|8 August 2011
GSA Release No. 11-52
Director of Education, Communication, & Outreach
GSA BULLETIN HIGHLIGHTS: NEW RESEARCH POSTED 8 August 2011
Boulder, CO, USA – GSA BULLETIN has posted new research online in pre-issue publication. Locations highlighted in this new research include four areas of California, USA: the Poison Lake chain, the Cuyama River, Mono Lake, and the Mojave Desert; as well as the High Plains of Kansas, USA; and the Kyushu region of southwest Japan.
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Diverse, discrete, mantle-derived batches of basalt erupted along a short normal fault zone: The Poison Lake chain, southernmost Cascades
L.J.P. Muffler et al., U.S. Geological Survey, Volcano Science Center, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/B30370.1.
L.J. Patrick Muffler of the U.S. Geological Survey and colleagues present integrated studies of the Poison Lake chain in northeastern California (USA). The chain consists of 39 small, calc-alkaline basaltic volcanoes in a zone trending NNW parallel to nearby Quaternary normal faults. The volcanoes fall into nine coherent groups based on stratigraphy, field characteristics, petrography, and major-element compositions. Precise 40Ar/39Ar determinations show that the lavas erupted between 100,000 years and 110,000 years ago. Paleomagnetic investigations suggest that the entire Poison Lake chain could represent three short-lived episodes of volcanism within a period as brief as 500 years. Each of the nine groups represents a small, discrete magma batch generated in the mantle and stored briefly in the lower crust. The basalt groups cannot be related by crystal fractionation at different pressures, and compositions have not been affected significantly by incorporation of low-degree silicic crustal melt, interaction with sialic crust, or interaction with each other. Compositional diversity of these primitive magmas argues against widespread, long-lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting, assimilation, storage, and homogenization in the lower crust.
Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan
S.H. Mahony et al., Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK; doi: 10.1130/B30408.1.
How do volcanic and tectonic processes interact, particularly during rapid plate boundary evolution? S.H. Mahony of the University of Bristol and colleagues attempt to unravel the interactions between tectonism and volcanism in the Kyushu region of southwest Japan. They use new data to reconstruct the movements of the tectonic plates for the past 15 million years, and evaluate these reconstructions in the context of the volcanic history and volcano geochemistry in the same period. Mahony and colleagues suggest that subduction of the Kyushu-Palau ridge (a remnant arc) beneath Kyushu Island has introduced abundant fluids to the magmatic system and caused more voluminous volcanism at places like Aso caldera volcano. They also suggest that the volcanic and tectonic evolution of Kyushu is strongly tied to rapid migration of the triple junction between the Philippine Sea, Pacific, and Eurasian plates along the southwest Japan plate boundary over the past 15 million years.
Late Holocene alluvial history of the Cuyama River, California, USA
Stephen B. DeLong et al., Biosphere 2, University of Arizona, PO Box 8746, Tucson, Arizona 85723, USA; doi: 10.1130/B30312.1.
Rivers sediments are archives of past climate changes, but unraveling the history of rivers is challenging because the deposits they leave behind have complicated geometry. Distinct packages of river deposits can be separated by boundaries in three dimensions because rivers deposit and erode both vertically and laterally. These deposits are sometimes younger upward in a layered sequence like deposits in lakes or oceans, but younger material can also be deposited inset into, and lower than, older sediments if the river first erodes deeply into its bed. Along the Cuyama River, these geometries are particularly complex. Also, many dryland rivers have gone through cycles of erosion and deposition that are very similar in their timing across a region. The most famous example of this is the formation of widespread arroyos across the deserts of the southwestern United States. The arroyos emerged when rivers eroded into their beds during the late 19th and early 20th centuries after several hundred years of sediment deposition. The Cuyama River, studied by Stephen B. DeLong of the University of Arizona and colleagues, follows this regional synchroneity, supporting the idea that regional climate changes lead to changes in river behavior. The Cuyama River generally eroded during periods of extreme flooding after droughts, and deposited material during relatively dry periods.
High-resolution chemostratigraphic record of late Pleistocene lake-level variability, Mono Lake, California
Susan R.H. Zimmerman et al., Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA; doi: 10.1130/B30377.1.
Mono Lake (California, USA) is famous for its tufa towers, bird migrations, and 20th-century water wars with Los Angeles. Over the past glacial period, ~67,000 to 14,000 years ago, the Mono Lake basin was mostly hydrologically closed, and thus was a sensitive recorder of changing paleoclimatic conditions. In this paper, Susan R.H. Zimmerman of Lawrence Livermore National Laboratory and colleagues describe lake-level changes from the stratigraphy of the Wilson Creek Formation sediments, exposed in multiple outcrops around the basin. Three deep lake and two (relatively) shallower lake periods are indicated by the sediment facies, with a final highstand likely approximately 200 m higher than the modern lake level. The millennial-scale variability of calcium carbonate content in three outcrops of deep-lake silts is interpreted as a high-resolution record of lake-level changes, based on comparison with the stratigraphy. The high carbonate-high lake level relationship indicated is the opposite of the high carbonate-low lake level relationship inferred in nearby Owens Lake, a difference attributable to extreme differences in basin geometry affecting the frequency of spilling conditions and resultant lake chemistry.
Late Quaternary environmental reconstructions of playa-lunette system evolution on the central High Plains of Kansas, United States
Mark W. Bowen and William C. Johnson, Dept. of Geography, University of Kansas, 1475 Jayhawk Boulevard, Lawrence, Kansas 66045-763, USA; doi: 10.1130/B30382.1.
Playas are small, internally drained wetlands, and lunettes are isolated ridges that form immediately downwind of playas. Playa-lunette systems have been identified in semi-arid to arid regions around the world and are particularly abundant on the High Plains of the central United States. They provide a host of essential ecological functions, including water storage, groundwater recharge, and wetland habitat. Mark W. Bowen and William C. Johnson of the University of Kansas utilize a combination of stratigraphic and environmental data collected from two representative playa-lunette systems on the High Plains of Kansas to reconstruct geomorphic responses of these systems to past climate changes. Data indicate that playa-lunette systems have been an integral part of the High Plains landscape for the past 40,000 years, and possibly much longer. Landscape processes alternated between dissolution and deflation as playas developed and as temperature and precipitation regimes changed. Due to the aggradational environment and their sensitive nature, playa-lunette systems preserve a high-resolution record of environmental change throughout their evolution, and records are more detailed in these systems than in other upland depositional environments within the High Plains. Evidence of rapid climate change events, such as the Younger Dryas (anomalous cold period), is also preserved within the systems, though impacts on playa hydrology and playa-lunette system evolution are yet to be evaluated.
Chronology, sedimentology, and microfauna of groundwater discharge deposits in the central Mojave Desert, Valley Wells, California
Jeffrey S. Pigati et al., U.S. Geological Survey, Denver Federal Center, Box 25046, MS-980, Denver, Colorado 80225, USA; doi: 10.1130/B30357.1.
Long-lived desert wetlands persisted in many broad valleys and basins of the American Southwest during the late Pleistocene. Geologic deposits representing the ancient wetlands contain information on the timing and magnitude of past changes in water-table levels. Jeffrey S. Pigati of the U.S. Geological Survey and colleagues identify two distinct wetland sequences at Valley Wells in the central Mojave Desert (California, USA), which are similar in age and environmental setting to wetland deposits elsewhere in the Southwest. Pigati and colleagues suggest that Pleistocene groundwater levels in the Southwest were likely driven by synoptic-scale climate processes.