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Find Your Science at GSA
1 April 2009
GSA Release No. 09-15
Christa Stratton
Director of Education, Communication, & Outreach

April Geosphere Media Highlights

Boulder, CO, USA - The April Geological Society of America e-journal, GEOSPHERE, is now online. This month’s papers investigate possible tectonic cycling of serpentized peridotites in the Mariana forearc; links between the Saddle Mountain and Seattle faults in the Olympic Peninsula, USA, as evidenced in maps created by airborne and marine geophysical methods; the presence of aquifers and oil and gas reservoirs within carbonate rock; and the depths of Earth using seismic reflection profiles and deep well drilling.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Christa Stratton for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific
Keiko Murata et al., Department of Literature, Kobe Women’s University, Kobe 654-8585, Japan, and Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan; corresponding author: Hirokazu Maekawa, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.

In the Mariana forearc, a zone of dome-shaped diapiric seamounts of serpentinized peridotites formed on the basement. Serpentine minerals in peridotites are mostly low-temperature (200-300 degrees Celsius) chrysotile and/or lizardite, but high-temperature antigorite is also found in peridotites recovered from some of the dome-shaped seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, suggesting high-temperature conditions of serpentinization at about 450-550 degrees Celsius. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift
Richard J. Blakely et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

The Saddle Mountain fault on the Olympic Peninsula of Washington state, USA, produced a magnitude 6.5 earthquake between 1000 and 1300 years ago, arguably at the same time as the well-known Seattle fault earthquake (magnitude 7.5). The timing of these two large earthquakes suggests that the two faults work together in some way. Blakely and colleagues used airborne and marine geophysical methods to map the Saddle Mountain fault and found evidence that it is connected with the Seattle fault. Global positioning system (GPS) measurements show that the Puget Lowland is compressing at the rate of about 4.5 mm per year in the north-south direction. We propose that the Saddle Mountain fault is part of a 45-km-long zone that helps to accommodate this north-south shortening of the Puget Lowland past the Olympic Mountains.

Improving fractured carbonate-reservoir characterization with remote sensing of beds, fractures, and vugs
Daniel Kurtzman et al., Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel

About 25% of the world's population drinks water extracted from carbonate-rock aquifers (mostly limestone, dolomite, and chalk). Carbonate-rock reservoirs contain about 60% of the world's oil reserves and hold enormous gas reserves. Nevertheless, the ability of geologists, hydrologists, and petroleum engineers to describe and predict the fluid flow in these reservoirs is far from what they hope it could be. This paper suggests that the characterization of important features controlling the flow in these aquifers and reservoirs can be improved significantly by using observations of the dissolved voids in the rock (holes, caves, and channels, etc., generally termed vugs). The paper describes how relationships between vugs, fractures, and rock beds that were scanned using remote sensing technology can be used for improving the characterization of flow in the carbonate host rock.

Unconformity-bounded seismic reflection sequences define Grenville-age rift system and foreland basins beneath the Phanerozoic in Ohio
Mark T. Baranoski et al., Ohio Department of Natural Resources, Division of Geological Survey, Columbus, Ohio 43224, USA

Seismic reflection profiles are used by earth scientists to image the geology deep beneath Earth's surface. Interpretation of reprocessed Ohio Consortium for Continental Reflection Profiling (COCORP) OH-1 seismic reflection profiles indicates complex Precambrian geology beneath relatively flat-lying Paleozoic strata. This new interpretation, along with analyses of sparse deep well samples in the region, suggests complex sedimentary strata in the Precambrian beneath this Paleozoic sedimentary cover. Other seismic profiles in the region and deep well core samples support this interpretation and proposed model. Because Paleozoic sedimentary cover exists and deep core samples are sparse or lacking in these regions, these new findings demonstrate the importance of using layered seismic sequences to image deep ancient Precambrian events more than one billion years old. Such work is beneficial because the ancient complex of Precambrian rocks controlled or affected deposition of younger Paleozoic rocks. For example, Paleozoic Cambrian saline reservoirs are utilized for industrial waste disposal and also have potential for hydrocarbon and CO2 sequestration. The use of seismic reflection data and deep well drilling allow delineation of deep subsurface areas with potential saline reservoirs and overlying capping or sealing rocks required by industry for modern society.

Review abstracts for these articles at