||February 7, 2002
GSA Release No. 02-08
|| Ann Cairns
DirectorCommunications and Marketing
February Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The February issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of 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. Contact Ann Cairns for copies of articles and for additional information or assistance.
- Permian sedimentary record of the Turpan-Hami basin and adjacent regions, northwest China: Constraints on postamalgamation tectonic evolution
Marwan A. Wartes, et al. Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 131-152.
- The remote interior of Asia remains one of the least understood geologic provinces on Earth. Our expeditions have documented the geologic history of northwest China during the Permian period, a time immediately following an episode of plate collisions and mountain building during the coalescence of the supercontinent Pangea. One of the more intriguing discoveries of our work was the existence of a vast lake, or lakes, which covered at least 300,000 square km, roughly equivalent to the largest modern lake on the planet (Caspian Sea).
- Paleoproterozoic arc magmatism imposed on an older backarc basin: Implications for the tectonic evolution of the Trans-Hudson orogen, Canada
Pete Hollings and Kevin Ansdell, Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada. Pages 153-168.
- This article reports new geochemical data from circa 1.83 billion year old granitic rocks that were intruded into sedimentary rocks and were themselves affected by metamorphism. This data suggests that the granitoids formed above a subduction zone in a continental arc environment, similar to the present-day Andes in South America, just prior to the collision with a continent called the Sask Craton. This collision ultimately led to the formation of Himalayan-type mountain belt, and prevented the further formation of granitic magmas.
- Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data
Richard J. Blakely, et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 169-177.
- Crustal faults are among the many earth hazards that confront the Puget Lowland of Washington. Some of these faults are believed to be capable of producing magnitude 7 earthquakes at shallow depth (<25 km), and knowing their location, length, and three-dimensional geometry is an important step in understanding their threat and mitigating their damage. This understanding is difficult to come by in the Puget Lowland because of concealing glacial deposits, vegetation, water, and urban development. The USGS in 1997 conducted an airborne magnetic survey of the entire Puget Lowland in order to learn more about crustal faults and the geologic underpinnings of the region.
- The pattern of magnetic anomalies over Seattle reflects the complex geology of the Seattle fault zone, which extends east-west along the southern margin of the Seattle basin beneath Puget Sound and Seattle. Geologic arguments, seismic-reflection data, and geophysical models indicate that the Seattle fault zone consist of several thrust faults, where crust to the south has moved up relative to crust northward. The Seattle fault zone was responsible for a magnitude 7 earthquake 1100 years ago, and a magnitude 4.9 earthquake in June 1997 near Bremerton suggests that part of it is active today. We have mapped various strands of the Seattle fault zone and its spatial relation to the Seattle basin, using the aeromagnetic data to interpolate fault locations between widely spaced seismic-reflection profiles and sparse bedrock exposures indicated by geologic mapping. Thus mapped, we relate the active strand of the Seattle fault zone to the 1997 earthquake and to a young topographic scarp observed on Bainbridge Island. The aeromagnetic data confirm the existence of the Seattle fault zone and provide constraints on its location, length, and three-dimensional geometry.
- Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Part 1. Stratigraphic development
Sean P.S. Gulick, et al. Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015-3188, USA. Pages 178-191.
- Offshore northern California lies a sedimentary basin known as the Eel River basin that has throughout its history overlain the southern Cascadia subduction zone where the Gorda plate dives beneath North America. This basin, which stretches between from Cape Mendocino, California to Cape Sebastian, Oregon, records the tectonic history of the continental margin. A history that includes the switch around three to two million years ago from a margin where the material that overlay the Gorda plate were subducted along with the plate to a phase where the margin actively accreted sediments and therefore widened. This addition of sediment and widening of the margin caused the outer margin to start to uplift. Between two and one million years ago, the ancestral Klamath and Eel Rivers deposited large amounts of sediment further widening the northern California margin. Around 1 Ma, the outer portion of the margin, which had been continuing to uplift, reached near sea level and was eroded, while the center of the basin remained submerged as a coastal seaway. Around 500,000 years ago, the northward migrating intersection of the Pacific, Gorda, and North American plates known as the Mendocino triple junction approached Cape Mendocino, California causing the entire margin up to 20 km north to uplift, tilt northward, and erode, and causing the sediment delivered by the Eel River to bypass the southern parts of the basin up to 80 km north. The bypassing sediment in turn were likely deposited further north within the basin or flowed down the Eel Canyon to pile up on the abyssal plain as the Gorda fan.
- Paleoseismic evidence for time dependency of seismic response on a fault system in the southern Arava Valley, Dead Sea rift, Israel
Rivka Rivka Amit, et al. Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel. Pages 192-206.
- The Elat fault system in the southern Arava Valley the Dead Sea Rift, Israel, is a complex fault zone, characterized by marginal normal faults and central sinistral strike-slip faults. Paleoseismic evidence shows that it has generated at least fifteen earthquakes of magnitudes (M) larger than 6 during the Late Pleistocene and the Holocene (80 ka). The late Pleistocene is characterized by earthquakes that displace the surface by 1-1.5 m, with magnitudes between M6.7 to M7 and an average recurrence interval of 2.8±0.7 ka. The Holocene is characterized by a higher frequency of tectonic events with a recurrence interval of 1.2±0.3 ka, but lower displacement amounts (0.2-1.3 m) and lower earthquake magnitudes (M5.9 - M6.7). Historical records document the last seismic event in the Elat fault zone as of about 1000 years ago. The decrease in tectonic activity with time is inferred from the concentration of tectonism in the central part of the Elat fault zone and decreased seismicity in the eastern and western margins. The magnitude range determined for the central zone (M6.1 - M6.7) was likely not high enough to activate the marginal faults. The decrease in earthquake magnitudes with time, combined with the observations that the last large event occurred in 1068 A.D. and that no microseismicity has been detected during the last 15 years, might signal locking of the Elat fault zone. This may result from episodic global reorganization of the system's mode of strain energy release, reflected in the configurational entropy of stress states on the fault. These results have significant implications for seismic hazard assessment in the southern Arava, southern Israel, and underscore the possibility that the Elat fault may be a site of major faulting in the near future.
- The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: New constraints on the evolution of the Alps
Romain Bousquet et al. Department of Earth Sciences, Universitšt Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland. Pages 207-225.
- The European Alps were formed during a long process of convergence and collision between the European and Adria (related to the Africa) plates during Mesozoic and Cenozoic times (from about 110 million years ago to today). The intervening oceanic crust progressively deformed and accreted to the continental margins and is now part of the Penninic nappes, which are sandwiched between units of the overlying Austroalpine and the underlying European domains. One major question is to know how many oceans were closed during the building of the Alps. From the metamorphic evolution of the metasediments that experienced high-pressure metamorphic conditions, we discuss the nature of the second oceanic domain (the Valaisan) and it's geodynamic significance in the Alpine evolution. So we show first that the Valaisan is really an ocean, and secondly that the high-pressure metamorphic event of the Valaisan domain took place late in the evolution of the Alps, but occurred before the collision between the European and Apulian continents.
- Weathering rinds and rock coatings from an Arctic alpine environment, northern Scandinavia
John C. Dixon et al. Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA. Pages 226-238.
- The breakdown of rocks and minerals at the Earth's surface is an extremely important geologic process as it prepares Earth materials for both the formation of soil as well as for movement across the Earth's surface by rivers, ice, wind, and gravity. Traditionally, weathering of rocks and minerals and the transportation of weathered products in cold climates has been regarded as being very slow or non-existent. Results of recent research by Dixon et al. have shown the contrary however. These researchers have shown that measurable weathering of rocks in an Arctic environment has occurred since the disappearance of glacial ice some 10,000 years ago. They recognize the importance of sulfuric acids derived from local bedrock in rock weathering as well as in the transportation of weathering products to form a variety of chemical coatings on rocks in the landscape.
- Multivariate hierarchical analyses of Miocene mollusk assemblages of Europe: Paleogeographic, paleoecological, and biostratigraphic implications
Michal Kowalewski et al. Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. Pages 239-256.
- In the Middle Miocene, 15 million years ago, large areas of Europe were flooded by shallow seas, including the Boreal Province in the north and the partly isolated bays and seaways of the Paratethys Province in the southeastern and central parts of the continent. Abundant fossil shellfish, found in many regions, provide an intriguing testament to diverse marine ecosystems that thrived during Miocene times in Europe. In an international project, aimed at rigorously exploring diversity and ecology of the Miocene marine life, a total of over 8,000 specimens of fossil shellfish were collected from the Netherlands, Belgium, Germany, Poland, Austria, and Hungary. Computer-intensive statistical analyses show that the diversity of the fossil shellfish was much higher in the north in the Boreal Province (the Netherlands, Belgium, Germany) than in the Paratethys (Austria, Hungary, Poland, Romania). This is consistent with the general tendency of partly isolated seaways and bays to display impoverished fauna (perhaps due to geographic barriers and environmental instabilities inherent to marginal bays and seaways). The two marine bioprovinces differed dramatically in type of dominant shellfish. This is consistent with recent paleogeographic reconstructions that suggest a lack of a direct marine connection between the Paratethys and Boreal Province in the Middle Miocene. The study shows also that the value of the biological information entombed in the fossil record can be improved by employing an hierarchical approach, in which differences among samples of fossils are evaluated quantitatively as a function of the geographic and temporal scales of observation.
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