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1 INTRODUCTION IMPETUS FOR THE WO~HOP Plate tectonic theory created a revolution in our understanding of both the present ocean basins and the ancient continental interiors. The basic tenets of the theory have been confirmed during two decades of intense exploration of the earth using increasingly sophisticated techniques, but it remains essentially a kinematic theory describing the motion of rigid plates on a sphere and thus provides only a crude outline of how plate boundarie-~--and hence continental margins--actually evolve. These boundaries are, by their nature, a deforming continuum governed by processes not directly addressed by plate tectonics theory. The last decade has seen a gradual transition in the focus of continental margins research from an observation-oriented endeavor to a more process-oriented study. ~ . . . . . Phenomenological and Descriptive studies are being complemented and extended by physical, quantitative, and analytic models. Such models provide specific predictions of measurable parameters, which can be compared with field or experimental observations. In many cases , this approach has challenged or overturned older theories of margin evolutions The last decade has also seen a number of important advances that have established a strong foundation for current and future research at continental margins. Some of these advances are listed here. 1. Seismic reflection methods and deep ocean drilling have produced startling discoveries about the large-scale architecture of continental margins. 2. Field studies of continental orogenic belts, both extensional and contractional, have provided important insights 8
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into tectonic processes operating within the submerged parts of modern continental margins. 3. Geodynamic models are now capable of modeling in a realistic fashion the mechanical and thermal processes associated with lithospheric extension and contraction. 4. Research in rock mechanics has outlined first-order processes governing frictional faulting and ductile flow of rocks. 5. Seismology has developed inverse methods capable of resolving the time evolution of the rupture process, which provides direct information about the local structure of seismically active faults. 6. Studies of trace element and isotope systematics in igneous rocks have produced a set of tools for monitoring the fluxes of mantle, crust, and sediment at volcanic arcs and at magmatically active rifts. 7. Advances in the understanding of continental margin depositional systems and the formation of large-scale stratigraphy sequences now allow us to extract from the stratigraphic record a detailed history of past climate and tectonism. These advances have led us to the brink of a new era in our understanding of continental margins. Many of the advances, however, have been made largely from within the confines of a specific discipline , with little contribution from other fields. Recognizing the current and future potential of continental margins research, the Ocean Studies Board and the Board on Earth Sciences jointly sponsored a workshop focused on this field. The workshop was designed to take full advantage of the recent advances. Participants were brought together from a broad range of disciplines to discuss common interests, new developments, and future directions for margins research. The objectives of the workshop were: (1) to assess the state of knowledge and current directions in continental margins research; (2) to identify areas in which research is poised to make significant progress; and (3) to design a strategy for the construction of a ~ong-term science plan for margins research. ORGANIZATION AND SUPPORT OF THE WORKSHOP The Continental Margins Committee was formed as a steering committee for the workshop e In advance of the workshop, background papers were solicited and then distributed to the participants. The papers summarized the current state of 9
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research in specific areas and were used as springboards for discussion (see Part III). The workshop was organized into two sections: Pa~ssive Margins and Active Margins. Each section in turn formed three working groups, which were assigned specific topics within the broader context of continental margins research. The focus of each group was as follows: Passive Margins Group 1: Mechanics of Rifting and Associated Magmatism Group 2: Rift and Passive Margin Basins: The Sedimentary Record Group 3: Post-Depositional Processes: Internal versus External Processes in Passive Margin Sediments Active Margins Group 1: Mechanics of Plate Motion Group 2: Geologic Evolution of Active Continental Margins Group 3: Mass and Chemical Transfer The opening afternoon of the workshop consisted of an informal poster session in which recently acquired data and new research developments were presented in a format conducive to discussion. The second day began with a joint plenary session of all participants, during which the objectives of the workshop were defined. The passive and active sections then met separately for presentations by selected keynote speakers. Each speaker was allotted 20 minutes for presentation, which was followed by 20 minutes of open discussion. The participants then reconvened in a joint plenary session in which the chairman issued a set of charges to the working groups in the form of a list of questions. CHARGES TO THE WORKING GROUPS 1. What is the single most important scientific objective within the focus of your working group? a. What are the critical problems? b. What are the specific processes? 10
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2. What studies are needed? a. be 3. What data should be collected? What laboratory, theoretical, and numerical developments are needed? What measurement capabilities are needed? What strategies must be developed to achieve your stated goals? For the rest of the second day the participants split into the six working groups, each of which also interacted extensively with the others. On the third day, the participants continued to meet in the working groups and began writing the working group reports included as Part TI of these proceedings. A combined session of all participants was held at midday for presentation and discussion of preliminary results. The remainder-of the day was spent in the working groups. - The final morning of the workshop was divided between worming group meetings and a plenary session of all participants. In the plenary session, the results of each working group and the links between them were presented and discussed. . · — These proceedings reflect the views expressed by the workshop participants. The overall goals stated throughout the report were derived from a consensus of the workshop but not necessarily of the National Research Council. 11
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