July GSA BULLETIN highlights

June 27, 2003

Boulder, Colo. - The July issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics of particular interest include: development of Florida's coral reefs over the past 6000-7000 years; and evidence for an oceanic plate, now completely subducted, that stretched from Oregon to Alaska more than 50 million years ago. 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.

Fe-Ti basalts and propagating-rift tectonics in the Josephine Ophiolite
Gregory D. Harper, Department of Earth and Atmospheric Sciences, State University of New York, Albany, New York 12222, USA. Pages 771-787.
Keywords: ophiolite, basalt, geochemistry, tectonics, propagating rift.

The Josephine Ophiolite of northwestern California is a rock complex representing ocean crust and upper mantle formed 165 million years ago. The ophiolite formed along the western margin of North America by seafloor spreading in a deep water basin situated above a subduction zone, where the ancient Pacific plate was thrust beneath western North America. Some of the lava flows of the ophiolite are unusual iron- and titanium-rich basalts. Such basalts are erupted today where a divergent plate boundary propagates into older ocean crust. Modern analogs occur in the western Pacific, west of the Tonga trench and south of the Fiji Islands.

Kilometer-scale kinking of crystalline rocks in a transpressive convergent setting, Central Sierra Nevada, California
Matthew A. Pachell, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA, et al. Pages 817-831.
Keywords: left-lateral, faults, kinks, transpressive, Sierra Nevada.

We examine the relationship between the emplacement of magma bodies, fracturing, faulting, and kinking in the high Sierra Nevada range, California. Our results show that even though granites are not normally thought of as a rock that can bend, the timing and development of fractures creates a "layering" that is then kinked during the last stages of tectonic convergence. These data further help us understand the development of the Sierra Nevada, and how convergent margins work worldwide.

Provenance of plutonic detritus in cover sandstones of Nicoya Complex, Costa Rica: Cretaceous unroofing history of a Mesozoic ophiolite sequence
Claudio Calvo, Anna-Peters-Strasse 51/C, 70597 Stuttgart, Germany. Pages 832-844.
Keywords: provenance, modal analysis, plagiogranites, ophiolite complex, Costa Rica.

This paper presents a new petrologic analysis of selected forearc sandstones from the cover sequence of Nicoya ophiolite complex in NW Costa Rica, made on the basis of relative abundance of identifiably plutonic detritus. Evidence of plutonic source includes fragments composed of micrographic intergrowths of quartz and plagioclase feldspar and uralitized pyroxene. Both of these grain types are typical of plagiogranite bodies intruding the underlying Nicoya Complex. About 9% of the total lithic fragment population was derived from such a source. This estimate is in general agreement with the volumetric proportion of plagiogranites in the basement complex. In addition to providing an example of plutonic provenance, the major result of this study is to document the relatively deep levels of Cretaceous erosion of the basement complex recorded by the cover sandstones analyzed.

Regional Quaternary submarine geomorphology in the Florida Keys
Barbara H. Lidz, et al.,U.S. Geological Survey, 600 Fourth Street South, St. Petersburg, Florida 33701, USA. Pages 845-866.
Keywords: contour maps, coral reefs, Florida Keys, geophysical investigations, outliers, regional sea-level changes.

The study fills a major gap in understanding the development of coral reefs in Florida during the past 6,000 to 7,000 years. New and historical information provided data for constructing new color contour maps of the reefs and the older rock surface on which they grew. The study indicates that the old rock surface is 10 or more feet deeper to the southwest than off the upper Florida Keys. The 6,000-year-old sediments average about 10 ft in thickness, and individual reefs and sand bodies are as much as 35 ft thick. The new maps show that when rising sea level first flooded the platform about 7,000 years ago, the lower, older reef areas were the first to become submerged in Florida. Aerial photographs reveal current-swept underwater sand dunes west of Key West and bands of narrow reefs surrounded by lime sands seaward of the Florida Keys. The map data reveal a region-wide upper-slope terrace in 100 to 130 ft of water that extends seaward for up to 1.5 miles from the base of an 80,000-year-old shelf-margin reef. The terrace extends from offshore Key Largo into the Gulf of Mexico beyond Key West. As many as four massive elongate 80,000-year-old reefs rise up about 100 ft from the terrace. These reefs, called outlier reefs, are thought to have been coastal dunes that were capped with coral when they first became submerged beneath a rising sea during the Ice Ages tens of thousands of years ago. Beyond being of scientific interest, information in this study will be useful to managers and decision makers who need to place fixed navigation markers and implement ecosystem restoration projects.

Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time
Peter J. Haeussler, U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA, et al., Pages 867-880.
Keywords: tectonics, Eocene, Kula plate, Farallon plate, North America, magmatism.

The plate tectonic paradigm provides predictive tools for earth scientists to understand the different kinds of geologic processes expected at convergent, divergent, or transform plate boundaries. However, there is no oceanic crust on Earth older than 180 Ma, and our understanding of the plate mosaic even 50 million years ago is not very clear. This paper, "Life and death of the Resurrection plate" by Haeussler and others, presents evidence that there was an additional oceanic plate that lay along the continental margin of North America, between Oregon and Alaska, prior to 50 million years ago. This plate was subducted into the mantle around 50 million years ago, which resulted in the modern plate tectonic configuration. The paper interprets numerous geologic events as being consistent with their hypothesis. If their hypothesis is correct, it makes numerous predictions that will be useful, among other things, for understanding the mineral resources and hydrocarbon potential of the continental margin.

Detrital-zircon geochronology of the northeastern Tibetan plateau
George E. Gehrels, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 881-896.
Keywords: tectonics, continental accretion, continental-margin sedimentation, island arcs, China.

This study uses U-Pb geochronology of detrital zircons from sandstones to place constraints on the tectonic evolution of the northeastern Tibetan Plateau. The main conclusions are that (1) the region is underlain by a mid- to Late Proterozoic assemblage of accretionary complexes, igneous rocks that formed in oceanic magmatic arcs, and shallow-marine strata, (2) a broad early Paleozoic magmatic arc was constructed across the Proterozoic basement, apparently in response to southwest-directed (in present coordinates) subduction, and (3) these assemblages have been offset by ~370-400 km along the left-lateral Altyn Tagh fault during Tertiary India-Asia collision.
To view abstracts for the GSA BULLETIN, go to www.gsajournals.org. Representatives of the media may obtain a complimentary copy of any GSA BULLETIN article by contacting Ann Cairns at acairns@geosociety.org.

The Geological Society of America: www.geosociety.org

Geological Society of America

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