May GEOLOGY media highlightsBoulder, Colo. - Topics in the May issue of GEOLOGY include: origins of the Afar hotspot in the horn of Africa; hypervelocity impacts and demagnetization of geologic materials; discovery of splash plumes; sulfate minerals and the search for Martian biomolecules; Permian sedimentary rocks in Kansas as a Martian analog; new insights into North American plate boundaries; dynamics of glacier formation in Oligocene Antarctica; fossil land snails and origins of a 2.6-million-year-old loess; and atmospheric nitrogen and the nitrogen cycle in the Phanerozoic.
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Gravitational potential energy of the Tibetan Plateau and the forces driving the Indian plate
Attreyee Ghosh, Department of Geosciences, State University of New York, Stony Brook, NY 11794, USA; et al. Pages 321-324.
What drives the Indian plate? It is well known that the Tibetan Plateau, the highest plateau in the world, was created by the collision between the Indian and Eurasian plates. However, geologists still don't fully understand the gigantic force or forces that are behind the massive collision. In their study, Ghosh et al. show that the lithospheric contribution of stress, such as ridge push, falls short of explaining the stress field across Tibet and the Indian plate, and they conclude that driving basal tractions from mantle flow is a major cause in driving India into Tibet.
Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?
J. Brendan Murphy, St. Francis Xavier University, Earth Sciences, Antigonish, Nova Scotia B2G 2W5, Canada; et al. Pages 325-328.
There is growing evidence that a series of supercontinents have assembled and dispersed over the last 3 billion years. The youngest of these supercontinents, Pangea, formed about 300 million years ago and its dispersal over the past 200 million years resulted in the formation of the Atlantic Ocean. Although the existence of Pangea is a cornerstone of plate tectonics, geologists still do not understand the mechanisms responsible for its amalgamation. Scientists know when and where, but not why and how. One of the chief obstacles is the lack of understanding of the origin and evolution of an ocean called the Rheic Ocean, which formed about 490 million years ago. The demise of the Rheic Ocean was associated with terminal continental collision and the formation of Pangea. Murphy et al. propose that the Rheic Ocean rift may have been located along pre-existing weak structures in the crust along the Gondwanan margin. Murphy et al. compare the isotopic characteristics of terranes rifted from Gondwana with adjacent regions that remained along the margin. The terranes rifted from Gondwana have isotopic characteristics indicating that they originally represent recycled oceanic lithosphere formed between ~1,000 and 750 million years ago that were accreted to the Gondwanan margin about 650 million years ago. Murphy et al. suggest that the rift that formed the Rheic Ocean was located near the boundary between the accreted terranes with oceanic heritage and the old margin of continental northern Gondwana. This conclusion has important implications for geodynamic models of the formation of oceanic crust.
Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot
Margaret H. Benoit, Pennsylvania State University, Department of Geosciences, University Park, PA 16802, USA; et al. Pages 329-332.
The Afar hotspot region, located in the horn of Africa, is the site of extensive volcanism, continental rifting, and plateau uplift. The origin of the hotspot is controversial, and the volcanism, rifting, and uplift have commonly been attributed to one or more starting mantle plumes in the mantle beneath this area. Recently, however, some geologists have suggested that the hotspot may be part of the African Superplume. Benoit et al. provide new constraints on the origin of the hotspot using data recorded by the Ethiopia Broadband Seismic Experiment, a network of seismic stations that were operated by Pennsylvania State University between 2000 and 2002. Results from a P-wave travel time tomographic study suggests that a broad (> 500 kilometers wide) and deep-seated (at least 400 kilometers deep) upwelling exists beneath the Afar hotspot. These results are not consistent with a starting mantle plume, but suggest that the hotspot may be part of the African Superplume.
Investigating impact demagnetization through laser impacts and SQUID microscopy
Jérôme Gattacceca, CNRS/Univ. Aix-Marseille 3, Geophysics and Planetology, CEREGE, BP 80, Aix-en-Provence Cedex 4,13545, France; et al. Pages 333-336.
Hypervelocity impacts are phenomena of major importance for the evolution of the solid bodies of the solar system. In particular, they may play a crucial role in the magnetic records of many extraterrestrial bodies. Impact demagnetization is invoked to explain specific features of the crustal magnetization of Mars, the moon, and asteroids. Gattacceca et al. describe an innovative approach for investigating the effects of impacts on the magnetization of geologic materials. It consists of the combination of pulsed laser impacts and high-resolution magnetic measurements with a spatial resolution of 100 micrometers. Gattacceca et al. present shock modeling and magnetic field data obtained for two laser impacts on a magnetite-bearing basalt sample. They obtain a continuous relation between the demagnetization intensity and the peak pressure suffered by the sample. This promising technique will allow for the investigation of the demagnetization behavior of a variety of geological materials upon impacts, with implications for scientific understanding of the magnetization of extraterrestrial materials and of terrestrial impact structures.
Correlation of lithotectonic units across the eastern Himalaya, Bhutan
Andy Richards and Nigel Harris (corresponding author), The Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, MK7 6AA, UK; et al. Pages 341-344.
Pinpointing the driving forces behind the uplift of mountains requires careful analysis of the sedimentary detritus deposited by rivers that rise amongst the mountains. Most of the eroded material from the Himalayan Mountains is carried southward by two rivers, the Ganges and the Brahmaputra, into the offshore fan in the Bay of Bengal. Drill core recovered from the fan can potentially provide information on which parts of the mountains were eroding when, and how fast this was occurring over the past 17 million years. Until now, the interpretation of this core has been based solely on what is known about the bedrock eroded from the mountains by the Ganges and its tributaries, despite the fact that the Brahmaputra contributes most of the deposited sediment. The study of Richards et al. redresses this imbalance by examining the isotope geochemistry of bedrock in eastern Himalayan tributaries of the Brahmaputra, where the monsoon is most intense. These data, combined with the first published ages for the deposition of the sediments, allow the lithologies of Bhutan to be correlated westward. For the first time, the offshore record can now be properly interpreted for the entire Himalayan mountain range.
Are splash plumes the origin of minor hotspots?
J.H. Davies, Cardiff University, School of Earth Ocean and Planetary Science, Cardiff, Wales CF10 3YE, UK; and H.-P. Bunge, Munich University, Geosciences, München, Bavaria D-80333, Germany. Pages 349-352.
Through numerical experiments of earth-like mantle convection, using very large computer clusters, Davies and Bunge have discovered an exciting new class of hot upwellings--splash plumes. Although notable regions of volcanism away from plate boundaries, such as Hawaii, are explained by many as resulting from hot cylindrical upwelling mantle originating from a hot layer, that explanation does not easily explain smaller volcanic provinces. These newly discovered splash plume upwellings do not originate from a hot layer, but as small fingers from hot bowls in the middle of the mantle. They can therefore provide a possible mechanism to explain the minor regions of volcanism such as Eifel (Germany), Massif Central (France), and Darfur (Sudan).
Does Kamchatka belong to North America? An extruding Okhotsk block suggested by coastal neotectonics of the Ozernoi Peninsula, Kamchatka, Russia
Joanne Bourgeois, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; et al. Pages 353-357.
The 20 April 2006 earthquake in far eastern Russia once again prompts the question: Where are the plate boundaries of North America? This major thrust earthquake, and the one that occurred in this region in 1969, are difficult to explain if the region is monolithic North America. For some years, an alternative view has suggested smaller plates such as the Bering block (Mackey et al., 1997) and the Okhotsk block or plate (Cook et al., 1986). The nature of this fundamental problem of plate tectonics was recently discussed in a December 2004 workshop sponsored by the National Science Foundation (http://pangea.stanford.edu/research/structure/nerussia/index.html). Recent research by Bourgeois and her colleagues in the Russian Far East, as reported in GSA Bulletin (March/April 2006) and Geology (May 2006) tends to support a model in which the Bering Sea, Okhotsk Sea, and the Kamchatka-Kuril region do NOT belong to the North American plate. Despite 40 years of plate tectonics studies, some plate boundaries remain poorly defined. The most outstanding of these is the boundary between North America and Eurasia in northeastern Russia-sometimes called the last frontier of plate tectonics. How does this plate boundary evolve from spreading in the Atlantic and Arctic to convergence in Siberia? Tectonic plate relationships are hard to picture on many maps because the northern polar region is either distorted or omitted. The simplest model, and the one presented in many textbooks and websites, assigns northeastern Russia, from Chukotka down through Kamchatka all the way to Hokkaido, to the North American plate. Typically, the data used for defining plate boundaries have been earthquake history and, more recently, GPS measurements. However, these records in the Russian Far East are quite short, so paleoseismology and neotectonics can contribute to the puzzle. This Geology paper documents evidence of uplift, tilting, and convergence on the Bering Sea coast of Kamchatka, contradicting the single-plate model. Moreover, historic and paleoseismic records provide evidence for recurring tsunami-producing thrust earthquakes offshore of the Ozernoi Peninsula, the most recent a Mw 7.7 earthquake in 1969. A multiplate model in which an eastward-moving Okhotsk block, including most of Kamchatka, is converging with a clockwise-rotating Bering block explains these observations better than the unbroken North American plate model.
Sulfate minerals and organic compounds on Mars
Andrew Aubrey, Scripps Institution of Oceanography, Geosciences Research Division, La Jolla, CA 92093-0212, USA; and Jeffrey L. Bada (corresponding author), University of California–San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093-0212, USA; et al. Pages 357-360.
The search for evidence of life on Mars involves the detection of organic molecules considered to be associated with either extinct or extant life. The recent finding of abundant sulfate minerals on Mars suggests that these minerals might be prime targets in the search for organic compounds. Aubrey et al. have determined the total organic carbon (C) and nitrogen (N) content, the stable C and N isotopic composition, and the amine and amino acid concentrations in 5 terrestrial sulfate minerals with ages up to 30 million years old. The interpretation of the results fits a model of amino acid degradation, and this model is used to estimate the degradation half-lives of amino acids in sulfate minerals on Mars. The Aubrey et al. model indicates that the degradation of amino acids in sulfate minerals is slow enough to be preserved in the sulfate mineral rock record on Mars for periods of at least 1 billion years. Thus, sulfate minerals provide excellent targets in the search for biomolecules derived from extinct or extant life on Mars.
A high-resolution late Holocene lake isotope record from Turkey and links to North Atlantic and monsoon climate
Matthew D. Jones, University of Nottingham, School of Geography, Nottingham, Nottinghamshire NG7 2RD, UK; et al. Pages 361-364.
Water is a politically sensitive resource in the eastern Mediterranean, and understanding long-term hydrological variability is therefore important for the sustainable management of regional water resources. Jones et al. provide the first high-resolution record of hydrological variability in the eastern Mediterranean through the last two millennia, showing that climatically driven changes in regional water balance would have had significant impact on water availability in the region in the past. Comparisons with adjacent regions indicate that Mediterranean climate is linked to changes in northern European and Indian monsoon climate systems throughout the last 2000 years.
Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust
John D. Fitz Gerald, Australian National University, Research School Earth Sciences, Canberra, ACT 0200, Australia; et al. Pages 369-372.
This contribution by Fitz Gerald et al. about strength in Earth's lithosphere relies, not on modeling, but on direct observation of rocks exhumed from deep shear zones. These fine-grained deformed rocks have been carefully characterized by backscattered electron diffraction and by light and electron microscopy. The key conclusion is that low water content and high strength keep parts of the mid-to-lower crust strong, even at the high temperatures characteristic of such depths.
Lateral periodic variations in the petrophyscial and geochemical properties of dolomite
David A. Budd, University of Colorado, Geological Sciences, Boulder, Colorado 80309-0399, USA; et al. Pages 373-376.
Dolomite rocks have been intensely researched for decades due to their importance as repositories of earth history and the economic resources (particularly oil and gas) that they contain. These rocks form as a complete replacement of limestones and thus represent an intriguing example of one of the many large-scale chemical mass transfers that occur within Earth's outermost crust. Prior geologic studies of dolomites have used representative but spatially uncorrelated samples. Budd et al. examine whether that traditional sampling strategy bypassed a hidden world of additional information. Budd et al. sampled at a regular 30 centimeter spacing for nearly 150 meters through a single layer of dolomite rock. This had not been done before. These samples were then analyzed for a number of physical and geochemical properties. The authors found very distinct oscillations in abundance of each property, with periods of 1.2–7.6 meters. A number of potential processes may explain this signal, and the best explanation is as yet unknown. Sorting out the origin and meaning(s) of these patterns represents a new approach to the study of dolomites. Understanding the lateral distribution of properties can also improve models of fluid flow in dolomite petroleum reservoirs as well as contaminant transport between matrix and conduits in dolomite aquifers.
Evidence for an earliest Oligocene ice sheet on the Antarctic Peninsula
Linda C. Ivany, Syracuse University, Earth Sciences, Syracuse, NY 13244, USA; et al. Pages 377-380.
The first appearance of glacial ice on Antarctica arguably marked the most significant climate transition on our planet since the extinction of dinosaurs, and heralded the beginning of today's "icehouse" world, which is characterized by more-or-less permanent polar ice caps. Geologists know that significant ice first appeared on Antarctica about 34 million years ago in response to changing greenhouse gases and ocean circulation, but had thought that those first glaciers were confined only to the eastern part of the continent. Ivany et al. describe rocks exposed on Seymour Island, just off the coast of the Antarctic Peninsula (western Antarctica), that they argue record this initial onset of glaciation. They recognized sediments that are typical of those carried along and left by ice, including till (a mixture of pebbles, sand, and mud that accumulates underneath glaciers as they flow) and dropstones (large rocks carried by glaciers, which then calve to become icebergs). Dropstones eventually melt out of icebergs once they reach the sea and fall into the muddy sediment on the sea bottom below. The tiny marine microfossils Ivany et al. found in the mud around the dropstones only lived about 34 million years ago. Their discovery is important because it suggests that ice covered the whole of Antarctica at this time, not just the eastern part, indicating that climate cooled fast enough to allow the growth of glaciers everywhere on the continent at the same time. This threshold response to progressively changing conditions makes clear that climate can shift rapidly and dramatically to a new state, and perhaps provides a harbinger for future change if we continue to alter our environment.
A Martian analog in Kansas: Comparing Martian strata with Permian acid saline lake deposits
Kathleen C. Benison, Central Michigan University, Geology, Mt. Pleasant, MI 48859, USA. Pages 385-388.
Permian (270 million years old) sedimentary rocks in Kansas that formed in extremely acid salt lakes are strikingly similar to strata recently documented on Mars. These similarities include similar gypsum, chloride, hematite, and quartz mineral assemblages. Sedimentary structures such as ripple marks, cross bedding, and mud cracks are present in both the Kansas rocks and the Martian rocks. Early alteration features found in both rocks are evaporite crystal molds and hematite concretions. These Kansas rocks may be the best-known terrestrial analog for Martian rocks and, therefore, give clues about past Martian environments.
What controls thickness of sediments and lithospheric deformation at a pull-apart basin?
Alexey Petrunin and Stephan V. Sobolev (corresponding author), GeoForschungsZentrum–Potsdam, Potsdam, Brandenburg D-14473, Germany. Pages 389-392.
Pull-apart basins are depressions that are formed as a result of crustal extension at transform faults and are often a locus of distractive earthquakes. The outstanding classic example of a seismogenic pull-apart basin is the 150-kilometer-long Dead Sea basin, which is located at the Dead Sea Transform and where more than 8 kilometers of sedimentary cover has accumulated during the last 15 million years. A three-dimensional thermomechanical modeling reported in this paper shows that the major parameter controlling the basin length, thickness of sediments, and deformation pattern beneath the basin is the thickness of the brittle layer, which is in turn controlled by the temperature and composition of the lithosphere. The model makes several predictions concerning crustal structure and surface heat flow at the Dead Sea, which will be soon tested by a new international geophysical project focused at the origin of the Dead Sea.
New pieces to the puzzle of reconstructing sediment paleofluxes from river dune deposits
Suzanne F. Leclair, University of Ottawa, Department of Earth Sciences, Ottawa, ON K1N6N5, Canada. Pages 401-404.
Modern and ancient river deposits reflect the variation of river flow conditions and the successive deposition/erosion events in time and space, which are strongly coupled to climate change and/or human impact. Leclair sets the basis of a new approach for the quantitative interpretation of fluvial deposits. A model to reconstruct the probability distribution of dune-bed surface elevation from the preserved stratigraphy is proposed and applied to ancient fluvial-dune deposits. The results are potentially important for a wide variety of geoscience studies.
Wind-blown origin of Dongwan late Miocene-Pliocene dust sequence documented by land snail record in western Chinese Loess Plateau
Fengjiang Li, Institute of Geology and Geophysics, Chinese Academy of Sciences, Cenozoic Geology and Environment, Beijing 100029, China; et al. Pages 405-408.
Fossil land snails are the most common fossil remains in loess deposits, and they have played an important role in early studies of the wind-blown origin of a 2.6-million-year loess. Li et al. found that land snail fossils preserved in the Dongwan loess-soil sequence, which was deposited in the time interval of about 7.1-3.5 million years ago, are highly comparable to those in the 2.6-million-year loess in the Chinse Loess Plateau. The close similarity of mollusk records in both the Dongwan deposits and the 2.6-million-year loess sequence in Loess Plateau supports that the Dongwan section is a typical loess-paleosol sequence, and is, without any doubt, of wind-blown origin.
Multidecadal drought and Holocene climate instability in the Rocky Mountains
Jeffery R. Stone (corresponding author) and Sherilyn C. Fritz, University of Nebraska, Geosciences, Lincoln, NE 68588, USA. Pages 409-412.
Fossil algae recovered from lake sediments were used to reconstruct fluctuations in lake levels from a lake in the northern Rocky Mountains. Statistical analyses of the recurrence of low lake-levels, interpreted to represent periods of severe drought, show two distinct patterns over the past 13,000 years. The main pattern begins occurring about 11,000 years ago and is characterized by severe drought repeated regularly over thousands of years, with drought cycles lasting roughly 50 to 70 years. These drought patterns recur with a frequency similar to a modern large-scale ocean-atmosphere phenomenon known as Pacific Decadal Oscillation, which may be partially responsible for the climate patterns observed. This pattern of drought recurrence dominates the sediment record from this lake until about 4,500 years ago, at which point the recurrence of severe drought appears to stop regularly recurring at these frequencies for nearly a thousand years. From 3,500 years ago until today, the dominant pattern of drought has appeared to oscillate between these two modes, periodically returning to the strongly cyclic drought patterns. These two modes of drought recurrence in the northern Rocky Mountains may tell us something about the strength and consistency of Pacific Decadal Oscillation throughout the last 13,000 years, particularly from about 9,500 years ago until about 4,500 years ago, when the recurrence of drought was extremely consistent and the climate was warmer than it is today.
Geological nitrogen cycle and atmospheric N2 over Phanerozoic time
Robert A. Berner, Yale University, Geology and Geophysics, New Haven, CT 06520-8109, USA. Pages 413-415.
Using the average nitrogen/carbon ratio (N/C) of sedimentary organic matter, coal, and volcanic/metamorphic gases, existing models of the carbon cycle have been used to calculate the fluxes of nitrogen to and from the atmosphere via the weathering, burial, and metamorphic/volcanic decomposition of organic matter. Results indicate no appreciable change, less than one per cent of that present, in the mass of atmospheric N2 over Phanerozoic time. This is in sharp contrast to atmospheric O2, which shows notable variation. Variation of the mass of O2, but not that of N2, must have led to variation in total atmospheric pressure with time. Results are not altered by considering N/C ratios ten times higher than the average for sedimentary rocks, nor by considering nitrogen release from silicate weathering. The residence time of N2 in the atmosphere, relative to exchange with and storage in crustal rocks, is estimated to be about one billion years.
To view the complete table of contents for the May issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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