November GEOLOGY and GSA TODAY Media Highlights

November 05, 2007

Boulder, CO, USA - Topics include: continental deformation and the San Andreas fault; interior heating of Mars; correlation of 200-million-year-old rocks between Britain and America; environmental stress and the end-Permian and end-Triassic mass extinctions; rock weathering in Canada's Mackenzie River basin as a CO2 source rather than sink; how stable continents split in the absence of active volcanism; and geologic evolution of Alaska and the northern Pacific Rim. The GSA TODAY science article explores conceptual uncertainty in geoscience interpretation.

Long-term continental deformation associated with transpressive plate motion: The San Andreas fault
James A. Spotila et al., Virginia Tech, Geosciences, Blacksburg, Virginia 24061, USA. Pages 967-970.

The San Andreas fault is perhaps the world's best known strike-slip fault, yet we still do not fully understand how it accommodates plate motion via continental deformation. Surprisingly, the fault is not parallel to plate motion, and more often than not it is transpressionally oblique. This means that convergence occurs along the fault, resulting in mountain building. But how should this mountain building be distributed along and away from the fault" Spotila et al. complete the first systematic review of how transpressional deformation is distributed along the entire plate boundary. They find that deformation increases towards the fault zone, but that it is very heterogeneous along the fault. This implies that local boundary conditions, such as erodibility of rocks, variations in climate (precipitation), and local structural complexity, largely dictate the pattern of transpressional strain along the fault.

Martian hydrogeology sustained by thermally insulating gas and salt hydrates
Jeffrey S. Kargel et al., University of Arizona, Hydrology & Water Resources, Tucson, Arizona 85721, USA. Pages 975-978.

The ability of certain hydrated salts to transmit heat by conduction is very limited. Therefore, thick deposits of these salts on Earth or Mars would tend to retain geothermal heat coming from the planetary interiors. Anomalous warm crustal temperatures may occur beneath and within these deposits. In the case of Mars, melting of ice and development of salty brines may occur. This mechanism of interior heating would have large consequences for possible Martian biology and geology.

Increased sediment accumulation rates and climatic forcing in the central Andes during the late Miocene
Cornelius E. Uba et al., University of Potsdam, Potsdam 14476, Germany. Pages 979-982.

Uba et al.'s research contributes to the better understanding of the interplay between tectonics and climate in the development of the Andes, in South America. Scientists have, hitherto, always explained the Andean evolution and sedimentation as a result of tectonics. Uba et al. have collected data that document increased sedimentation rate as a result of variability in climate between 8 and 6 million years ago. These results change the previous understanding of the Andean evolution and show climate is also an important factor in the development of the Andes.

Present-day temperatures in northern Scandinavia during the last glaciation
K.F. Helmens et al., Stockholm University, Physical Geography and Quaternary Geology, Stockholm, SE-106 91, Sweden. Pages 987-990.

The knowledge of changes in climate during the last ice age (between ca. 120,000 and 10,000 years ago) has enormously improved over the last decade through studies on Greenland ice cores and North Atlantic deep-sea sediments. On land, widespread erosion generally erases the geological record. Helmens et al. present results from a unique sediment sequence in northern Finland that significantly changes the present concept of climate variability on the northern European continent. Their data provide evidence of a period of rapid climate warming to present-day conditions next to the retreating northeastern margin of the Scandinavian Ice Sheet ca. 50,000 years ago.

A nonmarine record of eccentricity forcing through the Upper Triassic of southwest England and its correlation with the Newark Basin astronomically calibrated geomagnetic polarity time scale from North America
David B. Kemp and Angela L. Coe, Neftex Petroleum Consultants Ltd, 115BD Milton Park, Abingdon, Oxfordshire OX14 4SA, UK. Pages 991-994.

Recent work by British geologists has, for the first time, allowed approximately 200-million-year-old rocks to be accurately correlated between Britain and America at a potential resolution of just a few thousand years. Kemp and Coe's findings may allow ancient rapid global climate changes, recognizable in the geological record, to be precisely constrained in time in order to prove the synchronicity of events. This work may be beneficial in improving estimates into the timing, rates, and causes of ancient rapid global warming events and mass extinctions.

Bryozoan paleoecology indicates mid-Phanerozoic extinctions were the product of long-term environmental stress
Catherine M. Powers and David J. Bottjer, University of Southern California, Earth Sciences, Los Angeles, California 90089-0740, USA. Pages 995-998.

Marine life suffered two devastating extinctions, at the end of the Permian and the end of the Triassic, 252 and 200 million years ago, respectively. Through a global survey of Permian through Early Jurassic bryozoan assemblages, Powers and Bottjer show that the end-Permian and end-Triassic mass extinctions were part of two protracted intervals of environmental stress that initially affected deep-water settings before encroaching onto shallow shelves and killing marine organisms living in the photic zone. These results are important as they suggest a long-term oceanographic, rather than extraterrestrial, mechanism for these biotic crises, and shed new light on the long-term ecological influence of mass extinctions on the distribution and diversity of marine communities.

Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives
Damien Calmels et al., Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. Pages 1003-1006.

The chemical weathering of rocks is tightly coupled to the global carbon cycle because the dissolution of atmospheric carbon dioxide (CO2) in surface water is a main weathering agent of rock minerals. Dissolved weathering products as well as dissolved atmospheric carbon are transported by rivers to the ocean, where they combine to form carbonate deposits, trapping atmospheric CO2 for long periods of time. Although research into weathering has long focused on the role of CO2, there are other weathering agents that may disturb this carbon regulation loop. In addition to dissolved CO2, acidity of the Mackenzie River (one of the largest rivers in the world), Canada, is caused by sulfuric acid from the oxidation of sulfide minerals in the rocks underlying the catchment. Calmels et al. have shown that this mechanism contributes 60% of the river acidity, making sulfuric acid the most important weathering agent. Moreover, they have shown that its role increases with the intensity of physical erosion. In contrast to reactions involving CO2, the chemical weathering of rocks by sulfuric acid affects the global carbon cycle through the transitory release of CO2 from continental carbonate rocks to the atmosphere. In the Mackenzie River basin, weathering reactions are not a sink, but rather a source of CO2 to the atmosphere. If generalized, this conclusion may have interesting climatic perspectives.

New insights into the genesis of Indian kimberlites from the Dharwar Craton via in situ Sr isotope analysis of groundmass perovskite
Chad Paton et al., University of Melbourne, School of Earth Sciences, Victoria 3010, Australia. Pages 1011-1014.

Kimberlites are a rare and poorly understood form of volcano originating at great depths within the Earth. Unfortunately, their extreme nature makes them very fragile under surface conditions, and they quickly decay, a problem that conventional analytical techniques have been unable to overcome. By using a laser to drill holes of less than 0.1 mm in diameter, Paton et al. targeted particular minerals (perovskites) within the rock that are virtually immune to these weathering effects. These tiny crystals preserve reliable information about the composition of the magma that formed the volcano, providing new insights into kimberlite formation. The new strontium isotope data obtained from perovskites during this study suggest that kimberlites form at depths of hundreds of kilometers below the Earth surface and travel to the surface directly, without mixing with rocks of the Earth's crust. If this is the case, they may act as a "window" to the deep Earth, allowing us to better understand the processes at work deep within the mantle.

Age variation of pore water iodine in the eastern Nankai Trough, Japan: Evidence for different methane sources in a large gas hydrate field
Hitoshi Tomaru et al., Kitami Institute of Technology, New Energy Resources Research Center, Hokkaido 090-8507, Japan. Pages 1015-1018.

Gas hydrate is a large natural gas reservoir found commonly in marine sediments. Because of its high potential as a gas resource in the future and environmental impacts through Earth's history, an increasing number of scientific and industrial expeditions have been carried out to characterize gas hydrate occurrence and related geological phenomena. Tomaru et al. measured the concentration of a long-lived radioisotope of iodine (129I) in pore waters from the eastern Nankai Trough, offshore the main island of Japan, in order to determine the potential age of iodine as a proxy for the source of methane accumulated in the gas hydrate deposits. While iodine ages of as much as 50 million years are found in the landward basin area, those in the seaward ridge area are less than 32 million years old, with significant variation of less than 10 million years. This age contrast reflects methane accumulation from different source formations to an extensive gas hydrate filed; iodine and, by association, methane are derived from old Eocene subducted sediments in the landward region, while those in the seaward region originate preferentially from younger Miocene to Pliocene sediments. Iodine and methane are transported from the depths to the shallow gas hydrate through the active faults constrained by the subduction system.

'Vertically transferred' overpressures in Brunei: Evidence for a new mechanism for the formation of high-magnitude overpressure
Mark R.P. Tingay et al., University of Adelaide, School of Earth & Environmental Sciences, University of Adelaide, South Australia 5005, Australia. Pages 1023-1026.

Abnormally high fluid pressures (overpressures) are a very common phenomenon in sedimentary basins and are the primary cause of oil-field blowouts, such as the May 2006 East Java mud eruption. Overpressures are generally thought to be observed close to where they are generated. However, Tingay et al. use data from petroleum wells in Brunei to suggest that overpressures can migrate from their source to shallow depths. This new concept of overpressure transfer may explain the presence of anomalous overpressures in many other sedimentary basins worldwide.

New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc
Shuichi Kodaira et al., Japan Marine Science and Technology Center, Institute for Frontier Research on Earth Evolution, Yokohama 236-0001 Japan. Pages 1031-1034.

Kodaira et al. present three new constraints on the process of continental crust growth that are essential issues for understanding the evolution of Earth. They are: (1) Crust of continental composition is generated beneath basaltic volcanic centers in an intra-oceanic arc; (2) the bulk composition of the crust does not change during evolution from juvenile arc crust to mature arc crust; and (3) a process is required to return mafic to ultramafic components of the crust to the mantle to accomplish the evolution from arc crust to continental crust. These new findings resolve a longstanding paradox in the current understanding of the growth of continental crust in intra-oceanic arcs.

Mantle detachment faults and the break up of cold continental lithosphere
Roberto F. Weinberg et al., Monash University, School of Geosciences, Clayton, Melbourne, Victoria 3800, Australia. Pages 1035-1038.

Africa is currently in the process of splitting apart along its famous rift. The matching coasts of Africa and South America mark where these two continents, that now lie thousands of kilometers apart separated over 100 million years ago. There is considerable scientific interest in understanding why and how a stable continent like Africa splits into two, but there are also economic reasons for this interest: sediments deposited in the deep basins formed during continental rifting are currently our major source of oil and gas.

Weinberg et al. used a novel computational approach to investigate how stable continents split and become separated by newly formed oceans in the absence of active volcanism. The numerical models were designed to simulate the splitting of Europe from North America, so that the results could be checked against rock formations preserved offshore Spain and Portugal. Weinberg et al. found that flat-lying fault planes, known as detachment faults, develop naturally in the deeper part of rift systems. These detachments allow doming and exhumation of the deeply buried mantle rocks from underneath the continental crust. This mantle dome forces continents apart, and is ultimately exposed in ocean floors.

Escape tectonics and the extrusion of Alaska: Past, present, and future
T. F. Redfield et al., Geological Survey of Norway, Geodynamics, Trondheim, Trondelag 7491, Norway. Pages 1039-1042.

The geological evolution of Alaska is important to both science and economy. During the early days of North American geology, Alaska was viewed as part of a stable continent. By the middle of the previous century, came the first hints of tectonic unrest. In 1955, Professor Sam Carey suggested Alaska had been "oroclined"--bent nearly 60 degrees counterclockwise--at some point in the distant past, shaping the mountains into the arc we see today. Geologists of the 1970s interpreted much of southern Alaska from the perspective of plate tectontics, mapping individual terranes that were welded onto the margin by the Pacific Ocean crust diving beneath Alaska. But once attached, terranes were considered to be more or less fixed. Redfield et al. suggest instead that the northern Pacific Rim is a diffuse, wide, plate boundary zone--part of an inexorably continuous conveyor belt called the North Pacific Rim orogenic stream. From northern British Columbia to westernmost Alaska, whole landscapes are on the move, gliding serenely past interior, fixed North America on their way to an ever-growing terrane, parkeringsplass, in the Bering Sea. This is a new way to view the evolution of a significant part of the increasingly important circum-Arctic margin, and may have implications for our understanding of the development of the High Arctic basins.

Age constraints on the origin and growth history of a deep-water coral mound in the northeast Atlantic drilled during Integrated Ocean Drilling Program Expedition 307
Akihiro Kano et al., Hiroshima University, Department of Earth and Planetary Systems Science, Kagamiyama 1-3-1, Higashi-hiroshima, 739-8526, Japan. Pages 1051-1054.

Pliocene glaciation set up deep-water coral mounds. The Integrated Ocean Drilling Program Expedition 307 revealed that the growth of the 155-m-thick Challenger Mound in the northeast Atlantic was related to major changes in Plio-Pleistocene paleoceanography, because of the sensitivity of the mound-building corals to food supply, temperature, and sediment. Favorable conditions occur in the density gradient above the high-saline intermediate water from the Mediterranean. Mound establishment coincides with the start of modern northeast Atlantic stratification at ~2.6 million years ago. The mound then grew rapidly until ~1.7 million years ago and a second growth phase started at ~1.0 million years ago.

GSA TODAY Science Article

What do you think this is? "Conceptual uncertainty" in geoscience interpretation
C.E. Bond et al., Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK, and Midland Valley Exploration Ltd., 144 West George Street, Glasgow G2 2HG, UK.

If you ask 10 geoscientists to interpret a geological data set, you'll get 10 different answers. This is because geological data is inherently uncertain. Geoscientists piece together bits of information to build a picture of what they think is below our feet or of what happened many millions of years ago. To do this, they apply concepts that are based on rocks seen at Earth's surface and the systems that create them--from volcanoes to rivers and beaches. But how do the concepts we apply to scientific data sets affect our ability to correctly interpret them" This is the question addressed in the November 2007 GSA Today by Clare Bond and Zoe Shipton of Glasgow University and colleagues Alan Gibbs and Serena Jones at Midland Valley Exploration. They focus on the scientific contradiction of unbiased interpretation of data whilst applying known concepts. How do we know what concepts to apply" And by applying known concepts, are we biasing the interpretational outcome" In their research, the Scottish team demonstrates the extent of uncertainty in geoscience interpretation and consider the factors that influence the concepts applied, such as experience and training. Scientific uncertainty arising from concept-based interpretation is an important consideration for policy development in the twenty-first century because geoscience is a key component in the modeling of climate change and the prediction and management of resources.
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