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Formation of forearc basins and their influence on subduction zone earthquakes
Christopher W. Fuller, University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195, USA; et al. Pages 64-67.
Recent studies have demonstrated that the largest subduction zone earthquakes occur preferentially beneath thick, submarine sedimentary basins that develop along subduction margins. For this study, Fuller et al. conducted computer simulations of the forces experienced by the margin during subduction in order to determine how the deposition of sediment influences the forces that give rise to these earthquakes. They discovered that the weight of the sediment strengthens the edge of the margin above where the earthquakes occur. The stronger margin undergoes significantly less deformation than neighboring regions during the subduction process, and this lack of deformation influences several processes that increase the likelihood that large earthquakes will occur in these regions.
Earthquake-induced clastic dikes detected by anisotropy of magnetic susceptibility
Tsafrir Levi, Ben Gurion University, Department of Geological and Environmental Sciences, Beer Sheva 84105, Israel; Ram Weinberger (corresponding author), Geological Survey of Israel, Seismic Hazards, Jerusalem 95501, Israel; et al. Pages 68-71.
There is a growing need to identify structures induced by strong earthquakes in order to acquire a more complete paleoseismic record in seismically active zones. Such structures include injection dikes, which are probably the most impressive fluidization features occurring during strong earthquakes. However, these dikes are geometrically similar to clastic dikes formed by passive infill of materials into existing fissures, and, hence, inferring their mode of formation is commonly ambiguous. In order to distinguish between the two modes of formation, Levi et al. developed a novel application of the anisotropy of magnetic susceptibility analysis, assuming that clastic dikes of different origins have different indicative magnetic fabrics. They studied the mechanisms of clastic-dike formation within the seismically active Dead Sea Transform, where hundreds of dikes cross-cut the late Pleistocene lake sediments of the Lisan Formation. While this formation contains one of the longest and most complete paleoseismic records in the world, injection dikes were hitherto not recognized within it. Their study integrates considerations from stratigraphy, mineralogy, paleomagnetism, rock magnetism, and soil engineering. They demonstrate how the new magnetic analysis can be used as a petrofabric tool for distinguishing passively filled dikes from earthquake-induced injection dikes, hence identifying the latter as earthquake-induced structures. This identification provides a significant addition to the toolbox of paleoseismic studies.
Low temperature thermochronology: Resolving geotherm shapes or denudation histories?
T.J. Dempster, University of Glasgow, Geographical and Earth Sciences, Gregory Building, Glasgow, Scotland G12 8QQ, UK; et al. Pages 72-75.
The paper addresses the important concept of how scientists constrain the rates at which geological processes occur, such as the time scales over which mountain ranges are built and eroded. Earth's surface is always moving; mountain ranges are slowly built up by such processes as the collision between continental plates that generated the Himalayan Mountains over millions of years. Such mountains are then gradually worn away by erosion that acts like a potato peeler removing the surface layer, exposing the rocks underneath. Although rocks below Earth's surface are known to be at high temperatures, the way that temperature changes with depth, or geothermal gradient, is poorly known. This information is important, as many scientists base their calculations of rates of erosion on assumptions about the geothermal gradient. The paper compares long-term erosion histories from many areas in the world, which have been widely used to constrain rates of a variety of geological processes, and demonstrates that the geothermal gradients in Earth's crust are almost always wrongly estimated. This is due to the previously unrecognized thermal effects of fluids moving around within the crust and has resulted in rates of erosion of mountain ranges being overestimated by at least a factor of two. These errors have important implications for the way scientists estimate and predict the effect that climate change may have on the landscape. In many respects scientists' ability to accurately assess what the future may hold with a changing climate is dependant on the reliable assessment of the rates of processes in the past. The paper also shows how the problem can be solved by carefully choosing the location where to collect the rock samples that are analyzed to derive rates of erosion.
Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia
Birger Rasmussen, University of Western Australia, School of Earth and Geographical Sciences, Perth, Western Australia 6009, Australia; et al. Pages 76-79.
Determining the absolute age of mineralization is crucial to understanding the complex geologic events that lead to ore formation, but dating hydrothermal ore deposits is a difficult process. This is demonstrated by gold deposits in the Pine Creek Orogen of northern Australia where the lack of reliable ages for mineralization has led to a number of conflicting models. Using in situ U-Pb dating of phosphate minerals intergrown with gold-bearing sulfides, Rasmussen et al. show that neither intrusion-related nor recently proposed orogenic models are valid for this deposit, and they are probably not valid for the entire region. Instead, gold precipitation coincides with a period of regional deformation and heating about 1780 million years ago, an event not previously linked to mineralization. In situ phosphate dating stands to become one of the main tools for dating ore deposits; however, as our study demonstrates, it is vital to carry out careful sample selection, detailed petrography, and in situ dating of texturally constrained minerals to avoid confusion and misinterpretation. When correctly applied, in situ phosphate geochronology can not only provide precise dates for mineralization, allowing the validity of ore deposit models to be tested, but also advance our understanding of large-scale crustal fluid flow and the temporal relationship between ore formation and magmatism, metamorphism, and deformation.
Come a little bit closer: A high-resolution climate study of the early Paleogene Laramide foreland
Jacob O. Sewall, University of California, Santa Cruz, Earth Sciences, Santa Cruz, CA 95064, USA; et al. Pages 80-83.
As the warmest time interval in the Cenozoic and the most recent of the extreme "greenhouse" climates in earth history, the early Paleogene has long captured the attention of the paleoclimate community. Early researchers used plant fossils to develop a portrait of a climate vastly different from that of today. As increasing numbers and types of data became available, the picture of a time of global warmth solidified, and with the advent of computer models of climate, researchers began to investigate the causes of this intriguing climate. Fossil data do not, however, have uninterrupted spatial coverage and results from model studies are at a relatively coarse spatial resolution. Consequently, both data and model results fail to capture the spatial and temporal detail characteristic of the actual climate system. Sewall and Sloan have recently conducted a high-resolution, regional climate modeling study of the North American Laramide foreland. Their high-resolution simulation provides insight into the existence of snow and perennial ice in the Laramide highlands and northwestern Cordillera and describes the summer monsoon along the Rocky Mountain front in great detail. Monsoonal precipitation initiates in the southeastern Rockies in May, penetrates north and west as temperatures warm, and peaks at 1 cm/day along the southern and central mountain front; basins further to the north and west are drier.
Assessing decadal scale changes to a giant sand wave field in eastern Long Island Sound
Michael S. Fenster, Randolph-Macon College, Environmental Studies, Ashland, VA 23005, USA; et al. Pages 88-91.
This study analyzed the relatively long-term behavior of giant sand dunes or sand waves-up to 17 meters in height-that reside on the eastern Long Island Sound seafloor. Previous studies showed that the sand waves formed about 14,000 years ago as rising seas and strong tidal currents reworked a large delta. The delta developed as surging waters coming from glacial lake dam breaks in the Connecticut Valley deposited sediments into a young Long Island Sound. Whereas a one year study by the authors in 1987 showed that the sand waves moved very slowly, the study also gave indirect evidence that these giant dunes had migrated into Long Island Sound over longer periods of time. The results from this recent study present direct evidence that the sand waves migrated into Long Island Sound at a rate of about 2.2 meters per year over a 16 year time period (1987–2003). Moreover, these giant bedforms have complex migration mechanisms that include flexing, rotating, and migrating at different rates along their lengths. Finally, the results from this study suggest that the sand wave movement is slowing down and that the sand waves may eventually become preserved features of the seafloor as sea level rise rates and tidal current strength diminish.
Rapid sea-level movements and noneruptive crustal deformations in the Phlegrean Fields caldera, Italy
Christophe Morhange and Nick Marriner (corresponding author), CEREGE CNRS, Geomorphology and Tectonics, Europole Méditerranéen de l'Arbois, Provence 13545, France; et al. Pages 92-95.
Pozzuoli's Roman columns first attracted the interest of pioneer geologists during the 19th century, who believed the pillars could be used as a palaeotide gauge. Since this time the vestiges have come to symbolize a triumphant icon of actualism-the postulate that the present is a key to the past. The town lies at the centre of the Phlegrean Fields caldera (southern Italy), an active volcanic complex characterized by significant ground deformation. After the Roman market's construction, repeated magma chamber inflation and subsequent deflation caused the level of the land, recorded by the archaeological remains, to change significantly in relation to the sea. During periods of magma deflation, the level of the land fell, drowning the market area and its pillars. Conversely, during periods of magma inflation, the land rose relative to the sea. By radiocarbon dating the columns' marine remains, scientists have elucidated three 7 m relative sea-level highstands during the 5th century A.D., the early Middle Ages, and before the 1538 eruption of Monte Nuovo. The proposed reconstruction of the Phlegrean Fields' deformation history suggests that during the last 2000 years, noneruptive uplift episodes have been the rule rather than the exception.
Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone
Russell Drysdale, University of Newcastle, School of Environmental and Life Sciences, Callaghan, NSW 2308, Australia; et al. Pages 100-103.
About 4200 years ago, a severe drought of centennial-scale duration caused the abandonment of Old World civilizations in the low latitudes of northeast Africa and southwest Asia. Geochemical information extracted from a radiometrically dated cave flowstone in Italy reveals that the drought penetrated well into the middle latitudes of Europe.
Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California
James R. Hein, U.S. Geological Survey, Coastal and Marine Geology, Menlo Park, CA 94025, USA; et al. Pages 108-111.
A methane gas hydrate was recently discovered to occur only 24 km (15 miles) off the coast of Los Angeles in southern California. This is significant in that the hydrate occurs near the second largest urban area in the United States. Hydrates are important because of their potential contributions to mass wasting (landsliding), energy resources, abrupt global-climate change, and the global carbon mass balance. The newly discovered gas hydrate site is unique in that the composition of the calcite mineral deposits and shells of bivalves associated with the hydrate indicate the extreme flux of methane gas. The shells of the bivalves show an isotopic ratio of the contained carbon unlike that of any other marine macrofauna analyzed to date. The anomalous carbon composition of the shells likely reflects their habitat, with its unique chemical makeup. The source of the methane and the high quantities of heavy metals (such as mercury, cadmium, thallium, and silver) found in the sediment that hosts the gas hydrate is deep-seated; the gas and the metals are transported to the seafloor along a fault.
Large Holocene lakes and climate change in the Chihuahuan Desert
Peter J. Castiglia, University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87103-0001, USA; et al. Pages 112-115.
New evidence from relict lake shorelines in Chihuahua, Mexico, shows that millennially spaced wet periods caused currently dry basins in the U.S.- Mexico border region to fill with large lakes several times over the last 10,000 years. The evidence shows that moisture availability is not solely controlled by summer monsoon precipitation and alters the widely accepted paradigm for Holocene climate in southwestern North America by presenting evidence of a mid-Holocene wet period. Using a lake-level record from a closed basin in the modern North American monsoon moisture regime, Castiglia and Fawcett reconstructed changes in wet and dry periods. A series of lake highstands in the U.S.- Mexico borderlands region is synchronous with ice rafting events in the North Atlantic and glacial advances throughout the western United States, as well as wet and cold intervals recorded in other sediment, tree ring, and ice cores. This strong correlation with other wet and cold events documented in records from outside the monsoon moisture system shows that winter storms played a more important role in controlling regional climate than previously suspected. Understanding the magnitude, frequency, and mode of moisture trends in the borderlands region is a central component for water management policy and climate change research. The National Science Foundation, Los Alamos National Laboratories, and the University of New Mexico supported the study.
Rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism
Robert J. Musgrave, Geological Survey of New South Wales, Maitland, NSW 2320, Australia; et al. Pages 116-119.
Deep below the sea, in sediments on continental shelves around the world, are enormous concentrations of gas hydrate, an ice-like solid compound of methane gas and water. Gas hydrate is unstable, and readily decomposes to release the methane; only the enormous pressure of the overlying seawater keeps the gas hydrate stable. The zone in which gas hydrate is present extends down into the sediments, until the steady increase in temperature going down into Earth reaches a point where the hydrate is unstable again. Rising deep sea temperatures at the end of the last ice-age should have caused heat to diffuse down into the underlying sediments, with the result that hydrate near the bottom of the gas hydrate zone should have decomposed, and the base of the hydrate zone should have shifted upwards to a new stable level. Methane is a very powerful greenhouse gas, and some scientists have speculated that methane released to the atmosphere by the decomposed hydrate may have produced very rapid greenhouse heating. This idea could help explain why ice-ages end so suddenly. There is also the potential that current global warming might decompose more hydrate, leading to a runaway greenhouse effect. But gas hydrate leaves few traces behind, so testing whether the base of the hydrate zone did indeed jump upwards has been difficult. Now, scientists have used slight changes in the magnetic properties of the sediment, caused by bacteria that once lived on the now-vanished hydrate--in effect, magnetic fossils--to detect where the base of the hydrate zone lay at the end of the last ice age. The old base of the gas hydrate zone was tens of meters below its present-day position, confirming that large amounts of hydrate decomposed following the end of the ice age. The new magnetic technique for tracking hydrate migration has great potential for testing how melting hydrate might influence climate change.
Temperature change is the major driver of late-glacial and Holocene glacier fluctuations in New Zealand
Brian Anderson, Victoria University of Wellington, Antarctic Research Centre, Wellington, NZ 6001, New Zealand; et al. Pages 120-123.
Researchers have been debating for decades whether glacier advances in New Zealand are driven by cooler or wetter conditions. Now, Anderson and Mackintosh have shown that a glacial advance 13,000 years ago was caused by cooling of a similar magnitude to the Antarctic Cold Reversal, adding credence to an emerging school of thought that the Southern Hemisphere follows Antarctic, rather than Northern Hemisphere, millennial-scale climate signals.
Orbital time scale and new C-isotope record for Cenomanian-Turonian boundary stratotype
Bradley B. Sageman, Northwestern University, Geological Sciences, Evanston, IL 60208, USA; et al. Pages 124-127.
This paper reports on the use of advanced spectral analysis techniques to develop a high resolution orbital time scale for the late Cenomanian–early Turonian (C-T) interval at its proposed central Colorado stratotype in North America. The study interval is significant because it spans one of the most prominent of the Cretaceous oceanic anoxic events (OAE II) thought to represent an episode of massive organic carbon burial, drawdown in atmospheric carbon dioxide levels, and possible rapid climate change. The interval is also characterized by elevated levels of extinction among marine taxa. The new orbital time scale makes possible more precise quantification of rates of change in geochemical proxies and paleobiologic parameters across the OAE II interval. Correlation of the new time scale to other C-T sites will be aided by the publication, also in this paper, of a new high-resolution data set of stable isotopic data for organic carbon and carbonate.
To view the complete table of contents for the February issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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