|7 September 2011
GSA Release No. 11-57
Director of Education, Communication, & Outreach
October 2011 Geology Highlights
New research posted 2 September
Boulder, CO, USA - Topics in the October GEOLOGY include the mineralogical findings that indicate the possibility of water on Mars as recently as 2-2.5 billion years ago, instead of 4 billion years ago as previously believed; study of the 1944 Tonankai (M8.2) earthquake area by Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 316; and new EarthScope Project seismic observations in the U.S. Basin and Range province.
Keywords: Noctis Labyrinthus, Mars, China, Daly gap, Southwest Indian Ridge, sand dunes, Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 316, Tibetan Plateau, Western Mediterranean arc, Ronda peridotite, high-temperature rhyolites, Idaho, Amundsen Sea, West Antarctic Ice Sheet (WAIS), Pine Island Glacier basin, Japan, Basin and Range province, pyroclastic density currents, Canadas volcano, Tenerife, Abona landslide, Arabian-Nubian Shield, Israeli-Jordanian Paleozoic section, secondary ion mass spectrometer, Barberton granite greenstone terrane, South Africa, aragonite/calcite sea conditions, EarthScope, Barracuda Ridge, Tiburon Rise, Central Asia, SPICE event, trilobites
Highlights are provided below. Representatives of the media may obtain complimentary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Christa Stratton for additional information or assistance.
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Diverse mineralogies in two troughs of Noctis Labyrinthus, Mars
Catherine M. Weitz et al., Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, Arizona 85719, USA; doi: 10.1130/G32045.1.
Two small depressions on Mars found to be rich in minerals that formed by water could have been places conducive to life relatively recently in the planet's history. The locations at Noctis Labyrinthus, near the western end of the Valles Marineris canyon system, show many kinds of minerals, more than just about anywhere else on Mars, that formed by water activity. The observed minerals indicate that water varied in pH level in time, in one trough from acidic to neutral, and in the other trough from neutral to acidic and back to neutral. The clays identified in the troughs, called iron/magnesium (Fe/Mg) smectites, are much younger at Noctis Labyrinthus than those found in the ancient rocks on Mars, which indicates a different, more habitable water environment in these troughs relative to what was happening elsewhere on Mars. Catherine M. Weitz of the Planetary Science Institute and colleagues assert that this could mean the presence of liquid water on Mars, and a potentially habitable environment, as recently as 2-2.5 billion years ago, instead of 4 billion years ago as previously believed.
Formation of high delta18O fayalite-bearing A-type granite by high-temperature melting of granulitic metasedimentary rocks, southern China
Hui-Qing Huang et al. (Xian-Hua Li, corresponding), State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; doi: 10.1130/G32080.1.
The genesis of A-type granites has been controversial. Fayalite granite is a member of the most reduced A-type granites, commonly thought to have been primarily sourced from tholeiitic rocks. Hui-Qing Huang of the Chinese Academy of Sciences and colleagues report petrography, whole-rock geochemistry, Sr-Nd isotope, and in-situ zircon Hf-O isotope results for a fayalite-bearing A-type granite suite at Jiuyishan in southern China. Based on research presented in this paper, Huang and colleagues emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity (fO2), low fH2O, and high temperature.
Large-scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the Daly gap
Bernard Charlier et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA; doi: 10.1130/G32091.1.
Although it is well established that crystallization of basalt may lead to formation of silica-rich liquids such as granite, intermediate compositions are conspicuously rare at Earth's surface. Bernard Charlier of MIT and colleagues present the first evidence for large-scale liquid immiscibility in a large terrestrial basaltic magma chamber, which has direct bearing on this controversial issue. Furthermore, identification of silicate immiscibility as a significant mechanism of magmatic differentiation also has implications for the dynamics of magma chambers in general, and is of considerable importance in the interpretation of mineralogical and compositional features described at the surface of other terrestrial planets and asteroids by space-based exploration.
From slow to ultra-slow: How does spreading rate affect seafloor roughness and crustal thickness?
Daniel Sauter et al., Institut de Physique du Globe de Strasbourg, IPGS-UMR 7516, CNRS, Université de Strasbourg/EOST, 1 rue Blessig, 67084 Strasbourg cedex, France; doi: 10.1130/G32028.1.
Many of the differences between slow and ultra-slow spreading ridges are attributed to the difference in spreading rate, but how sure are scientists that spreading rate plays such an important role? Their understanding of seafloor character, based on global data compilations using widely spaced sampling, predicts dramatic differences between seafloor generated at ultra-slow spreading ridges and seafloor generated at slow, intermediate, and faster rates. Other factors generally not taken into account, such as mantle temperature, can profoundly affect the character of the seafloor. So how sure are scientists that this ultra-slow critical limit exists? To find out, Daniel Sauter of Université de Strasbourg/EOST and colleagues test the relationship of seafloor roughness and crustal thickness before and after a well-constrained transition from slow to ultra-slow spreading at the Southwest Indian Ridge. At this same ridge, they are also able to examine how seafloor character changes with progressively cooler mantle temperature. Their findings suggest that mantle temperature may play a more important role in determining seafloor morphology than spreading rate, and that mantle upwelling beneath ultra-slow spreading ridges may be more focused than at slow or intermediate rates.
Surface moisture-induced feedback in aeolian environments
Joanna M. Nield, School of Geography, University of Southampton, Highfield, Southampton SO17 1BJ, UK; doi: 10.1130/G32151.1.
Sand dunes develop as sand transported by wind interacts with the surface over which it is moving. In coastal and semi-arid areas (e.g., Great Sand Dunes National Park, USA), the amount of water on the surface influences how well the wind can move sand. Over short time scales, this moisture helps the wind to form accumulations of wet and dry sand bands on beaches. As these bands migrate, further accumulation occurs, forming the beginning of a larger dune. On a much longer time scale, in areas where there are well-defined wet and dry seasons (e.g., Brazilian north coast), water may form pools between large dunes in the wet season, leaving behind a dune imprint in the dry season. These dune imprints can tell scientists something about climate and dune migration rates, helping to unlock the secrets of past climate change. Joanna M. Nield of the University of Southampton and colleagues include both the short- and long-term influence of surface water in dune environments in a computer simulation model for the first time, the outputs of which highlight the significance of the duration of sand transport and surface interaction, and the importance of considering surface moisture influence in sandy environments.
Episodic seafloor mud brecciation due to great subduction zone earthquakes
Arito Sakaguchi et al., IFREE (Institute for Frontier Research on Earth Evolution), JAMSTEC (Japan Agency for Marine-Earth Science and Technology), 3173-25 Showa, Kanazawa-ward, Yokohama 236-0001, Japan; doi: 10.1130/G32043.1.
Many submarine faults are developed at seismogenic zones in plate subduction zones. The Nankai Trough off southwest Japan has an approximately 1300-year historical record of great earthquakes, including the most recent 1944 Tonankai (M8.2) earthquake. Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 316 drilled and cored several holes in a shallow portion of the offshore Tonankai earthquake area, including sites in the hanging wall of large splay fault that has previously been interpreted to rupture co-seismically during mega-thrust earthquakes. X-ray computed tomography scanning revealed that the uppermost core at one site contains repeated occurrences of mud-breccia. Radioisotope dating of the uppermost mud-breccia indicates a deposition time consistent with the 1944 Tonankai earthquake, suggesting that the mud-breccia layers result from episodic brecciation caused by seismic shaking. Mud brecciation provides a potential new tool to reconstruct ancient earthquake history in subduction zones.
Fragments of hot and metasomatized mantle lithosphere in Middle Miocene ultrapotassic lavas, southern Tibet
Chuan-Zhou Liu et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; doi: 10.1130/G32172.1.
How and when did the Tibetan Plateau achieve such a high altitude, which influences global climate? Many models already have been proposed by earth scientists to answer this question. Chuan-Zhou Liu of the Chinese Academy of Sciences and colleagues report their work on peridotite xenoliths trapped by the Middle Miocene (17 million years old) ultrapotassic rocks in Sailipu, southern Tibet. Garnet has not been observed in Sailipu peridotite xenoliths, implying a thin lithosphere (i.e., less than 80 km). Compositions of these xenoliths suggest that the mantle lithosphere has been highly metasomatized by hydrous melts. The estimated temperatures indicate that the mantle lithosphere was very hot during the Middle Miocene. All these features support the theory that the thickened mantle lithosphere beneath southern Tibet has been significantly thinned, probably through the convecting removal mechanism. The convecting thinning process resulted in the upwelling of the hot asthenosphere, which finally caused the uplift of the Tibetan Plateau. The study also substantiates the previously proposed link between the plateau uplift and the Cenozoic ultrapotassic rocks.
Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean
Carlos J. Garrido et al., Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC and UGR, Avenida de las Palmeras 4, 18100 Armilla, Granada, Spain; doi: 10.1130/G31760.1.
Uplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc, are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination- or rollback-driven stretching. In the Ronda peridotite (southern Spain), the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel mylonite domain in this massif. Carlos J. Garrido of Spain’s Instituto Andaluz de Ciencias de la Tierra and colleagues report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel mylonite domain of the Ronda peridotite.
Strontium isotopes and magma dynamics: Insights from high-temperature rhyolites
John A. Wolff et al., School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA; doi: 10.1130/G32062.1.
Laser ablation analysis of feldspar crystals in high-temperature rhyolites from Idaho reveals that their strontium isotope ratios do not vary internally. This is a surprising result, because most volcanic rocks emplaced through old, isotopically variable continental crust show internal isotopic heterogeneity due to crustal contamination while crystals are growing. John A. Wolff of Washington State University and colleagues show that these magmas have an unusual combination of physical properties that promotes internal isotopic uniformity. The crystals are held within the viscous magma until any initial isotopic heterogeneity is eradicated by strontium diffusion. In contrast, other magmas (e.g., common rhyolites and dacites) are either too cold for diffusion to be effective, or so dynamic due to low melt viscosity (e.g., basalts) that crystals are rapidly transferred between different strontium isotopic environments within the magma system.
Holocene stability of the Amundsen-Weddell ice divide, West Antarctica
N. Ross et al., School of GeoSciences, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK; doi: 10.1130/G31920.1.
Satellite data show that the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is experiencing increased ice flow, thinning, and marginal retreat. To fully understand the significance of recent changes, however, the longer-term behavior of the WAIS must be determined. N. Ross of the University of Edinburgh and colleagues use ice-penetrating radar and GPS to establish the past and present-day ice flow configuration in the upper Pine Island Glacier basin, interior WAIS. The pattern of ice flow in this part of the WAIS has remained unchanged for at least 7,000 years, with the boundary between the basins of Pine Island Glacier and the Institute Ice Stream (the Amundsen-Weddell ice divide) experiencing little or no migration. Glacial geological evidence from the Ellsworth Mountains supports these results, and suggests that the Amundsen-Weddell ice divide is a long-term recurrent feature of the ice sheet. Ross and colleagues provide a long-term context for the pronounced changes occurring in other parts of the WAIS. Numerical models have previously suggested that these changes may propagate inland, whilst recent observations suggest that some WAIS ice divides are already experiencing divide migration and thinning. They show that the Amundsen-Weddell divide has not experienced significant divide migration during the past 7,000 years.
Permeability anisotropy of serpentinite and fluid pathways in a subduction zone
Seiya Kawano et al., Dept. of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan; doi: 10.1130/G32173.1.
Subduction zones are the only sites where water is transported into Earth's deep interior. Water that is transported into the mantle affects the melting temperature and mechanical strength; consequently, the presence of water is believed to influence arc volcanism and earthquake generation. Subducting plates release most of their water into the mantle wedge via dehydration reactions, and the expelled water is generally believed to ascend under buoyancy. However, it is possible that fluid movement is influenced by anisotropic permeability along the subducting plate interface. In order to test this hypothesis, Seiya Kawano of Hiroshima University and colleagues performed fluid flow experiments of highly sheared serpentinites, in directions parallel and perpendicular to the foliation. The experimental results revealed a significant permeability anisotropy, in which fluid flow parallel to the foliation is an order of magnitude or more higher than that normal to the foliation. This strong anisotropy in permeability results in preferential fluid migration along the subducting plate interface, rather than vertical movements into the mantle wedge. The calculated fluid velocity (7 cm per year) is faster than the subducting plate velocity in southwest Japan (~4 cm per year), and slower than that of northeast Japan (~10 cm per year). This implies that water can be returned to the surface in southwest Japan, but further transported to deeper mantle in northeast Japan, affecting the genesis of arc magmatism in Japan.
Diffuse Pacific-North American plate boundary: 1000 km of dextral shear inferred from modeling geodetic data
Tom Parsons and Wayne Thatcher, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/G32176.1.
Geodetic measurements tell scientists that the eastern part of the Basin and Range province expands in an east-west direction relative to stable North America, whereas the western part of the province moves to the northwest. In this study, Tom Parsons and Wayne Thatcher of the U.S. Geological Survey develop 3-D finite element representations of the western United States lithosphere in an effort to understand the global positioning system (GPS) signal. The models are constrained by known bounding-block velocities and topography, and Basin and Range province deformation is represented by simple plastic (thermal creep) rheology. Parsons and Thatcher show that active Basin and Range spreading by gravity collapse is expected to have a strong southward component that does not match the GPS signal. They also reconcile the gravitational component of displacement with observed velocity vectors if the Pacific plate applies northwest-directed shear stress to the Basin and Range via the Sierra Nevada block. This effect reaches at least 1000 km east of the San Andreas fault in their models.
Effects of volcano profile on dilute pyroclastic density currents: Numerical simulations
Greg A. Valentine et al., Dept. of Geology & Center for GeoHazards Studies, University at Buffalo, 411 Cooke Hall, Buffalo, New York 14260-1350, USA; doi: 10.1130/G31936.1.
Explosive volcanic eruptions can produce fast-moving, hot, damaging pyroclastic flows (composed of volcanic gasses and particles). A major goal of volcanology research is to be able to predict the behavior of these flows, technically termed "pyroclastic density currents" or PDCs. Greg A. Valentine of the University at Buffalo and colleagues report computer simulations that take into account complex, interacting processes such as the drag between PDCs and the ground, mixing with the atmosphere, and loss of volcanic particles through sedimentation, to explore the effects of different types of volcano profiles on a certain type of PDC. For example, with volcanoes with straight profiles (slopes that do not vary with height), PDCs initially accelerate, but then gradually decelerate as they move downslope because as they lose particles, the flows become less dense. PDCs that move down concave slopes can accelerate greatly on upper, steep slopes, and maintain their particle load farther down the volcano flanks. These factors promote high damage potential for the flows. PDCs on convex slopes lose much of their particle load on the relatively flat upper slopes and travel much slower toward the lower volcano flanks. Valentine and colleagues suggest that systematic studies of such processes can greatly aid our ability to mitigate PDC effects.
Large eruption-triggered ocean-island landslide at Tenerife: Onshore record and long-term effects on hazardous pyroclastic dispersal
Pablo Davila Harris et al., Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Queretaro 76230, Mexico; doi: 10.1130/G31994.1.
A Quaternary debris-avalanche deposit, caused by a large landslide on Canadas volcano, Tenerife (Canary Islands), records the onshore component of the 733,000 (plus or minus 3,000) year-old Abona landslide. Pablo Davila Harris of UNAM and colleagues present three lines of evidence that show that the avalanche was triggered by an explosive eruption: (1) the deposit is enclosed by ignimbrites and is draped by a fallout tephra layer, all within a single eruption unit; (2) it contains blocks that were hot during emplacement; and (3) these blocks yield the same Ar/Ar date as the associated ignimbrite and pumice fall deposit. Landslide hummocks dammed surface water, forming ephemeral lakes perched on the volcano flank. A dome growth destabilized the southeast sector of a mid-Pleistocene Canadas caldera wall, and created a major breach that affected the passage of destructive pyroclastic density currents on Tenerife for 0.5 million years, showing that landslides can have enduring consequences for pyroclastic dispersal and hazards.
Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian-Ordovician sandstones
Navot Morag et al., Institute of Earth Sciences, Hebrew University of Jerusalem, Giv'at Ram, Jerusalem 91904, Israel; doi: 10.1130/G32184.1.
Paleozoic sandstones cover an extensive area along northern Africa and Arabia, from the Atlantic Ocean to the Persian Gulf. These sandstones were deposited at the peripheries of Gondwana supercontinent following its assembly in the Precambrian-Cambrian transition. Uranium-lead dating of detrital zircons from these sandstones indicate their derivation from mountain belts formed along the continental sutures further to the south, but their exact provenance and transportation distance are not well constrained. In Israel and Jordan, the Paleozoic sandstones overlie the northern tip of the Arabian-Nubian Shield, which comprises juvenile continental crust formed during the Neoproterozoic Era (1000-542 million years ago), and bears typical isotopic composition for a number of trace elements including hafnium. Navot Morag of Hebrew University and colleagues reveal that hafnium isotopic composition in most detrital zircons from the Israeli-Jordanian Paleozoic section is very different from that of the underlying Arabian-Nubian Shield basement, indicating that these zircons were derived from areas comprising pre-Neoproterozoic crust, up to 3.5 billion years old. This finding implies massive sand transport across the Arabian-Nubian Shield (a distance of over 2000 km, which requires substantial leveling of the Arabian-Nubian Shield topography prior to the onset of Cambrian deposition.
High-resolution P-T-t paths from delta18O zoning in titanite: A snapshot of late-orogenic collapse in the Grenville of New York
Chloe E. Bonamici et al., WiscSIMS, Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA; doi: 10.1130/G32130.1.
Individual mineral crystals record information about temperatures in the roots of mountain belts formed by plate-tectonic collisions and the rates at which rocks cool following collisions. Minerals are like tiny airplane flight recorders that sense large tectonic events, but the information they hold can only be accessed with specialized, high-precision geochemical instruments. Chloe E. Bonamici of the University of Wisconsin and colleagues use a secondary ion mass spectrometer (SIMS) to measure zoning of oxygen isotope ratios within the mineral titanite. These oxygen isotope ratio variations indicate "chilling" of rocks that were once buried at or more than 20 km inside a large mountain range at the southeast edge of the proto-North American plate more than 1 billion years ago. Rapid cooling suggests that these deep rocks moved toward Earth's surface quickly, carried upward along a large crustal fault as the mountain range collapsed under its own weight, much like the Himalayan Mountains are doing today. This is the first study to use high-precision SIMS oxygen measurements to reveal the tectonic collapse of an ancient mountain range.
Diversity in Earth's early felsic crust: Paleoarchean peraluminous granites of the Barberton Greenstone Belt
Cynthia J.M.G. Sanchez-Garrido et al., Center for Crustal Petrology, Dept. of Earth Sciences, University of Stellenbosch, South Africa Private Bag X-1, Matieland 7602, South Africa; doi: 10.1130/G32193.1.
Earth's continents arose during the period between 3.6 and 2.5 billion years ago, as sodium-rich granitic rocks were formed in subduction zones where oceanic crust melted. The granitic rocks that characterize our world today are fundamentally different from these early granitic rocks, having more potassium and less sodium in their compositions, and are considered to have arisen by re-melting of this primitive crust. Cynthia J.M.G. Sanchez-Garrido of the University of Stellenbosch and colleagues demonstrate for the first time that potassic granites were produced concurrently with the primitive sodic granites during each of three episodes of magmatism recorded in the Barberton granite greenstone terrane in South Africa. The potassic granites formed through melting at high pressure of a thin veneer of subducted sediment that was derived through weathering and erosion of both oceanic crust and continental crust. This demonstrates a mechanism for recycling of continental material that was perhaps unique to the Archean period.
Relic aragonite from Ordovician-Silurian brachiopods: Implications for the evolution of calcification
U. Balthasar et al., School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK; doi: 10.1130/G32269.1.
Most shelled marine organisms form their shells from one of the two main forms of calcium carbonate, the minerals aragonite or calcite. Fluctuations in marine ocean chemistry through Earth's history resulted in prolonged periods during which either aragonite or calcite were the favored minerals, and it is a long-standing question if and how ocean chemistry influenced the evolution of calcification in marine organisms. A problem for the correlation of evolutionary patterns of biocalcification is the unstable nature of aragonite, which reverts to calcite over geological time scales. U. Balthasar of the University of Glasgow and colleagues extend the known record of aragonite by more than 130 million years by discovering microscopic inclusions of aragonite in shells that have almost entirely reverted to calcite. Their results show that a major group of very successful aragonitic organisms evolved during a calcite sea interval. Results also suggest that water temperature is an important factor that influences the choice of aragonite or calcite in the evolution of biocalcification. Balthasar and colleagues propose that calcite sea conditions can shift to aragonite conditions in warm tropical-water temperatures. In order to resolve the influences of ocean chemistry on the evolution of biocalcification, it is necessary to resolve aragonite/calcite sea conditions at an environmental scale.
Melt in the mantle beneath the amagmatic zone, southern Nevada
Christina J. Rau and Donald W. Forsyth, Dept. of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02906, USA; doi: 10.1130/G32179.1.
Over the past 30 to 40 million years, nearly all the Basin and Range province in the southwestern United States has been affected by volcanic activity. There is one area in southern Nevada, however, termed the amagmatic zone or magmatic gap, in which no eruptions or intrusions are known to have occurred. Researchers in the past have suggested that the lack of volcanic activity was due to a piece of ancient, cold continental lithosphere in the mantle beneath the crust in this area. New seismic observations by Christina J. Rau and Donald W. Forsyth of Brown University presented here are made possible by the transportable array of seismometers of the EarthScope Project allow a test of this hypothesis. Instead of high seismic velocities that would be expected if there is an unusually thick or cold lithospheric plate, the velocities in this area are unusually low, indicating high temperatures and the present of melt. Rau and Forsyth note that the lack of eruptions must be caused by either trapping of the melt in the mantle or sideways migration of magma to erupt elsewhere, not by suppression of melting due to the presence of old, cold lithosphere.
Evidence for Quaternary convergence across the North America-South America plate boundary zone, east of the Lesser Antilles
M. Patriat et al., Ifremer (Institut Francais de Recherche pour l’Exploitation de la Mer), Geosciences Marines, BP70, 29280 Plouzane, France; doi: 10.1130/G32474.1.
While it appears on most global tectonic maps, the location and functioning of the North America-South America plate boundary remains unknown, despite significant past efforts to decipher its precise position and associated deformation. M. Patriat of Ifremer and colleagues analyzed geophysical and geological data, including multibeam bathymetry and seismic reflection profiles acquired in 2007, at the western end of this diffuse plate-boundary zone where it connects to the subduction zone beneath the Lesser Antilles island arc. Two major oceanic basement ridges, the Barracuda Ridge and the Tiburon Rise, enter the subduction zone and are known as to be out of isostatic equilibrium. Patriat and colleagues' work, including a reevaluation of paleontological associations first described in a core drilled in 1970 by the Deep Sea Drilling Project, has enabled them to propose an improved timing of the recent deformation and to locate the major structural features. Most of the uplift of the Barracuda Ridge and Tiburon Rise is Pleistocene in age, more recent than previously thought, and is associated with compressive deformation spread over an area more than 200 km wide that acts as the diffuse plate boundary.
Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift
Ralf Hetzel et al., Institut fur Geologie und Palaontologie, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 24, 48149 Munster, Germany; doi: 10.1130/G32069.1.
Deciphering the uplift history of Tibet -- the highest and largest plateau on Earth -- is key to understanding the geologic and paleoclimatologic evolution of Central Asia. Hitherto, it was unresolved whether Tibet reached its present-day elevation of 5000 m before or after India collided with Asia 50 million years ago. By constraining the age and stability of a flat bedrock surface (a so-called peneplain), which extends for about 100 km east-west in south Tibet, Ralf Hetzel of Westfalische Wilhelms-Universitat Munster and colleagues are able to answer this question. With a new combination of analytical tools, they show that the peneplain is 50-60 million years old and was formed at an elevation close to sea level. In addition, their data demonstrate that the peneplain is a remarkably stable feature that has survived the entire uplift history of the present-day plateau. These new findings, in combination with previously published data, indicate that Tibet rose from sea level to an elevation of at least approximately 4000 m between 50 and 35 million years ago. This rapid phase of uplift was most likely a direct response to the continent-continent collision between India and Asia.
Oxygen-isotope trends and seawater temperature changes across the Late Cambrian Steptoean positive carbon-isotope excursion (SPICE event)
Maya Elrick et al., Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA; doi: 10.1130/G32109.1.
During the past half billion years of Earth's history, there have been more than a dozen major pertubations in the global carbon budget, which are recorded by widespread and large peaks in carbon isotope ratios measured in marine limestones. One of the oldest of these events, the so-called SPICE event in Late Cambrian time (approximately 500 million years ago), is bracketed by global extinctions of many trilobite species. To better understand seawater temperatures associated with the SPICE event, small brachiopods composed of the mineral apatite (the same mineral as in mammal teeth) were collected from three different sites across the United States to analyze for their oxygen isotope ratios. The results suggest that the SPICE event was associated with seawater warming, while the bracketing extinction events were linked to seawater cooling. This first documented evidence for seawater warming provides support for the hypothesis that the SPICE event was triggered by low oxygen levels in ocean waters, which promotes the preservation and burial of fine particulate organic matter in seafloor sediments, which in turn leads to peaks in carbon isotopes.