||January 7, 2002
GSA Release No. 02-02
|| Ann Cairns
DirectorCommunications and Marketing
January Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The January issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes research on the rate the Dead Sea is subsiding, stable isotope evidence that a major carbon cycle perturbation helped propel Earth into the late Paleozoic Ice Age, the use of new technology to study the interaction of outside faults with an active fault zone in the Caribbean, and insights into the composition of oceanic crust based on a global survey of Precambrian ophiolites found in Earth's orogenic belts.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.
- Early Mississippian climate based on oxygen isotope compositions of brachiopods, Alamogordo Member of the Lake Valley Formation, south-central New Mexico
Robert J. Stanton Jr., et al. Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA. Pages 4-11.
- Our ability and will to react effectively to predictions of future climate change depend on the confidence we have in the predictions of what those changes will be. One way to test the accuracy of those predictions is to determine how effective they are in determining past climates. In this study of late Paleozoic (Early Mississippian) climate, the isotopic, lithologic, and paleontologic criteria available for paleoclimatic analysis are each applied and the results compared. An evaluation of the criteria and of the assumptions implicit in each provides a stronger framework for paleoclimatic determinations.
- The lowest place on Earth is subsiding An InSAR (interferometric synthetic aperture radar) perspective
Gidon Baer et al. Geological Survey of Israel, Jerusalem 95501, Israel. Pages 12-23.
- During the past decade, sinkholes and wide shallow subsidence features have become major problems along the Dead Sea shores in Israel and Jordan. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the ERS-1 and ERS-2 satellites. Subsidence is observed in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt diapir margins. Subsidence rates are in the range of 0-20 mm/year, with exceptionally high rates that exceed 60 mm/year in two specific regions. During the study period, the level of the Dead Sea and of the associated groundwater has dropped by about 6 meters. This water level drop caused compaction within the fine-grained marl layers, resulting in gradual subsidence. Such subsidence is unlikely to be related directly to the sinkholes (which are basically a consequence of salt dissolution at depth), excluding the use of gradual subsidence features as precursors to sinkhole formation.
- Structural and tectonic setting of the Charleston, South Carolina, region: Evidence from the Tertiary stratigraphic record
Robert E. Weems and William C. Lewis, U.S. Geological Survey, Reston, Virginia 20192, USA.
- Eleven late Eocene through Pliocene stratigraphic units in the subsurface of the Charleston, S.C., region contain a wealth of information concerning the long-term tectonic and structural setting of that area. Over geologic time, changes in sea level generate patterns of deposition and erosion that are geographically unique for the time of each transgression. Such patterns fail to persist when the basal surfaces of stratigraphic units are compared sequentially over time. In some areas around Charleston, however, there has been persistent, repetitive net downward or upward movement over the past 34 million years. These repetitive patterns of persistent motion are most readily attributable to tectonism. The spatial pattern of these high and low areas is complex, but it appears to correlate well with known tectonic features of the region. This correlation suggests that the tectonic setting of the Charleston region is controlled by scissors-like northeast-southwest compression on a crustal block located between the north-trending Adams Run fault west of Charleston and the northwest-trending Charleston fault north of Charleston.
- Ages of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon
P.R. Hooper, et al. Department of Geology, Washington State University, Pullman, Washington 99164, USA. Pages 43-50.
- By correlating basalt flows in east-central Oregon using field, major, and trace element composition of the flows, and argon/argon absolute ages, it is confirmed that the Steens Basalt is the earliest manifestation of the Columbia River flood basalt province. Flood basalt eruption started at 16.6 million years ago immediately above the calculated position of the Yellowstone hot spot, and ceased abruptly in this part of Oregon at 15.3 million years ago, the main eruption subsequently migrating northward to form the Columbia River basalt plateau. In east-central Oregon, cessation of flood basalt volcanism was immediately followed by east-west extension to form the 50-km-wide Oregon-Idaho graben, which was accompanied by small scale and chemically diverse calc-alkaline volcanic eruptions. These eruptions are similar to magmatism associated with east-west extension in the inland northwest from British Columbia to Nevada from the Eocene to the present.
- Control of internal structure and fluid-migration pathways within the Barbados Ridge décollement zone by strike-slip faulting: Evidence from coherence and three-dimensional seismic amplitude imaging
Christopher G. DiLeonardo,et al. Earth Sciences Board, University of California, Santa Cruz, California 95064, USA. Pages 51-63.
- New 3-D imaging technology, used primarily in the oil industry, was employed by scientists to study an active fault zone at the edge of the Caribbean. This new technology allowed researchers to "look" into the fault zone some 2,000 feet within the crust, beneath the ocean floor. The study reveals that fluids within the fault zone and the mechanical properties of this fault may, in turn, be controlled by other, previously unrecognized, faults cutting the zone from beneath. Interactions of these faults vary in both time and space and may ultimately prove important in understanding the behavior of subduction zone thrusts, a type of fault responsible for the majority of the world's largest magnitude earthquakes.
- Pre - 1 Ga (pre-Rodinian) ophiolites: Their tectonic and environmental implications
Eldridge M. Moores, Department of Geology, University of California, Davis, California 95616, USA.
- Ophiolites (from the Greek words ophis--snake, and lithos--rock) are fragments of oceanic crust and mantle formed at spreading centers and preserved on land. Ophiolite studies provide crucial insight into how oceans spread. Their emplacement on land is also a tectonic event of major importance. Because no oceanic crust in the oceans is older than about 200 million years, ophiolites also provide us with the only information preserved for oceanic spreading processes prior to 200 million years ago, that is, for the first 96% of Earth history. This paper reports on a global survey of Precambrian (more than 550 million years old) ophiolites from the Earth's orogenic belts (scars of continental collisions) that formed prior to 1 billion years ago (1 Ga). The chemical composition and structure of these very old ophiolites suggest that the nature of the oceanic crust was quite different prior to 1 Ga than at present. It was thicker, and at times was of considerably different composition than at present. Thinning of the oceanic crust may have led to major changes in tectonic style recorded in the world's orogenic belts. It also may have given rise to emergence of previously developed continents about 600-900 million years ago, which could explain the increase in atmospheric oxygen at that time, which led to the development of animals.
- Carbon and oxygen isotope stratigraphy of the Lower Mississippian (Kinderhookian lower Osagean), western United States: Implications for seawater chemistry and glaciation
Matthew R. Saltzman, Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210, USA. Pages 96-08.
- The late Paleozoic ice age is the earliest major glacial period for which we have a complete record. It now appears, based on evidence from stable isotopes presented in this paper, that a major carbon cycle perturbation helped drive the Earth into this ice age. The change in carbon cycling resulted from tectonic changes taking place in western North America, which deepened the ocean basins and allowed for storage of large quantities of organic matter. This in turn may have lowered atmospheric carbon dioxide levels and led to the massive Permo-Carboniferous glaciation of Gondwana.
- The Round Mountain serpentinite mélange, northern Coast Ranges of California: An association of backarc and arc-related tectonic units
F. Huot and R.C. Maury, Domaines Océaniques, Institut Universitaire Européen de la Mer (IUEM) and Université de Bretagne Occidentale, Plouzané, France. Pages 109-123.
- We report new field observations together with geochemical and structural data from the Round Mountain serpentinite mélange, west of Paskenta (northern California). This mélange (a lithological unit characterized by numerous rock fragments enclosed in a fine-grained matrix) was formed by dismemberment of the adjacent units known as the Franciscan Complex and the Coast Range ophiolite following their collision along faults during Mesozoic times. This study, although done in a specific area, has implications on the understanding of the geological evolution of western North America.
To view abstracts for the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, go to www.gsajournals.org.
To obtain a complimentary copy of any BULLETIN article, contact Ann