||May 28, 2002
GSA Release No. 02-32
June Media Highlights:
Boulder, Colo. - The Geological Society of America's June issue of GEOLOGY contains a number of newsworthy items. Topics include seafloor-crust "breathing" with ocean tides; recent earthquakes in the Mojave Desert and their impact on stresses on the nearby south San Andreas and adjacent fault systems; a challenge to the most widely accepted model for dating the rise of atmospheric oxygen on Earth; and the role of tectonic controls in generation of greenhouse gases and global warming.
The June issue of GSA TODAY does not include a science article. Science articles will resume with the July issue.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns for copies of articles and for additional information or other assistance.
- The breathing of the seafloor: Tidal correlations of seismicity at Axial Volcano
Maya Tolstoy Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, New York 10964-8000, USA et al. Pages 503-506.
- Recordings of small earthquakes on an active seafloor volcano show that seismic activity is correlated with ocean tidal lows. In addition, low-level harmonic tremor signals, attributed to superheated water movement in cracks, are also seen to correlate with ocean tides. These results suggest that the seafloor crust is essentially breathing with the ocean tides, allowing more movement of water through the crust and release of seismic energy on a regular tidal schedule. This may be an important phenomenon for the supply of nutrients to the biological communities living in these extreme environments, away from sunlight. Tidal deformation of the crust itself (called Earth tides) is seen to lag the ocean tides by 2 hours in this location. Therefore movement of water appears to be a more important force in day-to-day earthquake triggering than the subtle deformation of the crust caused by Earth tides. However, there is some indication that Earth tides may be a more important influence for the deeper crustal movement of magma, as seen at land volcanoes.
- Accelerated stress buildup on the southern San Andreas fault and surrounding regions caused by Mojave Desert earthquakes
Andrew M. Freed, Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA, (Present address: Berkeley Seismological Laboratory, 215 McCone Hall, University of California, Berkeley, California 94720, USA), and Jian Lin, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. Pages 571-574.
- Scientists have hypothesized for decades that one major earthquake can trigger another earthquake on a nearby fault through stress interaction. More recent studies have further suggested that this interaction may be delayed by the slow viscous creeping of rocks in Earth's lower crust and upper mantle. This is best illustrated by the 1999 magnitude 7.1 Hector Mine earthquake, which occurred only 30 km away from the 1992 magnitude 7.3 Landers quake, but 7.5 yr later. The delay between these events can be explained by viscous flow consistent with observations of continuous ground deformation following the Landers quake. In this study, the authors further calculated how the Landers, Hector Mine, and two other earthquakes in the Mojave Desert have changed stresses on the nearby southern San Andreas and adjacent fault systems. They calculated that these earthquakes and continuous viscous creeping at depth are causing a rapid increase of stresses on a section of the San Andreas fault called the San Bernardino Mountain segment, which is located only 80 km from Los Angeles. The San Bernardino Mountain segment is worthy of special attention because it is capable of producing major earthquakes with magnitude greater than 7. Since the last major earthquake on this segment was more than 190 years ago, the fault may be late in its earthquake cycle, and thus the calculated ongoing stress increase on the fault is of added significance. In addition, the authors calculated that parts of the San Jacinto, Elsinore, and Calico faults are also experiencing accelerated stress buildup. In particular, the Calico fault, which lies just north of the Landers rupture (near Barstow), appears to have the calculated stress patterns and the observed post-Landers aftershock clustering quite similar to the Hector Mine region before the 1999 quake. This makes the Calico fault another candidate for a potential earthquake in the future, and a location where seismic activity should be watched closely.
- Tropical laterites, life on land and the history of atmospheric oxygen in the Paleoproterozoic
Jens Gutzmer, Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa, et al. Pages 491-494.
- The history of oxygen in Earth's atmosphere has long been the subject of debate. At present the most widely accepted model predicts an oxygen-deficient, reducing atmosphere on early Earth and a rapid rise of the oxygen concentration at ca. 2.2-2.0 Ga. Two laterally widely separated fossil soil horizons-or paleosols-preserved in the Transvaal Supergroup on the Kaapvaal Craton in South Africa have provided the most important argument to time this apparent rise. Based on field geological evidence and a new stratigraphic correlation, the study reveals that these two important paleosols indeed redeveloped along the same ca.-2.2-Ga erosion surface that once covered most of the Kaapvaal Craton. A different degree of preservation is the only difference between the two paleosols. These findings imply that the paleosols cannot be used to time the rise of atmospheric oxygen and, consequently, that the history of atmospheric oxygen in the early Precambrian requires careful reexamination.
- Tectonic controls on greenhous gas flux to the Paleogene atmosphere from the Gulf of Alaska accretionary prism
Travis Hudson, Applied Geology, 1432 Fox Hollow Road, Sequim, Washington 98382, USA, and Leslie Magoon, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 547-550.
- Tectonic processes produced a tremendous amount of the greenhouse gas methane (8.35 x 105 kg) in the Gulf of Alaska accretionary prism during a time that overlapped the major period of global warming between 61 and 56 Ma. Much of this methane appears to have escaped to the atmosphere and, if accompanied by polar stratospheric cloud development, may have been sufficient to have forced global warming during this time. The Gulf of Alaska accretionary prism is an example of tectonic controls on greenhouse generation and the recycling of large amounts of carbon from the geosphere.
- Sedimentary pyrite: A window into the microbial past
Juergen Schieber, Department of Geology, University of Texas at Arlington, Arlington, Texas 76019, USA. Pages 531-534.
- Microbially driven geochemical processes are recognized to have profoundly influenced the evolution of the atmosphere and oceans. For microbes that live in sediments, the mineral pyrite (FeS2) is a common metabolic byproduct. Newly formed pyrite is widely observed in contemporaneous sediments, and forms small grains or concretions of variable size (millimeters to centimeters). Electron microscope examination of sedimentary pyrite grains from a wide range of geologic ages has revealed that they can contain fossilized microbes that once lived in the sediment. Thus, through a study of sedimentary pyrite we may be able to learn more about the microbial biosphere of the past. Whereas traditionally microbial fossils were recovered from certain rare occurrences of sedimentary chert, the common occurrence of sedimentary pyrite affords a broader and more systematic sampling of the microbial fossil record.
- Reassessment of Lake Victoria-Upper Nile River paleohydrology from oxygen isotope records of lake sediment cellulose
Kristina Beuning, Department of Biology, Room 360, Phillips Science Hall, University of Wisconsin, Eau Claire, Wisconsin 54701, USA, et al. Pages 559-562.
- This article addresses the re-initiation of Nile River flow from its equatorial source, Lake Victoria, following desiccation of the lake 15 ka. The authors analyze the oxygen isotopic composition of aquatic cellulose preserved in Lake Victoria sediments. From this they are able to infer the timing of the shift from a closed lake basin with no outflow to an open basin with overflow into the White Nile River. By combining analyses from two sediment cores, the authors conclude that basin overflow was established by 13 ka.
- Major depositional events under the deep Pacific inflow
Ian Hall, Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF10 3YE, UK, et al. Pages 487-490.
- The world's largest deep western boundary current and major branch of the global thermohaline circulation system enters the Pacific Ocean via the southwest Pacific gateway off eastern New Zealand. As the current passes through the gateway it receives a substantial influx of terrigenous sediment delivered mainly by three large submarine channels. This influx, together with the pervasive rain of pelagic biogenic detritus, has been transported and deposited by the deep western boundary current to form several prominent drifts that were cored during Leg 181 of the Ocean Drilling Program (ODP). Two drifts, North Chatham and Rekohu, yielded detailed and well-dated sedimentary records that provide a new insight into drift accumulation. These high-resolution records of terrigenous and carbonate burial fluxes spanning the past 3 m.y. have made it possible for us to unravel the various influences on the drift deposition. In particular, the flux records reveal the interplay between large-scale paleoclimatic-paleoceanographic influences and more local controls, such as terrestrial sediment supply and submarine-channel evolution. Furthermore, by comparing the flux records between the North Chatham and Rekohu drifts, the authors were able to assess flux variability along the deep western boundary current path.
- Yakutat collision and strain transfer across the northern Canadian Cordillera
Stephane Mazzotti and Roy D. Hyndman, Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada, and School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8W 3P6, Canada. Pages 495-498.
- In northwestern Canada and southeastern Alaska, seismicity is distributed mostly along the Pacific margin and within the Cordillera eastern front. The authors present a tectonic, kinematic, and mechanical model of deformation for this region that explains why there is an exceptional concentration of earthquakes along the eastern front of the northern Cordillera, ~600 km away from the main plate boundary. On the basis of earthquake and Global Positioning System data, the authors show that strain and stress are transmitted eastward from the Pacific margin (Yakutat collision zone) across the Cordillera. This strain transmission is allowed by a decoupling of the crust over the upper mantle, due to the very high temperature of the Cordillera.
To review the abstracts for these articles, go to www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns.
To review the complete table of contents for the June issue of GEOLOGY, go to www.gsajournals.org/.