We ’re A lmo S st Vol. 8, No. 12 December 1998 ee Th INSIDE p. 3 ere! • Support Your Society, p. 3 • 1999 Section Meetings— GSA TODAY South-Central, p. 23 A Publication of the Geological Society of America Northeastern, p. 26 Southeastern, p. 32 If the Strong Crust Leads, Will the Weak Crust Follow? Figure 1. View northeast in the central Sierra El Mayor showing the detachment fault that separates heavily intruded, light-colored, lower- plate migmatites (center and right) from dark-colored middle plate metasedimentary rocks (left). Middle-plate country rocks were shortened east-west and vertically thickened by isoclinal folding at greenschist conditions, while lower-plate country rocks and melanosome were mildly elongated east-west and vertically shortened. Gary J. Axen, Department of Earth and Space Sciences, University of California, Los Angeles, 90095-1567, [email protected] Jane Selverstone, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131 Timothy Byrne, Department of Geology and Geophysics, University of Connecticut, Storrs, CT 06107 John M. Fletcher, Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Baja California, México ABSTRACT Contemporaneous deformation at different levels of the conti- ° 11 30'E M nchen nental crust can be strongly heterogeneous, resulting in disparate 100 km A ZürichZ rich Austroalpine bulk deformation patterns between crustal levels. In each of three Innsbruck examples from diverse tectonic settings, exposed rocks from differ- Brenner Tauern Line Helvetic ent crustal levels differ greatly from one another in strain geome- AAm ° try. Such heterogeneity of deformation is likely to be controlled by 47 N Penninic rheological differences and boundary conditions. If strong three- Southern Alps dimensional heterogeneity of strain in deforming continental crust Milano Venezia is the norm rather than the exception, many assumptions com- AAg AAsn Brenner monly used in interpretation of vertical profiles of modern and ancient crust, in dynamic and kinematic modeling, and in infer- ence of ancient plate motions could be inappropriate. TWz INTRODUCTION TWls It has long been known that rock deformation patterns vary AAsb TWus greatly with rock type, temperature, pressure, strain rate, differential Sterzing N stress, and fluid conditions, among other controlling factors. Spatial AAg 10 km and temporal variability of any of these factors leads to heteroge- neous strain on a variety of scales, ranging from that of lithospheric plates to individual thin sections. rigid Austroalpine Brenner Line Tauern B hanging wall Window Crust continued on p. 2 AAsn TWus Figure 2. Tectonic map (A) and block diagram (B) showing key features of the Schneeberg syncline Brenner Line footwall (Tauern window) and hanging wall (Austroalpine units); (Cretaceous ) yellow box in inset shows location. Footwall units are Zentralgneis basement 25 km TWz (TWz) and Lower and Upper Schieferhülle cover sequences (TWls and TWus); Austroalpine units are gneisses (AAg), Mesozoic cover (AAm), metasedimentary AAg rocks of the Schneeberger syncline (AAsb), and overthrust Steinach nappe (AAsn). Deformation and metamorphism in AA units predate 70 Ma. Ductile mylonites, upright folds, and high-angle normal faults in the Tauern window developed Tertiary N-S shortening and during Oligocene-Miocene extrusion. eastward extrusion of footwall IN THIS ISSUE Women and Men in the Geosciences ...... 22 GSA TODAY December GSA Today Student Correspondent ......... 22 Vol. 8, No. 12 1998 If the Strong Crust Leads, South-Central Section Grants .............. 22 Will the Weak Crust Follow? ........ 1 1999 Section Meetings— GSA TODAY (ISSN 1052-5173) is published monthly In Memoriam ............................ 2 South-Central ............................ 23 by The Geological Society of America, Inc., with offices at 3300 Support Your Society—The Sequel ......... 3 Northeastern ............................. 26 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box GSA On the Web ......................... 7 Southeastern ............................. 32 9140, Boulder, CO 80301-9140, U.S.A. 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Written permission is required from GSA for all other forms of capture, reproduction, and/or distribution of any item in this publication by any means, including posting John L. Burnett Ruth Hopson Keen James B. Rucker on authors’ or organizational Web sites, except that permis- Sacramento, California Portland, Oregon Carriere, Mississippi sion is granted to authors to post the abstracts only of their September 1998 October 17, 1998 science articles on their own or their organization’s Web site William V. Sliter providing the posting includes this reference: “The full paper was published in the Geological Society of America’s news- Willard C. Gere Gerhard W. Leo Menlo Park, California magazine, GSA Today, [include year, month, and page num- Menlo Park, California Los Gatos, California October 1997 ber if known, where article appears or will appear].” GSA September 20, 1998 September 14, 1998 provides this and other forums for the presentation of William G. Wahl diverse opinions and positions by scientists worldwide, Clyde T. Hardy Ronald J. Lipp Corbyville, Ontario regardless of their race, citizenship, gender, religion, or polit- ical viewpoint. Opinions presented in this publication do not Logan, Utah Long Island, New York July 10, 1998 reflect official positions of the Society. October 13, 1998 September 11, 1998 SUBSCRIPTIONS for 1998 calendar year: Society Members: GSA Today is provided as part of membership dues. Contact Membership Services at (800) 472-1988, Crust continued from p. 1 vertical profiles, or used to infer major (303) 447-2020 or [email protected] for member- orogenic motions and past plate motions. ship information. Nonmembers & Institutions: Free with paid subscription to both GSA Bulletin and Geology, The most important boundary at the We discuss three examples of con- otherwise $50 for U.S., Canada, and Mexico; $60 else- plate scale could be the rheological gradi- trasting coeval deformation patterns where. Contact Subscription Services. Single copies may be requested from Publication Sales. Also available on an ent that decouples rigid lithosphere from between different crustal levels and con- annual CD-ROM, (together with GSA Bulletin, Geology, GSA weaker underlying mantle asthenosphere sider their implications for vertical pro- Data Repository, and an Electronic Retrospective Index to (e.g., Karato and Wu, 1993) and allows files, dynamic models, and inferences of journal articles from 1972); $89 to GSA Members, others call GSA Subscription Services for prices and details. Claims: major differences between their motions. relative plate motions. Our examples are For nonreceipt or for damaged copies, members contact This nearly complete decoupling leads to from diverse settings, including a conti- Membership Services; all others contact Subscription Ser- a situation where three-dimensional litho- nental collision zone (Alps), an accre- vices. Claims are honored for one year; please allow suffi- cient delivery time for overseas copies, up to six months. spheric-plate velocity fields are known tionary prism (Japan),