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Find Your Science at GSA
1 October 2009
GSA Release No. 09-48
Christa Stratton
Director of Education, Communication, & Outreach
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October 2009 Geosphere Highlights

Boulder, CO, USA - The October GEOSPHERE is now online. Articles deal with the composition and character of Earth's crust, documenting and analyzing "the birthplace of a new subduction zone" in Fiordland, southwest New Zealand; following eclogites through serpentinite channels from subducted ocean crust to Earth's surface; and using multiple approaches to illuminate the nature of the Yukon-Tanana terrane, including finding that it represents predominantly crustal recycling with only minor new, juvenile crustal growth.

Highlights are provided below. Review abstracts for these articles at

Representatives of the media may obtain complementary copies of GEOSPHERE articles by contacting Christa Stratton at the address above. 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 assistance.

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

Regional exhumation history of brittle crust during subduction initiation, Fiordland, southwest New Zealand, and implications for thermochronologic sampling and analysis strategies
Rupert Sutherland et al., GNS Science (Institute of Geological and Nuclear Sciences), P.O. Box 30-368, Lower Hutt, New Zealand.

Sutherland et al. present an unusually large and varied dataset used to determine the history of growth and erosion of an active mountain belt in Fiordland, southwest New Zealand. The location is unique because it is the birth place of a new subduction zone - a place where one tectonic plate is starting to sink beneath another. Subduction zones provide most of the forces that pull plates around and move continents, but their destructive nature, which includes earthquakes, volcanoes, and the erosion of mountains, means that evidence about how they form is not usually preserved. For the first time, this study allows the pattern and timing of subduction birth to be tracked and understood. Another interesting outcome of the study is that it shows there to be many fractures and faults, and the study suggest that the crushed rock is pushed up into mountains by movement on these fractures. Conventional methods for analyzing uplift have not previously considered this "geological noise."

Geochemical evidence for exhumation of eclogite via serpentinite channels in ocean-continent subduction zones
Ulyana Horodyskyj et al., Dept. of Earth Science, MS-126, Rice University, 6100 Main St., Houston, Texas 77005, USA

Oceanic crust subducts back into the mantle, but occasionally fragments of this material come back to the surface in the form of garnet-pyroxene rocks called eclogites. How these fragments come back to the surface has remained a mystery. This paper provides one little step toward solving this mystery. It shows that these eclogites have chemical signatures inherited from hydrated mantle rocks, called serpentinite (California's state rock). The authors interpreted this signature as circumstantial evidence that the eclogites were carried to the surface by serpentinite channels, perhaps aided by the fact that serpentinite is a very low-density rock and therefore has the propensity to rise buoyantly to the Earth's surface.

Composition and provenance of the Snowcap assemblage, basement to the Yukon-Tanana terrane, northern Cordillera: Implications for Cordilleran crustal growth
Stephen J. Piercey, SJPGeoConsulting, 11 First Avenue, St. John’s, Newfoundland A1B 1N3, Canada, and Dept. of Earth Sciences, Memorial University, St. John’s, Newfoundland A1N 3X5, Canada, and Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada; and Maurice Colpron

This paper is a field- and laboratory-based study that examines metamorphosed sedimentary rocks that formed the substrate to ~365 million year old volcanic rocks of the Yukon-Tanana terrane along the western edge of the ancient Pacific margin of North America. Despite forming along the ancient continental margin, there have been questions regarding the paleogeographic origin of the Yukon-Tanana terrane, specifically whether the terrane originally formed at northern versus southern latitudes along this margin. Field relationships, trace element geochemistry, radiogenic tracer isotopes, and detrital zircon U-Pb data illustrate that the terrane originated in northern latitudes close to its present position and did not form in southern latitudes and subsequently translate northwards. The paper also provides critical information regarding the crustal growth of the Cordilleran orogen, showing that the Yukon-Tanana terrane represents predominantly crustal recycling with only minor new, juvenile crustal growth.