Vol. 6, No. 4 April 1996 1996 Annual GSA TODAY Meeting Call for A Publication of the Geological Society of America Papers Page 17 Electronic Dipping Reflectors Beneath Abstracts Submission Old Orogens: A Perspective from page 18 the British Caledonides Registration Issue June GSA Today John H. McBride,* David B. Snyder, Richard W. England, Richard W. Hobbs British Institutions Reflection Profiling Syndicate, Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, United Kingdom A B Figure 1. A: Generalized location map of the British Isles showing principal structural elements (red and black) and location of selected deep seismic reflection profiles discussed here. Major normal faults are shown between mainland Scotland and Shetland. Structural contours (green) are in kilome- ters below sea level for all known mantle reflectors north of Ireland, north of mainland Scotland, and west of Shetland (e.g., Figs. 2A and 5); contours (black) are in seconds (two-way traveltime) on the reflector I-I’ (Fig. 2B) pro- jecting up to the Iapetus suture (from Soper et al., 1992). The contour inter- val is variable. B: A Silurian-Devonian (410 Ma) reconstruction of the Caledo- nian-Appalachian orogen shows the three-way closure of Laurentia and Baltica with the leading edge of Eastern Avalonia thrust under the Laurentian margin (from Soper, 1988). Long-dash line indicates approximate outer limit of Caledonian-Appalachian orogen and/or accreted terranes. GGF is Great Glen fault; NFLD. is Newfoundland. reflectors in the upper-to-middle crust, suggesting a “thick- skinned” structural style. These reflectors project downward into a pervasive zone of diffuse reflectivity in the lower crust. This zone of diffuse reflectivity corresponds to a theoretical depth interval of low strength that represents distributed shearing separating upper crustal and uppermost mantle lay- ABSTRACT ers of greater strength. Prominent dipping reflectors also occur After two decades of deep seismic reflection profiling, our within the high-strength uppermost mantle beneath the Cale- understanding of dipping reflectors in the crust and upper donian orogen. Reflectors within the stronger upper-to-middle mantle beneath old orogenic belts, and especially their rela- crust and upper mantle are not connected across the Moho tion across the crust-mantle boundary, remains incomplete. discontinuity and thus seem kinematically distinct, although The Caledonian orogenic front (Moine thrust) and the Iapetus mantle reflectors in places appear to continue upward into the suture in Great Britain, two of the world’s most studied com- lowermost crust. Dipping crust and mantle reflectors probably pressional belts, show a consistent pattern of discrete dipping originated from different geologic events: dipping reflectors in the crust first took form as thrusts during the early Paleozoic Caledonian orogeny, whereas those in the upper mantle *Present address: Illinois State Geological Survey, Natural Resources Building, 615 East Peabody Drive, Champaign, IL 61820. Dipping Reflectors continued on p. 2 IN THIS ISSUE GSA TODAY April Vol. 6, No. 4 1996 Dipping Reflectors Beneath Old 1996 GSA Annual Meeting Orogens: A Perspective from the Call for Papers ......................... 17 GSA TODAY (ISSN 1052-5173) is published British Caledonides .................. 1 monthly by The Geological Society of America, Inc., Penrose Conference Report ............. 33 with offices at 3300 Penrose Place, Boulder, Colorado. Geology and Geologists in the Former Mailing address: P.O. 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Per- mission is granted to individuals to photocopy freely all Correction: Dinofest .................... 11 1996 Committees and Representatives . 44 items other than the science articles to further science and education. Individual scientists are hereby granted Forum ................................. 12 GSA Meetings ......................... 45 permission, without royalties or further requests, to About People .......................... 13 Classifieds ............................. 46 make unlimited photocopies of the science articles for use in classrooms to further education and science, and to make up to five copies for distribution to associates in the furtherance of science; permission is granted to Dipping Reflectors continued from p. 1 (1) Can dipping crustal reflectors beneath make more than five photocopies for other noncom- mercial, nonprofit purposes furthering science and edu- the Caledonides be unequivocally linked cation upon payment of the appropriate fee ($0.25 per have an even older history associated with compressional structures at the sur- page) directly to the Copyright Clearance Center, 27 with an as yet unrecognized episode face, and how are compressional struc- Congress Street, Salem, Massachusetts 01970, phone (508) 744-3350 (include title and ISSN when paying). of subduction or Late Proterozoic rifting tures distributed deeper within the crust? Written permission is required from GSA for all other of the Laurentian supercontinent. Crust (2) Does dipping reflector structure in the forms of capture, reproduction, and/or distribution of and mantle reflectors were likely reacti- upper mantle beneath the Caledonides any item in this journal by any means. GSA provides this and other forums for the presentation of diverse vated by Mesozoic and younger North have any relation to Paleozoic or younger opinions and positions by scientists worldwide, regard- Sea rifting, which may also have pro- features in the crust above it—what really less of their race, citizenship, gender, religion, or politi- duced, or at least enhanced, the diffuse are the admissible interpretations for upper cal viewpoint. Opinions presented in this publication do not reflect official positions of the Society. reflectivity in the low-strength lower mantle reflectors? (3) Finally, we argue that crust. the crust and upper mantle were mechani- SUBSCRIPTIONS for 1996 calendar year: cally decoupled during Caledonian com- Society Members: GSA Today is provided as part of INTRODUCTION pression, and that crust and mantle defor- membership dues. Contact Membership Services at mation structures beneath the Caledonides (800) 472-1988 or (303) 447-2020 for membership Since its inception in 1981, the British have separate origins. information. Nonmembers & Institutions: Free with Institutions Reflection Profiling Syndicate paid subscription to both GSA Bulletin and Geology, (BIRPS) has been accumulating deep seis- otherwise $45 for U.S., Canada, and Mexico; $55 else- DIPPING CRUSTAL REFLECTORS where. Contact Subscription Services. Single copies mic reflection data over the offshore parts may be ordered from Publication Sales. Claims: For of the Caledonian orogen. To date, about Moine Thrust nonreceipt or for damaged copies, members contact Membership Services; all others contact Subscription 17,000 km of profiles have been recorded The Moine thrust (Fig. 1A) is Services. Claims are honored for one year; please allow around the British Isles. The major crustal the classic structure of the Caledonide- sufficient delivery time for overseas copies. framework structures of the Caledonian Appalachian system, and is probably anal- orogen, namely the Caledonian orogenic ogous to the Taconian suture in the North STAFF: Prepared from contributions from the GSA staff and membership. front (locally Moine thrust) and the Iape- American Appalachians. Where it crops Executive Director: Donald M. Davidson, Jr. tus suture, are arguably the best studied out in mainland Scotland, the thrust Science Editor: Suzanne M. Kay Paleozoic orogenic features in the world, floors a thick east-dipping mylonite Department of Geological Sciences, Cornell University, Ithaca, NY 14853 particularly from the standpoint of deep zone that separates metamorphic rocks Forum Editor: Bruce F. Molnia reflection data. These two structures of the thrust belt to the east from the less U.S. Geological Survey, MS 917, National Center, together effectively delimit compressional deformed Precambrian foreland and cover Reston, VA 22092 Managing Editor: Faith Rogers deformation resulting from the Caledo- sequence to the west (Barr et al., 1986). Production & Marketing Manager: James R. Clark nian orogeny (Silurian–Early Devonian) Deep seismic data over the offshore pro- Production Editor and Coordinator: Joan E. Manly in Britain (Fig. 1A). The goal of this report jection of the thrust reveal a thick wedge Graphics Production: Joan E. Manly, Adam S. McNally is to summarize results of our recent work of east-dipping reflections in the middle- on the British