Continental and Oceanic Geodynamics Series Inter-Union Commission on Geodynamics Editorial Board A. L. Hales, Chairman R. D. Russell, Secretary O. L. Anderson F. M. Delany C. L. Drake J. Sutton American Geophysical Union/Geological Society of America Editorial Board Kees Dejong C. L. Drake D. H. Eckhardt E. Irving W. I. Rose Rob Van der Voo The Final Reports of the International Geodynamics Program sponsored by the Inter-Union Commission on Geodynamics Geodynamic Series The Final Reportso f the International Geodynamics Program sponsored by the Inter-Union Commission on Geodynamics Dynamics of Plate Interiors A. W. Bally, P.L . Bender, T.R . McGetchin, and R.I . Walcott (Editors) 2 Paleoreconstruction of the Continents M.W. McEIhinny and D.A. Valencio (Editors) 3 Zagros ßH indu Kush' Himalaya Geodynamic Evolution H. K. Gupta and F. M. Delany (Editors) 4 Anelasticity in the Earth F.D . Stacey, M. S. Patterson,a nd A. Nicholas (Editors) 5 Evolution of the Earth R.J. O'Connell and W. S. Fyfe (Editors) 6 Dynamics of Passive Margins R.A . Scrutton [Editor] 7 Alpine-Mediterranean Geodynamics H. Berckhemer and K. Hs(] Continental and Oceanic Rifts Edited by G. P(cid:127)lmason Geodynamics Series Volume 8 American Geophysical Union Washington, D.C. Geological Society of America Boulder, Colorado Final Report of Working Group 4, Continental and Oceanic Rifts, coordinated by G. Palmason on behalf of the Bureau of Inter-Union Commission on Geodynamics American Geophysical Union, 2000 Florida Avenue, N .W. Washington, D.C. 20009 Geological Society of America, 3300 Penrose Place, P.O. Box 9140 Boulder, Colorado 80301 Library of Congress Cataloging in Publication Data Inter-Union Commission on Geodynamics. Working Group 4. Continental and oceanic rifts. (Geodynamics series; v. 8) Includes bibliographies. 1. Rifts (Geology)--Addresses, essays, lectures. I. Palmason, Gudmunder. II. Title. III. Series. QE606 .I57 1982 551 .8 '7 82-3991 ISBN 0-87590-504-8 AACR2 Copyright 1982 by the American Geophysical Union, 2000 Florida Avenue, N .W., Washington, D.C. 20009 Figures, tables, and short excerpts may be reprinted in scientific books and journals if the source is properly cited. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the American Geophysical Union for libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided that the base fee of $1 .00 per copy, plus $0.10 is paid directly to CCC, 27 Congress Street, Salem, MA 01970. 0065-8448/82/$01.10. This consent does not extend to other kinds of copying, such as copying for creating new collective works or for resale. The reproduction of multiple copies and the use of full articles or the use of extracts, including figures and tables, for commercial purposes requires specific permission from AGU. First Printing: 1982 Second Printing: 1985 Printed in the United States of CONTENTS Foreword A. L. Hales and C. L. Drake Preface G. P(cid:127)lmason History of Rift Studies C. L. Drake and R. W. Girdler Recent Ground Deformation in Continental and Oceanic Rift Zones 17 E. Tryggvason The Role of Shear in Rifting 31 R. Freund The Seismicity of the East African Rift Systema nd ComparisonW ith Other 41 Continental Rifts J. D. Fairhead and G. W. Stuart Seismicity of Oceanic Rifts 63 R. C. Lilwall Seismic Structure of the Lithosphere in Rift Zones 81 K. Fuchs Gravity Anomalies Over Oceanic Rifts 99 A. B. Watts Heat Flow in Rift Zones 107 P. Morgan Physical Aspects of Hydrothermal Systems 123 V. Stefdnsson and S. Bj(cid:127)rnsson Magma Chambers in Rifts 147 A. R. Sanford and P. Einarsson Magnetotelluric and Geomagnetic Deep-Sounding Studies in Rifts and Adjacent 169 Areas: Constraints on Physical Processes in the Crust and Upper Mantle J. F. Hermance Physical Aspects of Magmatism in Continental Rifts 193 L. A. J. Williams Chemical Aspects of Rift Magmatism 223 F. Barberi, R. Santacroce and J. Varet Metallogenesis in Relation to Rifting 259 F. Sawkins Terrestrial Energy Currents and Transfer in Iceland 271 G. Bodvarsson Mass Displacement and Geothermics Within the Upper Mantle of the Rhinegraben 283 Rift System: A Model to Interpret Data From Gravimetry and Seismology D. Werner, H.-G. Kahle, J. Ansorge and St. Mueller Musings on Continental Rifts P. Mohr FOREWORD After a decade of intense and productive Region, East scientific cooperation between geologists, WG 7 Geodynamics of Plate Interiors geophysicists and geochemists the Inter- WG 8 Geodynamics of Seismically Inactive national Geodynamics Program formally Margins ended on July 31, 1980. The scientific WG 9 History and Interaction of Tectonic, accomplishments of the program are Metamorphic and Magmatic Processes represented in more than seventy scientific WG 10 Global Syntheses and Paleorecon- reports and in this series of Final Report struction volumes. These Working Groups held discussion meetings The concept of the Geodynamics Program, as and sponsored symposia. The papers given at a natural successor to the Upper Mantle the symposia were published in a series of Project, developed during 1970 and 1971. Scientific Reports. The scientific studies The International Union of Geological were all organized and financed at the national Sciences (IUGS) and the International Union level by national committees even when multi- of Geodesy and Geophysics (IUGG) then sought national programs were involved. It is to the support for the new program from the Inter- national committees, and to those who partici- national Council of Scientific Unions (ICSU). pated in the studies organized by those As a result the Inter-Union Commission on committees, that the success of the Program Geodynamics was established by ICSU to manage must be attributed. the International Geodynamics Program. Financial support for the symposia and the The governing body of the Inter-Union meetings of the Commission was provided Commission on Geodynamics was a Bureau of by subventions from IUGG, lUGS, UNESCO and seven members, three appointed by IUGG, ICSU. three by IUGS and one jointly by the two Information on the activities of the Unions. The President was appointed by ICSU Commission and its Working Groups is available and a Secretary-General by the Bureau from in a series of 17 publications: among its members. The scientific work of Geodynamics Reports, 1-8, edited by F. the Program was coordinated by the Commission, Delany, published by BRGM; Geodynamics composed of the Chairmen of the Working Groups Highlights, 1-4, edited by F. Delany, and the representatives of the national published by BRGM; and Geodynamics Inter- con(cid:127)nittees for the International Geodynamics national, 13-17, edited by R. D. Russell. Program. Both the Bureau and the Commission Geodynamics International was published met annually, often in association with the by World Data Center A for Solid Earth Assembly of one of the Unions, or one of the Geophysics, Boulder, Colorado 80308, USA. constituent Associations of the Unions. Copies of these publications, which contain Initially the Secretariat of the Commission lists of the Scientific Reports, may be was in Paris with support from France through obtained from WDC A. In some cases only BRGM, and later in Vancouver with support microfiche copies are now available. from Canada through DEMR and NRC. This volume is one of a series of Final The scientific work of the Program was Reports summarizing the work of the coordinated by ten Working Groups. Commission. The Final Report volumes, WG 1 Geodynamics of the Western Pacific- organized by the Working Groups, Indonesian Region represent in part a statement of what WG 2 Geodynamics of the Eastern Pacific has been accomplished during the Program and Region, Caribbean and Scotia Arcs in part an analysis of problems still to WG 3 Geodynamics of the Alpine-Himalayan be solved. This volume from Working Group 4 Region, West (Chairman, J. Sutton) was edited by G. WG 4 Geodynamics of Continental and P(cid:127)imason. Oceanic Rifts At the end of the Geodynamics Program it WG 5 Properties and Processes of the is clear that the kinematics of the major Earth's Interior plate movements during the past 200 million WG 6 Geodynamics of the Alpine-Himalayan years is well understood, but there is less understanding of the dynamics of the Dynamics and evolution of the litho- processes which cause these movements. sphere: The framework for earth resources Perhaps the best measure of the success and the reduction of the hazards. of the Program is the enthusiasm with To all of those who have contributed which the Unions and national committees their time so generously to the Geodynamics have joined in the establishment of a Program we tender our thanks. successor program to be known as: C. L. Drake, President ICG, 1971-1975 A. L. Hales, President ICG, 1975-1980 Members of Working Group 4: G. PRimason P. Gouin F. Aumento J.H. Illies R.J. Bridwell D.E. Karig R.W. Decker N. Logatchev A. Dimitriev P.A. Mohr L.V. Dmitriev D. Peck N.A. Florensov I.A. Ramberg J. Francheteau L. Rykunov J.G. Francis R. Schlich T. Francis G.E. Sigvaldason I.G. Gass M. Talwani R.W. Girdler L.A.J. PREFACE During the past decade of the International Geo- from the early work in East Africa at the end of dynamics Program many publications dealing with the 19th century to the present time, when the specific rift areas of the Earth's surface have main features of the World Rift System have been appeared. Most of them followed symposia that recognized. Subsequent papers deal with various were held in the respective rift zones. Among physical aspects of rifts, including ground defor- the rifts thus treated are the Rhinegraben, Afar, mation, seismicity, gravity and heat flow. One Iceland, the Baikal rift, the Oslo paleorift, the paper deals with hydrothermal systems, which seem Rio Grande rift and the Dead Sea rift. Further- to be as integral a part of active rift systems more, publications dealing with studies of the as e.g. volcanism and seismicity. Magmatism is ocean floor have discussed many oceanic rift the subject of three papers, and metallogenesis in areas. relation to rifting is discussed in one paper. In view of the recent growth of publications Two papers deal with certain aspects of the Ice- dealing with specific rift areas, the Working land rift zone and the Rhinegraben system. Final- Group 4 (Continental and Oceanic Rifts) decided ly, one paper gives some thoughts on unsolved at its meeting in Santa Fe, New Mexico, in 1978, problems of the continental rifts. that this volume should emphasize comparative It is hoped that this collection of papers will studies on topical subjects rather than focus on prove a valuable source of information for rift geographical areas. It is hoped that this will studies during the decade of the International prove to be a useful approach to reviewing the Lithosphere Program. present state of knowledge of the rift zones of the Earth. The papers in this volume deal with many aspects of rift systems that have received attention in Gudmundur PRimason recent years. An introductory paper gives a much Chairman, ICG Working Group 4, Geodynamics of needed overview of the history of rift studies Continental and Oceanic