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GSA TODAY Conference, P Vol. 10, No. 2 February 2000 INSIDE • GSA and Subaru, p. 10 • Terrane Accretion Penrose GSA TODAY Conference, p. 11 A Publication of the Geological Society of America • 1999 Presidential Address, p. 24 Continental Growth, Preservation, and Modification in Southern Africa R. W. Carlson, F. R. Boyd, S. B. Shirey, P. E. Janney, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, D.C. 20015, USA, [email protected] T. L. Grove, S. A. Bowring, M. D. Schmitz, J. C. Dann, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA D. R. Bell, J. J. Gurney, S. H. Richardson, M. Tredoux, A. H. Menzies, Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa D. G. Pearson, Department of Geological Sciences, Durham University, South Road, Durham, DH1 3LE, UK R. J. Hart, Schonland Research Center, University of Witwater- srand, P.O. Box 3, Wits 2050, South Africa A. H. Wilson, Department of Geology, University of Natal, Durban, South Africa D. Moser, Geology and Geophysics Department, University of Utah, Salt Lake City, UT 84112-0111, USA ABSTRACT To understand the origin, modification, and preserva- tion of continents on Earth, a multidisciplinary study is examining the crust and upper mantle of southern Africa. Xenoliths of the mantle brought to the surface by kimber- lites show that the mantle beneath the Archean Kaapvaal Figure 2. Bouguer gravity image (courtesy of South African Council for Geosciences) craton is mostly melt-depleted peridotite with melt extrac- across Vredefort impact structure, South Africa. Color scale is in relative units repre- senting total gravity variation of 90 mgal across area of figure. Yellow circular pattern tion accompanying crust formation in the Archean. close to geometric center reflects local gravity high that suggests that the core of the Eclogitic xenoliths from the craton record subduction of structure is underlain by dense mafic or ultramafic rock. Borehole located close to peak altered oceanic crust beneath the craton at ca. 3 Ga. Protero- of gravity anomaly indicates that rocks beneath surface consist mainly of peridotite. zoic age peridotite found beneath the surrounding Protero- zoic accretionary belts provides evidence for crust- mantle coupling and long-term stability of the upper 150 km of the lithosphere. Petrologic examination of Archean ultramafic magmas (komatiites) from South Africa indicates that some komatiitic magmas con- tain substantial quantities of water (>4 wt%). This finding strengthens the possibility that the cratonic lithosphere formed initially in a subduction zone set- ting, the demise of which led to accretion of the arc crust and thickening of the lithospheric mantle to create a stable, thick, continental lithosphere. Geochronologic studies of lower crustal xenoliths from the craton show a prolonged thermal evolution of the lower crust extending to 1 Ga. This thermal evolution is also reflected in ca. 1 Ga ages of some eclogitic diamond inclusions from the lithospheric mantle. INTRODUCTION To explore the causes of continent formation and Figure 1. Geologic sketch map of southern Africa. Brown indicates areas underlain by preservation, a multidisciplinary study involving geol- Archean basement; green indicates Proterozoic metamorphic belts; blue dots indicate ogy, geochemistry, geochronology, petrology, and major kimberlites (Gurney et al., 1991); red square marks Vredefort impact structure. Portable, broad-band seismometer deployment covers area outlined in red. Continental Growth continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 10, No. 2 2000 Continental Growth, Preservation, Employment Matching Service ......... 18 and Modification in Southern Corrections—Southeastern Section GSA TODAY (ISSN 1052-5173) is published monthly Africa ................................ 1 by The Geological Society of America, Inc., with offices at 3300 Meeting . .............................. 18 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Dialogue .............................. 3 Section Meetings To Feature 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage paid at Boulder, Colorado, and at additional mailing offices. Karlstrom Appointed GSA Today Mentor Workshops . ............ 18 Postmaster: Send address changes to GSA Today, Member- Science Co-Editor . 7 2000 Annual Meeting . 19 ship Services, P.O. Box 9140, Boulder, CO 80301-9140. GSA President Named to Committee 2000 GSA Section Meetings ............ 19 Copyright © 2000, The Geological Society of America, Inc. on National Medal of Science .......... 7 (GSA). All rights reserved. Copyright not claimed on content 2000 GeoVentures . ............ 20 prepared wholly by U.S. Government employees within the SAGE Remarks—Teaching Companion scope of their employment. Permission is granted to individ- to This Dynamic Planet ................. 8 Letters ................................. 21 uals to photocopy freely all items other than the science arti- Book Reviews . ..................... 22 cles to further science and education. Individual scientists are Toward a Stewardship of the Global hereby granted permission, without royalties or further Commons, Part II . ...... 10 Call for Nominations . ........ 23 requests, to make unlimited photocopies of the science arti- 1999 Presidential Address—Geologists cles for use in classrooms to further education and science, Subaru–GSA Partnership ............... 10 and to make up to five copies for distribution to associates in Penrose Conference—Terrane Probe Hominid Environments ........... 24 the furtherance of science; permission is granted to make Accretion along the Western Student News and Views . 26 more than five photocopies for other noncommercial, non- profit purposes furthering science and education upon pay- Cordilleran Margin . 11 Biggs Award Nominations .............. 27 ment of a fee ($0.25 per page-copy) directly to the Copy- New Associated Societies ............... 14 right Clearance Center, 222 Rosewood Drive, Danvers, Call for Applications and Nominations MA 01923 USA, phone (978) 750-8400, http://www. Forensic Scientists Association .......... 14 for GSA Bulletin Editor . 27 copyright.com; when paying, reference GSA Today, ISSN 1052-5173. Written permission is required from GSA for all GSA Foundation . 16 Classifieds ............................. 30 other forms of capture, reproduction, and/or distribution of any item in this publication by any means, including posting on authors’ or organizational Web sites, except that permis- sion is granted to authors to post the abstracts only of their CORRECTION: Cordilleran Section Field Trips 8 and 9 science articles on their own or their organization’s Web site providing the posting includes this reference: “The full paper Field trip 8 for the Cordilleran Section meeting has NOT been canceled, contrary was published in the Geological Society of America’s news- to what appears on page 31 of January 2000 GSA Today. Trip 8, Tertiary Geology of the magazine, GSA Today [include year, month, and page num- ber if known, where article appears or will appear].” GSA Eastern Flank of the Central Cascade Range, Washington, is still on the schedule and provides this and other forums for the presentation of should not be crossed out on the preregistration form. diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or polit- Field trip 9, Quesnel Terrane—Always There?, IS canceled, and should be crossed ical viewpoint. Opinions presented in this publication do not out on the preregistration form. reflect official positions of the Society. SUBSCRIPTIONS for 2000 calendar year: Society Members: GSA Today is provided as part of membership dues. Contact Membership Services at (800) 472-1988, (303) 447-2020 or [email protected] for member- Continental Growth continued from p. 1 interest, the southern African lithosphere ship information. Nonmembers & Institutions: Free with paid subscription to both GSA Bulletin and Geology, has been penetrated by hundreds of kim- otherwise $50 for U.S., Canada, and Mexico; $60 else- seismology was initiated 3 years ago with berlite diatremes (Fig. 1) that have brought where. Contact Subscription Services. Single copies may be requested from Publication Sales. Also available on an support from the National Science Foun- xenoliths of lower crust and upper mantle annual CD-ROM, (together with GSA Bulletin, Geology, GSA dation Continental Dynamics Program. to the surface (Gurney et al., 1991). Data Repository, and an Electronic Retrospective Index to The study focuses on the current structure journal articles from 1972); $89 to GSA Members, others call GSA Subscription Services for prices and details. Claims: and geologic history of southern Africa. A CRUSTAL GROWTH AND For nonreceipt or for damaged copies, members contact general description of the project and list MODIFICATION Membership Services; all others contact Subscription Ser- of participants can be found at vices. Claims are honored for one year; please allow suffi- Whereas the geology of the shallow www.ciw.edu/mantle/kaapvaal/. The cen- cient delivery time for overseas copies, up to six months. crust of the Kaapvaal craton is relatively terpiece of the study is an array of 55 well known, the deeper crust is not well STAFF: portable broadband seismometers placed Chief Executive Officer: Sara S. Foland exposed. One exception is the large circu- in 82 sites
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