||May 7, 2003
GSA Release No. 03-14
May Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The May issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published. Members of the press should contact Ann Cairns for copies of articles and for additional information or assistance. All others should contact GSA Sales and Service, 1-888-443-4472.
- The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps
Giulio Viola, Department of Earth Sciences, ETH, Sonneggstrasse 5, 8092 Zürich, Switzerland, et al. Pages 515-532.
- Keywords: Alpine orogeny, structural geology, tectonics, fault zones, fission-track dating.
- The manuscript "The Pejo fault system: an example of multiple tectonic activity in the Italian Eastern Alps" describes in detail the process of structural reactivation of a major fault zone in the Italian Alps. As long as a pre-existing fault remains mechanically weaker than its surroundings, strain is preferentially concentrated in the zone. This occurs irrespective of whether the deformation is continuous or interrupted by periods of little activity, because pre-existing faults are surfaces along which the cohesive strength and the friction coefficient are lower than those of intact rocks. The study by Viola et al. is a clear field example of multiple reactivation and deformation of a major shear zone, from the Creataceous onward during the Alpine orogenic cycle. Geochronology is also used to constrain the absolute age of the multiple tectonic pulses.
- Ultrahigh-temperature metamorphism in the lower crust during Neoarchean Ventersdorp rifting and magmatism, Kaapvaal Craton, southern Africa
Mark D. Schmitz and Samuel A. Bowring, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Pages 533-548.
- Keywords: U-Pb geochronology, zircon, monazite, UHT metamorphism, sapphirine, granulite, lower crust, Ventersdorp Supergroup, Kaapvaal craton, rifting.
- Ultrahigh-temperature (UHT) metamorphism in the lower crust of the Kaapvaal craton of southern Africa has been dated 2.7 Ga by U-Pb zircon and monazite geochronology of sapphirine-bearing metasedimentary xenoliths entrained in Cretaceous kimberlites. Preservation of peak and prograde monazite dates through the UHT conditions support a very high volume diffusion closure temperature for this mineral, similar to that of zircon (>1000 °C) (>1000 °C). The 5-10 m.y. duration of UHT metamorphism in the lower crust is correlative with the rapid eruption of the Ventersdorp flood basalts and associated crustal melting, plutonism, and widespread extensional tectonics in the central Kaapvaal craton. The links between upper and lower crustal manifestations of 2.7 Ga magmatism and extension are used to explore their effects on the crustal architecture of the Kaapvaal craton, the degree of crust-mantle decoupling during intracratonic rifting, and the generation and preservation of cratonic lithospheric mantle keels.
- 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming
Michael E. Smith, Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA, et al. Pages 549-565.
- Keywords: Green River Formation, sedimentation rates, lacustrine, 40Ar/39Ar, Wasatchian-Bridgerian, Early Eocene Climatic Optimum
- The Green River Formation is world famous for fully articulated fossils of fish that lived in ancient Lake Gosiute. Its age has been the subject of controversy for many years. We have dated volcanic phenocrysts from six airfall tuffs using the 40Ar/39Ar method. Our ages indicate that the Green River Formation spans ~5 m.y. from 53.5 to 48.5 Ma. This indicates that Lake Gosiute coincided with the latest period of greenhouse conditions.
- High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa
L. Paul Knauth, Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404, US,) and Donald R. Lowe, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA. Pages 566-580.
- Keywords: 18O/16O, chert, Onverwacht Group, Swaziland Sequence, paleoclimatology.
- Flint-like rocks known as cherts occur in the world's oldest sedimentary rocks and are remarkably depleted in the heavier isotope of oxygen relative to cherts that formed in the past 540 million years. It has never been clear if this is an alteration phenomenon, a result of cherts having formed in a fundamentally different way, changes in the isotopic makeup of the ocean, or much higher climatic temperatures. After a thorough evaluation of the geologic context of 3.5 billion year old samples from South Africa, it appears that higher climatic temperatures provide the only satisfactory explanation for the oxygen isotope data. It now seems likely that Earth's oceans, at the time when widespread life was emerging, were in the temperature range of 55 °C (degrees C) to 80 °C, the temperature of hot tap water.
- Rapid generation of both high- and low-d18O (low-Delta 18), large-volume silicic magmas at the Timber Mountain/Oasis Valley caldera complex, Nevada
Ilya N. Bindeman and John W. Valley, Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA. Pages 581-595.
- Keywords: Paintbrush tuff, Timber Mountain tuff, oxygen isotopes, isotope zoning, zircon, zone refinement melting.
- The paper presents an oxygen isotope and petrologic study of four voluminous zoned ash-flow sheets of the SW Nevada Volcanic Field (SWNVF): Topopah Spring (TS, >1200 km3, 12.8 Ma), Tiva Canyon (TC, 1000 km3, 12.7 Ma), Rainier Mesa (RM, 1200 km3, 11.6 Ma), and Ammonia Tanks (AT, 900 km3, 11.45 Ma). Each tuff is characterized by a distinct range of d18O values of minerals and melt that suggest that tuffs can not be related by in situ fractionation and assimilation in a single zoned magma chamber, but rather suggest that TS, TC, RM, and AT represent independent magma batches which were rapidly generated, fractionated, and erupted from shallow, sheet-like magma chambers, a result of extensional tectonics in the Basin and Range. The AT tuff and associated pre- and post-caldera lavas are 2.5" lower in d18O than RM, and represent the largest known low-d18O magma. All units of AT cycle contain isotopically-zoned zircons with up to 2" core-to-rim zoning and correspondingly smaller out-of-equilibrium quartz-zircon and melt-zircon fractionations that are similar to zoned zircons in low-d18O lavas at Yellowstone (Bindeman and Valley, 2001). This discovery suggests that normal-d18O zircons have been inherited from precursor volcanic rocks in the volcanic roof, in which matrix suffered depletion in d18O (down to +4%-+5%), but zircons and quartz survived hydrothermal alteration. These precursor rocks were later rapidly remelted to form low-d18O melt and caused progressive exchange oxygen on a 10,000 year time scale.
- Late Quaternary fluvial landscape evolution in desert grasslands of northern Chihuahua, Mexico
Lee Nordt, Department of Geology, Baylor University, Waco, Texas 76798-7344, USA. Pages 596-606.
- Keywords: late Quaternary, alluvial stratigraphy, paleosol, climate, grassland, Mexico.
- River evolution during the past 15,000 years is poorly understood in desert grasslands of northern Mexico. Based on radiocarbon dating and landscape plant ecology, two rivers from the region, the Casas Grandes and San Pedro, were studied in detail using floodplain alluvial deposits. Seven periods of floodplain construction and six episodes of paleosol formation and channel erosion occurred during this interval. From 15,000 to 10,000 years ago, channel erosion ensued in response to a general warming trend and landscape instability. During the past 10,000 years, warm season grasses began to dominate and channel erosion was typically initiated by a shift from relatively warmer and drier to cooler and wetter climates. The timing of erosion events exhibits a remarkable correlation to arroyo formation and paleosol formation of ephemeral streams in the North American Southwest. In contrast to arroyos, the Casas Grandes and San Pedro rivers created floodplains with mixed to bedload meander belts that persisted throughout the late Quaternary. Although intervals of landscape instability occurred numerous times in the past 15,000 years, river evolution was largely governed by the dominance of warm season grasslands, which appears to have maintained sufficient upland vegetation cover to mitigate flash flooding typical of arid environments.
- Eocene to Miocene magnetostratigraphy, biostratigraphy, and chemostratigraphy at ODP Site 1090 (sub-Antarctic South Atlantic)
J.E.T. Channell, Department of Geological Sciences, University of Florida, Post Office Box 112120, Gainesville, Florida 32611, USA et al. Pages 607-623.
- Keywords: Cenozoic, magnetostratigraphy, stable isotopes, strontium isotopes, biostratigraphy, South Atlantic.
- Three normal polarity subchrons (5Dr.1n, C7Ar.1n, and C13r.1n), not included in the standard GPTS, are recorded at ODP Site 1090 in the Sub-Antarctic South Atlantic. The base of the sampled section is correlated to C19n (middle Eocene). The top of the sampled section is correlated to C5Cn (late early Miocene), although, in the uppermost 10 m of the sampled section, a foraminifer usually associated with the Messinian and early Pliocene (Globorotalia sphericomiozea) has been identified. 87Sr/86Sr, d13C (Delta 13C), and d18O values measured on foraminifera, including the d18O and d13C shifts close to the Eocene-Oligocene boundary, support the correlation to the GPTS. For the interval spanning the Oligocene-Miocene boundary, benthic d13C, d18O, and 87Sr/86Sr records from Site 1090 can be correlated to isotope records from ODP Site 929 (Ceara Rise) providing support for the Oligocene-Miocene boundary age (22.92 Ma) of Shackleton et al. (2000).
- Number and size of last-glacial Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon
Gerardo Benito, Consejo Superior de Investigaciones Científicas, Centro de Ciencias Medioambientales, Serrano 115 bis, 28006 Madrid, Spain, and Jim E. O'Connor, U.S. Geological Survey, 10615 SE Cherry Blossom Drive, Portland, Oregon 97216, USA. Pages 624-638.
- Keywords: Quaternary, Columbia Basin, Missoula floods, fluvial features, radiocarbon dating.
- Perhaps the largest freshwater floods in Earth's history were from rapid drainage of ice-dammed glacial Lake Missoula during the last ice age. New field evidence and radiocarbon dating from the Columbia River Gorge in Washington and Oregon show that between about 19,000 and 12,000 radiocarbon years ago, more than 25 floods had discharges greater than 1.0 million m3/s. At least 15 floods had discharges greater than 3.0 million m3/s, and at least 6 or 7 of them had peak discharges greater than 6.5 million m3/s. At least one flood had a peak discharge of about 10 million m3/s.
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