Boulder, Colo., USA – New Geology postings discuss a vanished link between Antarctica and Australia; the West Salton Detachment fault in California, USA; chemical interaction between peridotite and intruding melts in the Northern Apennines, Italy; calving barchan dunes; the nature of black shale in the Late Devonian Appalachian Basin; the August 2008 avulsion belt of the Kosi River, India; reef island formation; and a one-year record of eight quakes within dune deposits of the Navajo Sandstone, Utah, USA.
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The Mesozoic Victoria Basin: Vanished link between Antarctica and Australia
Frank Lisker (corresponding) and Andreas L. Läufer, Dept. of Geosciences, University of Bremen, PF 330 440, 28334 Bremen, Germany. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G33409.1.
The Transantarctic Mountains divide the Antarctic continent between the two embayments of Ross Sea and Weddell Sea into East and West Antarctica. With a total length of about 3,500 km and altitudes up to 5 km, they are one of the largest mountain chains in the world. Their origin is traditionally related to episodic uplift since 180 million years ago, with the final, most important uplift episode commencing about 65 million years ago. This concept of a long-lived morphological high constitutes the base of virtually all Gondwana reconstructions and global climate models. This study by Frank Lisker and Andreas L. Läufer demonstrates that crossover age relationships between thermochronological data and stratigraphic information contradict this established interpretation. Instead these data, together with a wealth of independent thermal indicators and geological evidence, require the existence of a vast basin that formed within Gondwana prior to the breakup of the supercontinent. Fast erosion of this "Mesozoic Victoria Basin" about 35 million years ago was isostatically compensated and triggered the uplift of the Transantarctic Mountains. The recognition of the long-lived Mesozoic Victoria Basin has primary consequences for the general understanding of the landscape of Gondwana, the breakup between Antarctica and Australia, the uplift of the Transantarctic Mountains, and global long-term climate evolution and faunal radiation.
Alternating extensional and shortening stress fields on the West Salton detachment fault, Southern California
Amy Luther (corresponding) and Gary Axen, New Mexico Institute of Mining and Technology, 801 S. Leroy, Socorro, New Mexico 87801, USA. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34404.1.
Footwall exposures of the West Salton Detachment fault (WSDF) in southern California record the tectonic transition from eastward extension to north-south strike-slip motion on the San Andreas fault. Amy Luther and Gary Axen performed paleostress inversion analyses on minor faults and fractures at several locations along the WSDF to explore the relationship between stresses and the earthquake cycle during this transition. They found that the paleostress fields alternated from extensional to compressional during this transition at one location on the fault. This change in orientation of the maximum stress direction is much larger in magnitude than those reported in studies of modified stress fields from recent earthquakes. Luther and Axen also found that the angle between the WSDF and the maximum compressive stress during extension is large, suggesting it was weak and slipped at low shear stresses.
Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability
Giulio Borghini et al., DISTAV, Università di Genova, corso Europa 26, 16132 Genoa, Italy; and Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, USA; and Dipartimento di Scienze della Terra, Università di Milano, via Botticelli 23, 20133 Milan, Italy. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34438.1.
Mantle rocks outcropping as ophiolitic terrains in mountain belts can provide important information on the mineralogical and chemical structure of Earth's upper mantle and on the processes that drive its modifications and dynamics. Giulio Borghini and colleagues studied a pyroxenite-peridotite mantle sequence from the Northern Apennines (Italy) that well preserves some of these petrologic processes that occurred at deep mantle levels. These rocks represent an excellent "geological window" to improve knowledge of the mechanisms controlling the chemical and isotopic modification of the upper mantle, potential source of MORB. High-detailed spatially controlled chemical and isotope profiles allowed Borghini and colleagues to map the extension and amplitude of mantle heterogeneities. Remarkably, they found the Nd isotopic compositional variation of a single outcrop covering most of the global Nd isotopic variability of abyssal peridotites; i.e. the upper mantle below the oceanic crust. Their investigations demonstrate that this extreme isotopic heterogeneity is the results of an ancient event (430-450 million years ago) of chemical interaction between peridotite and intruding melts at high pressure. Deep intrusions (pyroxenites) may thus significantly modify the upper mantle introducing several small-scale isotopic heterogeneities. Meter-scale domains of such modified mantle would be able to generate the entire spectrum of Nd isotopic compositions observed in MORB.
Modeling emergent large-scale structures of barchan dune fields
Stacey L. Worman et al., Division of Earth and Ocean Sciences, Nicholas School of the Environment, Center for Nonlinear and Complex Systems, Duke University, Box 90227, Durham, North Carolina 27708, USA. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34482.1.
Crescent-shaped sand dunes sprawling across vast deserts represent an iconic image, familiar even to people who have never seen such a dune in person. Fields of these "barchans" dunes form in many other environments, including sea floors and other planets. However, despite the widespread nature and familiarity of this phenomenon, even a basic understanding of how fields of barchans can exist in nature has remained elusive. Studies explaining how individual barchans grow and take on their characteristic shape lead to the prediction that individual dunes should either shrink and disappear, or grow without bound. Yet, in nature, vast collections of barchans, all with similar, apparently stable sizes somehow persist. Other recent studies have suggested that under the right circumstances, repeated collisions between slow moving dunes in a large field could explain the stability. In this paper, Stacey L. Worman and colleagues offer an alternative explanation: that the birth, or "calving," of small sand dunes from the downstream ends of large dunes alters the way sand is exchanged within a field of dunes, allowing dune sizes to stabilize. Their computer-modeling results also suggest that these relatively simple interactions between individual dunes collectively lead to the spontaneous emergence of previously enigmatic patterns within dune fields.
Basinward nitrogen limitation demonstrates role of terrestrial nitrogen and redox control of δ15N in a Late Devonian black shale
Michael L. Tuite Jr. and Stephen A. Macko, Dept. of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904, USA. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34549.1.
Depleted nitrogen (N) isotope values observed in Devonian black shales have been ascribed to the dominant role of microbial N fixation in providing this vital nutrient for the productivity that generated the organic matter preserved in these organic-rich facies. However, the emergence of substantial terrestrial ecosystems in the Middle to Late Devonian introduced an additional source of reactive N to shallow epicontinental seas. Michael Tuite and Stephen Macko examined three sites along a deepening transect of the Late Devonian Appalachian Basin employing element ratios and stable isotopes to determine whether basinward changes in N biogeochemistry reflect the dominant source of nutrient N. The sediment ratio of nitrogen to phosphorus rises basinward, indicating that the N limitation of primary production rose with increasing distance from the shore. Nitrogen isotope values, however, are very consistent despite the evidence of distally increasing N limitation. These observations suggest that terrestrially derived reactive N was as an important source of reactive N and that marine N fixation was not sufficient to address the stoichiometric deficit. Therefore, depleted N isotope values in these black shales do not provide a reliable record of N fixation; rather, they are likely a product of the oxygen-deficient state of the water column and underlying sediments.
Exploring the channel connectivity structure of the August 2008 avulsion belt of the Kosi River, India: Application to flood risk assessment
R. Sinha et al., Dept. of Civil Engineering, Indian Institute of Technology, Kanpur 208016, India. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34539.1.
The Kosi River is an important drainage of the Ganga Basin that frequently floods large tracts of North Bihar in India impacting millions of people. This river has been jacketed since 1955-1956 between embankments on both the eastern and western sides. In August 2008, a major breach of the eastern embankment at Kusaha (Nepal) resulted in an eastward shift of the Kosi by more than 100 km at its maximum near Purnea, India, and the formation of an accompanying inundation belt of 2722 square kilometers. The August 2008 avulsion (sudden shift) was the first major eastward shift of the river during the post-embankment period with 80% to 85% of the total flow entering the new course, reoccupying one the preexisting paleo-channels in the fan landscape. This research by R. Sinha and colleagues captures the topography-driven channel connectivity from Kusaha downstream into the inundation belt based on 2000 Shuttle Radar Topography Mission (SRTM) data, and compares this with the pre-flood and post-flood channel network connectivity on Landsat satellite imagery. The results demonstrate that adequate connectivity information is embedded in the digital elevation model (DEM) to forecast major flow paths and the approximate extent of the inundation belt, thereby raising the possibility of using this approach as an effective tool for flood risk assessment.
Time scales and modes of reef lagoon infilling in the Maldives and controls on the onset of reef island formation
C.T. Perry et al., Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK. Published online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34690.1. Open access.
Coral reef islands are considered to be among the most vulnerable environments to climate change on Earth (a view endorsed in the IPCC 4th Assessment). However, such prognoses of high vulnerability (and potential future loss) are based on only a cursory understanding of the developmental history and timing of reef island establishment. In this study, C.T. Perry and colleagues use data from a range of reef island sites in the Maldives to examine the relationship between coral reef growth (as a foundation for island development) and the timing of island establishment. They show that a strong relationship exists between reef size, reef lagoon infilling, and the timing of reef island initiation and expansion. These new observations, when combined with previously published data on Maldivian reef island development, suggest that whilst the lagoons of the largest reefs are unlikely to fill over the next few centuries (at least), other smaller reefs with near-infilled lagoons may provide important foci for future reef-island building, even under present high (and slightly rising) sea-levels.
Jurassic earthquake sequence recorded by multiple generations of sand blows, Zion National Park, Utah
David B. Loope et al., Dept. of Earth & Atmospheric Sciences, 214 Bessey Hall, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Posted online ahead of print on 12 Aug. 2013; http://dx.doi.org/10.1130/G34619.1.
Earthquakes don't happen in isolation, but instead occur in sequences composed of many events -- fore-shocks, main-shocks, and after-shocks. The geologic record of these sequences, however, is likely to be incomplete. Recent work carried out by University of Nebraska geologist David B. Loope and colleagues demonstrates that migrating sand dunes can record the separate events within ancient earthquake sequences. During the Jurassic period, at the edge of the North American continent in what is now Zion National Park in southern Utah, a cluster of powerful earthquakes caused water and sand to vent repeatedly to the land surface, making small "sand blows." These are conical features (recently videotaped during earthquakes in Japan and New Zealand) that form when quakes liquefy subsurface, water-saturated sands. When water and sand from one ancient quake erupted onto the slope of a sand dune, the deposit was quickly buried as winds pushed the dunes southward. As each earthquake in the ancient sequence struck, the moving dune accumulated another distinct sand-blow record, separated from the preceding one by a layer of undisturbed sand. At one site, researchers found a record of eight quakes within dune deposits of the cliff-forming Navajo Sandstone that they estimate took only a single year to accumulate. The record in rock suggests that recent earthquakes may be recorded in the loose sand of modern dunes.
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