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News Release November 25, 2002
GSA Release No. 02-56
Contact: Christa Stratton
+1-303-357-1093
FOR
IMMEDIATE
RELEASE

December Media Highlights:
The Geological Society of America Bulletin

Boulder, Colo. — The December issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics of particular interest include new analysis of the San Andreas fault system in the San Francisco Bay area as well as the unstable intersection of three tectonic plates off the northern California coast. 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.

Please note that GSA will close for the Thanksgiving holiday at 12:00 p.m. MST on Wednesday, November 27. The office will reopen Monday, December 2.

Controls on timing and amount of right-lateral offset on the East Bay fault system, San Francisco Bay region, California
R.W. Graymer, et al., U.S. Geological Survey, 345 Middlefield Road, M.S. 975, Menlo Park, California 94025, USA. Pages 1471-1479
Keywords: offsets, San Andreas fault, San Francisco Bay region, strike-slip faults, tectonic models.
Scientists at the U.S. Geological Survey have deduced the geologic history of faults in the San Andreas fault system east of San Francisco Bay by matching up distinctive rock bodies separated by fault motion, mapping out other rock bodies that have not been offset, and looking at the pattern of the age of eruption of ancient volcanoes in the area. The 12 million year fault history revealed shows that, even though the driving force of plate motion stayed generally constant, motion on the faults started, stopped, sped up, and slowed down without noticeable trend or pattern. It also shows that one fault, the Hayward fault, dominates the system of faults, having accumulated more than half the total 175 kilometers of fault offset. Understanding the history of these faults will help us understand how the landscape in the San Francisco Bay Area was formed, and may also shed light on the processes that will cause the next big earthquake in the region.
Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone
Kenneth D. Ridgway, Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397, USA, et al. Pages 1480-1503.
Keywords: Alaska Range, Cantwell Formation, Kahiltna assemblage, suture zone, Usibelli Group, Wrangellia.
New structural, stratigraphic, and metamorphic data from the Alaska Range document the progrssive growth of a suture zone from its inception as a late Mesozoic deep-marine sedimentary basin to one of the highest active mountain ranges on Earth. The Alaska Range represents a suture zone that formed due to the collision of an island arc assemblage with the former North American continental margin. Results of this study show that sedimentary basins may continue to form and evolve along suture zones long after collision has ceased because changes in plate motion periodically reactivate the suture zone.
Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Structural evolution
Sean P.S. Gulick and Anne M. Meltzer, Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania, USA. Pages 1504-1518.
Keywords: Mendocino triple junction, Cascadia subduction zone, microplates, seismic reflection, profiles, strike-slip faults, thrust faults.
Offshore northern California, the Gorda tectonic plate dives obliquely beneath North America forming the southern end of what is known as the Cascadia subduction zone. The southern Cascadia subduction zone and overlying Eel River sedimentary basin lie just north of the unstable intersection of the Gorda, North American, and Pacific plates centered at Cape Mendocino, which is known as the Mendocino triple junction. The Neogene layers of rock contained in the Eel River basin record deformation caused by the underthrusting of the Gorda plate as well as deformation generated by northward migration and encroachment of the Mendocino triple junction. Three distinct areas of deformation are present in the Eel River basin. Along the western margin of the basin and within slope from the deep ocean to the continental shelf, thrust faults and folds record Plio-Pleistocene deformation caused by underthrusting of the Gorda plate. The southern part of the basin has rotated counterclockwise in the late Pleistocene resulting in both modern northeast-southwest shortening and northwest-southeast sliding (strike-slip faulting) offshore Humboldt Bay. The rotation and deformation of the southern part of the basin are caused by a rigid Pacific plate at the triple junction pushing northward across its boundary with the southernmost part of the more deformable Eel River basin. The northeastern margin of the Eel River basin is deformed by steep faults with a component of strike-slip motion that may represent the beginnings of San Andreas style faulting moving north of the triple junction at Cape Mendocino.
Post-Nevadan detachment faulting in the Klamath Mountains, California
Susan M. Cashman, Department of Geology, Humboldt State University, Arcata, California 95521, USA, and Don R. Elder, Klamath National Forest, 1312 Fairlane Road, Yreka, California 96097, USA. Pages 1519-1533.
Keywords: detachment fault, extension tectonics, faults and faulting, Klamath Mountains, massive-sulfide deposits.
Detachment faults are shallowly dipping normal faults that accommodate crustal extension. These faults are important structures throughout the eastern two-thirds of the North American Cordillera, where they record widespread mid-Tertiary extension. They are not commonly found in the accreted terranes along the western margin of the continent, where thrust faults and other structures that record plate convergence predominate. The authors describe a group of faults overprinted on accreted terranes of the Klamath Mountains of northern California that display detachment fault geometry. The faults dip shallowly outward from the central Trinity terrane. The presence of cataclastic rocks and of epithermal alteration and associated small gold ± copper massive sulfide deposits on these structures indicates that faulting occurred at shallow crustal levels. The detachment faults cross-cut Late Jurassic thrust faults, and one detachment fault is intruded by a 126 ± 3 Ma dike. These relationships indicate that detachment faulting in the Klamath Mountains occurred in Early Cretaceous time.
Valley-fill alluviation during the Little Ice Age (ca. A.D. 1400-1880), Paria River basin and southern Colorado Plateau, United States
Richard Hereford, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001, USA. Pages 1548-1561.
Keywords: alluvial deposits, arroyos, climate effects, El Niño, geomorphology, Holocene.
Alluvium deposited from A.D. 1400-1880 is widespread in valleys of the southern Colorado Plateau. Deposition coincided with the Little Ice Age epoch of global climate variability and was probably related to a sustained decrease in the number of high magnitude floods, which enhanced sediment storage in stream channels and on floodplains. Deposition in the valleys ended around A.D. 1880 with the beginning of historic arroyo cutting. This shift from deposition to valley entrenchment coincided with an episode of frequent large floods that was probably caused by an increased frequency and intensity of flood-producing El Niño events.
Detrital zircon provenance analysis of the Great Valley Group, California: Evolution of an arc-forearc system
Kathleen DeGraaff-Surpless, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA, et al. Pages 1562-1578/
Keywords: California, forearc basins, Great Valley, provenance, Sierra Nevada, zircon.
The composition of grains in sandstone can be diagnostic of the original source of those grains, as in the case of volcanic grains eroded from volcanic arcs. Conventionally, such analyses are conducted with a standard optical microscope, but the broad compositional similarity of sand from all volcanic arcs often results in ambiguity as to specific sources. The ability to determine the ages of individual grains provides greater interpretational leverage, and that capability exists in the Sensitive High Resolution Ion Microprobe (SHRIMP). The application of SHRIMP technology to a geologically well-characterized arc-related sedimentary basin could serve as a standard against which studies of poorly known but analogous basins might be gauged. Because the sources of the Mesozoic strata of the Great Valley basin of California are generally known to be in the adjacent Sierra Nevada and Klamath arcs (based on sediment bulk-grain and trace element compositions), our SHRIMP study of the ages of Great Valley zircon sand grains forms the basis of a model of sediment source relations in an ancient arc-related basin, and provides insights into the long-term evolution of volcanic arcs and their related sedimentary basins. From our results, we can infer the position and development of submarine canyons entering the Great Valley basin, document shifting river systems within the Sierra Nevada arc during basin development, and postulate a rapid rate of arc erosion which removed as much as 6 kilometers thickness of volcanic rock in as little as 3 million years.
Variation in glacial erosion near the southern margin of the Laurentide Ice Sheet, south-central Wisconsin, USA: Implications for cosmogenic dating of glacial terrains
Patrick M. Colgan, Department of Geology, Northeastern University, 14 Holmes Hall, Boston, Massachusetts 02115, USA, et al. Pages 1581-1591.
Keywords: cosmogenic isotopes, dating, exposure age, glacial erosion.
In this paper the authors discuss the use of cosmogenic isotope methods to determine the timing of ice sheet retreat in Wisconsin during the end of the last glaciation about 15,000 years ago. This dating method uses the accumulation of the isotopes 10Be and 26Al, produced by cosmic radiation in surface rocks, to estimate the time during which those rocks were exposed at Earth's surface following glacial erosion. The evidence in the paper suggests that the rate at which the isotopes 10Be and 26Al accumulate may be overestimated in previous studies because glacially eroded bedrock may contain a significant quantity of pre-existing isotopes from earlier periods of exposure and the production of isotopes at depth.
The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A >675 km3 compositionally zoned felsic lava of Mesoproterozoic age
S.R. Allen and J. McPhie, Centre for Ore Deposit Research and School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia. Pages 1579-1596.
Keywords: Gawler Craton, large-volume rhyolitic lava, Mesoproterozoic, South Australia.
In this article we describe the Eucarro Rhyolite, a remarkable lava that is voluminous (>675 km3), thick (300 m), extensive (> 225 km) and compositionally heterogeneous (SiO2 varies from 74 wt% at the base, 71 wt% in the interior, and >74 wt% in the outer part). The Eucarro Rhyolite was erupted within a Mesoproterozoic (ca. 1592 Ma), continental, intraplate volcanic province (Gawler Range Volcanics) in South Australia. Textural characteristics are consistent with transport as lava, and cooling and degassing as a single emplacement unit. The main Eucarro magma was compositionally zoned, and several smaller batches of magma were tapped during the eruption. The lava was presumably derived from several source vents located along a highly elongate E-W vent system. Although very widespread, outflow of the lava was probably no more than a few tens of kilometers from source vents that are most likely now concealed directly below the unit.

To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.
To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns.

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