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2 RESULTS OF THE WORKSHOP MARGIN PROCESSES: A NEW RATIONALE FOR RESEARCH INTO LITHOSPHERIC CONVERGENCE AND DIN7ERGENCE Each of the six working groups identified within its subject area the major scientific objective whose achievement would represent a significant step towards realization of the overall goal stated above. Those objectives are: Divergent Margins - Mechanics of riftina and associated maamatisme To understand how the thermal and mechanical evolution of rift systems, at crust to lithosphere scales, controls the variability of continental margins in space and time. - Rift and passive margin basins: the sedimentary record. To understand the relations between the stratigraphy of sedimentary sequences, the processes that form stratification at all scales, and the geologic events that triggered the processes. - To understand the causes and interactions that control fluid flow in post-rifting divergent margins. To understand the dynamics of short-term deformation at continental margins. Post-denositional Processes in passive margin sediments. Convergent and Translational Margins - -Dynamics of short-term deformation at continental marring To understand the dynamics of short-term deformation at continental margins. - Geologic evolution of active continental marainse To understand how convergent and transcurrent plate motions 12

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fabricate, deform, redistribute, and waste lithosphere at continental margins. - Mass and chemical transfer. To understand the geochemical fluxes responsible for formation and modification of crust and the associated evolution of the mantle at convergent margins. Many of the scientific problems identified by the individual working groups were in fact common to all these margins, despite the diversity of topics considered and the range of Investigators from different disciplines. The commonality of direction belies traditional boundaries--based on discipline, geography or methodology--that divide the community. The principal reason for the similarities is that many of the processes are not unique to either convergent or divergent margins. Results of the workshop shower] that a science plan guided by a process-oriented, interdiscipl inary philosophy could take advantage of these commonalities to foster a rapid and more accurate understanding of margin processes. Processes that were identified by the participants as being at the foundation of divergent and convergent margin evolution, and some of the important questions and research objectives associated with them r are discussed below. Deformational Processes We have yet to formulate a realistic friction law for slip on large faults at plate boundaries. The gentle dip associated with subduction thrusts at convergent margins and with detachment faults at some continental rifts suggests low shear stresses across these types of fault. Heat flow and stress direction measurements at the well-studied San Andreas fault, a translational plate boundary, also indicate low shear stresses. Fluid pressures or the generation of low friction clay gouge is one hypothesis that could explain the low-stress paradox. Resolution of the paradox is critical for understanding the mechanics of margin formation. The mechanics of seismic rupture and aseismic creep on large plate-boundary faults remain poorly understood. Seismologists have shown that the maximum magnitude earthquake on a subduction thrust is related to how that fault ~ ruptures during an earthquake. Large-magnitude events on the subduction thrust tend to rupture in a steady fashion, whereas moderate and small events rupture in a more irregular fashion. This behavior has been attributed to differences in sizes and distributions of asperities, or locked regions, on the fault surface. Rock mechanicians have concluded that earthquake instability at large faults is probably caused by a velocity-weakening mechanism that 13

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operates on a preexisting fault surface, rather than by rupture of unfaultec3 rock. We have yet to reconcile this interpretation w ith the phenomena ~ og i ca ~ concept o f a sperit i es . The sl ip behavior of large faults, whether stick-slip or steady slip, also remains poorly understood. For example, the San Andreas fault has alternating ~ ocked and creeping segments. Some subduction thrusts slip only curing maj or seismic events, whereas others slip with ~ ittle release of seismic energy. This problem also has important implications for extensional faults. Seismological studies have not identified earthquake-generating normal faults with low dips, suggesting that these faults are initiated with moderate dips and rotate to shallower dips after formation. Alternatively, low-ang~ e normal faults may slip aseismically . Plate-boundary deformation at many active margins is distributed across a zone several hundred kilometers wide. In contrast, it is not clear how the distribution of deformation across such a broad plate boundary zone is related to lateral variations either in the stress fief ~ or in rheological properties of the overlying platee Vertical motion of the crust is one of the most easily measured expressions of lithospheric clefonnation. Geodetic and geologic observations, together with reconstructions of tectonic subsidence derived from backstripping basin strata, provide records of these vertical motions. These types of data, which are increasing in quality and availability, are essential to developing a better understanding of deep-seated Reformational processes. The data will become an increasingly important constraint on dynamic models of deformation at continental margins. The manner in which the lithosphere deforms in response to stresses is controlled mainly by rheology. This property-in turn is controlled by a variety of factors, one of the most important of which is temperature. Thus, the lithosphere is likely to react quite differently when deformation is associated with magmatism as opposed to the absence of magmatism, but magmatism itself is an expected outcome of lithospheric deformation, so the two processes cannot-- be treated separately. Apart from the effect of temperature, the presence of even a small percentage of melt drastically alters the rheological properties of the mantle. Magmatic Processes Magmatism at continental margins is the primary source of new continental crust e A long-standing problem is that the continents have an average composition of andesite, whereas melts from the mantle at convergent margin arcs and at continental 14

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rifts are dominantly basal tic. How does the average composition cuff continental crust become more silicic? One possibility is a periodic delamination of a more mafic lower crust, but lithe e is known about this process. Discrete spacing of volcanic centers along-strike is a common feature of arcs and continental rifts. In continental rifts, a relationship has been recognized between the spacing of magmatic centers and structural segmentation, a relationship reminiscent of that for oceanic spreading centers. Many investigators studying the dynamics of oceanic crusty accretion believe that segmentation reflects the distribution of magma generation and ascent, so that magmatic processes actually control segmentation, but it is equally plausible that global stress patterns associated with plate motions govern segmentation, which in turn controls magma transport. Spatial and temporal variations in magmatism are found at both divergent and convergent margins e For instance, tectono- magmatic segmentation at rifted margins is probably influenced by prior tectonic history. Models of magmatism in both settings consider enhancement of melting by the effects of small-scale convection, and the extraction and focusing of melt by mantle flow. Volume and composition of magmatism are related to the thermal structure of the lithosphere and the percentage of melting as controlled by adiabatic decompression of the mantle. In divergent margins, rifts have been classified as amagmatic or magmatic, based on the presence or absence of an intense phase of magmatism in their initial stages. The amount of magmatism in this setting is believed to reflect not only the prior tectonic history, but also the thermal structure of the lithosphere and mantle and the evolution of the lithosphere during extension. In convergent zones' the source and contamination of arc magmas can be distinguished using certain trace elements and isotopes. Radiogenic isotopes, like Abe, with half lives of a few million years, measure the contribution of the uppermost ocean sediments to a melt. Certain isotopes (e.g., Pb, Sr, and Nd) and possibly some element suites may be able to measure the contributions to the melt of subducted sediment and/or crust of the overriding plate. Other elements (e.g., B and K) can detect contributions from altered basaltic crust of the subducting plate. Surprisingly uniform ratios of some of these elements (e.g., Abe to B) have been observed along entire arcs, but these ratios do vary between arc systems. The reason for this uniformity is not understood, but there is clearly the potential for resolving the sources (e.g., sediment, continental crust, hydrothermally altered rock, and mantle) for magma genesis at convergent margins. 15

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Mantle f ~ uins are important in initiating melting and controlling the composition of magmas in both convergent margins and continental rifts. Factors such as the redox state of the mantl e and compos iti on and spec iation o f the f luid are important in control! ing the chemistry of the magma, the extent of melting , anc] segregation of melt in both environments . Arguably, the principal dif ferences between convergent and divergent margin magmatism are clue to clif ferences in the composition and rot e of associated fig aids. Fluids beneath continental rifts appear to be dominated by methane and carbon dioxide r whereas those beneath convergent margins are dominated by water released from dehydration of the subducting slab. The character of continental mantle and, ultimately, the evolution of the whole mantle, are largely defined by processes occurring at continental margins and rifts. The creation and modification of the crust at these margins result in fluid mobility and element transport, as well as melt migration and extraction. The residual mantle has different trace-element signatures, which evolve through time to form distinct isotopic signatures. Mantle modified at convergent margins can be later remelted at divergent margins, which superimposes the ef feet of one process on the other. Recyc' ing of continental crust through subduction or by lower crustal delamination adds processed crustal material back to the mantle, where it is gradually mixed into the asthenosphere by convection. Fluid Flow Fluids infl uence al most all the processes operating at continental margins in some fashion. The dewatering of a subducting slab is the primary driving force for magmatism and volcanic activity at convergent margins. Without water, subduction of a cold slab would freeze the mantle rather than melt it, and there would be no basaltic magmatism. Once a melt is generated, water influences the petrologic evolution of the melt, and the development of mad or ore deposits (porphyry copper, molybclenu~n, and gold) . Fluic] pressures also influence seismicity. Subducted fluids clearly affect, and perhaps even control, the tectonics at convergent margins. Fluids also play an important role in the accretionary prism. Overpressured fluids are probably required to reduce the sediment shear strength sufficiently to maintain the shape of the accretionary prism. Fluid transport may affect heat flow, cause mud diapirism, and promote rock alteration. In the evolution of divergent continental margins, excess pore pressures are generated through compaction and burial metamorphism (diagenesis) of organic and inorganic components in sediments. Some of the escaping volatiles accumulate as 16

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clathrates in very shallow and cold ocean sediments. Thermal or tectonic disturbances may cause the breakdown of the clathrates, generating pore fluid overpressures that-may result in surficia] slumping. Other escaping volatiles (especially hydrocarbons) tend to be trapped in subsurface structures. Understanding and predicting these structures is the basis of the petroleum industry. At the same time' topographically driven flow in hydrostatical ly pressured zones on divergent continental margins (overlying the geopressured zones or existing later in time) s igr~if icantly alters the crust and can dissolve large amounts of rock at stylo~ites. The discovery of abundant seeps at the base of some continental margins suggests that deep circulation may be common there. Convection of saline fluids could be the geological agent for dolomitization, a long-standing problem in the earth sciences. Thus, fluids participate in continental margin processes in a pivotal and pervasive way. For example, the physical and chemical behavior of overpressured fluids in porous media is fundamental to an understanding of primary oil migration, porphyry ore deposits, the fate of subduction-zone magmas, the shape of accretionary prisms, and the tectonics and fluid flow in divergent margin basins e Fluid-based processes also illustrate the potential synergism of ocean and continent observations. Submarine settings best display large-scale fluid! fluxes (sal ine seeps , heat f low perturbations , clathrate accumulations ), whereas alteration resulting from fluid flow is best appreciated from observations of large, on-lane} outcrops Sedimentation Processes Sediments represent composite records of past climate, sea level fluctuations, ocean circul ation and chemistry, ant} variations in sediment supply. The margin depositional system responds to all these local, regional, and global geological processes, which transcend both shorelines and continent-ocean structural boundaries. The fundamental problem lies in deconvolving the multiple inputs to margin stratigraphic development, while taking into account the imperfect nature of preservation in the geologic record. Development of such a complex inversion requires knowledge of the forward problem: the construction and preservation of margin sedimentary prisms. Progress is being made at various observational scales. For example, at one end of the spectrum, realistic mathematical models now appear capable of simulating depositional patterns produced during storms and tidal surges. Furthermore, within the next decade, computational models should make it possible to synthesize stratigraphic sequences formed by a variety of Prepositional and erosional inputs, such as sediment flux and 17

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subsidence. Finally, devel opment of the systems tracts concept provides new descriptive power for the study of depositions systems from outcrop to basinal dimensions, and thereby holds promise for the del ineation of the controls of sedimentary processes e A multifaceted approach to understanding the dynamics of stratigraphic development is crucial to the use of sediments as tape recorders of margin evolutionary processes. For example, any attempt to use stratigraphy to study modes of vertical motion on margins relies upon a thorough understanding of the coupled processes of erosion and deposition. Similarly, recovering the kinematic history of motion on low-angle normal or thrust faults from associated sedimentary units requires knowledge o f the interplay of tectonism and sedimentation. Along active margins , sorting out the geologic implications of commonly thick, of ten rapidly formed sedimentary sequences is critical for reconstructing plate-margin history and the associated development of continental crust. Major increases in sediment supply can be keyed to pulses of arc volcanism and margin uplift due to changes in the position of the underthrusting plate or to collision events e In summary, virtually all tectonic' oceanographic and climatological factors leave their combined imprint on margin stratigraphy. Learning to make effective use of this complex but genetically significant record is a major challenge. Mantle Dynamics A fundamental control on deformation at continental margins is exerted by temperature and density differences within the mantle and the lithosphere. The temperature variations exist mainly because the asthenosphere and the plates are moving. The stresses may be related to plate motions. Examples are ridge- push and slab-pull forces e The great challenge is to discern which of these forces are due-to local effects (such as a hot spot or a cold spot) and which are transmitted from great distances. One view has been that rifting is initiated by the uplift of the earth's surface caused by mantle plumes, which heat and thin the overlying lithosphere. Above active hotspots, we do see large uplifts. The stresses associated with these uplifts may be capable of causing extension of the lithosphere, but uplift does not precede rifting in all cases. For example, stratigraphic data indicate that the continental surface was close to sea level for tens of millions of years before rifting of the Gulf of Suez. It appears that both local and far field stresses can lead to the creation of a continental margin. 18

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- Recently, many workers have recognized that deformation of . the lithosphere may induce flow in the mantle , which then affects further deformation of the lithosphere. The thickening of the lithosphere may produce instabilities of the lower lithosphere. This instability may cause the lower part of the lithosphere to be removed, resulting in heating of the remaining lithosphere . Thinning of the lithosphere by rifting produces lateral density differences, which lead to flow in the mantles This flow may have topographic and maqmatic consequences, FUTURE DIRECTIONS FOR RESEARCH ON MARGIN PROCESSES: REQUIREMENTS FOR SUCCESS The most important result of the workshop was the development of consensus that the focus of margins research should shift to processes-oriented studies under a completely interdisciplinary organizational and funding structure. An integrated, process-based approach represents a significant change in direction in current research, and it will require a major shift in research organization and management. Currently, despite the fact that many scientists are studying the same physical processes of margin evolution' their perspectives vary enormously according to discipline. One perspective may be represented by the petroleum industry and another, wholly different' by academia. The outlook, for instance, of the field geologist studying outcrops often differs markedly from that of the geophysicist studying seismic reflection profiles collected at sea. These disparate perspectives tend to inhibit understanding the cross ~ inked processes that operate at continental margins. A focusing of these different approaches will be required to achieve a major increase in our understanding of the continental margins. This can be done if we marshal our individual physical and intellectual resources for a truly interdisciplinary process-basec] assau] ~ on the problems of margin Avon ution. To initiate this change, a number of specific areas of science organization and facil ity support need to be addressed. Communication and Collaboration Among Scientists Our success is dependent upon the development of projects involving a collaboration of scientists from many disciplines and from industry, government, and academia, including those who study land geology and marine seismology, those for whom the earth exists largely inside a computer and those who measure the chemical composition of rocks dredged or drilled from the deep sea. Projects developed from the Margins Initiative must involve participants from the broadest discipline and professional base in the planning, execution, and interpretation of margin 19

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research. Focusing of research along process-oriented lines capitalizes on the fact that margins are a natural meeting ground of the disciplines. Furthermore, because continental margins are by their nature international, experiments will need to be conducted at locations worldwide, wherever the critical processes can best be addressed. Databases Progress toward the objectives of the Margins Initiative would be greatly facilitated if investigators could have greater access to digital databases of geological, geochemical, geophysical and satellite-derived observations of the continental margins. Access and organization are factors critical to the success of such databases. This represents a major effort, because the amount of data held by investigators is known to be very large, and it could not be integrated without a substantial commitment of funds and manpower. Nonetheless, the existence of such databases would prove cost effective in the long term by enhancing the exchange of data between disciplines. An excellent example of this is the DNAG compilation of geological and geophysical data from the North American continent, currently available on CD-ROM. Computational Facilities The NSF made an outstanding commitment to computer support of science in the United States by developing a network of supercomputers. Nonetheless, the present availability of these advanced facilities is not completely adequate to support a major effort in geoscience modeling' data processing, and analysis. Support for a wide range of computational facilities, including mini-supercomputers, minicomputers, workstations and even personal clesk-top machines, is essential for scientists engaged in continental margins research. Many scientists would opt f or less powerful but more available computers. Computational methods in modeling physical processes will continue to play a critical role in research at continental i margins. These range from basin-wide fluid flow to deformation of the deep mantle. There is virtually no area in earth science today in which computational methods using high-speed computers have not been immensely valuable, and many in which advances in understanding simply would not have taken place at all were it not for the application of computer modeling. The development of subsurface images using seismic reflection techniques cannot be done without large high-speed computers, and the study of mantle convection would be extremely limited if computational facilities were not available. It has been the exploration of the earth using modern digital computers that has taken many scientists 20

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toward] the process approach that we regarc] as fundamental to the Margins Initiative. Scientif ic Drill ing Scientif ic drilling--and industry drilling as well--has made extremely important contributions to our understanding of ~ ntinental margins ~ Nonethel ess r scientific drilling capabil ities need to be extended to address many of the process- related problems discussed in this report. Many of the targets of interest in modern and ancient continental margin settings are deep. Examples include Plating unconformities in thick basin sequences or intersecting low-angle faults, such as the subduction decollement or potential low-angle normal faults at passive margins (e. ge ~ Galicia). Such problems demand broader capabilities than are currently available through the scientific dril 1 ing programs presently operating at sea and on land. For instance, the Ocean Dril ~ ing Program (ODP) generally cannot drill holes in excess of 1 km, yet this represents only a tenth of the sediment thickness found at many margins. The lack of riser capability also means that even these hod es must be located away from structure and any ~ ikely fig uid accumu~ ations. Seismic Facilities Seismic ref lection and refraction studies provide one of the principal methodologies for investigating the subsurface of the earth. Future research at continental margins will require a substantial long-term commitment to support seismic facilities, both on land and at sea, that are equal to or exceed those available in the exploration industry. Seismic acquisition also generates enormous quantities of data, wherein archiving and retrieval of these data in eff icient ways present an increasingly complex problem of information management for all concerned parties. Seismic acquisition carried out or proposed by universities or consortia that do not have suitable capabilities (the BIRPS program in the U.K., LITHOPROBE in Canada, COCORP and EDGE in the United States) employ seismic contractors who normally work for the petroleum exploration industry. This commercialization of geophysical research has benefited academia in that advanced MCS instruments, currently used by industry throughout the world, are wholly compatible with academic research needs. Rapid growth in instrument development has made 'blast year's model" available to universities at a fraction of the original cost, and occasionally as gifts. In rare cases, university researchers are able to acquire not only these special instruments, but also the platform for deploying them. In addition to leasing commercially available platforms, these spin-offs allow universities to obtain 21

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and operate advanced seismic acquisition capabilities at comparatively modest costs. The total amounts involved are still large, so that a major commitment to continuing long-term support by the granting agencies is required. Chemical Analytical Facilities In order to evaluate elemental fluxes at convergent and divergent margins, and to use these fluxes to trace mantle and crusty evolution, increasingly more precise, rapid, and sensitive analytical facilities are needed to analyze very low abundance tracers as well as compositions of smaller sample volumes (e.~., mineral separates or fluid inclusions). As is case with computational facilities, these analytical needs are generic to almost all the earth sciences. However, because of the importance of fluid and magma processes at continental margins, microanalytical tools are particularly important. Theoretical and Computational Studies The key element in research aimed at understanding the physical and chemical processes that affect the evolution of margins is the development of self-consistent models of these processes. In general, this means computer simulations. Considerable advances have been made recently in constructing such models, and the exciting results they have brought to light provide an important impetus for the Margins Initiative. Most models of Reformational and magmatic processes of margin formation remain largely kinematic in nature e Progress is being made toward the development of self-consistent, dynamic models of mantle flow and melt migration under mid-oceanic ridges, but similar models for continental extension and convergence are still in their infancy. The development of realistic models of earth structure using seismic data is a problem of staggering proportions, especially for three- dimensional problems. This statement applies equally to description of the geometry of mantle flow developed from tomographic inversion of earthquake data, the production of seismic-reflection images, and the determination of velocity structure of the crust. Field experiments aimed at defining fundamental processes of margin evolution also rely critically on insights gained from computational simulations, so that the critical location for these experiments is chosen properly, and is followed by optimum analysis of the resultant data. Thorough integration of data from laboratory experiments will also be required to determine 22

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the rheologica~ properties of the lithosphere under real earth conditions. Sustained support of theoretical and computational programs is an essential component of future margins research. # 23