November-December GSA BULLETIN media highlights

10/27/05

Boulder, Colo. – The November-December issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include: environmental changes in Death Valley over the past 190,000 years; analysis of the San Andreas fault zone at San Gorgonio Pass; the developing Coso geothermal field in east-central California; new insights into the dynamics of desert dune systems; new data on the Antarctic Peninsula ice sheet during the last ice age; and revised dating of the Patagonian Andes.

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An ostracode based paleolimnologic and paleohydrologic history of Death Valley: 200 to 0 ka
Richard M. Forester, U.S. Geological Survey, Denver, Colorado 80225, USA, et al. Pages 1379-1386.
Keywords: Death Valley, paleohydrology, paleolimnology, ostracodes, climate.

This paper examines fossil ostracodes (a small shelled crustacean) found in samples from a sediment core. Ostracode fossils offer a way to determine what kinds of environments existed in Death Valley in the past, in this case during the last 190,000 years. Interpretation of the ostracode data suggests that in the past there were large fresh to slightly saline lakes, shallow saline lakes, brine lakes that precipitated halite (table salt), and flowing spring–supported wetlands. The large lakes were common during the full extent of past ice ages, while salt lakes and wetlands existed during the waxing or waning phases of ice ages. The modern hot and dry setting was common between ice ages. The study documents how climate change can radically affect even a place like Death Valley.


Geochemistry of Neoarchean (ca. 2.55–2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt, central orogenic belt, North China craton: Implications for geodynamic setting and continental growth
Ali Polat, Department of Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada, et al. Pages 1387-1399.
Keywords: Archean, ophiolite, greenstone belt, continental crust, North China craton.

We report geochemical data for Archean (2.55-2.50 billion years old) volcanic and intrusive rocks exposed in the Wutaishan greenstone belt, North China. This study has provided new information on the geodynamic origin of this belt and its mineral deposits. We propose that anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have played an important role in the formation and preservation of this belt. Our model has important implications for the origin of Archean continents. The origin of the Archean continents is crust genetically linked to eclogites through partial melting of the accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction.


Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater
Peter S. Mozley, Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA, and J. Matthew Davis, Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA. Pages 1400-1412.
Keywords: calcite, concretions, sandstone, groundwater, New Mexico, Santa Fe Group.

A recent study by Peter Mozley (New Mexico Tech) and J.M. Davis (University of New Hampshire) provides further evidence for the importance of microbes in controlling early chemical reactions in Earth's crust. Mozley and Davis studied elongate calcite concretions commonly found in nonmarine sandstones. (Concretions are localized concentrations of a particular cement that form in a wide variety of shapes and sizes.) Such elongate concretions are known to form parallel to the direction of ancient groundwater flow, but the mechanism of their formation had not been previously determined. The authors conclude that the concretions form in elongate zones of altered water chemistry that formed downstream from particles of organic matter undergoing microbial decay. The elongate zones are comparable in many ways to plumes associated with point-source contaminants found in aquifers, but formed at a much smaller scale. The results of this study indicate that the influence of microbes on sandstone cementation may be far more important than previously supposed.


Geochemical constraints on the origin of the Annieopsquotch ophiolite belt, Newfoundland Appalachians
C. Johan Lissenberg, Department of Earth Sciences, University of Ottawa, and Ottawa-Carleton Geoscience Centre, Ottawa, Ontario, K1N 6N5, Canada, et al. Pages 1413-1426.
Keywords: ophiolite, suprasubduction zone, geochemistry, boninite, Appalachians, Newfoundland, Iapetus.

This paper discusses the origin of an extensive ophiolite belt that occurs near the Iapetus suture zone in the Newfoundland Appalachians. Geochemical data of the ophiolites indicate that they comprise three magmatic phases. Along with regional tectonic constraints, these data indicate the ophiolite belt formed during initiation of west-directed subduction. The Annieopsquotch ophiolite belt thus constrains the timing of initiation of west-directed subduction outboard of Laurentia, thereby providing a framework for the tectonic evolution of the Annieopsquotch Accretionary Tract and the peri-Laurentian portion of the Newfoundland Appalachians


Magmatic and tectonic history of Iceland's western rift zone at Lake Thingvallavatn
Jonathan M. Bull, School of Ocean and Earth Science, National Oceanography Centre, Southampton University, Southampton SO14 3ZH, UK, et al. Pages 1451-1465.
Keywords: neotectonics, dikes, subsidence, normal faults, Iceland, liquefaction.

In a study of a lake called Thingvallavatn in the Western Rift Zone of Iceland, Bull and coauthors demonstrate how the large-scale morphology of the lake was largely formed during an eruption from Hengill volcano around 2000 years before present. The authors used a small research vessel to collect under-way-remote acoustic observations of the lakebed and sub-lakebed. Analysis of this data showed that during the eruption of lava into the southern part of the lake 2000 years ago there was simultaneous emplacement of a scoria cone, movement on normal faults, and the liquefaction of soft sediments within the lake.


Late Pleistocene eolian history of the Liwa region, Arabian Peninsula
Stephen Stokes and Helen E. Bray, Saint Catherine's College, University of Oxford, Oxford OX1 3QL, UK. Pages 1466-1480.
Keywords: Arabian Peninsula, chronology, eolian, dune, optical dating.

Optical dating of some of the world's largest sand dunes from the Rub Al Khali have provided important insights into the dynamic of desert dune systems. The dunes and oases of the Rub Al Khali, made famous in the west by travelers such as Lawrence of Arabia and Wilfred Thesinger, have always been thought to potentially provide a record of how the deserts of the old world have responded to changing global climates. The long-standing conventional wisdom has been that the desert expands and builds large dunes over long periods of so called "glacial" conditions, which correspond to phases of recent earth history in which high latitude glacial ice has expanded on land in Greenland and Antarctica. This study has cast an important new light on this issue. By using new dating techniques to directly date the sand dunes, it has been demonstrated that the bulk of the dune landscape of the Arabian Peninsula was actually preserved during interglacial periods of high-latitude warmth. These interglacial periods result in globally high sea levels and increases in rainfall in the intertropical areas. The determining factor for the construction of the desert landscape is one of preservation instead of dryness. Preservation results from the combination of high sea groundwater levels and the import of carbonate-rich dust, which serves to cement ancient dunes. It is likely that during the glacial dry periods dunes were also active in the region, but conditions were not favorable for their preservation in the geologic record.


Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA
Timothy G. Fisher, Department of Earth, Ecological, and Environmental Sciences, University of Toledo, Toledo, Ohio 43606, USA. Pages 1481-1496.
Keywords: Lake Agassiz, glacier, spillway, strandline, Pleistocene, Quaternary.

Glacial Lake Agassiz, a large Pleistocene lake, left behind beaches at multiple elevations within its basin. Prior researchers have used these beaches to identify four relatively stable water level stages, but tracing them over long distances is often problematic. In this study, the elevation of the Tintah beach is returned to its original designation, and a newly interpreted stable lake level at 1020–1030 ft (310.9–313.9 m) is referred to as the Upham level. Cores from beaches and lagoon complexes reveal a decreasing thickness of beach sediment with lower elevation beaches. All but the Campbell beaches at the southern outlet are interpreted as the result of lake level lowering driven by incision of the southern outlet spillway.


Ice-sheet extent of the Antarctic Peninsula region during the Last Glacial Maximum (LGM)-Insights from glacial geomorphology
David C. Heroy and John B. Anderson, Department of Earth Science, Rice University, Houston, Texas 77005, USA. Pages 1497-1512.
Keywords: Antarctic Peninsula, deglaciation, geomorphology, glacial geology, glacial sedimentation, ice sheets, ice streams, swath bathymetry.

The global rise in sea levels that accompanied the melting of glaciers may owe more to the Antarctic Peninsula than had been thought. New data show that the peninsula's ice sheet extended about 220 km (~135 mi) beyond its current shoreline-more than twice as far out as earlier estimates-during the last Ice Age, before it began retreating about 16,000 years ago. In addition, these data show that the ice sheet was sitting on bedrock between 400-1200 m below sea level. The past extent of the ice sheet can be determined from the presence of glacial till-accumulations of silt and rock deposited by glaciers-in sediment samples and by seismic data from the sea floor. In addition, using a state-of-the-art seafloor imaging technique called multibeam swath bathymetry, we are able to discern the telltale grooves and humps scoured by glaciers. We also found ridges up to 70 km long, called mega-scale glacial lineations, left by ice streams draining the ice sheet's interior. These huge features were only discovered in Antarctica over the past five years, and are similar to grooves discovered by satellite imagery of North America in 1993. These new data indicate that the North American ice sheets were not the only big players in determining global sea level during the last Ice Age.


Insights from the Talysh of Azerbaijan into the Paleogene evolution of the South Caspian region
Stephen J. Vincent, CASP, Department of Earth Sciences, University of Cambridge, Cambridge, CB3 0DH, UK, et al. Pages 1513-1533.
Keywords: Talysh, South Caspian Basin, rifting, Paleogene, Tethys, Arabia-Eurasia collision.

This paper describes the Early Tertiary geology of the Talysh Mountains of Azerbaijan. It provides evidence for a major Middle Eocene rift event that affected basins across Iran, Azerbaijan, Armenian, Georgia, and Turkey and may constrain the age of the South Caspian Basin opening.


The Coso geothermal field: A nascent metamorphic core complex
F.C. Monastero, Geothermal Program Office, Naval Air Weapons Station, China Lake, California 93555-6108, USA, et al. Pages 1534-1553.
Keywords: metamorphic core complex, brittle-ductile transition, geothermal, eastern California, transtension.

On the basis of structural geology, geochemistry, geophysics, seismology, and petrology data, the Coso geothermal field, located in east-central California, is characterized as a nascent metamorphic core complex that is in the early stages of development. The feature is located in the midst of a Pleistocene-Holocene bimodal volcanic field that has numerous surface hydrothermal manifestations and is situated in a tectonically active dextral strike-slip fault zone. A shallow (4-km-deep) brittle-ductile transition with temperatures in excess of 350 °C is likely the result of crustal thinning that is compensation for horizontal plane strain. This transition zone is the future mylonitic shear zone that is commonly found in fully exhumed metamorphic core complexes.


Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California
V.E. Langenheim, U.S. Geological Survey, Menlo Park, California 94025, USA, et al. Pages 1554-1572.
Keywords: San Andreas fault, wedging, gravity data, magnetic data, Banning fault, San Gorgonio Pass.

Analysis of geophysical data in San Gorgonio Pass (20 km [12 mi] northwest of Palm Springs, California) shows a very complicated San Andreas fault zone, not only at the surface, but at depth. This analysis indicates how vertical deformation, as evidenced by the two tallest peaks in southern California that straddle San Gorgonio Pass, may be manifested in the upper and middle crust. Interpretation of small differences in Earth's magnetic field indicate rocks north of the fault extend at depth south across the San Andreas fault, forming a wedge-like structure. An even deeper level of convergence is suggested by seismicity. The multilayered nature of deformation in this part of the fault suggests that future earthquakes along the San Andreas fault will exhibit complex rupture patterns through the pass.


Stratigraphic record across a retroarc basin inversion: Rocas Verdes-Magallanes Basin, Patagonian Andes, Chile
Andrea Fildani, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA, and Angela M. Hessler, Department of Geology, Grand Valley State University, Allendale, Michigan 49401, USA. Pages 1596-1614.
Keywords: Patagonian Andes, orogenic evolution, backarc basin, Magallanes foreland basin, provenance, geochemical modeling.

The spectacular southern Andes, one of the most popular naturalistic destinations, are much older than previously thought. By analyzing a continuous ~6000-ft-long (~2000 m) section of sediment that was deposited on the ocean floor for over 40 million years, Fildani and Hessler discovered that the important tectonic transition from extension to mountain-building compression occurred more than ~100 million years ago, implying that the Patagonian Andes are not a recent feature. They also found that the main source for the ocean sediments was the rapidly growing Andean mountains, not the nearby remnants of the supercontinent known as Gondwana (an ancient continent composed by South America, Africa, Antarctica, and Australia). This has implications for how and when Gondwana broke apart to form the modern distribution of continents and clarifies how the southernmost Andes formed.


Debris avalanche deposits associated with large igneous province volcanism: An example from the Mawson Formation, central Allan Hills, Antarctica
O. Reubi, Department of Geology, University of Otago, Dunedin 9001, New Zealand, et al. Pages 1615-1628.
Keywords: debris avalanche, Mawson Formation, Ferrar, large igneous province, Allan Hills, Antarctica.

Debris avalanches are catastrophic landslides that normally occur on the flanks of volcanic edifices or other mountains. Several cubic kilometers of debris can be mobilized and come down the mountain very quickly, representing a significant hazard to nearby people and property. In volcanic settings, it is thought that the intrusion of magma in the volcanic edifice can trigger debris avalanches. It has been proposed, for instance, that such an intrusion-controlled sector collapse might occur in the Canary Island and cause a huge tsunami, which would cross the Atlantic Ocean to reach the North American coastline within hours. Reubi et al. (2005) have identified a new type of setting for debris avalanches, in relationship with the opening stages of magmatism in Large Igneous Provinces (LIPs). LIPs represent an unusual type of volcanism, occurring far away from the boundaries of tectonic plates; they produce voluminous lava flows that can reach hundreds of kilometers in extent and tens of meters in thickness. They also produce voluminous intrusions at shallow levels in Earth's crust, and these can trigger debris avalanches if steep slopes are present. Reubi et al. (2005) worked in the Transantarctic Mountains (Antarctica), where they identified the deposits of such an event.

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