Accelerator mass spectrometry in geologic research
Paul Muzikar, Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA, et al. Pages 643-654.
Keywords: accelerator mass spectrometry, cosmogenic nuclides, geochronology, surface-exposure dating, isotope geology.
Accelerator mass spectrometry (AMS) is an experimental technique that is having an increasingly important impact on many areas of geological research. AMS combines aspects of traditional mass spectrometry and nuclear physics, and allows very small numbers of certain radioactive nuclides to be counted. The production of these nuclides by cosmic rays in the atmosphere and in rocks, and their subsequent behavior, allows geologists to study many processes. Carbon dating was the first main application of AMS; now, nuclides such as 10Be, 26Al, and 36Cl are being used to: (1) establish chronologies for glaciations, (2) infer erosion rates, (3) deduce the history of variations in Earth's magnetic field and in the solar output, (4) date volcanic eruptions, and (5) date tectonic events. The time covered ranges to a few million years before the present. New developments in the AMS technique continue, which should allow for smaller and cheaper instruments, and faster and more precise measurements.
Segmentation of the Laramide Slab--evidence from the southern Sierra Nevada region
Jason Saleeby, Division of Geological and Planetary Sciences, California Institute of Technology, M.S. 100-23, Pasadena, California 91125, USA. Pages 655-668.
Keywords: tectonics, subduction, delamination, Cordillera, Laramide.
The Laramide orogeny has long been recognized as a distinct latest Cretaceous–early Paleogene tectonic event that deformed the North American continental crust along a northwest-trending corridor extending from the Arizona to Wyoming regions. A commonly cited plate tectonic mechanism is the shallowing of the subducting slab beneath North America resulting in basal shear of the continent and the attendant deformation and uplift. Deformation zones like this are well represented today along the Andean subduction zone where buoyant submarine ridges or rises are being subducted. The result is the segmentation of the subducting slab into shallow domains, which correspond to the subducted ridges or rises, and normal domains where normal oceanic crust is being subducted. Geologic and geophysical studies of the deep crust and upper mantle of the southern Sierra Nevada and Mojave Desert region have revealed the existence of a segmentation structure that corresponds in time and in spatial relation to Laramide deformation of the continental interior. Fragments of the counterpart of the Hess-Shatsky rise system, which is presently submerged in the northwest Pacific basin, constitute part of the Franciscan subduction complex of the California Coast Ranges. These fragments were accreted to the subduction complex at the time of the Laramide orogeny. Together these relations suggest that the Laramide orogeny formed over a shallow slab segment much like what is observed in the Andes today.
Strontium isotope stratigraphy of the Comanchean Series in the north Texas and southern Oklahoma
Rodger E. Denison, Department of Geosciences, University of Texas at Dallas, Richardson, Texas 75083-0688, USA, et al. Pages 669-682.
Keywords: Sr isotopes, Cretaceous, Comanchean, Texas, trace elements.
Pioneering descriptions of the stratigraphic section exposed in north Texas profoundly influenced geological understanding of the Cretaceous in North America. Fourteen rock formations were ultimately defined and grouped into the Comanchean Series, representing the middle division of the Cretaceous. The strontium isotope variation in seawater for this 19 million year interval has been defined using results from 135 samples collected at 43 sites in north Texas and adjacent Oklahoma. Variation of the seawater strontium isotope ration is controlled by competing influences of Sr derived from oceanic crust production, having low ration strontium, and high ratio riverine strontium derived from continental weathering. The Comanchean record shows alternation of these global crustal influences; namely, an early dominance of continental sources followed by an extended period when oceanic influences were greater by much more closely balanced. The isotopic results from this well-studied stratigraphic section allow precise correlation with rocks of this age anywhere in the world.
New K-Ar ages and the geologic evidence against rejuvenated-stage volcanism at Haleakalâ, East Maui, a postshield-stage volcano of the Hawaiian island chain
David R. Sherrod, U.S. Geological Survey, P.O. Box 51, Hawaii National Park, Hawaii 96718, USA, et al. Pages 683-694.
Keywords: K/Ar, shield volcanoes, Hawaiian Islands, Hawaii, Maui, Haleakalâ volcano.
Scientists have long thought that when Hawaiian volcanoes end their shield- and postshield stages of growth, they enter a lengthy dormancy lasting hundreds of thousands of years. Some volcanoes then erupt anew. These renewed eruptions, called the rejuvenated stage of volcanism, have formed well-known features such as Diamond Head on Oahu and, until now, the many youthful cinder cones and lava flows that mantle Haleakala volcano on Maui. Using the potassium-argon method of dating, however, scientists from the U.S. Geological Survey in Hawaii and Kyoto University, Japan, have shown that Haleakala is still in its postshield stage, not the rejuvenated stage. The ages, which range from about 1 million years to less than 50,000 years indicate that Haleakala never had a lengthy dormant period. During the past 200,000 years the volcano's eruptive frequency has diminished substantially, compared to more vigorous postshield volcanoes such as Hualalai on the Big Island of Hawaii. These findings broaden our basis for evaluating volcano hazards, which over the very long term will continue to diminish for Haleakala and the island of Maui.
Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region
Paul M. Myrow, Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA, et al. Pages 695-713.
Keywords: Ordovician, Cambrian, Colorado, Transcontinental Arch, biostratigraphy.
Standard reconstructions of central and western North America during the Paleozoic include a mountain range known as the Transcontinental Arch that extended from present-day Canada to Mexico. The Colorado region was considered the one region along the length of this range that breaching of the arch took place and where the paleo-Pacific Ocean was contiguous with a seaway in the central part of the continent. This topographically low region of flooding was termed the Colorado Sag. High-resolution sedimentologic, biostratigraphic, and stable isotope data from numerous measured sections across Colorado reveal a complex architecture for lower Paleozoic strata in the central Cordilleran region. Differences in stratigraphy on either side of the state were interpreted as the result of deposition on either side of a basement high that existed within the Colorado Sag. We show that this paleogeographic reconstruction is an artifact of miscorrelation. Biostratigraphic data show that there is little or no overlap in age between Lower Ordovician strata that were presumed to be coeval. This miscorrelation resulted in part from removal of strata by a series of unconformities. Deciphering of these complex stratal geometries invalidates the existence of the Colorado Sag for the early Paleozoic. In fact, regional reconstructions of earliest Paleozoic paleogeography along the entire length of the purported Transcontinental Arch should be reevaluated with similarly precise biostratigraphic data to reconsider all potential causes for missing strata and to eliminate topographic elements not supported by multiple stratigraphic techniques. This study illustrates how seriously paleogeographic reconstructions can be biased by the presumption that missing strata represent periods of non-deposition rather than subsequent episodes of erosion, particularly in thin successions where stratigraphic gaps are common and often inconspicuous.
Eruptive history and geochronology of the Mount Baker volcanic field, Washington
Wes Hildreth, Volcano Hazards Team, U.S. Geological Survey, Menlo Park, California 94025, USA, et al. Pages 714-749.
Keywords: Washington, stratovolcano, geochronology, caldera, volcanology, Mount Baker.
The Mount Baker volcanic field in northwest Washington was mapped and studied in detail by a U.S. Geological Survey team during the summers of 1992–1999. The steaming ice-clad stratocone, Mount Baker, highest peak in the North Cascades, is only the most conspicuous component of a multi-vent complex active for the past 1.3 million years. More than 70 packages of lava flows and 110 odd shallow dikes were mapped, 500 samples chemically analyzed, and 80 K-Ar and 40Ar/39Ar ages determined. Main components are (1) Kulshan caldera, which filled with rhyolite tuff during a Crater Lake–sized eruption at 1.15 Ma, and many pre- and post-caldera rhyodacite lavas and dikes (1.29–0.99 Ma); (2) remnants of a large postcaldera andesite center represented by 60 intracaldera dikes that cut hydrothermally altered caldera fill (1.1–0.6 Ma); (3) unaltered, intracaldera andesite lavas like Table Mountain and Coleman Pinnacle (0.5–0.2 Ma); (4) the Chowder Ridge focus west of the caldera, an andesite-to-rhyodacite complex now glacially reduced to 50 dikes and remnants of ten lava flows (1.3–0.1 Ma); (5) Black Buttes stratocone and several contemporaneous andesitic satellite centers (0.5–0.2 Ma); and (6) Mount Baker and small peripheral volcanoes (0.1 Ma to Holocene). Glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field. Volume estimates for 77 increments of known age yield cumulative curves and eruption rates in the range 0.17–0.43 km3/k.y. for the main episodes and 0.02–0.07 km3/k.y. as long-term background. Andesite and rhyodacite each make up nearly half of the 161 ± 56 km3 of magma erupted, whereas basalt and dacite each amount to only 1%–3% of the total. The magmatic focus has migrated stepwise 25 km southwestward over the past 4 m.y., from Hannegan caldera to Mount Baker.
Time vs. composition trends of magmatism at Sunlight volcano, Absaroka volcanic province, Wyoming
T.C. Feeley, Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA, and M.A. Cosca, Institute of Mineralogy and Geochemistry, University of Lausanne, BFSH 2, 1015 Lausanne, Switzerland. Pages 750-764.
Keywords: Absaroka Supergroup, 40Ar/39Ar, shoshonite, geochemistry, petrogenesis.
On the basis of high precision 40Ar/39Ar ages, eruptions at the volcano began around 49.6 million years ago and continued for about 1.5 million years. The exposed volcanic rocks at Sunlight volcano record three principal periods of eruptive activity. We speculate, based on geochemical data, that these periods are associated with the birth, maturation, and death of a major magma chamber system beneath the volcano.
To view abstracts for the GSA BULLETIN, go to http://www.gsajournals.org. Representatives of the media may obtain a complimentary copy of any GSA BULLETIN article by contacting Ann Cairns at acairns@geosociety.org.
The Geological Society of America: http://www.geosociety.org
Geological Society of America
Release No. 03-15
Contact: Ann Cairns
303-357-1056
acairns@geosociety.org