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Find Your Science at GSA
21 September 2009
GSA Release No. 09-45
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
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November/December 2009 GSA BULLETIN Highlights

Boulder, CO, USA – November/December GSA Bulletin articles are available now online “ahead of print.” Topics include an addition to the "subduction factory" study series; analysis of the ANDRILL-1B drillcore, the most complete single record to date of Late Neogene and Quaternary Antarctic Ice Sheet oscillations; detailed mapping of the North Spirit Lake greenstone belt; what fossils and minerals in the Tlayua Quarry, Mexico, reveal about past precipitation; and details of a 108-meter drillcore from Maxwell Bay, Antarctica, an area considered to be a "global hot spot."

Highlights are provided below. To review abstracts, go to http://gsabulletin.gsapubs.org/content/early/recent. Representatives of the media may obtain complementary copies of GSA Bulletin articles by contacting Christa Stratton at the e-mail 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.

Non-media requests for articles may be directed to GSA Sales and Service, .

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Cenozoic tectonics of the Nicaraguan depression, Nicaragua, and Median Trough, El Salvador, based on seismic-reflection profiling and remote-sensing data
Justin Funk et al., Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Pages 1491-1521.

Lakes Nicaragua and Managua are the two largest lakes in Central America, and they cover a combined area of ~9000 square kilometers of the presently active Nicaraguan depression and Central America volcanic front. As part of the subduction factory focus area of the U.S. National Science "MARGINS" program, ~1925 km of shallow geophysical data were acquired over Lakes Nicaragua and Managua in May 2006 to establish their late Quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western Nicaragua, the Gulf of Fonseca, and the Median Trough in El Salvador. In order to investigate regional, upper-crustal deformation resulting from forearc sliver transport and/or slab rollback of the Cocos plate, these new data were integrated with relocated earthquake epicenters, earthquake focal mechanisms, high-resolution digital topography from the NASA Shuttle Radar Topography Mission published GPS vectors, onland geologic maps, previous maps of lake bathymetry and bottom sediment types, a previously unpublished regional aeromagnetic data set, and multichannel seismic-reflection profiles from the Gulf of Fonseca and Pacific Ocean. These new data sets have improved bathymetric, bottom sediment, and recent fault maps for both Nicaraguan lakes and can be used as new constraints on the regional geology and tectonics. Three regional structural cross sections across the Nicaraguan depression indicate that the basin is a highly asymmetrical half-graben bounded to the southwest by northeast-dipping, oblique-slip normal faults. Late Oligocene to Holocene extension resulted in footwall uplift along the elevated and folded area of the Nicaraguan Isthmus, and the area of greatest subsidence occurs on the footwall block underlying Lake Nicaragua in the southeast. A similar but younger pattern of footwall uplift adjacent to the downthrown footwall block is present to the northeast beneath Lake Managua and the Gulf of Fonseca. Funk et al. interpret this structural pattern as a time-transgressive rift opening, where the oldest extension (late Oligocene-early Miocene) began in the southeast and migrated to the northwest. Global Positioning System data indicate that this earlier phase of intra-arc normal rifting is presently being superimposed by arc-parallel, right-lateral shear related to the northwestward transport of the Central America forearc sliver.

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Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate
Uri S. ten Brink et al., USGS, Woods Hole, Massachusetts 02543, USA. Pages 1522-1536.

Most volcanic island chains are separated by a belt of compressed rocks from the nearby ocean trench where one tectonic plate goes under (or subducts) the island chain. However, there are several island chains, where a second active belt of compression is found away from the trench. The secondary compressive belts in these locations are often associated with destructive earthquakes and tsunamis, sometimes more so than the primary compression belt, as attested by the 1992 M7.9 Flores (Indonesia) earthquake and tsunami, the 1999 M7.5 Vanuatu earthquake and tsunami, the 1991 M7.7 Limon (Costa Rica) earthquake and the 1882 San Blas (Panama) earthquake and tsunami. The secondary belts of compression have sometimes been explained by an inception of a secondary plate subduction in an opposing direction to the primary plate subduction, and this hypothesis is of interest to the study of the forces what drive plate tectonics. In this paper ten Brink et al. show that the development of the secondary belt of compression does not necessitate the development of a secondary plate subduction, but can be generated by the subducting plate pushing the island chain backwards, and because the island chain is rigid, it does not deform internally, but is pushed against the Earth crust behind it. That push forms the secondary belt of backward compressed rocks. The implication of this model is that the island chain should have relatively few active faults within it, and the backward belt should have shallow active compressive faults. This analysis helps explain the compressive belt south of Puerto Rico and predicts that this belt is particularly active south of eastern Dominican Republic where moderately large earthquakes and tsunamis could take place in the future.

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The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment
Robert McKay et al., Antarctic Research Centre, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Pages 1537-1561.

The history of the Antarctic Ice Sheets since mid-Miocene times (about 14 million years ago) is poorly constrained due to a sparse on-land geological record, and a lack of high-quality and continuous marine drillcore records for this time period. The paper by McKay et al. presents a sequence stratigraphic scheme for the ANDRILL-1B drillcore, the most complete single record to date of Late Neogene and Quaternary Antarctic Ice Sheet oscillations. Lithofacies are used to track changes in glacial and marine depositional environments. Three vertical facies successions, or "motifs," occur repeatedly in the ANDRILL-1B drillcore, and are related to glacial-interglacial oscillations of the ice sheet grounding line under three different styles of glaciation, or thermal regimes in the Ross Embayment. Comparisons of these motifs are made to modern analogues of glaciation from a range of polar to sub-polar settings to interpret the long-term evolution of the Antarctic Ice Sheet over the past 13 million years. Cold, "polar" type glacial conditions associated with the marine based ice sheet in the Ross Embayment occurred during the late mid-Miocene, followed by warmer sub-polar to polar glacial type conditions that occurred during the Late Miocene and Pliocene before reverting back to a cold polar glacial regime in Pleistocene times.

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Oceanic spreading center-generated basaltic crust and associated sulfidic and carbonate-rich hydrothermal deposits in the Archean (ca. 3 Ga), North Spirit Lake greenstone belt, Ontario, Canada
H.W. Nesbitt et al., Dept. of Earth Sciences, University of Western Ontario, London, Ontario, Canada, N6A 5B7. Pages 1562-1569.

In this study by Nesbitt et al. detailed mapping of a portion of the North Spirit Lake greenstone belt reveals a basal basaltic volcanic unit (about 3 billion years old) with a komatiitic component and strong major and trace element geochemical affinity with typical mid-ocean ridge basalts. The overlying transitional unit includes basaltic tuffs, which have close geochemical affinity to the underlying basalts, and to typical mid-ocean ridge basalts. These supracrustal rocks of the North Spirit Lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive Earth, including evidence for the existence of ocean-floor basalts, proximal and distal hydrothermal activity, and possibly a protracted cycle of ocean opening and destruction.

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High-resolution seismic and resistivity profiling of a buried Quaternary subglacial valley: Northern Alberta, Canada
Jawwad Ahmad et al., Institute for Geophysical Research, Dept. of Physics, MS 615, University of Alberta, Edmonton, Alberta T6G 2G7, Canada. Pages 1570-1583.

Waters running beneath ice-age glaciers carved out numerous valleys and channels. Some of these valleys remain unfilled and may still be seen today but many were filled by sediments as the ice sheets retreated. Often, there is no hint at the surface that such features exist and they are usually found serendipitously by drilling for water or deeper petroleum resources. Geophysics, however, can be used to better delineate these features and can provide further information on their internal structure. In this paper, Ahmad et al. carried out near surface geophysical surveys of a known buried valley near Rainbow Lake, Alberta. This valley is as deep as 350 m from today's surface. The electrical conductivity of the differing sediments allowed the valley to be distinguished from the surrounding bedrock. The seismic images further allowed numerous features, such as flat-lying lake sediments, to be seen at various depths throughout the valley fill material; indicating a complex history of filling of this valley. Such studies are important as they can help in the location of potable water, small commercial gas deposits used by local communities, and potential pathways for groundwater pollutants.

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Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous
Marina B. Suarez et al., Dept. of Geology, University of Kansas, 1475 Jayhawk Blvd., Lawrence, Kansas 66045, USA. Pages 1584-1595.

The response of the hydrologic cycle to global greenhouse conditions is important to our understanding of future climate change, and to the calibration of global climate models. Conditions during past periods of greenhouse world climates, such as those during the Cretaceous, the last age of the dinosaurs, can be used as an analogue. This study by Suarez et al. utilized stable isotope geochemical analyses of rocks from an important fossil locality in southern Mexico called the Tlayua Quarry, where marine, freshwater, and terrestrial fossils are found. Isotopes are different varieties of the same element that have differing numbers of neutrons, and thus differing weights. This study finds that the oxygen isotopic composition of precipitation during the Cretaceous was lighter than previously expected. The lighter values are likely due to greater amounts of precipitation. The data can be used along with data from other latitudes in computer models to calculate the annual precipitation rates for different latitudes during the Cretaceous. In addition, aspects of the rocks, such as the former presence of evaporite minerals like gypsum suggest high amounts of evaporation. The implication of this ongoing research is that during greenhouse climates, precipitation rates in some areas were much higher, and to balance this, evaporation was also higher. Similar changes can be expected as a result of future global warming.

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Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India
R. Sinha et al., Dept. of Civil Engineering, Indian Institute of Technology, Kanpur 208016, Uttar Pradesh, India. Pages 1596-1610.

The dominance of Himalayan rivers draining from the north in the modern configuration of the Ganga plains is more than obvious and this situation has often been extrapolated to the geological past without much supporting data. As a corollary, the role of cratonic rivers that join the axial Ganga and Yamuna rivers from the south has been minimized, both in terms of hydrological input and sediment contribution to one of Earth’s largest alluvial successions. However, the cratonic rivers, including the Chambal, Betwa, Sindh, and Ken, drain large upland catchments in northern peninsula India and are dynamic monsoonal rivers with large discharges of water and sediment. Sinha et al. demonstrate that the cratonic rivers were an important sediment source for the Ganga plains until 60,000 years before present or later. An extensive subsurface wedge of cratonic sediments extends well beyond the modern axial drainage of the Ganga and Yamuna rivers and is onlapped by sediments of Himalayan origin. Their interpretation is based on analysis of dense minerals in sandstones and clay minerals, which allow them to distinguish Himalayan and cratonic sources, and is supported by studies of alluvial architecture and age dates across the southern Ganga plains. Subsequent drainage reorganization appears to have affected the contribution of the cratonic rivers to foreland basin filling. An important element of this drainage reorganization may have been a relatively recent eastward avulsion of the Yamuna river into the Ganga plains from a previous westward course below its mountain exit. This westward course may have contributed water to the ancient Saraswati river during the period of the Indus Civilization. The demise of the Indus Civilization has often been linked to the relocation or drying up of the ancient Saraswati and, although the debate is still on, they believe that such drainage reorganization must greatly have affected the hydrological budget of river systems in the foreland basin, which would have been further modified by monsoonal fluctuations during the late Quaternary. With an increased Himalayan contribution to the southern Ganga plains in the recent past, the cratonic rivers appear to have become tributary to the Yamuna river, and Himalayan systems have pushed the cratonic systems to the basin’s feather edge. This is the first comprehensive and integrated study on subsurface sediments of the Ganga plains and demonstrates basin-scale competition of the Himalayan and cratonic rivers over a period in excess of 100,000 years. The study has important implications for the sediment budgets of foreland basins worldwide. It extends the debate on the role of Quaternary drainage reorganization in controlling hydrological budgets and cultural development in the Himalayan foreland basin.

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Structural architecture and active deformation of the Nankai Accretionary Prism, Japan: Submersible survey results from the Tenryu Submarine Canyon
Kiichiro Kawamura et al., Fukada Geological Institute, 2-13-12 Honkomagome, Bunkyo, Tokyo 113-0021, Japan; kichiro@fgi.or.jp. Pages 1629-1646.

Two research cruises that deployed submersible surveys were undertaken along the Tenryu Submarine Canyon to directly observe the structural architecture of the eastern Nankai Accretionary Prism of 2000-4000 meters in water depth, off the coast of southern Japan. From the leading edge of the prism near the trench toward the arc, the following deformation zones have been identified within the prism: Frontal Thrust zone, Prism Toe zone, Imbricate Thrust zone, and Tokai Thrust zone (or out-of-sequence thrust or OOST zone). Kawamura et al. conclude that out-of-sequence thrust faults play a major role in transporting deeply buried, deformed rocks in accretionary prisms to the shallower depths and even to the seafloor during ongoing subduction.

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An inter-ice-stream glaciated margin: Submarine landforms and a geomorphic model based on marine-geophysical data from Svalbard
Dag Ottesen and Julian A. Dowdeswell, Geological Survey of Norway, Trondheim N-7491, Norway. Pages 1647-1665.

In a study by Ottesen and Dowdeswell, well-preserved submarine landforms from the continental shelf and fjords of northwesternmost Svalbard provide an example of ice-sheet deposition in an inter-ice-stream. EM1002 swath-bathymetric imagery covering 1280 square kilometers was examined. At the shelf edge, a distinctive and continuous belt of hummocky topography represents the grounding zone of a slow-moving Late Weichselian ice sheet. Active ice on the outer shelf is inferred from subtle lineations orientated parallel to flow. Low-amplitude transverse moraines crosscut the lineations, suggesting ice retreat across the outer shelf with brief stillstands. On the middle and inner shelf, large moraine ridges indicate multiple stillstands during deglaciation. There are arcuate moraine ridges at fjord mouths. Streamlined crag-and-tail landforms are preserved from when active full-glacial ice flowed out of the fjords. Clusters of smaller transverse ridges indicate slow retreat of grounded ice through the fjords. Holocene sedimentation is by rainout from sediment-rich meltwater, producing smooth basin fill. Small slides from the fjord walls are common. Little Ice Age glacier re-advance produced another set of terminal moraines and smaller retreat moraines in the innermost fjords. A schematic model of this inter-ice-stream glacial landform assemblage summarizes the geomorphic record. It is compared with a model derived from several Svalbard cross-shelf troughs occupied by fast-flowing Late Weichselian ice streams. In general, the seafloor morphology of continental margins affected by ice streams is dominated by streamlined, subglacially produced landforms oriented in the former ice-flow direction, interrupted by major grounding-zone wedges formed during temporary halts in ice retreat. By contrast, between ice streams, shelf and fjord morphology records submarine landforms of various dimensions oriented mainly transverse to ice flow, produced at slowly retreating, grounded ice-sheet margins. There is little evidence for channeled subglacial water flow in the form of eskers and ice-contact fans on the Svalbard margin, implying that basal water is drained or advected mainly within soft subglacial sediments.

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Eruption recurrence rates in a basaltic volcanic field based on tephra layers in maar sediments: Implications for hazards in the Auckland volcanic field
Catherine Molloy et al., School of Geography, Geology & Environmental Science, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. Pages 1666-1677.

Basaltic volcanic fields consist of numerous small volcanoes that erupt once. The fields are known to be long-lived but frequency of activity is low. As human activities and infrastructure spread into such regions, it is important to assess the likelihood of future activity. Auckland City, New Zealand, is a large urban area of about 1.3 million people built on a basaltic volcanic field. New insight by Molloy et al. into the past 80,000 years of activity in the field has been gained by examining layers of volcanic ash (tephra) preserved in lake sediments. The tephra layers reveal recurrence times between eruptions have varied from less than 500 years to 20,000 years. There was a major "flare-up" in explosive activity at about 32,000 years ago. This is related to a period of simultaneous eruptions from several volcanoes across the field. The field has been relative quiet during the past 20,000 years, punctuated by the construction of a shield volcano 700 years ago. This highlights the difficulty in hazard forecasting for basaltic volcanic fields around the world.

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Geology and geochronology of Paleozoic rocks in western Acatlan Complex, southern Mexico: Evidence for contiguity across an extruded high-pressure belt and constraints on Paleozoic reconstructions
Carlos Ortega-Obregon et al., Departamento de Geologia Regional, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510, Mexico. Pages 1678-1694.

Ortega-Obregon et al. describe Paleozoic rocks from the western Acatlan Complex of southern Mexico that were previously thought to have been deposited on the opposite side of an ocean from those in the eastern Acatlan Complex. The data show that the geological record from both sides are very similar, a finding that supports the idea that the oceanic remnants traversing the Acatlan Complex represent rocks extruded into the complex above a subduction zone.

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Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)
Wolfgang Kuhnt et al., Institute for Geosciences, Christian-Albrechts University, 24118 Kiel, Germany. Pages 1695-1710.

In this study by Kuhnt et al., reconstruction of sea level fluctuations in a well-dated and expanded sedimentary succession from a marginal basin off SW Morocco provide new insights into the tempo and origins of sea-level variations in the middle Cretaceous greenhouse climate. The waxing and waning of ephemeral ice caps provide the only mechanism that can account for major sea level fluctuations in the order of several tens of meters such as the early-middle Cenomanian transition (96 million years ago). Major changes in the Earth's orbit appear to have provided the pacemaker for periodic glaciations in the middle Cretaceous greenhouse world.

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High-resolution Holocene climate record from Maxwell Bay, South Shetland Islands, Antarctica
K.T. Milliken et al., Rice University, Dept. of Earth Science, MS 126, PO Box 1892, Houston, Texas 77251-1892, USA. Pages 1711-1725.

Milliken et al. present results from the analysis of a 108 meter drill core from Maxwell Bay in the South Shetland Islands, Antarctica. This area is a "global hot spot" that has experienced warming of nearly 3 degrees Celsius this century. The objective of the study was to examine the climate history of the region during the past several thousand years and provide a framework for interpreting current climate change. The timing of climate events recorded at this location varies from that previously measured at four other sites in the Antarctic Peninsula region and reflects differences in factors such as topography, precipitation, and sea-ice coverage. The rapid regional warming and glacial retreat observed during the last century appear to be unprecedented during the Holocene epoch in both breadth and synchroneity.

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