Following are highlights from the July issue of Geology and a summary of the science article from the July issue of GSA Today, published by the Geological Society of America. Stories written regarding these articles are embargoed until July 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to Geology in stories published. Please contact Anika Burkard for additional information and assistance.
Geology
Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore southern California by M.S. Marlow et al., p. 587
This paper demonstrates the power of new multibeam bathymetric scanning systems that are being used to image the seafloor in great detail. The resulting images are equivalent to aerial photographs, which geologists have used for years to map onshore surficial geologic features such as faults, stream patterns, landslides, mountain ranges, etc. Now, for the first time, marine geologists are able to map offshore features along continental margins and in deep oceans. The authors describe how these images were used to map a newly discovered fault that offsets the seafloor near Catalina Island offshore southern California. The multibeam imagery allows a new and detailed look at offshore hazards such as faults and landslides that may directly impact onshore facilities during major earthquakes.
Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea by David Wiprut and Mark D. Zoback, p. 595
Long-dead geologic faults can be reactivated by changes in the stresses in the earth and changes in fluid pressure. In the particular case studied, the authors examine natural gas leakage from a hydrocarbon reservoir in the northern North Sea along a reservoir-bounding fault that seems to have been recently reactivated. They show that elevated pore pressures resulting from the accumulated column of gas pushing against the fault appear to have caused portions of the fault to slip, thereby releasing the gas from the reservoir. This work has implications for hydrocarbon leakage and migration in other areas, as well as broader implications, including the triggering of earthquakes by gas accumulations in the crust and the potential limits on plans for carbon dioxide sequestration in abandoned oil and gas reservoirs.
Chuar Group of the Grand Canyon: Record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma by Karl E. Karlstrom et al., p. 619
Recent work in the remote areas of the Chuar Valley in the eastern Grand Canyon provide insight into the interactions of climate, tectonism, and biological activity in the Late Precambrian. The Chuar Group is now known to be 742 Ma at its top. It was deposited at low latitudes during east-west rifting of Laurentia, apparently during dispersal of the Precambrian supercontinent Rodinia. Dramatic fluctuations in C isotopes in marine rocks appear to mark early changes in the linked biogeochemical, tectonic, and climatic conditions that led to global glaciations, and organic-rich shales contain evidence for early heterotrophic eukaryotes and increased complexity of the ecosystem.
May 5, 1998, debris flows in circum-Vesuvian areas (southern Italy): Insights for hazard assessment by M.T. Pareschi et al., p. 639
On May 5, 1998, after 30 hours of continuous rainfall, large areas of volcanic-rich soil east of Vesuvius volcano (Italy) failed, generating a series of mud avalanches that ravaged inhabited areas and killed more than 150 people in the Sarno area. This was the last episode of catastrophic debris-flow activity in the Vesuvius region. On the basis of the 1998 case history, the authors discuss the possibility of using the shape of the debris flows to predict the degree of hazard. The most vulnerable setting is characterized by basins with a steep average slope and a triangular shape with the base up-valley.
Tempo of the end-Permian event: High-resolution cyclostratigraphy at the Permian-Triassic boundary by Michael Rampino et al., p. 643
The boundary between the Permian and Triassic Periods (~245 million years ago) is marked by the most severe mass extinction in the geologic record, when ~96% of fossil marine species became extinct along with many terrestrial animals and plants. In order to estimate the time interval over which these extinctions took place, the authors used sedimentary cycles that record known astronomical cycles of Earth's orbit. The sediment layering in a long core from the Austrian Alps shows evidence for cycles in the ratio of ~40:10:4.7:2.3 m that can be correlated with orbital cycles of ~412:100:40:20 thousand years. Using the improved resolution provided by the cycle analysis, the authors find that the dramatic change in the faunal record that marks the P-T boundary took place over less than 60,000 years, and perhaps as rapidly as less than 8,000 years. These results are compatible with a catastrophic cause for the extinctions.
Active faulting at Delphi, Greece: Seismotectonic remarks and a hypothesis for the geologic environment of a myth by Luigi Piccardi, p. 651
Historical data are fundamental to the understanding of the seismic history of an area. At the same time, knowledge of the active tectonic processes allows us to understand how earthquakes have been perceived and recorded in legends and myths by past cultures. Delphi, positioned above the surface expression of one of the major active faults that controls the tectonic depression of the Gulf of Corinth, is a key area for improving our understanding of the seismotectonic behavior of this region. Integration of geological data, archaeological evidence, and historical sources suggests that in the earthquake of 373 B.C., ground ruptures, due to slip on the fault plane, opened not only at Helice on the principal fault of the tectonic depression, but also on the opposite fault at Delphi. Such a mechanism would have been similar to what observed in the earthquake of 1981 near Corinth. The Delphic oracle, by far the most venerated oracle of the Greek ancient world, has been worshiped since its remote origins almost 2000 years ago. According to tradition, the prophetic properties of this oracle were obtained from a gas-exhaling, open "chasm" in the earth. Existence of this chasm has been a subject of debate ever since. Re-examination of the myth on the basis of the local geological setting, and the fact that the temples of the oldest sanctuary are positioned directly above the active fault which ruptured in 373 B.C., indicate that the mythological oracular chasm might well have corresponded to a prehistoric event of surface faulting at the same location.
Modeling the behavior of the continental mantle lithosphere during plate convergence by Russell N. Pysklywec et al., p. 655
While increasingly improved near-surface geological and geophysical observations have allowed for relatively detailed study of the crust, the evolution of the lower lithosphere (i.e., the mantle lithosphere) beneath the continents remains enigmatic. The authors use numerical models to investigate the behavior of the mantle lithosphere during episodes of continental plate collision and mountain building. Previous studies have suggested that in these tectonic environments deformation of the mantle lithosphere occurs primarily by processes of viscous dripping. The authors find such dripping events in their numerical experiments, but also observe a range of alternate modes of deformation of the mantle lithosphere. These include asymmetric underthrusting and plate consumption by scraping, slab detachment, and various combinations of these modes. The style of deformation is controlled by the viscous and plastic behavior of the system as well as the tectonic plate convergence imposed by the parameters of the model.
Middle Pleistocene sea-surface temperature change in the southwest Pacific Ocean on orbital and suborbital time scales by Alexandra L. King and William R. Howard, p. 659
The discovery of past intervals during which global climate was similar to today unlocks important clues about the behavior of the climate system. One such interval occurred approximately 425,000 years ago during an interglacial interval known as stage 11. Geological evidence suggests that Earth's climate during this period was slightly warmer than today with global sea levels up to 20 m higher, at a time when astronomical forcing mechanisms were very similar to today. A new data set from the southwest Pacific Ocean, however, indicates slightly cooler sea-surface temperatures than are experienced today. Although the global extent of these relatively cool conditions remains unclear, they imply that ice-sheet collapse, and its associated rise in global sea level, may not require temperatures warmer than today. This new southwest Pacific record shows see-saws in temperature occurring every 5,000 to 10,000 years during the stage 12 glacial interval (423,000 to 478,000 years ago). The timing of these events appears to match those recorded in the North Atlantic during this same interval, and during the last glacial. The possibility that climate fluctuations occurred globally implies that rapid reorganizations in global climate are a common and may be a predictable feature of Earth's climate system.
Delta 13C values and radiocarbon dates of microbial biomarkers as tracers for carbon recycling in peat deposits by Richard D. Pancost et al., p. 663
There is considerable concern regarding the climatic impact of anthropogenically influenced greenhouse gas emissions. Consequently, it is vital to understand the natural controls on the emission of such gases and how those controls have varied through time. Because peat bogs are important sources of both carbon dioxide and methane, the authors applied several new techniques to the study of carbon cycling in an ancient peat deposit. The results indicate that methane-producing organisms were abundant. The tools the authors used can be useful in studying changes in methane emission and establishing the history of methane production in other ancient peat deposits.
Activation of Pacific mantle plumes during the Carboniferous: Evidence from accretionary complexes in southwest Japan by Yoshiyuki Tatsumi et al., p. 580
Island-arc basalt alkali ratios: Constraints from phengite-fluid partitioning experiments by Stefan Melzer and Bernd Wunder, p. 583
How active is a passive margin? Paleoseismicity in northeastern Brazil by Francisco H.R. Bezerra and C. Vita-Finzi, p. 591
Seismic evidence for a deep upper mantle thermal anomaly beneath east Africa by Andrew A. Nyblade et al., p. 599
Magmas in collision: Rethinking chemical zonation in silicic magmas by John C. Eichelberger et al., p. 603
Autopsy on a dead spreading center: The Phoenix Ridge, Drake Passage, Antarctica by Roy Livermore et al., p. 607
New constraints in the limits of the Barents-Kara ice sheet during the Last Glacial Maximum based on borehole stratigraphy from the Pechora Sea by Leonid Polyak et al., p. 611
Devonian, orogen-parallel, opposed extension in the Central Norwegian Caledonides by Alvar Braathen et al., p. 615
New C isotope stratigraphy from southwest China: Implications for the placement of the Precambrian-Cambrian boundary on the Yangtze Platform and global correlations by Yanan Shen and Manfred Schidlowski, p., 623
Rare earth element chemistry of zircon and its use as a provenance indicator by Paul W.O. Hoskin and Trevor R. Ireland, p. 627
Apatite weathering and the Phanerozoic phosphorus cycle by Michael W. Guidry and Fred T. Mackenzie, p. 631
Reversal of the regional-scale flow system of the Williston basin in response to Pleistocene glaciation by Stephen Grasby et al., p. 635
Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight by Noel P. James et al., p. 647
Overthrusting and sediment accretion along Kilauea's mobile south flank, Hawaii: Evidence for volcanic spreading from marine seismic reflection data by Julia K. Morgan et al., p. 667
FORUM
High radiogenic heat-producing granites and metamorphism -- An example from the western Mount Isa inlier, Australia
Comment: Damien Foster and Mike Rubenach, p. 671
Reply: Sandra McLaren, Mike Sandiford, and Martin Hand, p. 672
GSA Today science article
Hydrothermal Systems: Doorways to Early Biosphere Evolution by Jack D. Farmer, p. 1 - 9.
Hydrothermal systems may have provided favorable environments for early life on Earth. They could have also provided a refuge for thermophilic (heat-loving) microorganisms during giant-impact events. The author studied the microbial biosedimentology of Mammoth Hot Springs in Yellowstone National Park, resulting in important constraints for interpreting the fossil record of thermophilic ecosystems. Hydrothermal processes appear to be inextricable linked to planetary formation and evolution and are likely to have existed on other bodies in the solar system. Thus, hydrothermal systems are considered primary targets in the search for fossil evidence of life elsewhere in the solar system.
June 22, 2000
GSA Release No. 00-18
*Full text Geology articles and the science article from GSA Today are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.