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GEOLOGY
Formation of Martian outflow channels by catastrophic dewatering of evaporite deposits
David R. Montgomery and Alan Gillespie, University of Washington, Earth and Space Sciences, Seattle, WA 98195, USA. Pages 625-628.
The processes responsible for forming Valles Marineris, the largest valley in the solar system, and its associated outflow channels have proven controversial ever since these features were first recognized during the Mariner 9 mission. Although many hypotheses have been offered to explain channels on Mars, most workers now consider the major outflow channels to have been formed by huge discharges of flowing water. However, the source of enough water to account for the tremendous discharges required to carve the outflow channels remains controversial, as does the genetic relation of the outflow channels to the troughs of Valles Marineris. On the basis of new geological mapping and topographic analyses that used Mars Orbiter Laser Altimeter data, interpretation of Mars Orbiter Camera images, previously reported evidence for evaporite deposits on Mars, and thermodynamic and heat-flow considerations, Montgomery and Gillespie propose a novel hypothesis for the formation of Martian outflow channels: catastrophic dewatering of evaporite deposits by heating during the development of the Tharsis volcanic province triggered significant volumetric expansion and catastrophically released tremendous amounts of overpressured water.
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Greg A. Valentine, Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM 87545, USA; et al. Pages 629-632.
Scoria cones are one of the most common volcanic landforms on Earth. This article shows that the formation of such cones is more complex than previously thought. Rather than being formed simply by the accumulation of large fragments of material ballistically thrown from a vent, the cones can have a significant proportion of material that accumulates by fallout from a high-standing plume. Such phenomena have important implications for volcano dynamics and hazard assessments.
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Yemane Asmerom, University of New Mexico, Department of Earth and Planetary Sciences, Radiogenic Isotope Laboratory, Albuquerque, NM 87131-1116, USA; et al. Pages 633-636.
Volcanic eruptions have altered both the physical and ecological landscape of Earth. The most violent eruptions occur in geologic settings called arcs, where one tectonic plate is subducted beneath another. Among these, eruptions involving silica-rich lavas are particularly destructive. Understanding the time involved for magmas to evolve from primitive silica-poor parental magma to differentiated silica-rich compositions is an essential component for building future eruption models that will have predictive capabilities. In this contribution Asmerom et al. describe a novel approach for constraining the time scale of magma differentiation in arcs based on short-lived daughter isotopes of uranium-238 and urnium-235 and discuss results from the Taal volcano in the Philippine archipelago.
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Richard J. Davies, Cardiff University, 3DLab, School of Earth, Ocean and Planetary Science, Cardiff, Wales CF10 3YE, UK; et al. Pages 641-644.
Geologists now have at their disposal a new tool for investigating the structure of Earth's crust. It's the equivalent of the medical CAT scan and is known as three-dimensional seismic reflection data. The oil industry has been acquiring three-dimensional seismic data for the past 15 years in the hunt for oil and gas and is now collecting it over deepwater continental margins. It can provide unprecedented images of the boundaries between Earth's tectonic plates. In this case, Davies et al. present spectacular images of a fossil fracture zone, known as a "continent-ocean fracture zone" that forms the boundary between oceanic and continental crust. The Chain Fracture Zone is located in the Equatorial Atlantic and can actually be traced across this ocean but in this paper they just examine its eastern termination, offshore from Nigeria. Surprisingly, the boundary is very sharp and can be pinpointed to the nearest 100-200 m. The oceanic crust on one side of the fracture zone is some of the first to have formed as South America separated from Africa and the Atlantic Ocean developed. It is cross cut by distinctive faults that have curious hooked ends, adjacent to the continent-ocean fracture zone itself. The fracture zone is also marked by submarine volcanoes that erupted as the new oceanic crust was produced. In addition to these insights into what this type of plate boundary looks like, these data sets are providing new three-dimensional images of oceanic crust, the structure of which is still much debated, as this type of imagery is still quite rare in oceanic settings. Three-dimensional seismic data should have a significant role to play in imaging active and inactive tectonic plate boundaries.
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Matthew J. Kohn, University of South Carolina, Department of Geological Sciences, Columbia, SC 29208, USA; et al. Pages 649-652.
Stable isotope compositions of fossil teeth from South Carolina were used to reconstruct paleoecologies and paleodiets ~400,000 years ago-a time when global climate is thought to have been quite similar to today's climate. These data confirm the presence of mixed grassland-forested ecosystems, and suggest mixed C3/C4 grass types, similar to modern ecosystems and grass species proportions. Different animal species occupied different ecological niches and exploited different food resources. For example, horses fed in grasslands, consuming variable amounts of C4 grasses, whereas "camels" fed in open woodlands or woodland/grassland areas, and tapir, deer, and "llamas" fed in dense forests. Data for saber-toothed cats show they had a strong preference for marginal areas between forests and grasslands, consistent with their ambush predator morphology. In contrast, wolves principally lived in denser forests, but had greater dietary flexibility.
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Guillaume Matton, University of Quebec at Montreal, Earth and Atmospheric Sciences, Montreal, QC H3C 3P8, Canada; et al. Pages 665-668.
The Richat dome is one of the most spectacular natural structures that can be seen from space. Often used as a landmark for space shuttle crews, it appears as a series of large concentric rings (50 km) located in the Mauritanian part of the Sahara desert. The center of this bull's-eye structure is marked by a giant breccia and is intruded by alkaline volcanic and magmatic rocks. Several hypotheses have been presented to explain this feature (including meteorite impact) but its origin remains enigmatic. A model is proposed in which the intrusion of an underlying magmatic body resulted in the bulging of the overlying crust and production of fluids, thus creating a favorable setting for the dissolution of sedimentary rocks. Formation of voids has led to the collapse and brecciation of overlying units. Thus, Matton et al. suggest that this unique structure had a terrestrial origin, ultimately forming from the effects of an intrusion originating from Earth's mantle.
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Thomas R. Watters, Smithsonian Institution, Washington, DC 20013-7012, USA; et al. Pages 669-672.
Thirty years ago Mariner 10 made three flybys of Mercury and discovered puzzling troughs forming giant polygons in the floor of the Caloris Basin, one of the largest impact basins in the solar system. No landforms analogous to the Caloris polygonal troughs have been found in the floors of impact basins on either the Moon or Mars. New analysis and modeling of the polygonal troughs in Caloris suggests that they are the result of extension caused by late-stage uplift of the basin floor due to lateral flow of the lower crust. Modeling also suggests that the amount of late-stage uplift depends on the crustal thickness and that the extensional troughs may be unique to the Caloris Basin because of Mercury's relatively thick crust.
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Sandra McLaren, Australian National University, Research School of Earth Sciences, Acton, ACT 0200, Australia; et al. Pages 673-676.
The Proterozoic was a time of great global change and substantial mineral wealth. Australia is endowed with rocks that are significantly enriched in the heat producing elements (Uranium, Thorium, and Potassium) that provide a unique view of how Earth worked at this time. McLaren et al. suggest that this hot-crust provided an extra control on tectonic activity that helps to explain many of the features of Australian crustal evolution at this time. It also helps to explain the long-term evolution of Earth's continental crust. The progressive redistribution of the heat producing elements into the upper crust through tectonic processing may reflect a fundamental control on the organization of the continents and help to explain the emergence of modern-day plate tectonics.
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K.T. Lawrence and T.D. Herbert, Brown University, Geological Sciences, Providence, RI 02912, USA. Pages 677-680.
The Australian Great Barrier Reef is the largest coral reef system in the world. Recent geologic evidence suggests that the reef system is a geologically young feature forming within the last 800,000 years. However, the mechanism responsible for this growth remains unclear. After studying ocean sediments recovered from the northwestern margin of the Australian continent, several paleoclimatic investigators hypothesized that a warming of sea surface temperatures in the region during the last 700,000 years was responsible for the recent rise of the reef. Using an independent method to reconstruct past sea surface temperatures, Lawrence and Herbert find that there were only small fluctuations (~1.5 ºC or less) in ocean surface temperatures in the vicinity of the Australian Great Barrier Reef over the past 800,000 years. Their results, therefore, refute surface water temperature change as a cause for recent Great Barrier Reef growth. Further examination of the sediment fraction on which the initial sea surface warming hypothesis was based indicates that these sediments likely suffered alteration after deposition on the sea floor, which has undermined their usefulness as indicators of past climate.
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Marc-André Gutscher, CNRS, IUEM, Univ. Bretagne Occ., Domaines Oceaniques, Plouzane, Brittany F-29280, France. Pages 685-688.
Of the many theories proposed to explain the Atlantis legend, a small submerged island (Spartel Bank) in the western Straits of Gibraltar is among the most plausible. Plato's account in the Timaeus and Critias (360 B.C.) describes catastrophic destruction by "violent earthquakes and floods in a single day and night." This is consistent with the impact of a great earthquake (magnitude 8–9) and tsunami, like the famous Great Lisbon earthquake which ravaged the southern Iberia region in 1755. In this paper, the sedimentary record of great earthquakes and tsunami is examined and suggests this type of event occurs here roughly every 1500-2000 years. A thick, coarse grained sedimentary deposit dated at 10,000 BC may correspond to the event described in Plato's account. The results of recently conducted bathymetric mapping of the seafloor of Spartel paleo-island are also reported.
GSA TODAY Science Article
Red rock and red planet diagenesis: Comparisons of Earth and Mars concretions
Marjorie A. Chan, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111, USA; et al. Pages 4-10.
Since Italian astronomer Schiaparelli suggested that the curious linear features on Mars were canals, humans have wondered about the presence of water on the Mars. In the August issue of GSA Today, Marjorie Chan and co-authors use a simple yet elegant approach to examine evidence for a watery history on Mars: comparing red rocks on Earth to red rocks on Mars. Chan's research on Earth has examined the origin and evolution of the red color so prominent in sandstones of the southwestern United States; during this research, she developed an explanation for the unusual concentrations of minerals called concretions (technically) and marbles (colloquially). The similarity between Earth marbles and Martian "blueberries" observed by the Mars Exploration Rover Opportunity team led Chan to use the genesis of marbles on Earth as a means to understand the Martian examples. Her research suggests that the Red Planet has indeed had a history that involved water and, importantly, included water that moved in the subsurface of the planet, much like what has been deciphered for the Colorado Plateau.
To view the complete table of contents for the August issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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