||May 31, 2002
GSA Release No. 02-34
June Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The June issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest are: the geologic history of Crater Lake, Oregon, and implications for improved assessment of volcanic hazards; and new evidence that the southern Sierra Nevada were formed by a "bottom-down" delamination process within Earth's lithosphere. 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.
- Southeastern margin of the middle Paleozoic shelf, southwesternmost Appalachians: Regional stability bracketed by Acadian and Alleghanian tectonism
James F. Tull, Department of Geological Sciences, Florida State University, Tallahassee, Florida 32306, USA. Pages 643-655.
- The southwesternmost part of the Appalachian orogen contains a metamorphic thrust sheet representing the most outboard fragment of the ancient North American continent in this region. This thrust sheet includes a thick Devonian to earliest Mississippian (?) metamorphic chert/black slate starved basin sequence representing a period of continental margin stabilization following significant tectonic instability represented by formation of a thick, early Acadian clastic wedge. The starved basin sequence formed while the main manifestations of the Acadian orogeny were occurring in the central and northern Appalachians. Subsequent deformation and metamorphism of the middle Paleozoic starved basin sequence may mark the earliest effects of the protracted Alleghanian orogeny in the southern Appalachians. These relationships suggest that first effects of both the Acadian and the Alleghanian orogenies may have been felt in the extreme southern Appalachians, but that they were separated by a period of continental margin stability.
- A geologic test of the Kula-Pacific Ridge capture mechanism for the formation of the West Philippine Basin
Jonathan C. Lewis, Department of Geology, University of California, Davis, California 95616, USA, et al. Pages 656-664.
- The tectonic setting that led to the formation of the oceanic crust underlying the Philippine Sea remains poorly understood. Indeed, the Philippine Sea plate is one of the most enigmatic of the major tectonic plates on earth, partly because it is entirely submarine and surrounded by deep trenches. In this paper we test the hypothesis that the core of this plate, the West Philippine Basin, formed at the time of a reorganization of plate motions in the Pacific Basin. In particular, we explore the idea that the West Philippine Basin formed as a stranded rift-segment in response to the transition from north-south spreading to northwest-southeast spreading along the now extinct Kula-Pacific rift. Sea-floor data from a surviving segment of this rift indicate that the change in motion occurred ca. 53 million years ago. This time can be correlated with an analogous change in the direction of faulting in rocks exposed along the coast of southwest Japan. Using the change in motion recorded by these faults in combination with the sea-floor record of Kula-Pacific spreading, we develop constraints on the position and motion of the Kula plate in the western Pacific basin from ca. 64 to 39 million years ago. In so doing we demonstrate that the Philippine Sea plate may have originally been part of the Kula-Pacific divergent plate boundary.
- Modeling the subglacial hydrology of the late Pleistocene Lake Michigan Lobe, Laurentide Ice Sheet
Christopher W. Breemer, et al., Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA. Pages 665-674.
- The results of this study suggest that water pressure beneath the ice was great enough that the ice sheet encountered little resistance when flowing over underlying beds. Since bed resistance partially controlled ice sheet growth and recession, this study suggests that factors other than climate may have affected ice growth and recession in the Lake Michigan area.
- Morphology, volcanism, and mass wasting in Crater Lake, Oregon
Charles R. Bacon, Volcano Hazards, U.S. Geological Survey, 345 Middlefield Road, M.S. 910, Menlo Park, California 94025-3591, USA, et al. Pages 675-692.
- Crater Lake was surveyed in the summer of 2000 by high-resolution multibeam echo sounding to define its geologic history and provide an accurate base map for research and monitoring surveys. The new survey reveals the character of landforms in detail and leads to a chronology for volcanism, debris movement, and filling of the lake within the approximately 7,700 year old caldera. Andesitic volcanoes erupted as the lake was filling during the first roughly 500 years of the caldera's history. The youngest volcanic feature is a rhyodacite lava dome that was emplaced about 4,800 years ago. Major debris avalanches containing blocks nearly 300 meters (1,000 feet) long occurred sometime after the early volcanic eruptions. The lake rose until it reached relatively permeable layer in the caldera wall after about 650 years, and than gradually stabilized. A wave-cut platform, commonly 40 meters (130 feet) wide, indicates that Crater Lake has remained within a few meters of its present level for a long time. The new survey documents landforms that result from volcanism in rising water, delineates avalanching and sediment transport into a restricted basin, and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards.
- Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction
Jeffrey M. Trop, Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA, et al. Pages 693-717.
- The manuscript presents new scientific data that demonstrates how a fragment of the Earth's crust was translated northward from its origination near the equator about 200 million years ago to a modern position in southern Alaska. Changes in rocks types, deformation features, and fossils provide important clues toward understanding this history and how the Earth's crust grows through time.
- Holocene strath terraces, climate change, and active tectonics: The Clearwater River basin, Olympic Peninsula, Washington State
Karl W. Wegmann, Division of Geology and Earth Resources, Washington State Department of Natural Resources, Olympia, Washington 98504, USA, and Frank J. Pazzaglia, Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA. Pages 731-744.
- Terraces are bench-like landforms common to many river valleys. In the Olympic Mountains of Washington State, terraces along the Clearwater River contain clues about river channel changes, watershed hydrology, climate change, and the uplift of the Olympic Mountains. A robust data set of 38 radiocarbon dates from these terraces indicates that the terraces were created during descrete periods over the past 10,000 years when the Clearwater River carved laterally, widening its flood plain. These flood-plain-widening events alternate with times when the Clearwater River carves vertically into bedrock. Understanding these changes in river behavior helps geologists discern the range of natural river processes from those caused by human uses of the watershed.
- Did lithospheric delamination trigger late Cenozoic potassic volcanism in the southern Sierra Nevada, California?
G. Lang Farmer, Cooperative Institute for Research in Environmental Sciences and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA, et al. Pages 754-768.
- Recent studies have suggested that the southern Sierra Nevada in California may be a "bottom-down" mountain range, that is, the mountains exist because deep portions of the continental lithosphere peeled off the bottom of the continent, sometime over the past 10 million years, and sank deeper into the Earth. "Delamination" of the deep lithosphere allowed the remaining portions of the continental mantle and crust to bob up to higher elevation forming at least a portion of the current Sierra Nevada. In this study, we investigate the geochemistry of some compositionally unusual, relatively young (less than 12 million years old), volcanic rocks that occur within the southern Sierra Nevada to determine if the formation of these rocks, and the magmas from which they were originally derived, was related to a delamination event. Interestingly, Pliocene age volcanic rocks about 3.5 million years old in age have unique geochemical characteristics relative to other Sierran volcanic rocks and apparently represent the products of melting of a very shallow (about 40 kilometers -25 miles deep) portion of the sub-Sierran lithospheric mantle. Provoking melting in shallow, and relatively cold, portions of the mantle beneath continental crust is a difficult task. However, the conditions necessary to trigger melting in the shallow portions of the continental mantle could have been achieved beneath the southern Sierra if this mantle were uplifted and/or exposed to hot upwelling mantle from greater depths in the Earth. Both conditions are the likely result of delamination of the lithospheric mantle originally present beneath the Sierra at depths greater than 40 km. As a result, our data provide further evidence supporting a delamination model for the origin of the southern Sierra Nevada, and restricts to the timing of delamination to the interval between about 8 and 3.5 million years ago.
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