||28 March 2007
GSA Release No. 07-12
Boulder, CO, USA - Topics include: global climate change and biotic recovery following the end-Permian mass extinction; evidence of warming during the Neoproterozoic; new insights into structure and dynamics of the San Andreas fault; origin of some of Earth's largest natural crystals; and discovery of Martian rock layers that illuminate the planet's hydrological history. The GSA TODAY science article addresses relationships between Northern Cordilleran terranes and tectonic evolution of western North America.
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- Smithian-Spathian boundary event: Evidence for global climatic change in the wake of the end-Permian biotic crisis
Thomas Galfetti, University of Zurich, Paläontologisches Institut, Zurich, ZH 8006, Switzerland; et al. Pages 291-294.
- Using carbon isotope data from the Tethyan and the Boreal realms, together with independently dated palynological records from the Barents Sea area and global ammonoid distribution patterns, Galfetti et al. present evidence for a global climatic change at the Smithian-Spathian boundary (Olenekian, Early Triassic). This change profoundly affected terrestrial and marine environments during the recovery phase after the end-Permian biotic crisis.
- Climatic cycles during a Neoproterozoic "snowball" glacial epoch
Ruben Rieu, Philip A. Allen (corresponding author), Imperial College, Earth Science & Engineering, South Kensington Campus, London, London SW7 2AZ, UK; et al. Pages 299-302.
- The profound glaciations of the Neoproterozoic Cryogenian period (roughly 850-544 million years ago) represent an extreme climatic mode when, it is claimed, Earth was fully or almost completely covered with ice for millions of years. Rieu et al. show that the geochemistry and mineralogy of fine-grained Neoproterozoic sedimentary rocks in Oman are best explained by climatic oscillations that drove variations in the intensity of chemical weathering on contemporary land surfaces. The cold climate modes of the Cryogenian were therefore cyclical, punctuated with well-defined warm-humid interglacial periods. The hydrological cycle and the routing of sediment were active throughout the glacial epoch, which requires substantial open ocean water. This reconstruction represents a significantly different target for numerical climate models at this critical time in the evolution of Earth's biosphere.
- Restoring dense vegetation can slow mountain erosion to near natural benchmark levels
Veerle Vanacker, Université Catholique de Louvain, Geography, Waals Brabant 1348, Belgium; et al. pages 303-306.
- Tropical mountain areas suffer from severe soil erosion, which often limits their agricultural growth potential and induces major socio-economic and environmental problems. Modern, elevated sediment fluxes in tropical mountain areas are commonly attributed to improper land management, although natural process rates may already be high due to steep slopes, tectonic activity, and the erosive climate. To examine the impact of human-induced land use on erosion, Vanacker et al. compared current erosion trends with natural, benchmark erosion rates for a total of 37 sites located in the Ecuadorian Andes. These sites have similar topography and soils, but differ in land use history. They found that vegetation cover exerts first order control over current erosion rates at the catchment scale. Catchments with high-vegetation density erode at rates that are similar to those of the natural erosion benchmark, regardless of their specific land use history. When reducing their surface vegetation cover, catchment-wide erosion rates increase by up to 100 times. Vanacker et al.'s data show that accelerated erosion rates as a result of human activities can be slowed to near their natural benchmark levels with suitable revegetation programs. This suggests that stabilizing and restoring degraded land by increasing surface vegetation cover is indeed possible, even in steep mountain environments.
- Thermal structure of oceanic transform faults
Mark D. Behn, Woods Hole Oceanographic Institution, Department of Geology and Geophysics, 360 Woods Hole Road - MS 22, Woods Hole, MA 02543, USA; et al. Pages 307-310.
- Oceanic transform faults are strike-slip fault systems that separate spreading segments along the global mid-oceanic ridge spreading system. Transform faults represent an optimal environment for studying the mechanical behavior of strike-slip faults (e.g., the San Andreas fault) because of the relatively simple thermal, kinematic, and compositional structure of the oceanic lithosphere. Previous numerical modeling studies have shown that the upper-mantle beneath transform faults is anomalously cold relative to adjacent intra-plate regions. However, these studies used highly simplified laws to simulate the material behavior of the crust and mantle. Here Behn et al. show that using a more realistic treatment of brittle deformation results in enhanced upwelling and significantly warmer temperatures along the transform, with the warmest temperatures occurring near the center of the fault. This warmer upper mantle temperature structure is consistent with a wide range of geophysical and geochemical observations from ridge-transform environments, including the depth of transform fault seismicity and geochemical anomalies along adjacent ridge segments. In addition, the elevated temperatures at the center of the fault may provide an explanation for the tendency of long transforms to break into a series of small intra-transform spreading centers during changes in plate motion.
- Diffuse intersesimic deformation across the Pacific-North America plate boundary
Shimon Wdowinski, Division of Marine Geology and Geophysics, University of Miami, Division of Marine Geology and Geophysics, Miami, FL 33149-1098, USA; et al. Pages 311-314.
- Crustal movements and deformation within the diffuse Pacific-North America plate boundary are dominated by the right-lateral motion between the two plates. By using the Pacific-North America pole of rotation (PoR) spherical coordinate system, Wdowinski et al. decompose observed crustal movements into parallel and normal components to the Pacific-North America plate motion. They transformed the 840 velocity vectors of the Southern California Earthquake Center (SCEC) 3.0 velocity field into the Pacific-North America PoR system in order to characterize the interseismic velocity across the plate boundary. Their results show that despite the very different deformation styles occurring across the San Andreas fault, the fault trace follows the half plate motion contour. Deviation occurs in the southern section, where the half motion contour correlates with the San Jacinto and Imperial fault segments. Wdowinski et al.'s analysis yields interesting asymmetric patterns in both parallel and normal components. The parallel components show asymmetrical velocity gradients across the San Andreas fault, and the normal components indicate compression southwest of the Big Bend, but not northeastward. The observations are compared with viscoelastic modeling results, which show a similar velocity field. The main disagreements between the observations and the model are in a narrow band along the San Andreas fault and in the Mojave block, suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models.
- Amplitude and timing of temperature and salinity variability in the subpolar North Atlantic over the past 10 k.y.
Rosemarie E. Came, California Institute of Technology, Geological and Planetary Sciences, Pasadena, California 91125, USA; et al. Pages 315-318.
- Subpolar North Atlantic near-surface winter water temperature and salinity increased over the course of the Holocene. Insolation-forced northward retreat of the boundary between polar and North Atlantic subsurface waters probably caused these trends. Temperature fluctuations superimposed on the orbital trend do not appear to be periodic, but tend to recur within a broad millennial band. The records provide evidence of an open-ocean cooling (nearly 2 °C) and freshening during the cold event 8200 years ago, and suggest similar conditions at 9300 years ago. However, two temperature oscillations (~2 °C) during the last 4000 years were the largest of the last 10,000 years, suggesting a recent increase in temperature variability relative to the mid-Holocene, perhaps in response to neoglaciation, which began at about this time.
- Formation of natural gypsum megacrystals in Naica, Mexico
Juan Manuel García-Ruiz, Consejo Superior de Investigaciones Científicas, Laboratorio de Estudios Cristalográficos / IACT, IACT, Fac. Ciencias, Av. Fuentenueva s/n, Granada 18002, Spain; et al. Pages 327-330.
- The recently discovered Cave of Crystals in Naica (Chihuahua, Mexico) contains some of the largest and most fascinating natural crystals ever found: transparent gypsum beams up to 11 meters long. This natural wonder poses some very interesting questions related to both the preservation of such crystals and, much more puzzling, the conditions and mechanisms that could explain their origin. A cross-disciplinary team of Spanish and Mexican scientists worked to address these questions using geochemical evidence collected onsite and a set of crystal growth physics models. Their research shows that these megacrystals formed by a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the formation of these giant crystals, yet only when operating within the very narrow range of temperature identified by García-Ruiz et al.'s fluid inclusion study.
- Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source
J. Brendan Murphy, St. Francis Xavier University, Earth Sciences, P.O. Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada; and Jaroslav Dostal, . Pages 335-338.
- Most studies of the chemistry of ancient volcanic rocks focus on suites with a limited range in age. In this paper, Murphy and Dostal show that additional insights may be gained by focusing on regions where magma generation occurred repeatedly over a long period of time. In the Antigonish Highlands of Nova Scotia, Canada, four episodes of magmatism occurred in discrete intervals between 620 and 360 million years ago. The oldest episodes occurred when the region was situated along the northern margin of an ancient continent called Gondwana. The youngest volcanic activity occurred after the region became attached to North America. Murphy and Dostal show that the volcanic compositions display a remarkable degree of inheritance,which allows them to constrain the evolution of the region in the mantle where these magmas were formed. Their results additionally imply that the mantle and the overlying crust must have been attached to one another during that entire time interval, even though the magmatism occurred on different continents. Murphy and Dostal can also identify a time when the mantle source was chemically modified some 150 million years or more before the oldest volcanic eruption in the region. At that time, the source region was probably in an oceanic setting.
- Diamond, subcalcic garnet, and mantle metasomatism: Kimberlite sampling patterns define the link
V.G. Malkovets, W.L. Griffin (corresponding author), Macquarie University, GEMOC, Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia; et al. Pages 339-342.
- Pyrope garnet and chromite are two minerals commonly found together with diamonds in Earth's upper mantle, and in the kimberlite volcanoes that deliver diamonds to the surface. In Siberian kimberlites, diamonds are known to come mainly from a layer 140-190 kilometers deep in the Earth. Using trace-element data, Malkovets et al. have shown that chromite is ubiquitous in this layer, but garnets and diamonds are patchily distributed and occur together. This pattern, and the composition of the chromites in the diamond-bearing and barren pipes, suggest that diamond is formed by the oxidation of methane-rich fluids that penetrate the upper mantle along well-defined conduits and react with the wall-rocks to deposit both garnet and diamond. The diamond grade of later kimberlites is determined by these earlier processes; kimberlite magmas that intersect one of the former fluid conduits will have high diamond contents, but those that miss the conduits will carry few diamonds. This research suggests new approaches to diamond exploration within known diamond provinces.
- Dynamic single-thread channels maintained by the interaction of flow and vegetation
Michal Tal and Chris Paola, University of Minnesota, Geology and Geophysics, St. Anthony Falls Lab, 2 3rd Ave, SE, Minneapolis, Minnesota 55414, USA. Pages 347-350.
- Tal and Paola describe a set of experiments designed to explore how vegetation can alter a river's shape and flow dynamics. The experimental methodology consists of repeated cycles of high flow alternating with longer periods of low flow and accompanied by the continuous addition of alfalfa sprouts to simulate vegetation. This simple laboratory reproduction of a natural river demonstrates how the continuous interaction of flow and vegetation fosters a multi-thread system's evolution to a single-thread channel with well defined banks and a floodplain, and offers insight into the mechanisms associated with this change.
- High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt
Martin J. Streck, Portland State University, Geology, Portland, Oregon 97207-0751, USA; et al. Pages 351-354.
- High-magnesian andesites (HMA) have been proposed to represent an important near-primary magma in many volcanic arcs despite their rare occurrence. Their origin has been attributed to processes mainly occurring in Earth's mantle beneath volcanic arcs. HMA from the Mt. Shasta area have been represented as the 'type example' of HMA. Prior investigations of these unusual rocks have produced conflicting interpretations. Here, Streck et al. present results of a detailed petrographic study to clarify the origin of these rocks and argue that HMA from Mt. Shasta represents a hybrid magma produced by magma mixing and the incorporation of crystal debris from wall rocks; thus, it is not a close proxy for a mantle magma.
- Geometry and slip rate of the Aigion fault, a young normal fault system in the western Gulf of Corinth
L.C. McNeill, National Oceanography Centre, Southampton, School of Ocean and Earth Science, University of Southampton, Southampton, Hampshire SO14 3ZH, UK; et al. Pages 355-358.
- The first stage of a continent rifting apart, potentially leading to the formation of a new ocean basin, is taking place at the Gulf of Corinth in central Greece, with accompanying high rates of deformation and earthquake activity. The Aigion fault is one of the youngest faults within the rift, probably forming within the last 200-300 thousand years. McNeill et al. examine how the fault has developed relative to more established, neighboring faults and record the rate of slip along the fault, focusing on the eastern offshore tip of the fault by applying high-resolution marine geophysical techniques. The fault appears to have developed very rapidly during its short geological history, contributing to debates of both fault and rift development, with potential significance for the evolution of hydrocarbon reservoirs and traps.
- Identification and preservation of landforms diagnostic of past ice-sheet activity on continental shelves from three-dimensional seismic evidence
Julian A. Dowdeswell, University of Cambridge, Scott Polar Reseach Institute, Cambridge CB2 1ER, UK; et al. pages 359-362.
- Ice Ages, which have occurred a number of times in Earth's history, are important to understanding changes in long-term climate. However, it is difficult to demonstrate the presence of ice sheets in the ancient record because their sedimentary products can resemble those from non-glacial processes (e.g., mass wasting). Diagnostic large-scale glacial landforms, produced at the base of ice sheets and preserved on continental shelves after deglaciation, can establish a glacial origin. Three-dimensional seismic evidence from the 2.7 million-year-old Naust Formation, Norwegian margin, illustrates several types of spectacular streamlined glacial landforms that are also commonly occurring features on modern high-latitude shelves: A) streamlined mega-scale lineations produced by fast-flowing ice streams; B) ploughmarks formed by iceberg keels; and C) regularly spaced transverse ridges or push moraines formed during ice retreat. Dowdeswell et al. have found, for example, buried ice-keel ploughmarks on a paleo-shelf dating to about 2 million years ago. Norwegian-margin seismic stratigraphy shows that ice advanced many times, and that some paleo-shelf surfaces were preserved. Such paleo-surfaces must be preserved for up to hundreds of millions of years for glacial landforms to be useful in identification of ancient depositional environments.
- Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars
Gilles Dromart, ENS Lyon, Laboratoire Sciences Terre, Lyon 69364, Cedex 07, France; et al. Pages 363-366.
- Any findings about the past existence of water on Mars are of direct relevance to NASA's objectives in astrobiology and are critical to understanding Mars' volatile climate history. To date, all the orbital observations that attest to fluvial activity have been limited to surficial landforms (e.g., valley networks); consequently, there is no answer to such pivotal questions as whether such flows were persistent or how long it took for the landforms to develop. In addition, some of the landforms have been erroneously interpreted to be of lacustrine origin, e.g., much of the floor of Gusev Crater (MER Spirit). However, in Melas Chasma, Valles Marineris, Dromart et al. discovered an outstanding exposure of layered rocks showing stratal architectures representative of long-lived, sub-lacustrine depositional systems, including a fan delta and a giant channel-levee system. This discovery provides an unprecedented record of the hydrological history of Mars.
- Valley asymmetry and glacial versus nonglacial erosion in the Bitterroot Range, Montana, USA
Shawn Naylor, University of Montana, Geosciences, Missoula, Montana 59812, USA; and Emmanuel J. Gabet (corresponding author), University of California, Department of Environmental Sciences, Geology Building, Riverside, CA 92521, USA. Pages 375-378.
- Theories that propose feedbacks between climate, tectonics, and surface processes commonly assume that erosion is enhanced by glacial activity. Naylor and Gabet test these assumptions by comparing relative efficiencies of glacial and non-glacial erosion within multiple Bitterroot Mountain watersheds where lithology and precipitation are held constant. The east-west-trending valleys of the Bitterroot Range present an ideal scenario for this study because the north-facing sides of the valleys were glaciated, whereas the south-facing slopes were not. The different erosional regimes operating on either side of the valleys have created strongly asymmetric ridges. Ridgelines separating the east-west-trending valleys have been pushed southward by glacial headwall retreat such that ridge-to-valley distances are approximately 50% greater on the north-facing slopes than on the south-facing slopes. In addition, mean hillslope angles are 6 degrees lower on the glaciated slopes than on the unglaciated slopes, and calculations of relief production suggest that, on average, glaciers removed nearly twice as much rock as nonglacial processes. The findings confirm the dominant role of glacial erosion in leveling mountains under the appropriate climatic conditions.
- New constraints on the seismic structure of West Australia: Evidence for terrane stabilization prior to the assembly of an ancient continent?
A.M. Reading, Australian National University, Research School Earth Sciences, Research School Earth Sciences, Australian National University, Canberra ACT 0200, Australia; et al. Pages 379-382.
- Earthquake waves from distant regions of the planet can help us work out the structure of continents. Seismic waves travel faster in some rock types than others, making it possible to 'map' the deep Earth in three dimensions by analyzing the differences in the waveforms ('wiggles') from each seismic recording station. In this study, Reading et al. deployed many extra seismic stations across western Australia—one of the oldest regions on Earth. Their results show that the deep continental structure changes according to the major geological boundaries on the surface. Land areas did not just form by cooling lava—vast regions of the ancient Earth must have collided to build the first continents.
- Northern Cordilleran terranes and their interactions through time
Maurice Colpron, Yukon Geological Survey, Whitehorse, Yukon Y1A 2C6, Canada; et al.
- Reining in Roving Real Estate: The concept of geologic terranes, distinct parts of the crust of mountain belts that may be far traveled and unrelated to presently adjacent crust, has been a first-order part of the analysis of mountain belts the world over but particularly in the Cordilleran mountain belt of western North America, the birthplace of the terrane concept. The patchwork nature of mountain belt geology was popularized by Pulitzer Prize winning author John McPhee in books such as In Suspect Terrain and Assembling California. In a recent GSA Today article, authors Maurice Colpron, JoAnne Nelson, and Don Murphy pool their considerable experience in the northern Cordillera (Canada and Alaska) to establish geologic relationships between terranes and show that what was formerly considered as a collection of isolated and unrelated fragments now shows linkages that span back to their birth as a chain of island-arc volcanoes and slivers of continental crust that fringed western North America more than 350 million years ago. Their work resolves some fundamental problems in the geology of the Cordilleran mountain range and is an important step forward in understanding the tectonic evolution of western North America.
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