similar to today's rain shadow (wet on the east, dry on the west) about 15 to 11 million years ago. This improved history provides an important new context for understanding the evolution of the Andes and the Amazon basin, the most biologically diverse region on Earth.
Crustal structure and rheology of the Longmenshan and Wenchuan Mw 7.9 earthquake epicentral area from magnetotelluric data
Guoze Zhao et al., State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China. Posted online 18 Sept. 2012; doi: 10.1130/G33703.1.
The Longmenshan (LMS) in the eastern margin of the Tibetan Plateau is anomalous since it has formed despite low convergence and slip rates and without the development of a foreland basin. The devastating 2008 Wenchuan (Mw = 7.9) earthquake has renewed debate about the tectonics of the LMS. A magnetotelluric (MT) study reveals a high conductivity layer (HCL) at a depth of ~20 km beneath the Eastern Tibetan Plateau that terminates ~25 km west of the LMS faults, which is fluid-rich and mechanically weak. Beneath the LMS, a high resistivity zone extends through the entire crust, but with a zone of low resistivity in the vicinity of the Wenchuan hypocenter. MT data, combined with other geological and geophysical observations, support geodynamic models for the uplift of Eastern Tibet being caused by southeast directed crustal flow that is blocked by stable lithosphere beneath LMS leading to inflation of the Eastern Tibetan Plateau. This rigid high resistivity backstop not only provided a block to flow, but also may have accumulated stress prior to the earthquake. The MT observations provide new insights into the generation of Wenchuan earthquake, which occurred in a region with low convergence rates prior to the earthquake.
Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
R. Sutherland et al., GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand. Posted online 18 Sept. 2012; doi: 10.1130/G33614.1.
Scientific boreholes into geological faults that have ruptured during recent earthquakes reveal a damaged zone of fractured rock around the principal slip surface, but little is known of how faults look just before an earthquake. To address this question, a team of international scientists drilled into the Alpine Fault in southern New Zealand, where there is a 30% chance of a magnitude 8 earthquake during the next 50 years, according to Berryman et al. 2012 (Science). New scientific-drilling results from New Zealand show that the Alpine Fault has repeatedly shattered during earthquakes, then healed, then shattered again. The fault is currently in its fully-healed pre-earthquake state. The chemical reactions between rock and water that heal the fault have created a low-permeability alteration zone that is suggested to play a fundamental role in the mechanics of fault movement and earthquakes. The low-permeability rock in the Alpine Fault is a barrier that stops water passing through it, and also creates a pressure chamber around the fault. If water pressure within the fault were to build up for some reason, water could not escape and the pressure would stay high. Increased water pressure weakens the fault and may cause slip, which causes the water pressure to increase even further. This feedback mechanism means that once a fault rupture starts, it may grow into a large earthquake that releases all of the energy stored in surrounding rock. One intriguing suggestion from the drilling is that the Alpine Fault may be a massive subterranean dam, holding back high-pressure water within adjacent mountains. The water pressure drop across this dam is comparable to stress drops during earthquakes, and may, therefore, be an important factor in earthquake nucleation that has not been fully considered.
Rock uplift rates in South Africa from isochron burial dating of fluvial and marine terraces
Erica D. Erlanger et al., Dept. of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA. Posted online 18 Sept. 2012; doi: 10.1130.G33172.1.
Although southern Africa has not experienced active tectonics since the breakup of Gondwana more than 100 million years ago, it has a very high continental interior and a steep escarpment. Some have argued that this high and steep topography indicates ongoing uplift. To test this idea, Erica Erlanger and colleagues use a new method based on isotopes produced by cosmic rays to date a set of river terraces and a raised beach on the South African coast. They show that uplift over the past several million years has been slow and steady, consistent with the persistence of high topography over 100 million year time scales.
Mild Little Ice Age and unprecedented recent warmth in an 1800 year lake sediment record from Svalbard
William J. D´Andrea et al., Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33365.1.
The Arctic region is currently undergoing large changes due to anthropogenic global warming. Anticipating future changes requires understanding how the arctic climate system has behaved in the past. William D'Andrea and colleagues generated a summer air temperature record for the west coast of Svalbard, Norway (High Arctic). The temperature record is based on trans-fat molecules (alkenones) that are produced by algae in the lake and are preserved in lake sediments. Volcanic ash layers from known eruptions were used to determine the age of the lake sediments; neither approach had previously been used at such a high latitude site. The results indicate that the summer warmth of the past 50 years, recorded by both man-made thermometers and the alkenones, was unmatched in Svalbard in the course of the past 1,800 years, including during the so-called "Medieval Warm Period." Furthermore, the Little Ice Age (LIA) summers of the 18th and 19th centuries on Svalbard were not particularly cold, even though glaciers occupied their maximum Holocene extent. The results suggest that increased wintertime snowfall, rather than cold temperatures, was responsible for LIA glaciations on Svalbard and that increased ocean heat transport into the Arctic via the West Spitsbergen Current began around 1600 A.D.
Late glacial fluctuations of Quelccaya Ice Cap, southeastern Peru
Meredith A. Kelly et al., Dept. of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA. Posted online 18 Sept. 2012; doi: 10.1130/G33430.1.
During the last glacial-interglacial transition, approx. 18,000-11,000 years ago, there were rapid, millennial-scale climate changes in both the Northern and Southern Hemispheres. However, there are relatively limited data as to how these changes affected tropical regions. What happened in the tropics is important for defining the mechanisms that influenced these climate events. Meredith A. Kelly and colleagues provide a precise radiocarbon chronology of the re-advance and rapid recession of Quelccaya Ice Cap, located in southeastern Peru, during the last glacial-interglacial transition. These new data demonstrate that the southern tropical Andes experienced a rapid change from relatively warm and dry conditions to colder and wetter conditions between 12,800 and 12,400 years ago. The timing of this cold/wet event overlaps with, but does not correlate directly to, rapid climate changes documented in the middle-to-high Northern and Southern latitudes. However, the warming about 12,400 years ago at Quelccaya is contemporaneous with a slight warming over the Greenland Ice Sheet and in the northern tropical Atlantic Ocean.
Twentieth-century warming revives the world's northernmost lake
Bianca B. Perren et al., Laboratoire Chrono-Environnement, UMR CNRS 6249, UniversitÃ© de Franche-ComtÃ©, 25030 BesanÃ§on cedex, France. Posted online 18 Sept. 2012; doi: 10.1130/G33621.1.
This paper by Bianca Perren and colleagues examines evidence of recent climate change and atmospheric pollution from the northernmost lake in the world, Kaffeklubben SÃ¸, on the north coast of Greenland. Lake sediment cores show that siliceous algae (diatoms and chrysophytes) were initially present in the lake 3,500 years ago but disappeared around 2,400 years ago with colder conditions, permanent lake ice cover, and inferred sub-zero local summer temperatures. At the same time, local glaciers readvanced, landfast sea ice became permanent, and the northernmost culture in the world (Independence II Paleoeskimo culture) disappeared from the area. Starting around 1980 A.D., diatoms and chrysophytes reappear in the lake in unprecedented numbers, which track rising summer temperatures. However, Perren and colleagues find no evidence for atmospheric pollution, which suggests that recent climate change is solely responsible for the profound biological changes underway in North Greenland. This paper underscores the importance of biological thresholds in response to climate change (both cooling and warming) and adds an important geographic and climatic end member to our understanding of ongoing climate change
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