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2010 GSA Annual Meeting & Exposition • Denver, Colorado, USA VOL. 20, No. 9 A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA SEPTEMBER 2010 Subduction of the Chile Ridge: Upper mantle structure and flow Inside: ▲ First Announcement and Call for Papers: 2011 GSA Northeastern & North-Central Joint Section Meeting, p. 48 ▲ First Announcement and Call for Papers: 2011 GSA Southeastern Section Meeting, p. 52 ▲ Groundwork: The Internet as a resource and support network for diverse geoscientists, p. 59 VOLUME 20, NUMBER 9 ▲ SEPTEMBER 2010 SCIENCE ARTICLE GSA TODAY publishes news and information for more than 4 Subduction of the Chile Ridge: Upper 22,000 GSA members and subscribing libraries. GSA TODAY mantle structure and flow (ISSN 1052-5173 USPS 0456-530) is published 11 times per R.M. Russo, J.C. VanDecar, D. Comte, year, monthly, with a combined April/May issue, by The V.I. Mocanu, A. Gallego, and R.E. Murdie Geological Society of America®, Inc., with offices at 3300 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Cover: Elephant Rock, an evocatively shaped sea stack 9140, Boulder, CO 80301-9140, USA. Periodicals postage paid ~1 km off the west coast of the Taitao Peninsula, Chile at Boulder, Colorado, and at additional mailing offices. (visible in background), is made up entirely of the Taitao Postmaster: Send address changes to GSA Today, GSA Sales and Service, P.O. Box 9140, Boulder, CO 80301-9140, Ophiolite, an obducted sliver of Nazca Plate oceanic USA. GSA provides this and other forums for the presentation crust. This portion of the Nazca Plate crust formed at of diverse opinions and positions by scientists worldwide, the Tres Montes segment of the Chile Ridge spreading regardless of their race, citizenship, gender, religion, or political center prior to its subduction beneath South America. viewpoint. Opinions presented in this publication do not reflect Photo by R.M. Russo. See “Subduction of the Chile official positions of the Society. Ridge: Upper mantle structure and flow”, p. 4–10. Copyright © 2010, The Geological Society of America (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within the scope of their employment. Individual scientists are hereby granted 2010 GSA Annual Meeting & Exposition: Reaching New Peaks in Geoscience permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in 12 Presidential Address & Awards Ceremony other subsequent works and to make unlimited photocopies of 13 GSA Gold Medal Lectures items in this journal for noncommercial use in classrooms to further education and science. For any other use, contact 14 Five MORE Reasons to Come to the 2010 GSA Annual Meeting & Exposition Permissions, GSA, P.O. 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Short Courses 38 Graduate School Information Forum ADVERTISING: 40 Exhibitors by Category Classifieds & Display: Ann Crawford, +1-800-472-1988, ext. 1053, +1-303-357-1053, Fax +1-303-357-1070; 42 Childcare [email protected] GSA ONLINE: www.geosociety.org 44 Field Forum Report: Significance of Along-Strike Variations for the 3-D Architecture of Orogens: The Hellenides and Anatolides in the Eastern Mediterranean Printed in the USA using pure soy inks. 46 Penrose Conference Announcement: Neotectonics of Arc-Continent Collision 48 First Announcement and Call for Papers: 2011 GSA Northeastern & North- Central Sections Joint Meeting 51 2011 GSA Section Meeting Calendar 52 First Announcement and Call for Papers: 2011 GSA Southeastern Section Meeting 54 GSA Foundation Update 55 Classified Advertising 59 Groundwork: The Internet as a resource and support network for diverse geoscientists 61 Call for Applications: 2011–2012 GSA-USGS Congressional Science Fellowship Subduction of the Chile Ridge: Upper mantle structure and flow R.M. Russo, Dept. of Geological Sciences, P.O. Box 112120, concept of ridge subduction was developed to explain Neo- 241 Williamson Hall, University of Florida, Gainesville, Florida, gene tectonics and magmatism in western North America that USA, [email protected]; John C. VanDecar, Dept. of Terrestrial were difficult to ascribe to Farallon plate subduction alone (At- Magnetism, Carnegie Institution of Washington, 5241 Broad water, 1970; DeLong and Fox, 1977; Dickinson and Snyder, Branch Road NW, Washington, D.C. 20015, USA; Diana 1979; Thorkelson and Taylor, 1989), and since then a host of Comte, Depto. de Geofisica, Universidad de Chile, Blanco observations from Central America (Johnston and Thorkelson, Encalada 2002, Santiago, Chile; Victor I. Mocanu, Dept. of 1997) to Baja (e.g., Rogers et al., 1985; Michaud et al., 2006; Geophysics, University of Bucharest, 6 Traian Vuia Street, Pallares et al., 2007) to British Columbia (Groome et al., 2003; Bucharest, Romania; Alejandro Gallego, Dept. of Geological Audet et al., 2008) to Alaska (e.g., Sisson and Pavlis, 1993; Sis- Sciences, P.O. Box 112120, 241 Williamson Hall, University of son et al., 2003; Breitsprecher et al., 2003; Cole et al., 2006; Qi Florida, Gainesville, Florida, USA; and Ruth E. Murdie*, et al., 2007) have been associated with spreading ridge subduc- Comprehensive Test Ban Treaty Organization, Vienna Centre, tion beneath western North America in some way. Vienna, Austria Beyond the clear effects on the overriding plate, ridge sub- duction is the last stage of destruction of one of the two oce- anic lithospheres involved in the process, and, depending on ABSTRACT the exact geometry of the ridge with respect to the consuming We deployed 39 broadband seismometers in southern Chile trench, may mark the introduction of new oceanic plate into from Dec. 2004 to Feb. 2007 to determine lithosphere and the subduction system. In most cases, ridge subduction seems upper mantle structure in the vicinity of the subducting likely to result in wide separation of the subducted lithospheres Chile Ridge. Body-wave travel-time tomography clearly that were once contiguous at their intervening ridge-transform shows the existence of a long-hypothesized slab window, a boundary, at least at depths greater than a few hundred kilo- gap between the subducted Nazca and Antarctic lithospheres. meters. Such divergence between the trailing edge of the com- P-wave velocities in the slab gap are distinctly slow relative to pletely consumed plate and the leading edge of the conjugate surrounding asthenospheric mantle. Thus, the gap between plate opens slab windows and provides gaps through which slabs visible in the imaging appears to be filled by unusually asthenospheric mantle can flow and mix (Thorkelson, 1996). warm asthenosphere, consistent with subduction of the Chile The implications for geochemical cycling in the mantle, at arcs Ridge. Shear wave splitting in the Chile Ridge subduction (Gutiérrez et al., 2005), and even at unsubducted portions of region is very strong (mean delay time ~3 s) and highly vari- the spreading ridge may be profound (Klein and Karsten, 1995; able. North of the slab windows, splitting fast directions are Karsten et al., 1996). Given the possible effects of ridge sub- mostly trench parallel, but, in the region of the slab gap, split- duction on the geology of overriding continental plates, and on ting fast trends appear to fan from NW-SE trends in the north, mantle mixing, some direct imaging of how ridge subduction through ENE-WSW trends toward the middle of the slab win- actually works is desirable. Currently, the Chile Ridge, a long- dow, to NE-SW trends south of the slab window. We interpret lived wide ocean basin spreading ridge, is subducting beneath these results as indicating flow of asthenospheric upper mantle southern South America, affording a perfect opportunity to ex- into the slab window. amine exactly what happens when a ridge meets a trench and is recycled into