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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 heterogeneous 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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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

yy LOWER Middle PLATE Plate Peninsular Ranges Lower 5 km Plate Basin & Range yy Figure 4. Kinematic interpretation for extrusion C P El and exhumation of the Sanbagawa belt. Move- ment of the Sanbagawa from west to east (pres-

ent coordinates), slightly up the dip of the sub- Pacific Ocean N ducting slab to shallower structural levels, is inferred to have been driven by a combination of S M O Mayor oblique plate convergence, underplating, tectonic thinning, and partial closure of the subduction channel (see also Wintsch et al., 1999). Regional- scale recumbent fold deforms peak metamorphic 32° isograds and is based, in part, on Wallis (1998). WEST Figure 5. Tectonic map of Sierra El Mayor, Baja California, showing distribution of upper-, Structural and thermochronologic middle-, and lower-plate rocks and of late Ceno- Figure 6. Schematic Cretaceous structures and data from the Sanbagawa belt suggest a zoic detachment faults (heavy lines with tick deformation (orange arrows) of the Sierra El progressive 60 m.y. cooling history as it marks) that separate them. Yellow is sedimentary Mayor middle and lower plates, which are now strata and red and pink are crystalline basement. juxtaposed across a younger detachment fault was exhumed and extruded from west to (heavy line with tick marks). Middle plate was east. High-grade rocks of the Besshi unit Inset of southwestern North America shows location (black square); CP is Colorado Plateau; SMO thickened vertically and shortened east-west. cooled through ~500 °C at 94 Ma, through is Sierra Madre Occidental. Lower-plate metamorphic rocks were flattened ~350 to 400 °C at 86 to 76 Ma (Takasu and vertically, which was at least partly offset by verti- Dallmeyer, 1990), and were at the surface cal inflation owing to sill emplacement, and by ~50 Ma, because they are overlain by stretched slightly east-west. unmetamorphosed Eocene sedimentary seems to dominate high structural levels, rocks. Whole-rock 40Ar/39Ar (Takasu and whereas top-to-east shear appears to domi- Dallmeyer, 1990) and zircon fission-track nate lower structural levels (Wallis, 1995), 1995). The Shimanto belt comprises (Shinjoe and Tagami, 1994) ages from the suggesting that the middle of the Sanba- coherent turbidite sequences and interlay- Oboke unit indicate Late Cretaceous cool- gawa belt was extruded from west to east ered belts of shale-rich tectonic melange. ing. Penetrative, retrograde fabrics, includ- (Fig. 4). Extrusion was apparently driven Metamorphism was relatively low grade, ing east-trending stretching lineations, by underplating of rocks represented by and illite crystallinity, vitrinite reflectance, asymmetric shear fabrics, and sheath folds the modern Oboke and Shimanto belts. and zircon fission-track studies document (Faure, 1985; Hara et al., 1977; Hara et al., The Shimanto belt is latest Early Cre- peak metamorphic conditions of ~225 °C 1990; Toriumi, 1985; Wallis and Banno, taceous to latest Cretaceous in age (Taira, (DiTullio and Hada, 1993; Hasebe et al., 1990a), document lateral flow, although 1985) and was being accreted and meta- 1993). Other zircon fission-track data indi- the dominant flow direction is debated morphosed as the Sanbagawa belt was cate peak metamorphism at about 75 to (e.g., Faure, 1985; Hara et al., 1992; Wallis cooling and being exhumed (Hasebe et al., 60 Ma (Hasebe et al., 1993), similar to K-Ar and Banno, 1990b). Top-to-west shear 1997; Hasebe et al., 1993; Tagami et al., Crust continued on p. 6

Strategic Long-Term Planning. Over the past two years, is within reach. We can all be proud not only of the achievement a special Committee on Long-Range Planning has developed a but of the activities that the achievement has made possible. series of ambitious strategies to build further on the gains made. As we approach the millennium, we now call on the mem- Recommendations adopted by Council in October reinforce bership to look ahead and to contribute to what GSA will be. GSA’s commitments The challenges to us, as geoscientists and as members of GSA, • to our science, expressed in professional meetings and publica- have not diminished. But the results of our program initiatives, tions and enhanced by extending electronic communications, and of our fund-raising efforts in support of those initiatives, give increasing research and educational grants, and focusing on us the confidence to anticipate that we can meet the challenges. promoting integrative systems science through collaborations And with your help, we will. The $9.3 million raised so far is with earth, life, planetary, and social scientists; proof that GSA’s efforts are worthy of support. I invite all of you • to society, expressed in fostering the education and outreach to contribute to the efforts. If you have already given, we thank that bring earth science and its professionals to ever wider, you most sincerely and urge you to renew your gift. If you have increasingly diverse audiences of students and the general not yet made a gift, now is the time when every dollar is a step public; closer to $10 million—or beyond! • to our members, by ensuring GSA’s vitality and effectiveness as You have recently received a mailing offering you an oppor- a respected and objective voice on behalf of the geosciences. tunity to contribute to the success of GSA’s Second Century campaign. I hope you will give careful thought to an investment to As GSA embarked on its second century in 1998, the leader- benefit our Society, our colleagues, and our science. ship looked back to assess what GSA had accomplished. The Sec- For further information about how you can make a gift, ond Century campaign evolved as the necessary means to aug- please contact the GSA Foundation office at 1-800-472-1988, ment the programs that defined GSA’s proven excellence. At the ext. 154. ■ outset, the goal of $10 million seemed daunting, yet attainment

GSA TODAY, December 1998 5 Crust continued from p. 5 fault between the middle and lower plates different: middle-plate strain is dominated removed at least 2 km of crust. In particu- by east-west shortening, whereas lower- ages of cleavage-forming micas (Agar et al., lar, andalusite + biotite assemblages in mid- plate strain is dominated by vertical flat- 1989; MacKenzie et al., 1990). Structural dle plate pelitic rocks indicate peak meta- tening. Middle-plate deformation proba- and kinematic data from the Shimanto morphic conditions at less than ~10 km bly records subhorizontal(?) maximum belt, however, indicate north-south short- depth (Spear, 1993), whereas the disap- principal stress of tectonic origin, whereas ening or north-directed underthrusting pearance of muscovite and appearance of lower-plate deformation apparently (Byrne and DiTullio, 1992) rather than migmatite in the lower plate suggest the records subvertical maximum principal east-west elongation as in the Sanbagawa reaction muscovite + plagioclase + quartz stress due to lithostatic load at tempera- belt (Fig. 3). Thus, structural fabrics from = sillimanite + K-feldspar + melt, which tures where much lower differential stress these two belts have essentially the same takes place above ~3.5 kbar or deeper than could be maintained. age but preserve very different kinematic ~12 km (Spear and Kohn, 1996). Also, pre- axes, suggesting substantial crustal-scale liminary 40Ar/39Ar, fission-track, and (U- DISCUSSION, IMPLICATIONS, heterogeneities in strain. Th)/He thermochronology of lower-plate AND CONCLUSIONS rocks (Axen et al., 1998) suggests 5 to 7 Our examples illustrate very different, EXAMPLE 3—SIERRA EL MAYOR, km of Neogene tectonic unroofing, which but contemporaneous, principal strains BAJA CALIFORNIA limits horizontal middle-plate translation from one crustal level to the next. In the over the lower plate to <20 km for reason- The Sierra El Mayor lies in the Brenner area in the Alps, the upper crust able fault dips. extended region just east of the stable was essentially rigid, while subjacent Middle-plate structures reflect Peninsular Ranges (Fig. 5; Gastil et al., levels were extruded laterally with a cigar- regional trends, so we are confident that 1975). Late Cenozoic, brittle, low-angle shaped bulk strain ellipse imparted in and lower-plate structures evolved beneath normal (detachment) faults juxtapose below a normal shear zone. In southwest structures like those in the middle plate. rocks from three different crustal levels Japan, structurally higher but hotter rocks Middle-plate folds probably formed in (Axen and Fletcher, 1998). The Cretaceous were unroofed and elongated east-west, response to east-west shortening and verti- deformation histories of the middle and while cooler, underlying rocks were under- cal thickening (Fig. 6) with minor north- lower plates are discussed here. The mid- plated and shortened north-south. In the south elongation. Although these folds are dle plate consists mainly of metasedimen- Sierra El Mayor in Baja California, mid- now largely recumbent, rotation of non- tary rocks and various mid-Cretaceous crustal rocks were shortened east-west and conformably overlying east-dipping sedi- granitic bodies, the lower plate exposes thickened vertically, while underlying ments back to horizontal brings their axial the roots of the plutonic-metamorphic crustal rocks were slightly elongated east- planes into line with the typical steeply suite (Fig. 1), and the upper plate is made west and shortened vertically at long-term dipping, north-striking folds of the region. up of Cenozoic sediments. geological rates and incrementally inflated In contrast, lower-plate country rocks The metamorphic grade of middle- vertically by sill intrusion. record mainly vertical shortening and plate metasedimentary rocks ranges from Strain heterogeneity between differ- minor east-west elongation in the absence middle greenschist to lower amphibolite ent crustal levels primarily reflects rheo- of noncoaxial shearing (Fig. 6). Where facies (Siem and Gastil, 1994; Axen and logical differences. In the Sierra El Mayor, the middle plate is absent and upper-plate Fletcher, 1998); higher-grade rocks are the lower plate was hotter and weaker sedimentary rocks rest directly above present near granitic intrusions. Compo- than the middle plate, and sill emplace- lower-plate gneiss along a brittle detach- sitional layering is isoclinally folded at all ment was controlled by rock strength ment fault, the sedimentary rocks gener- scales and overprinted by a penetrative anisotropy rather than regional stress. ally dip <30° (Siem and Gastil, 1994; cleavage that is axial planar to north- In the Alps, the Brenner footwall was Vásquez-Hernández et al., 1996), implying trending folds (Fig. 6). In pelitic units, weaker than the Austroalpine units due only minor rotation of the fault, its foot- euhedral, 1 to 3 mm garnets preserve to the combined effects of temperature wall, and the flattening foliation. The compositional gradients that probably and rock type. High strains and metamor- high-temperature lower-plate fabrics prob- reflect prograde growth. Granitic bodies phic grades, sheath folding, and regionally ably developed in the weak, hot, fluid-like are locally present, are unfoliated, and are folded metamorphic isograds in the San- lower part of the middle crust where the discordant to country-rock foliation. bagawa belt indicate that it was less rigid rocks could not support differential stress The lower plate comprises upper- than the underlying Shimanto belt. Meta- as high as that causing folding in the mid- amphibolite facies migmatitic gneiss morphic and thermal inversion within the dle plate. Concordant sills and leucosome (sillimanite + K-feldspar–grade) with Sanbagawa belt, and between it and the (partial melt) were emplaced or generated, subhorizontal flattening foliation (Fig. 6; underlying Shimanto belt could be the respectively, in rocks with anisotropic ten- Axen and Fletcher, 1998). Rare mineral cause. Fluid released from the subducting sile strength that was greater parallel to lineations and elongate pressure shadows Shimanto belt would have also hydrated foliation than perpendicular to it (e.g., around garnets trend east-west. Meso- and weakened the overlying Sanbagawa Lucas and St-Onge, 1995). The sills scopic folds are sparse. Granitic units that belt during retrograde conditions. inflated the column vertically, such are broadly coeval with similar units in Thus, flow geometries and strain that the bulk strain ellipse is difficult to the middle plate are common and occur in weak crustal levels can be largely con- characterize. as concordant sills, discordant dikes, and trolled by complex regional and local Existing data are consistent with centimeter-scale incipient melt segrega- boundary conditions. For example, east- middle- and lower-plate structures being tions parallel to foliation. Garnets in ward extrusion of the Brenner line foot- coeval. For example, regional east-west pelitic rocks are typically anhedral, 3 to wall is consistent with rigid boundaries to shortening was common in the Peninsular 15 mm across, strongly embayed by reac- the north, west, and south, and relatively Ranges during batholith emplacement tion with quartz and biotite, and compo- free boundaries above and to the east. (e.g., Todd et al., 1988), events we inter- sitionally homogeneous except for narrow Similarly, eastward lateral flow of the San- pret as recorded by middle-plate folds retrograde rims. Bands of hornblende- bagawa belt during northward underplat- and lower-plate migmatites, respectively. bearing amphibolite with calc-silicate ing of the Shimanto belt could have been However, better determination of local margins are common. directed by the geometry of the overlying, geochronology is needed. Preliminary analysis of metamorphic relatively rigid crust and mantle of the arc Thus, broadly coeval principal strain conditions indicates that the detachment (Fig. 4). directions in the two levels were very

6 GSA TODAY, December 1998 The heterogeneities we describe com- prise strain partitioning, which is a natural GSA ON THE WEB consequence of acquisition of strain in rocks given their heterogeneous and Visit the GSA Web Site at http://www.geosociety.org. evolving material properties. Strain parti- From our home page you can link to many information resources. tioning is currently used in at least three Here are some highlights: ways. The first describes coeval but spa- tially separate zones of orthogonal simple Visit our ’99 Denver Annual Meeting page … www.geosociety.org/meetings/99 shear, typically along oblique plate mar- to catch proposal information and electronic forms for Pardee Keynotes and for gins where strike-slip faults are separated Topical Sessions. Firm deadline: January 6, 1999. from genetically related dip-slip faults Also learn the newest on what’s happening by using our direct link to the web (e.g., Fitch, 1972; Stock and Hodges, sites of the Denver Convention and Visitors Bureau, Colorado Geological Survey, 1989). The second refers to different and the University of Colorado, Department of Geological Sciences. deformation mechanisms that contribute, coevally or not, to the bulk strain in a rock The handy 1998 database of searchable abstracts, authors, and sessions continues body (e.g., Ramsay and Huber, 1983). The to be available via the link to the 1998 Home Page. third involves zones in which strain magnitude is higher than in the surroundings, as in the cores of shear zones (e.g., fornia and the Salton Trough, California: Geological Complex vertical strain partitioning Mohanty and Ramsay, 1994). None of Society of America Abstracts with Programs, v. 30, could be the norm rather than the excep- no. 5, p. 4. these usages fits our examples well. To tion in geologically complex areas where Banno, S., 1986, The high-pressure metamorphic belts unify these various concepts, we suggest perturbations of regional and local bound- of Japan: A review, in Evans, B. W., and Brown, E. H., a general definition: strain partitioning is eds., Blueschists and eclogites: Geological Society of ary conditions, heat flow, tectonic and the natural division of strain into discrete America Memoir 164, p. 365–374. topographic load, and rock strength are parts in one or more of these mutually com- Banno, S., 1998, Pumpellyite-actinolite facies of the complex and evolving. Description and patible ways: (1) kinematic division of strain Sanbagawa metamorphism: Journal of Metamorphic modeling of active and inactive orogenic Geology, v. 16, p. 117–128. onto distinct structures, (2) spatial variation belts should take into account three- Behrmann, J., 1988, Crustal-scale extension in a conver- of strain orientation and/or magnitude, and dimensional heterogeneous strain, or gent orogen: The Sterzing-Steinach mylonite zone in (3) distribution of strain among different the eastern Alps: Geodinamica Acta, v. 2, p. 63–73. have strong reasons for its dismissal, deformation mechanisms within the same before two-dimensional models are given Blanckenburg, F. von, Villa, I., Baur, H., Morteani, G., body. Also, distinguishing between coeval, and Steiger, R. H., 1989, Time calibration of a P-T path total credence. progressive, and temporally distinct strain in the Western Tauern Window, Eastern Alps: The The strain patterns of the weak levels problem of closure temperatures: Contributions to partitioning is desirable. of the present middle to lower crust are Mineralogy and Petrology, v. 101, p. 1–11. If strong three-dimensional strain particularly poorly known, because of Brace, W. F., and Kohlstedt, D. L., 1980, Limits on heterogeneity is as common as we suggest lithospheric stress imposed by laboratory experiments: inaccessibility and the weak dependence (existing examples are too numerous to Journal of Geophysical Research, v. 85, p. 6248–6252. of most geophysical imaging techniques cite), then there are serious implications Buck, W. R., 1991, Modes of continental lithospheric on strain patterns. Analogy to ancient for characterization of continental crust. extension: Journal of Geophysical Research, v. 96, examples will likely be key to understand- p. 20,161–20,178. For example, various types of vertical pro- ing the distribution of heterogeneous Byrne, T., and DiTullio, L., 1992, Evidence for changing files are widely used in geophysical and deformation in continental crust, and plate motions in southwest Japan and reconstructions geological studies (e.g., seismic reflection of the Philippine Sea plate: Island Arc, v. 1, p. 148–165. to surficial processes that may affect or profiles, two-dimensional dynamic and be affected by such heterogeneity. Christensen, J. N., Selverstone, J., Rosenfeld, J., and kinematic models, balanced and restored DePaolo, D. J., 1994, Correlation by Rb-Sr geochronol- cross sections). These profiles can be very ogy of garnet growth histories from different structural ACKNOWLEDGMENTS levels within the Tauern Window, Eastern Alps: Contri- useful in upper crustal studies, where butions to Mineralogy and Petrology, v. 118, p. 1–12. deformation commonly may be ade- Supported by National Science Dewey, J. F., Helman, M. L., Turco, E., Hutton, D. H., quately characterized in two dimensions, Foundation grants EAR9526249 (Axen), and Knott, S. D., 1989, Kinematics of the western but their construction and interpretation EAR9219742 and EAR9526390 (Selver- Mediterranean, in Coward, M. P., et al., eds., Alpine tectonics: Geological Society of London Special Publica- typically hinges on a lack of motions into stone), and EAR9418344 (Byrne), by tion 45, p. 265–283. and out of the profile plane. These two- CONACYT grant 4345 PT (Fletcher), by DiTullio, L., and Hada, S., 1993, Regional and local dimensional analyses will not adequately a UC MEXUS grant (Axen), a Monbusho variations in the thermal history of the Shimanto Belt, represent the (typical?) tectonic evolution Fellowship (Byrne), and grants from the southwest Japan, in Underwood, M., ed., Thermal evo- of regions with strong three-dimensional Consejo Técnico de CICESE (Fletcher). We lution of the Tertiary Shimanto Belt, southwest Japan: Geological Society of America Special Paper 273, 172 p. strain partitioning. thank P. Bird, W. Moore, A. Snoke, J. Tullis, Faure, M., 1985, Microtectonic evidence for eastward Our examples also show the difficulty and B. Wernicke for insightful comments. ductile shear in the Jurassic orogen of SW Japan: inherent in inferring paleo–plate motions Journal of Structural Geology, v. 7, p. 175–186. from deformed rocks in ancient orogenic REFERENCES CITED Fitch, T. 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Journal of Structural Geology, v. 17, p. 1077–1093. Selverstone, J., Axen, G. J., and Bartley, J. M., 1995, Wallis, S., 1998, Exhuming the Sanbagawa metamor- Fluid inclusion constraints on the kinematics of foot- phic belt: The importance of tectonic discontinuities: wall uplift beneath the Brenner Line normal fault, Journal of Metamorphic Geology, v. 16, p. 83–95. MULTIMEDIA CD eastern Alps: Tectonics, v. 14, p. 264–278. Wallis, S., and Banno, S., 1990a, The Sambagawa belt— Destination Miri : A Geological Tour, Shinjoe, H., and Tagami, T., 1994, Cooling history of Trends in research: Journal of Metamorphic Geology, the Sambagawa metamorphic belt inferred from fission Northern Sarawak's National Parks v. 8, p. 393–399. track zircon ages: Tectonophysics, v. 239, p. 73–79. Wallis, S. R., and Banno, S., 1990b, Ductile extension and Giant Caves Siem, M. E., and Gastil, R. G., 1994, Mid-Tertiary to as a cause of exhumation of the Sanbagawa high P/T Holocene extension associated with the development metamorphic belt, Japan [abs.]: Eos (Transactions, of the Sierra El Mayor metamorphic core complex, Discover some of Nature's best American Geophysical Union), v. 71, p. 901. northeastern Baja California, Mexico, in McGill, S. F., kept secrets: and Ross, T. M., eds., Geological investigations of an Wintsch, R., Byrne, T., and Toriumi, M., 1999, Exhuma- active margin: Geological Society of America Cordil- tion of the Sanbagawa blueschist belt, SW Japan, by lat- Mulu National Park, the site of the leran Section Guidebook: Redlands, California, San eral flow and extrusion: Evidence from structural kine- world's largest cave chamber, the Bernardino County Museum Association, p. 107–119. matics and retrograde P-T-t paths, in Ring, U., et al., eds., Exhumation processes: Normal faulting, ductile Spear, F. S., 1993, Metamorphic phase equilibria and world's largest cave passage, South flow and erosion: Geological Society of London (in pressure-temperature-time paths: Washington, D.C., press). East Asia's longest underground river, Mineralogical Society of America, 799 p. Manuscript received May 12, 1998; revision received and much more... Spear, F. S., and Kohn, M. J., 1996, Trace element zon- September 13, 1998; accepted October 15, 1998 ■ ing in garnet as a monitor of crustal melting: Geology, A 30 million- year geological journey v. 24, p. 1099–1102. illustrated by maps, pictures, logs, videoclips and extensive narrations. Includes a historical overview on the oil field of Miri, discovered in 1910. Spend Your Summer in a National Park!

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8 GSA TODAY, December 1998