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Contact: Christa Stratton
cstratton@geosociety.org
303-357-1093
Geological Society of America

May/June 2010 GSA Bulletin highlights

Boulder, CO, USA – Highlights from the May/June 2010 GSA Bulletin include examinations of martian impact craters, the record of floods from Pleistocene Glacial Lake Missoula, "salt glaciers" in the Red Sea, the Santa María-Santiaguito volcanic complex in Guatemala, historical photographs of the Sacramento River floodplain, juvenile crust in the Canadian Shield, ice streams, the end-Permian mass extinction record offshore mid-Norway, and field mapping and high-resolution digital elevation model analysis of fault-scarp morphologies across a flower structure in Nevada.


What we know about Mars from its impact craters
Nadine Barlow et al., Dept. of Physics and Astronomy, Northern Arizona University, Flagstaff, Arizona 86011-6011, USA. Pages 644-657.

Impact craters are often the only way to estimate unit ages, infer subsurface structure, and investigate gradational histories on other planets and moons in our Solar System. Analysis of Martian impact craters from orbital and surface robotic missions reveals that Mars has experienced a complex geologic history involving liquid water and ice as well as volcanism, tectonism, and aeolian processes. Large meteorite impacts early in Martian history may have contributed to the formation of the planet's hemispheric dichotomy and atmospheric erosion. Current climatic conditions preclude the presence of liquid water on the surface, and precipitation only occurs seasonally over the polar regions. Temporal changes in the amount of degradation experienced by impact craters and the presence of ice-rich deposits on crater floors indicate that Mars has experienced episodic climatic change throughout its history. Impact craters also allow determination of subsurface structure, including estimation of the depths of present-day buried ice reservoirs. Impact craters therefore play a major role in unraveling the climatic and geologic histories of Mars.


40Ar/39Ar dating of Silurian and Late Devonian cleavages in lower greenschist-facies rocks in the Westminster terrane, Maryland, USA
R.P. Wintsch et al., Dept. of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA. Pages 658-677.

Understanding the origin of Earth's crust and its components is important for reconstructing plate tectonic processes, and for predicting the consequences of plate tectonic processes that are active today. As part of a regional study of the Appalachian Mountains, geologists working with the U.S. Geological Survey have discovered that the rocks underlying the low rolling hills of the western Piedmont of Maryland have a much more complicated history than previously understood. Wintsch et al. dated the micas in the slates locally exposed between the Great Falls of the Potomac and Frederick, Maryland, and discovered that the cleavage in these rocks developed not only in the Devonian period (about 360 million years ago) but also about 430 million years ago, in the Silurian period. They infer that strike-slip movement along a fault, unrecognized until this study, slid the eastern Devonian domain against the western Silurian one about 300 million years ago. The coarse-scale structure implied by this discovery supports the hypothesis that a crustal embayment existed under the Chesapeake Bay during the Silurian and Devonian periods.


Simulations of cataclysmic outburst floods from Pleistocene Glacial Lake Missoula
R.P. Denlinger and D.R.H. O'Connell, U.S. Geological Survey, Vancouver, Washington 98683, USA. Pages 678-689.

The glacial outburst floods from Pleistocene Glacial Lake Missoula are the largest known floods on Earth. The record of these floods is most evident in arid regions of Washington, Idaho, and Oregon, and provides constraints for sophisticated numerical modeling of these floods. Using a grid spanning the Pacific Northwest with a 250-meter posting, Denlinger and O'Connell simulate flooding from the sudden rupture of a 1250-meter high ice dam near Pend Oreille, Idaho. They find that much of Lake Missoula drained in a few days to broad Pasco, Yakima, and Umatilla basins, scarring much of the scablands in the process. Drainage of these basins was restricted by Columbia River Gorge, and a month or more was required to drain these basins to the sea.


The German Bank pluton, offshore SW Nova Scotia: Age, petrology, and regional significance for Alleghanian plutonism
Georgia Pe-Piper et al., Dept. of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada. Pages 690-700.

Three hundred million years ago, an ancient precursor of the Atlantic Ocean known as the Rheic Ocean lay between Europe and North America on one side and the southern continents on the other. As this ocean closed, Africa and North America collided to form the super-continent Pangea, with the suture marked by the mighty Alleghanian mountains. In the southeastern United States, abundant granites were intruded as a result of this collision. Previously, no correlative granites were known from the northern part of the Alleghanian mountains in Canada. Pe-Piper et al. have identified the first granite of this age in Canada. By making comparisons with the closing Mediterranean ocean remnants, the origin of this granite can be interpreted in terms of processes that took place in Earth's mantle 300 million years ago.


Submarine salt flows in the central Red Sea
Neil C. Mitchell et al., School of Earth, Atmospheric and Planetary Sciences, University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK. Pages 701-713.

Mitchell et al. examine giant glaciers of salt found beneath the Red Sea. The salt originally formed when the Red Sea dried out, when the sea was isolated from the Indian Ocean and evaporated by the dry Arabian climate. With the Arabian and African tectonic plates moving apart from each other, a rift valley has formed in the sea. Now rock salt, which is a very weak geological material, is flowing into those valleys, forming great tongues looking very much like ice glaciers. Similar salt glaciers occur in the Gulf of Mexico and other places where salt deposits are near the seabed. This Red Sea area is also an analogue for the older southern Atlantic Ocean, which similarly formed a narrow basin with salt basins at a time when the African and South American tectonic plates were much closer together. The images of the glaciers were created from data collected on the Italian research ship "Urania" using a multibeam echo-sounder installed in the ship's hull (an instrument that measures the topography of the sea bed using multiple sounding beams). Funding was provided by the Italian government (agency CNR) under a European Union collaborative programme EUROMARGINS.


Aerogravity evidence for major crustal thinning under the Pine Island Glacier region (west Antarctica)
T.A. Jordan et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK. Pages 714-726.

Jordan et al. use recent aerogravity data to reveal the West Antarctic Rift System under one of the fastest flowing and rapidly changing glaciers in Antarctica. They found that the crust beneath the Pine Island Glacier region is the thinnest observed so far under the West Antarctic Ice Sheet. Major crustal thinning is likely to have an impact on Pine Island Glacier. This new research makes a significant contribution towards understanding geological controls on the dynamics of the West Antarctic Ice Sheet.


Comparison of crustal thickening budget and shortening estimates in southern Peru (12-14°S): Implications for mass balance and rotations in the "Bolivian orocline"
Nicole Gotberg et al., Dept. of Geosciences, Princeton University, Princeton, New Jersey 08540, USA. Pages 727-742.

Convergence of tectonic plates results in deformation along the plate boundary. When the boundary of a plate is a continent, the resulting deformation, or orogen can be very broad and diffuse. An example of this is the Central Andean Plateau in Bolivia and Peru. The Central Andes are defined by two of South America's most distinguishing morphologic features, the ~4-km-high, 400-km-wide Andean Plateau, and a pronounced seaward concave bend of the western coast and cordillera between ~13-29S latitude. Although north-to-south changes in the magnitude of shortening has often been thought of as the primary control of both features, this assumption has never been fully tested along the arc of thickened crust and high elevations. Gotberg et al. present shortening estimates for the northern margin of the high Andean Plateau in southern Peru and show that the amount of shortening is not quite sufficient to account for a thick crust there (assuming isostatic equilibrium). Shortening estimates can account for some (but not all) of the curvature of the mountain range.


Large-scale reorganization and sedimentation of terrestrial ice streams during late Wisconsinan Laurentide Ice Sheet deglaciation
Colm Ó Cofaigh et al., Dept. of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK. Pages 743-756.

Ice streams are "arteries" of fast-flowing ice surrounded by slower-moving ice within ice sheets. They are present today in the large polar ice sheets of Greenland and Antarctica. Evidence for ancient or paleo-ice streams can be found in the geological record in the form of a range of distinctive glacial landforms, especially highly elongate subglacial bedforms. Ó Cofaigh et al. report on the results of a study from a ~140,000-square-kilometer area of the western Canadian Prairies which shows that during deglaciation of the last North American Ice Sheet, large paleo-ice streams up to almost 700 km in length underwent a major reorganization in flow involving a 90-degree shift in flow direction. This reorganization was accompanied by an increase in the control of topography on streaming flow. These results highlight the role of internal glaciological controls in triggering ice streams within large ice sheets and they demonstrate that large-scale flow reorganization of ice streams can occur over the time scale of a single deglaciation.


40Ar/39Ar and paleomagnetic constraints on the evolution of Volcán de Santa María, Guatemala
Rüdiger P. Escobar-Wolf et al., Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA. Pages 757-771.

Volcanoes are built over tens of thousands of years, erupting in a variety of styles and sizes and sometimes changing radically the composition of their products. The Santa María-Santiaguito volcanic complex in Guatemala grew over more than seventy thousand years, forming the cone shaped Santa María volcano. This was followed by thirty thousand years of repose in the volcanic building process that ended in 1902 with one of the largest eruptions of the 20th century, and the subsequent formation of the Santiaguito dome, which continues to erupt. Escobar-Wolf et al. report that major change in the chemical composition of the erupted magma happened after the long repose period, reflecting an important change in the processes happening inside the volcanic system during that time. The lava flows that erupted during the formation of the Santa María cone recorded snapshots of the earth magnetic field over tens of thousands of years, however their quest for finding a an "excursion" of the field (a rapid and erratic change in the direction and intensity of the magnetic field) that had been hypothesized to be recorded in these lavas resulted inconclusive.


Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions
Zheng-Xiang Li et al., Institute for Geoscience Research, Dept. of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. Pages 772-793.

Intraplate orogenic belts are the most obvious exception to the plate tectonic paradigm, which assumes that plate interiors are rigid and undeformable, and that orogen-scale deformation occurs exclusively at plate margins. It has long been established that intraplate orogens occur in the present Earth (e.g., the Mesozoic-Cenozoic Tian Shan orogenic belt in Central Asia), but they are uncommon in Earth's history in comparison to collisional or accretional orogenic belts. Li et al. document the spatial extent, timing, and pressure-temperature condition of a poorly known Early Paleozoic Wuyi-Yunkai Orogen in South China. They demonstrate that the orogen likely developed over a Neoproterozoic failed continental rift that was linked to the breakup of the supercontinent Rodinia. The orogenic event started before 460 million years ago and lasted until at least 420 million years ago, possibly as a result of far-field plate interactions with Gondwanaland. The orogen experienced crustal thickening and high-temperature decompressional melting, resulting in widespread syn- to late-orogenic granitic intrusions.


The Calderian orogeny in Wopmay orogen (1.9 Ga), northwestern Canadian Shield
Robert S. Hildebrand et al., Dept. of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA. Pages 794-814.

Hildebrand et al. present a study on Wopmay orogen - a nearly two-billion-year-old belt of rocks that formed when the leading edge of the Slave continent, now located in northwestern Canada, was pulled beneath a microcontinent, which contained a volcanic regime similar to the present-day Ring of Fire. The collision lasted about 10 million years and ended when the leading edge of the Slave continent failed and sank into the mantle. The duration and deformational styles are identical to those of younger collisional belts and demonstrate the presence of plate tectonics for at least the last two billion years of Earth's history.


U/Pb geochronology of Devonian and older Paleozoic beds in the southeastern Maya block, Central America: Its affinity with peri-Gondwanan terranes
Uwe Martens et al., Dept. of Geological and Environmental Sciences, 450 Serra Mall, Building 320, Stanford University, Stanford, California 94305, USA. Pages 815-829.

The first occurrence of lower Paleozoic sedimentary rocks on the Maya (Yucatan) Block of Central America is reported by Martens et al. Some of these sedimentary rocks are interbedded with volcanic beds with yielded an age of about 405 million years ago using the U/Pb technique on the mineral zircon. Field observations and dating of detrital minerals showed that some of the beds in the Maya Mountains are even older, having been deposited sometime in the Cambrian-Silurian eras. The ages of detrital zircon grains in these allowed identifying age-groups which are consistent with a paleoposition along the periphery of the Gondwana paleocontinent in the early Paleozoic. Furthermore, these age groups suggest that the Maya Block was not connected with Florida at that time.


Ice-sheet evolution in James Ross Basin, Weddell Sea margin of the Antarctic Peninsula: The seismic stratigraphic record
R. Tyler Smith and John B. Anderson, Earth Science Dept., Rice University, 6100 Main Street, MS 126, Houston, Texas 77005, USA. Pages 830-842.

The history of the Antarctic Peninsula Ice Sheet, its development and subsequent advance and retreat cycles, are recorded in the sedimentary sequences of the continental shelf. Large erosional unconformities caused by scouring at the base of the ice sheet are evidence of ice sheet grounding all the way to the continental shelf break. Smith and Anderson attempt to subdivide the first-order stratigraphic units the northwest Weddell Sea, and to constrain a basic chronostratigraphic model for the Neogene using the stratigraphic architecture. Detailed mapping of regional unconformities combined with their chronostratigraphic model reveal that the Antarctic Peninsula Ice Sheet first developed in the Late Middle Miocene to Early Late Miocene. They also identify at least 10 glacial/interglacial cycles during the Pleistocene, which is the most recorded using seismic methods on an Antarctic margin. These results show an earlier development of the Antarctic Peninsula Ice Sheet and a high frequency advance and retreat history as recent as the Pleistocene.


Eolian grain-size signature of the Sikouzi lacustrine sediments (Chinese Loess Plateau): Implications for Neogene evolution of the East Asian winter monsoon
Hanchao Jiang and Zhongli Ding, State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, P.O. Box 9803, Beijing 100029, China. Pages 843-854.

In order to explore the Neogene evolution of the East-Asian winter monsoon circulation, grain-size analysis was conducted by Jiang and Ding on the Neogene Sikouzi section, Guyuan, Ningxia, China. According to stratigraphic variations in the content of various fractions of the Sikouzi grain-size record, the East-Asian winter monsoon was weak during the period 20.1 to 12.0 million years ago and strengthened dramatically after 12.0 million years ago. From 4.3 to 0.07 million years ago, the winter monsoon strengthened further. This inference agrees well with the timing of glaciation development in the northern hemisphere.


A mechanism for chute cutoff along large meandering rivers with uniform floodplain topography
José Antonio Constantine et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA. Pages 855-869.

Chute cutoff is one of the most common processes that create oxbow lakes, which are segments of meandering rivers that have been removed and stored in the floodplain. Oxbow lakes function as important aquatic habitat and as sinks for sediment, but their production by chute cutoff has remained poorly understood because of the paucity of observations explaining how the process occurs across a range of natural settings. Constantine et al. report the discovery that chute cutoff occurs in a predictable way along large rivers without pronounced floodplain topography, such as along the Sacramento River in California. Using historical photographs, they observed that chutes, or floodplain channels cutting across the inside of meander bends, formed where the Sacramento River's floodplain was devoid of woody vegetation or farmed. Chutes consistently initiated where meander bends were most tightly curved because floodwaters most efficiently escaped onto the floodplain at these locations. Constantine et al. constructed a quantitative model that allows for predictions of chute incision based on the type and quantity of floodplain vegetation. In concert with their empirical observations, the model provides an explanation for why chute cutoff is prevalent along some meandering rivers but not others.


The extent of juvenile crust in the Grenville Province: Nd isotope evidence
A.P. Dickin et al., School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada. Pages 870-883.

This study works to determine the time of formation of parts of the Canadian Shield in order to better understand its geological evolution. The Shield is a mosaic of crustal terranes of different ages that were created over geological time as separate micro-continents, and later joined together by mountain building events that resulted from continental drift and collision of these fragments. The neodymium isotopic dating method used in this study gives the best estimate for the time when these fragments were first formed. The results show that the 1 billion-year-old Grenville mountain-building event did not lead to the creation of significant amounts of new crust, but welded together pre-existing continents with histories that went back a further 1.5 billion years. This work improves our understanding of the evolution of the Precambrian Shield, and will lead to the creation of more accurate geological maps of the North American continent.


Multiple climatic changes around the Permian-Triassic boundary event revealed by an expanded palynological record from mid-Norway
Peter A. Hochuli et al., Paleontological Institute and Museum, University Zürich, Karl Schmid-Strasse 4, CH-8006 Zürich, Switzerland. Pages 884-896.

The end-Permian mass extinction is considered to be the largest biotic and ecological crisis recorded in Earth's history. More than 90% of the marine organisms went extinct during this catastrophic event. Its impact on the terrestrial ecosystems is still a matter of dispute. Some authors have described an apocalyptical scenario claiming that also the great majority of plants became extinct. The study by Hochuli et al. of spore-pollen of a thick section (750 meters) recovered from shallow cores from offshore mid-Norway and previously published records from East Greenland allow a more differentiated view on the development of the floras of this time interval. Instead of the great extinction event we observe an increase in the diversity of pteridophyte spores. One group of gymnosperms experienced a sharp reduction in abundance and diversity whereas others survived without significant changes. The data presented in this study and the well-known fact that most plant groups show a continuous record across the Permo/Triassic boundary event suggest that plants must have survived in refuge areas where they faced less harsh conditions, or alternatively, conditions on land were not as hostile as proposed by some authors. Plausible scenarios for the changes in the plant ecosystems include climatic changes induced by increased levels of CO2 caused by volcanic activity and methane release as well as possibly temporarily increased UV radiation.


High-resolution spatial rupture pattern of a multiphase flower structure, Rex Hills, Nevada: New insights on scarp evolution in complex topography based on 3-D laser scanning
Ramona Baran et al., Dept. of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich, Luisenstr. 37, 80333 Munich, Germany. Pages 897-914.

Fault scarps represent the most manifested expression of tectonic activity at Earth's surface. Their morphology had been studied to constrain fault kinematics and scarp-degradation processes. Where fault scarps are exposed in simple topography they indeed facilitate data acquisition and interpretation. In contrast, little work had been done where scarps are superimposed on complex topography as observed along flower structures of strike-slip faults. Here, Baran et al. examine the detailed meter- to hundred meter-scale structure and surface expression of a flank of one fault-scarp bounded pressure ridge (Rex Hills) by combining field mapping with high-resolution digital elevation model (DEM) analysis. The dissected Rex Hills topography is characterized by alternating valleys and ridges. Based on terrestrial laser scanning they generated a detailed DEM and extracted high-resolution topographic cross-sections, which enabled them to identify fault scarps and to determine their relative ages and geometry. Baran et al. evaluated the height and slope angle of the scarps by using the topographic cross-sections to detect differences between the scarps. Their results indicate that scarp shape is influenced by fault dip, lithology, and degradation processes. Moreover, scarp degradation is stronger in the valleys and hence, the preservation potential of small, individual fault scarps is greater on the ridge crests. Their approach of analyzing high-resolution topographic data of closely-spaced fault scarps is promising especially when combined with subsurface data as well as geochronological and paleoseismic data, and it provides a basic scheme for analyzing scarp populations in a complex topographic region. Despite the absence of subsurface data, their approach allowed the study of complex, high-resolution fault-scarp morphologies across a flower structure for the first time.


Origin and emplacement of the Aguablanca magmatic Ni-Cu-(PGE) sulfide deposit, SW Iberia: a multidisciplinary approach
Rubén Piña et al., Departamento de Cristalografía y Mineralogía, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain; rpinagar@geo.ucm.es. Pages 915-925.

Piña et al. synthesize the existing geological data and provide a metallogenetic model for the Aguablanca deposit based on the structural and gravity data obtained from the Aguablanca stock and petrological, geochemical, and mineralogical data on the host igneous rocks and the ore bodies.


Structural evolution of the Neogene Gar Basin, western Tibet: Implications for releasing bend development and drainage patterns
Veronica I. Sanchez et al., Dept. of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA. Pages 926-945.

Recent field work by Sanchez et al. in the Gar Basin along the Karakoram fault in westernmost Tibet revealed interesting relationships that characterize the basin system as a releasing-bend basin. The uniqueness of this system is that it is bounded at a single margin by a listric fault that plays a crucial role in the depth, internal deformation, and width of the fault-basin system. The active margin is manifested as a series of basinward-stepping faults which disrupt old basin fill and incorporate it into the new basin fill. Field work, remote-sensing, geomorphologic analyses, and forward modeling were integrated to develop a structural model for the evolution of the Gar Basin that may be valuable in augmenting our understanding of the development of double releasing-bend basins along strike-slip systems.

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View abstracts at http://gsabulletin.gsapubs.org/content/current. Representatives of the media may obtain complimentary copies of GSA Bulletin articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA Bulletin in articles published. Contact Christa Stratton for additional information or assistance.

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