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. Periodicals postage Abstracts with Programs Order Form ...... 31 paid at Boulder, Colorado, and at additional mailing offices. 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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), and a convergent- with centimeters-per-year precision, but margin batholith (Baja California; Fig. 1). STAFF: Prepared from contributions from the GSA staff and membership. comparative motions of the underlying Executive Director: Donald M. Davidson, Jr. upper mantle are very poorly known EXAMPLE 1—TAUERN WINDOW, Science Editors: Suzanne M. Kay, Department of (e.g., Montagner, 1994). EASTERN ALPS Geological Sciences, Cornell University, Ithaca, NY 14853; Continental crust is also mechanically Molly F. Miller, Department of Geology, Box 117-B, Vanderbilt The Alps formed in response to University, Nashville, TN 37235 and rheologically stratified, some crustal roughly north-south convergence between Forum Editor: Bruce F. Molnia, U.S. Geological Survey, levels being weaker and less rigid than MS 917, National Center, Reston, VA 22092 Eurasia and the Adriatic microplate in lat- others (e.g., Brace and Kohlstedt, 1980). Director of Publications: Peggy S. Lehr est Cretaceous through mid-Tertiary time. Managing Editor: Faith Rogers Weak lower or middle crust can allow Convergence resulted in closure of the Assistant Editor: Vanessa Carney decoupling of upper crust from underlying Production Manager: Jon Olsen Tethys ocean and partial subduction of Joan E. Manly mantle (e.g., Molnar, 1988; Hopper and Production Editor and Coordinator: European continental crust. Syncolli- Graphics Production: Joan E. Manly, Leatha L. Flowers Buck, 1998) and probably affects deforma- sional, orogen-parallel extension, as in tion style(s) of the continental crust as a ADVERTISING: Classifieds and display: contact Ann the Brenner area (Fig. 2), was important Crawford, (303) 447-2020; fax 303-447-1133; acrawfor@ whole (Buck, 1991; Royden, 1996). (Behrmann, 1988; Selverstone, 1988; geosociety.org. Different strain magnitudes and Ratschbacher et al., 1991; Mancktelow Issues of this publication are available as electronic Acrobat geometries can develop coevally between files for free download from GSA’s Web Site, http://www. and Pavlis, 1994). The footwall of the different crustal levels as a result of rheo- geosociety.org. They can be viewed and printed on various Brenner Line underwent strong constric- personal computer operating systems: MSDOS, MSWin- logical stratification. These differences are tional strain during Tertiary orogenesis, dows, Macintosh, and Unix, using the appropriate Acrobat difficult to observe, but we believe that reader. Readers are available, free, from Adobe Corporation: whereas the hanging wall remained they are common, if not typical, in tecton- http://www.adobe.com/acrobat/readstep.html. essentially rigid. ically active regions. Nevertheless, strain This publication is included on GSA’s annual The Brenner Line normal shear zone CD-ROM, GSA Journals on Compact Disc. characteristics of one crustal level are Call GSA Publication Sales for details. 50% Total marks the western margin of the Tauern Recoverd Fiber commonly extrapolated to other crustal window, which is a metamorphic core Printed in U.S.A. using pure soy inks. 10% Postconsumer levels, compared in two-dimensional
2 GSA TODAY, December 1998 complex exposing middle and lower Support Your Society—The Sequel crustal schists and gneisses of European affinity beneath the structurally higher Gail Ashley, President, Geological Society of America Adriatic plate (Austroalpine nappes; Fig. 2A). The north and south margins of the window are dominated by left-lateral and Two years ago Eldridge Moores, • Geological Education through Intelli- right-lateral zones, respectively, and rocks who was then president of GSA, wrote gent Tutors has produced its first multi- of the western window were extruded eloquently about the vital importance of media earth science CD-ROM, “Energy upward and eastward (Ratschbacher et al., members’ support of GSA’s Second Cen- in the Earth Systems,” scheduled for 1991; Fig. 2B). The Brenner shear zone tury campaign. At that time, in September release in January 1999. excised >10 km of crust during 30 to 60 1996, contributions to the campaign • SAGE is collaborating with other geo- km of top-to-west slip (Axen et al., 1995). amounted to $4.7 million. Since then, the science organizations to support imple- Despite juxtaposition during collision, total has nearly doubled and now stands mentation of earth and space science Tauern and Austroalpine rocks record very at more than $9.3 million. curriculum standards in high schools different metamorphic histories. Footwall As your current president, I am privi- and to develop an earth systems science rocks reached high-pressure, greenschist- leged, both for myself and on behalf of my core curriculum for higher education to-amphibolite facies metamorphic condi- predecessors in office during the cam- non–geology majors. tions between 30 and 20 Ma (Blancken- paign, to applaud this wonderful commit- • The Partners for Education Project now burg et al., 1989; Christensen et al., 1994) ment to GSA’s activities and influence. The has 1,800 volunteers interacting with in response to Austroalpine overthrusting generosity of GSA’s members and friends science teachers and students from (Selverstone, 1985). In contrast, the hang- has brought us to within $700,000 of our kindergarten to university level, and ing-wall rocks attained medium-pressure campaign goal. More important, this gen- 600 of these volunteers are on-line as metamorphic conditions before 70 Ma erosity has had a measurable impact on e-mail Partners. (Frank et al., 1987). the growth and success of GSA’s programs • Plans have been developed for the Colo- Structural differences also exist across of education and outreach. rado Rock Park Project, an outdoor exhibit the Brenner Line normal fault. Footwall Two years ago, Eldridge Moores representing Colorado’s geology, geogra- rocks are highly sheared and recrystallized described GSA’s emerging plan to ensure phy, and history. The project is expected (mylonitized) for several kilometers (Fig. 2) better efforts and results in communicating to be a model for similar educational below the Brenner Line, with common the crucial importance of the geosciences to installations elsewhere in the country. top-to-west shear indicators (Selverstone, society. I am pleased to report that in every 1988; Axen et al., 1995). Rocks and early- Institute for Environmental area of targeted activity, the volunteer and formed mylonites (35 to 30 Ma; Chris- Education (IEE). IEE has successfully financial support of GSA’s membership has tensen et al., 1994) exposed in the western promoted the participation of the geosci- made measurable differences. The activities Tauern window are folded into two up- entific communities in the integration of Eldridge outlined were: right, large-amplitude antiforms (Lam- sound scientific information into policy merer and Weger, 1998; Selverstone, New efforts to enhance GSA’s discussions and decisions. 1988). These mylonites are overprinted publications. The fundamental goal of • IEE initiated and led a series of special- upward by younger top-to-west mylonites the publications program is to serve indi- focus workshops to facilitate the transition that become less folded as the Brenner vidual members as well as the academic, of the National Biological Service into the Line is approached, and the Brenner Line research, and applied geoscience commu- USGS/Biological Resources Division. itself is broadly warped by large folds nities. Owing to the dedication of GSA’s • A second mentorship program, the (Axen et al., 1995). These observations editors and the headquarters publications Mann Mentorships in Applied Hydroge- indicate that north-south convergence staff: ology, has been added to the Shlemon continued during east-west extension • Geology has held its place as the Mentors in Applied Geology to encour- and unroofing of the Tauern window. foremost journal in its field and draws age dialogue between students and pro- Syn- to postmylonitic, high-angle a wide range of manuscripts. fessional geologists from outside normal faults are abundant within the • The GSA Bulletin remains one of the academia. footwall near the Brenner Line (Fig. 2). most frequently cited journals of • To facilitate cooperative leadership in These faults probably formed in response geoscience. integrating the earth, life, and social sci- to buoyancy forces induced by unroofing, • GSA has successfully co-ventured ences, IEE partnered with the Ecological and they have both west- and east-down with the Association of Engineering Society of America and the USGS to pre- displacements (Axen et al., 1995). Fluid- Geologists in publishing the journal sent a specialized workshop, “Enhanc- inclusion data show that west-down faults Environmental and Engineering Geoscience. ing Integrated Science.” were active at 15–25 km depth, whereas • GSA Today has broadened its contents, • IEE has collaborated with other organi- east-down faults later affected the same and readers have responded zations to present two workshops focus- rocks at 3–8 km depth (Selverstone et al., enthusiastically. ing on predictive modeling for environ- 1995). Footwall exhumation processes • Increases in nonmember subscription mental policy making. from ~25 to 5 km were thus both ductile prices have put the publications • The Congressional Science Fellowship, and brittle. We infer that Brenner Line program on a sound fiscal footing. maintaining an effective voice for the slip, mylonitization, antiform growth, geosciences in Congress, has been Science Awareness through and high-angle faulting were coeval in increased to an 18-month tenure to Geoscience Education (SAGE). This mid-Oligocene to late Miocene time increase continuity. program’s ambitious plans have become (Selverstone, 1988; Selverstone et al., • A new program providing stipends for reality, and new plans are being made. 1995; Axen et al., 1995). summer internships at national parks • The Earth and Space Science Technolog- In contrast, the Austroalpine hanging supported two interns in its first year ical Education Project has completed wall is essentially unextended internally. and six interns in its second year; it is two series of highly successful summer Mylonites are locally present 200 m above set to grow to 10 interns for the summer workshops for middle school science the Brenner Line (Selverstone, 1988), but of 1999. teachers to help them integrate earth science into their curricula. Crust continued on p. 4 Support Your Society continued on p. 4
GSA TODAY, December 1998 3 Crust continued from p. 3 are absent elsewhere. Similarly, evidence for north-south Tertiary shortening is prevalent in the footwall, but absent in the hanging wall. For example, the Creta- ceous Schneeberg syncline in the hanging wall is on strike with one of the major Ter- tiary antiforms in the footwall (Fig. 2B), but was unaffected by formation of the antiform. Alpine-age fabrics are absent in hanging-wall rocks and their Cretaceous
+ + mica cooling ages preclude Tertiary heat- + + ++ + + + + ing or penetrative deformation (Frank, + 1987). + The upper and lower crust in this + + + region thus responded differently to ++ + + + + + + + + ++ + + + ++ ++ + + ++++ + + Tertiary plate convergence. The western + ++ Tauern rocks record east-west lower crustal flow and north-south shortening Figure 3. Location map and representative structural data (stretching lineations) from the Sanbagawa and Shimanto belts. Units locally present between these belts (no pattern) represent either transitional contemporaneous with semipenetrative, packages (e.g., Banno, 1998) or klippe derived from units exposed on Honshu Island (Isozaki and Itaya, mid-crustal, brittle faulting, whereas the 1991; Taira et al., 1992). Arrows show mean trends of lineations and sense of movement of footwall with overlying Austroalpine rocks underwent respect to hanging wall. Shimanto belt records consistent north-directed underthrusting, whereas San- only insignificant synchronous deforma- bagawa belt shows different senses of movement at different structural levels. tion (Fig. 2B). A geologist working in the Austroalpine units would infer an episode of north-south contraction associated recrystallization has been emphasized in (Fig. 3). The belt comprises two tectono- with moderate heating during the Late the region. Wintsch et al. (1999) have stratigraphic units: the Besshi and the Cretaceous, followed by cooling and rela- suggested that retrograde fabrics formed as Oboke (Takasu and Dallmeyer, 1990). tive quiescence until the present. In con- the Sanbagawa belt was extruded eastward The structurally higher Besshi unit is com- trast, a geologist working in the western during Late Cretaceous oblique plate con- posed largely of pelitic, mafic and siliceous Tauern window would infer extreme east- vergence. Thermal and biostratigraphic schists with deep marine protoliths. Peak west stretching and north-south contrac- data suggest that extrusion was driven, metamorphic conditions of this unit gen- tion from ~35 Ma until <10 Ma. Both are at least in part, by underplating of the erally range from epidote-glaucophane to correct, but each tells only a part of the younger, more seaward Shimanto belt epidote-amphibolite facies (~550 °C and 10 story. (Kimura, 1997). The kinematic histories kbar) (Banno, 1986; Miyashiro, 1961), the of the two belts, however, are strikingly highest-grade rocks occurring in the core EXAMPLE 2—SANBAGAWA different and suggest substantial crustal- of an east-striking, regional-scale fold AND SHIMANTO BELTS, scale heterogeneities in strain (Wintsch (Takasu et al., 1994; Wallis, 1998). The SOUTHWEST JAPAN et al., 1999). Oboke unit has a distinctly lower meta- High-pressure–low-temperature rocks The Sanbagawa belt forms a generally morphic grade, reaching only pumpellyite- of the Sanbagawa belt (Fig. 3) form part north-dipping package of regional-scale actinolite facies. Wintsch et al. (1999) and of a classic “paired metamorphic” belt nappes and folds below the Cretaceous Hara et al. (1992) proposed that the Oboke (Miyashiro, 1961). Recently, the impor- Ryoke magmatic arc and above the accret- represents a more deformed and deeply tance of retrograde metamorphism and ed rocks of the Cretaceous Shimanto belt buried equivalent to the Shimanto belt.
Support Your Society continued from p. 3 • To expand national and international perspectives, program chairs for the technical program and hot topic sessions will be Restructuring of GSA Meetings. During the past two chosen through a process of member-wide search and selection. years, the Annual Program Committee has initiated numerous • The chairs of the Annual Program Committee, the Penrose enhancements to provide more flexible opportunities and to pro- Conference Committee, and the Continuing Education Com- mote excellence in the scientific presentations at the annual mittee have met to form a professional development consor- meetings. Some of the changes were introduced in 1998; more tium to promote a coordinated approach to program planning will be implemented by 1999; and more still are being planned for professional geologists. for the future. Internationalization. In an increasingly active effort to • Pardee keynote symposia, supported with funds from the facilitate GSA’s broader outreach: Joseph T. Pardee bequest, present up to eight leading-edge • International Secretary Ian Dalziel has met with geoscientists topics, selected by a review panel, to illustrate the breadth abroad who have confirmed an interest in joint programming. and significance of the geosciences. • GSA has formed a task force on international activities and, for • Topical sessions offer up to 70 predetermined topics, combin- the past two years, has brought representatives of international ing both invited and volunteered papers. surveys to the annual meeting. • Hot topics, expressly chosen for their controversial aspects and • In a related initiative, a fund established in memory of Charles impact on the geoscientific community, are noontime debates Lum Drake will provide grants to young foreign geoscientists to available to all attendees. attend geoscientific meetings in the United States, forming a • Technical wizardry has enabled Web-based development of a counterpart to the 28th IGC Fund that sends American geosci- session proposal system, an abstracts scheduling system, and entists to meetings abroad. GeoTimer for on-line abstract and session searches in advance • GSA is supporting the 30th IGC, to be held in Brazil, and more of the meeting. Penrose Conferences are being held outside the United States.
4 GSA TODAY, December 1998 UPPER 115.5° PLATE
MIDDLE Sierra PLATE