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News Release 10 April 2008
GSA Release No. 08-16
Contact: Christa Stratton

April Media Highlights: Geosphere

Boulder, CO, USA – The April issue of Geosphere, published by the Geological Society of America, is now available online. Topics of interest include the major gold belts of north-central Nevada; California plate tectonics; GSA Penrose Conference-inspired recommendations for consistent terminology to describe geospatial data; imaging the new lakes of the Sahara and predictions for their 2011 evaporation; large crystals in the Sierra Nevada range; and the continuing hot-topic debate over a snowball versus slushball Earth.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Ann Cairns for additional information or other assistance.

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

Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah
Brian D. Rodriguez, U.S. Geological Survey, Box 25046, MS 964, Denver, Colorado 80225, USA; and Jackie M. Williams. Pages 315-328.
Keywords: suture zone, Archean, Proterozoic, gold, resistivity.

Results published in this paper suggest that major gold belts in north-central Nevada may be underlain by isolated blocks of Archean crust, which are known to contain major gold deposits. Later gold deposits in this region may be produced from recycled Archean gold. The authors’ results also suggest that future mineral exploration to the east may yield large gold tonnages, and exploration near edges of isolated blocks of Archean crust depicted with these results may yield large tonnages in areas of favorable ore controlling structure and stratified sediments.

Accretionary Mesozoic-Cenozoic expansion of the Cordilleran continental margin in California and adjacent Oregon
William R. Dickinson, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 329-353.
Keywords: accretion, California, Klamath Mountains, Sierra Nevada, tectonics.

Because California was the first place in the world where concepts of plate tectonics were applied in detail to questions of continental geology, the California segment of the Cordilleran mountain belt has served for decades as the global exemplar of accretionary tectonics whereby continental margins are expanded by the addition of wide tracts of new ground as a result of plate interactions. This paper offers a summary appraisal of accretionary tectonics in California over the past 200 million years based on fresh compilations of the ages and areal patterns of rock assemblages that underpin more than half the width of California. Many past inferences by a host of past investigators are confirmed and expanded, but some novel interpretations seem necessary in light of information developed from research during the last few decades.

Describing the dimensionality of geospatial data in the earth sciences—Recommendations for nomenclature
Richard R. Jones et al., Geospatial Research Ltd., Department of Earth Sciences, and e-Science Research Institute, University of Durham, Durham DH1 3LE, UK. Pages 354-359.
Keywords: geological representation, digital surveying methods, virtual outcrop models, dimension, coordinate axes.

At a recent GSA Penrose Conference, it became clear that there is much ambiguity and inconsistency in the nomenclature used by geoscientists to describe spatial data (i.e. datasets that include the position of datapoints in terms of geographical coordinates). In this paper, the conveners of the conference discuss current terminology and make recommendations to encourage more consistent usage.

Whole crustal response to late Tertiary extension near Prince Rupert, British Columbia
Lincoln S. Hollister et al., Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. Pages 360-374.
Keywords: crustal extension, seismic reflectivity, Moho, British Columbia, tectonic reactivation.

Crust-penetrating multichannel seismic (MCS) data imaged crustal features of Late Tertiary extension in east Dixon Entrance, British Columbia. The extension-related features extend from the brittle upper crust down through the ductile lower crust to the Moho. The data show grabens, mid-crustal west-dipping reflecting packages interpreted as normal sense shear zones, mid- to lower crustal subhorizontal reflecting horizons, a generally reflective Moho, and arches of the Moho with relief up to 3 km. The principal direction of extension was east-west, and it was likely concentrated between 25 and 20 million years ago. We interpret the features to be due to stretching of a relatively weaker crust over a stronger mantle lithosphere. Integration of our results with the geologic history across the study area suggests that preexisting zones of weakness may have controlled the pattern of crustal thinning.

Rise and demise of the New Lakes of Sahara
Mohamed G. Abdelsalam et al., Department of Geological Sciences and Engineering, Missouri University of Science and Technology, 129 McNutt Hall, 1400 N. Bishop, Rolla, Missouri 65401, USA. Pages 375-386.
Keywords: New Lakes of Sahara, Western Desert, Egypt, Nubian aquifer.

Satellite and space shuttle images are used to monitor the New Lakes of Sahara and predict when they will disappear. These lakes started forming in the Sahara Desert in southern Egypt in September 1998 when water spilled westward from Lake Nasser due to high precipitation in the Ethiopian Highlands. This region had not witnessed standing water for over 6000 years. Five new lakes were formed, and at their peak in September 1999, they covered an area of about 1600 km2 (20 times the area covered by the five boroughs of New York City), holding about 27 billion m3 of water in one of the driest places on Earth. The lakes are now rapidly drying out because of high rate of evaporation, and it is predicted that they will disappear completely by March 2011, leaving behind a crust of salt that will change the face of Sahara forever. This study suggests that recharge of the Nubian Aquifer by the lakes’ water is minimal since the lakes are spread over an impermeable layer.

Igneous phenocrystic origin of K-feldspar megacrysts in granitic rocks from the Sierra Nevada batholith
James G. Moore, U.S. Geological Survey, Menlo Park, California 94025, USA; and Thomas W. Sisson. Pages 387-400.
Keywords: Sierra Nevada, megacryst, barium zoning, K-feldspar.

Abundant large crystals (4–10 cm) of potassium feldspar typify four massive granitic intrusions along the crest of the Sierra Nevada, California, including the frequently visited Mount Whitney and Tuolumne Meadows in Sequoia and Yosemite National Parks. These granites, with their spectacularly large feldspar crystals, are also the youngest intrusions in the range. Why some granitic rocks contain such exceptionally outsized feldspar crystals, while most do not, has been a persistent issue in understanding how granites form. This study presents diverse evidence that the megacrystic granites formed by repeated surges of fresh hot magma recharged the underground magma reservoirs (magma chambers). These replenishments kept the magma bodies hot and in a partly molten state for extended time periods, and also provided necessary chemical components, thereby allowing growth of exceptionally large crystals. Other chemically similar granitic bodies that lack such large crystals probably were not replenished as frequently or as consistently, and therefore cooled and solidified too quickly for large crystals to grow. That all of the youngest granites of the Sierra Nevada contain these crystals and are among the largest granite bodies in the range suggests that the locus of magma formation stalled beneath what is now the crest of the range before migrating rapidly eastward and ending Sierran magmatism.

A snowball Earth versus a slushball Earth: Results from Neoproterozoic climate modeling sensitivity experiments
Arne Micheels, Senckenberg Research Institute, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany; and Michael Montenari. Pages 401-410.
Keywords: Neoproterozoic, snowball Earth, climate modeling, climate modeling sensitivity experiment.

The Neoproterozoic was characterized by an extreme glaciation, but up until now there has been no consensus on whether it was a complete (snowball Earth) or a softer glaciation (slushball Earth). For this period of dramatic global cooling, we performed sensitivity experiments with an Earth model of intermediate complexity. Our sensitivity simulations have boundary conditions that represent situations from a snowball to a slushball Earth. All Neoproterozoic model experiments represent much colder conditions than today and a widespread glaciation. In the case of an initial forcing representing a snowball Earth, the model maintains its complete glaciation, and temperatures are as low as -45 °C in equatorial latitudes. At the poles, the snowball experiments demonstrate annual average temperatures of less than -70 °C. If the initial model forcing is more moderate (slushball Earth), polar temperatures are below -50 °C, but temperatures in low latitudes stay well above the freezing point of water and therefore ice-free ocean areas remain. We conclude that our simulations support the rather moderate scenario of a slushball Earth instead of the extreme snowball Earth hypothesis. The experimental design and the model might, however, limit the interpretation of our results.

Characteristics of mantle fabrics beneath the south-central United States: Constraints from shear-wave splitting measurements
Stephen S. Gao et al., Department of Geological Sciences and Engineering, Missouri University of Science and Technology (formerly University of Missouri–Rolla prior to 1/1/2008), 129 McNutt Hall, Rolla, Missouri 65409, USA. Pages 411-417.
Keywords: shear-wave splitting, seismic anisotropy, continent-ocean transitional crust, mantle flow, south-central United States.

It has long been recognized that the Earth’s surface is divided into tens of pieces that are scientifically called “tectonic plates.” Relative movement between the plates creates mountain belts, volcanoes, and earthquakes. However, there are still debates about the origin of the force(s) that drive plate motion. One hypothesis is that the plates are driven by underlain currents, although the existence, direction, and strength of the currents are also topics of debates. Our measurements of the birefringent effect (“shear-wave splitting”) of elastic waves produced by distant earthquakes recorded by stations in Texas, Oklahoma, and Arkansas suggest the existence of mantle flow around the “keel” of the North American continent. The flow is in a plastic layer called the asthenosphere, which is beneath the mechanically stronger lithosphere. The measurements also imply that the magmatic dikes, which can be observed at the surface in southern Oklahoma, extend to a large depth, perhaps over a 100 km.

Automatic detection of anisotropic features on rock surfaces
Bodey R. Baker et al.; Klaus Gessner, corresponding author, Centre for Exploration Targeting, School of Earth & Geographical Sciences, The University of Western Australia, Crawley, Western Australia 6009, Australia, and Computational Geoscience, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Exploration and Mining, Kensington, Western Australia 6151, Australia. Pages 418-428.
Keywords: digital mapping, roughness, fault planes, photogrammetry, fractures.

Surface roughness is an important property of rock outcrops. We have developed an automatic technique to analyze the shape of natural rock surfaces formed by weathering, tectonic fault movements, and mining activities. We show applications of the method to outcrops in England, Turkey, Australia, and Germany. Our results indicate that geometric aspects of surface roughness change across the scale of observation. The method has potential to be applied in geology and engineering.

Geometry of the Neoproterozoic and Paleozoic rift margin of western Laurentia: Implications for mineral deposit settings
Karen Lund, U.S. Geological Survey, MS 973, Denver Federal Center, Box 25046, Denver, Colorado 80225, USA. Pages 4259-443.
Keywords: U.S. and Canadian Cordilleran miogeocline, Laurentia, Neoproterozoic–Paleozoic rift, sediment-hosted mineral deposits, sedimentary exhalative, Mississippi Valley type.

From about 709 to 360 million years ago, the western edge of the ancient North American (Laurentian) continent formed due to the rifting away of other continental fragments. Subsequently, the resultant western margin of Laurentia was obscured and complicated by the addition of younger rocks. The location and original setting of margin segments are well documented for the Canadian and southern U.S. Cordillera but are poorly understood for the northern U.S. Cordillera. This paper integrates new data for rift-related sedimentary and igneous rocks from central Idaho to north-central Nevada, interprets them with respect to original rift setting, and links resulting and published interpretations of the rift system along the Cordillera. The interpretations indicate that the margin formed as a zigzag system of northwest-striking extensional fault segments offset by northeast-striking fault systems. The thickness and depositional environment of sedimentary rocks and the location of rift-origin igneous rocks reflect the rift-segment geometry. Associated sediment-hosted, lead-zinc-silver (plus gold and barite) exhalative-type mineral deposits formed in sediments deposited along the continental slope. Post-rift deformation of the continental margin resulted in rift segment-parallel structures. Resultant circulation of hydrothermal fluids formed Mississippi Valley–type, lead-zinc-silver mineral deposits in rocks originally deposited along the continental shelf. These interpretations provide an integrated model for formation of the Cordilleran margin of Laurentia and for its associated metal endowments.

Evolution of Devonian carbonate-shelf margin, Nevada
Jared R. Morrow, Department of Geological Sciences, 5500 Campanile Drive, GMCS 237, San Diego State University, San Diego, California 92182-1020, USA; and Charles A. Sandberg. Pages 445-458.
Keywords: Devonian, Nevada, shelf-slope break, carbonate platforms, tectonism.

Incorporating data from published syntheses and ongoing research, this article summarizes the geologic history of the Nevada segment of an extensive Devonian carbonate-shelf margin that fringed western North America. This Devonian margin records a complex interplay of sea-level change, sedimentation, and tectonism, including the major late Middle Devonian transition from extensional to compressional tectonic modes associated with the earliest phase of the Antler orogeny. Fault-bound, offshore shelf-margin basins, which were present especially during the Early to early Middle Devonian, were sites of synsedimentary exhalative gold deposition that subsequently formed a significant part of the economically important Carlin trend reserves. During the Late Devonian, the Antler orogeny, together with the marine Alamo Impact Event, profoundly changed the position and morphology of the carbonate-shelf margin.


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