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EXECUTER STY OVERVIEW The geology of continents is to a large degree the geology of continental margins. The margins mark the site where continental crust is distilled from the earth's mantle and where continents grow through plate interactions that progressively incorporate these margins into the continental mass. Thus, much of the continents consist of the remnants of ancient margins that offer an extensive and detailed record of continental evolution over time and space. This record is essential for understanding processes of global change' its diversity, and its long-term history. . Approximately 70 percent of the worId's population is concentrated in the coastal borderlands, which geologists recognize to be the present continental margins. ~ . . . Virtuallv all of society depends on the products of the vast chemical reactors that underlie the margins wick transform organic matter into petroleum resources. Working against these benefits are a plethora of geologic hazards, ranging from the obvious destructive effects of earthquakes and volcanoes to subtle threats of sea level change and climate modification. The paradigm of plate tectonics developed in the 1960s created a revolution in our understanding of both the present ocean basins and the ancient continental interiors. It allowed us to place continental margins in their proper kinematic context. Its basic tenets have been conf irmed during two decades of intense exploration of the earth using increasingly sophisticated techniques. Plate tectonic theory r however, is essentially a kinematic model describing the motion of rigid plates on a sphere. It does not provide any direct insight into specific processes of pl ate boundary interactions, and hence of margin evolution. 3
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In an effort to better understand continental margins, there has been a recent shift in emphasis away from phenomenological descriptions to an approach that focuses on modeling the fundamental physical processes associated with margin evolution. The construction and testing of such models involves the same sort of innovation that spawned the plate tectonic revolution itself. Seeing the potential for rapid and dramatic advances in our understanding of the continental margins, the Ocean Studies Board and the Board on Earth Sciences jointly convened a workshop on continental margins. This report is the product of that workshop. The outcome of the workshop indicates widespread support and enthusiasm for a new direction in margins research that would focus on interdisciplinary studies of the fundamental processes of margin evolutions Interdisciplinary studies, as a new direction, are demonstrated by the working group reports, which show a remarkable similarity in focus despite the wide diversity o f top i as addres s ed and the degree of separation usually present between investigators from different disciplines. Many of the scientific problems that were identified, and their suggested solutions, were common to several working group topics. These similarities define a commonality of direction that belies traditional boundaries based on discipline, geography, or methodology. The principal reason for these similarities is that many of the processes, such as deformation, seismic rupture, fluid flow, magmatism, and sedimentation, are not unique to either divergent or convergent margins. Instead they are fundamental global processes that control the ways in which continents grow and deform with time. The Margins Initiative is proposed as a means of nurturing this new direction. The primary scientific objective of the initiative will be to develop programs aimed at understanding the processes that control the initiation and evolution of continental margins. The examples below, extracted from the working group reports, highlight some of the processes that might be studied as components of this initiative. Mechanics of Faulting Faults form a fundamental aspect of the architecture of all continental margins, as well as extensional and contractional orogenic belts that form within the continents. The mechanics of faults, and low angle faults in particular, remain poorly understood. For instance, high fluid pressures have often been invoked to explain how thrust sheets overcome the large frictional resistance to fault slip. This explanation is compelling where fluid-saturated sediments and oceanic crust are 4
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thrust rapidly beneath the continental margins, but, observations at some maj or strike-slip faults and ~ ow-angle normal fault ts indicate that faults can have very ~ ow shear resistance in the absence of anomalous fl uid pressures Another probe em is the slip behavior of low-ang~ e faults: some faults are seismically active, moving in a stick-slip fashion, whereas others appear to slip in a steady, aseismic fashion. Some seismologists argue that this behavior is controlled by plate parameters (erg., rate of subduction, age of the plate), whereas rook mechanicians argue that slip behavior is a consequence of the rheology of materials in the fault zone. Fluic] Flow Fluid flow affects almost all geologic processes active at continental margins. Continental margin sediments represent the main low-temperature chemical reactor at the earth's surface. The circulation of fluids in margin sediments is responsible for the extraction, transport, and concentration of much of the world's petroleum and mineral resources. High fluid pressure can drastically reduce the frictional strength of rocks. Rapid circulation of fluids can enhance pressure-solution and may result in dissolution and removal of a large portion of the deforming rock mass. Water from compaction of sediment (both tectonic compaction at subduction zones and depositional compaction at continental margin basins) and from subduction of hydrated oceanic crust is released at at estimated global rate of 1.5 km /yr (assuming a density of 1 g/cm ). Thud these expelled fluids are probably important for metamorphic processes in the subduction wedge and magmatic processes beneath the volcanic arc. The recent recognition of fresh- and saline-water seeps at the base of some continental margins suggests that gravity-driven flow from the Continents may be responsible for an average global flux of 100 km /yr. The interactions and effects of these different fluid fluxes remain virtually unknown. Growth and Modification of Continental Crust Magmatism at continental margins, such as basaltic magmatism at divergent margins and especially arc magmatism at convergent margins, is the primary mechanism for the formation of new continental crust. The process of magma genesis is strongly dependent on the interaction of several distinct chemical reservoirs, such as subducted oceanic crust and sedimentary cover and a variety of mantle reservoirs. We now have the ability to quantify the fluxes of mass and fluid between these different reservoirs using geochemical tracers. An excellent example is the short-lived cosmogonic isotope ,°Be, which provides a useful tracer for the presence of subducted sediment in arc magmas. s
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An important problem to be addressed is how continental crust attains its chemical character. The average composition of continental crust appears to be "ndesitic, whereas the melts extracted from the mantle wedge at convergent margins are dominantly basaltic. This observation suggests that the more mafic component of juvenile continental crust is stripped away. Deposit~onal Systems at the Edge of the Continents Sedimen is are the most abundant constituent of continental margins. They typical ly form thick, extensive deposits over ~ ying the zones of rifting or convergence that def ine the continental edges . The depositional processes respons ~ ble for transport =~d a ccumu ~ a t i on o f these s ed iments repre s ent a c omo ~ e x, dynam i c system that is controlled by eustatic, tectonic, and climatic conditions. The sedimentary record itself has great practical signif icance because it provides a direct measure of past deformation over a long interval of time. The record further provides a rich source of inf formation about evolution o f the environment and of life itself, which is Ingram uable in understanding current global change. An important and continuing goal is to develop realistic models of erosion, transport, and accumulation across the entire width of the continental margin. DEVELOPMENT OF A SCIENCE PLAN This report presents a scientific rationale for a new conceptual approach to continental margins research. The central thrust o f this approach is the acknowledgement that we have passed well beyond the main phase of the plate tectonics revolution in which the new paradigm was erected and its tenets were tested largely by studies that were discipline- and region- · . speck: : 1C. We have now reached a stage characterized by basic enquiry into the fundamental processes associated with plate interactions. Results of the workshop show that many of these processes are common to all margins, whether formed by extension, contraction, or translation. This conclusion suggests a unified approach to margins research that emphasizes the critical role of process-oriented inter-disciplinary programs. Several specific corollaries become apparent. One is that research on any one type of margin can be of significance to research on all margins. Another is that research in intra-continental erogenic belts, where continental extension is initiated and ancient margins are ultimately preserved, represents an essential component of the study of continental margins. Thus, marine and land-based studies need to be further integrated. Yet another corollary is that laboratory and theoretical studies of the processes of plate interaction provide a basic contribution to the study of 6
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continental margins. The process-based approach advocated here gives promise that a more aggressive and coherent strategy for margins research can be defined. The workshop took the first few steps toward defining this strategy by identifying the essential scientific objectives of a new margins research direction. Several further steps toward developing and implementing a concrete science plan are outlined below. Formation of a Margins Initiative Committee The workshop participants concluded that an independent scientific oversight committee, such as those organized under the auspices of the National Research Council, would be useful as a means to foster and ensure a greater degree of coordination and communication between land and marine communities in establishing process-oriented research programs. It would also advise U.S. funding agencies on mechanisms necessary for implementing such programs. Organization of Workshops The committee would be charged to organize some workshops by the end of 1990 involving a broad cross section of the community. The workshops would focus on the following topics: 1. Mechanics of Low Angle Faults 2. Fluids and Fluid Flow 3. Magmatism and the Growth of Continental Crust 4. Continental Margin Sedimentary Record The objective of the workshops would be to take the broad scientific objectives defined by the Irvine workshop and to develop and implement from them a focused program of research in the four areas named above. The plans formulated by these workshops and the results of discussions with the funding agencies will be combined into a so i ent i f ic pi an to be f ormul ated by the end of 1990 to chart a course for margins research for-the following decade. 7
Representative terms from entire chapter: