written by MARGINS Steering Committee
Margins are where the action is. Continental margins are the Earths principle loci for producing hydrocarbon and metal resources, for earthquake, landslide, volcanic and climatic hazards, and for the greatest population density. Despite the societal and economic importance of margins, many of the mechanical, fluid, chemical and biological processes that shape them are poorly understood. Progress is hindered by the sheer scope of the problems and by the spatial-temporal scale and complexities of the processes. To overcome these obstacles, the MARGINS community has identified the outstanding scientific problems in continental margins research and the MARGINS Program (a research initiative supported by the US National Science Foundation) is promoting research strategies that redirect traditional approaches to margin studies. The MARGINS Program seeks to understand the complex interplay of processes that govern continental margin evolution. The objective is to develop a self-consistent understanding of the processes that are fundamental to margin formation and evolution. The MARGINS approach involves concentration on several study areas targeted for intense, multidisciplinary programs of research in which an ongoing dialogue among field experiment, numerical simulation and laboratory analysis researchers is axiomatic. The plan is to investigate active systems as a whole, viewing a margin not so much as a "geological" entity of divergent, translational or convergent type, but more in terms of a complex physical, chemical and biological system, subject to a variety of influences. The processes that fundamentally govern the evolution of margins include lithospheric deformation, magmatism and mass fluxes, sedimentation, and fluid flow. The goal of the MARGINS Program is to provide a focus for the coordinated, interdisciplinary investigation of these processes. In the following sections we describe five fundamental scientific problems to be studied, outline the common research strategies, present four focused research initiatives, and describe the state of the planning process. Broad community input at three thematic workshops in 1991-1993 attended by over 120 researchers defined the scientific problems and research strategies outlined below. With this input, the MARGINS Steering Committee produced an Initial Science Plan 1996 that presented notional experiments for the scientific objectives. They deliberately eschew description of "place" but outline the scope and nature of process-oriented experiments that could be applied at any suitable location. Even so, hard choices had to be made to frame an initial science plan of achievable scope. Three parallel planning efforts further influenced and refined the MARGINS implementation plan that we presented to NSF in 1997: the 1994 JOI-USSAC workshop on Recycling Processes and Material Fluxes at Subduction Zones, the 1995 International Lithosphere Program workshop on Dynamics of Lithosphere Convergence, and the 1996 NSF Future of Marine Geophysics (FUMAGES) meeting.
Scientific Problems
Low-strength Paradox of Lithospheric Deformation
Very large fault structures (subduction thrusts, major transforms and perhaps normal detachments) accommodate a major component of strain at continental margins and produce nearly all the most destructive earthquakes. However, these structures move at resolved shear stresses far smaller than those expected to cause failure and we currently lack a viable theory to account for them. The apparent low-strength property of large faults may be corollary to an even more fundamental issue. When lithospheric strength (the integrated "yield stress envelope") and tectonic forces are compared, we find that the forces available are insufficient to rupture the lithosphere. Although mechanisms may exist that allow a strong lithosphere to be deformed by weak forces through concentration of stresses into narrow regions, those mechanisms are not yet understood. Similarly, the mechanical factors that control the rupture size and frequency of earthquakes are not understood.
Strain Partitioning During Deformation
Mounting evidence suggests that strain measured at the surface by geological techniques may be significantly different from that inferred to have taken place in the lower crust and upper mantle from geophysical observations. One explanation is that the rheology of the lower crust is viscous and weak relative to mechanically strong layers above and beneath. This "jelly-sandwich" model of lithospheric rheology allows upper and lower crust to behave essentially independently of one another, and provides appealing explanations for some problems. The jelly-sandwich rheology, however, remains little more than plausible conjecture and even a basic description of how the lower crust and upper mantle behave during deformation remains incomplete.
Magma Genesis and Crustal Recycling
Models of mantle flow, melt generation, and melt migration for margin settings have lagged behind those for mid-ocean ridges due to the more complex boundary conditions and uncertainties about the relative roles of subducted and upper plate material. Tracing and balancing mass, volatiles, and energy across a convergent margin holds promise to address the poorly understood mechanisms of thrusting and seismicity, the cycling of crustal material through the subduction zone, and the magmatic fluxes that ultimately lead to continental crust formation. Although subduction magmatism has long been thought of as the primary mode of continental growth, it is becoming clear that very large volumes of magma are brought to the Earth's surface in so-called Large Igneous Provinces in intra-plate and divergent margin settings. We lack a theory that can adequately explain the spatial and temporal aspects of melt generation and migration needed to account for even our most basic observations.
Stratigraphic Preservation of Geologic Events
Continental margins are the Earth's principal loci of sedimentary accumulations and contain one of the best preserved records of global sea-level variations, climatic fluctuations, lithospheric deformation, ocean circulation, geochemical cycles, organic productivity and sediment supply. Margins, therefore, record the variations in the solid earth-ocean-atmosphere system essential to evaluating models for today's global changes. Margin sediment prisms are also the principal low- to moderate-temperature chemical reactors that produce massive mineralogical changes in basin sediments, resulting in most of the world's oil and gas reserves and mineral deposits. The problem is to understand the complex and dynamic interplay of processes responsible for the erosion, transport, accumulation and preservation of margin sediments.
Fluid Fluxes
Large-scale fluid circulation is the most important chemical transport mechanism through margins. Geochemical processes such as diagenesis and metamorphism, and deformation processes such as stick-slip faulting versus creep, are strongly controlled by the rate of fluid flow, fluid composition and the rate of rock-fluid interactions. Flow driving mechanisms include compaction, compression, and thermal and gravitational circulation. Water/rock/organic matter interaction changes fluid composition and, by altering rock porosity and permeability, creates a feedback mechanism affecting fluid pathways and flow rates. These fluid flow and diagenetic processes represent important contributions to the global geochemical inventory. The problem is that many of these mechanisms, their rates, and the fluid pathways are still largely unknown.
Research Strategies
The fundamental scientific problems outlined above have been defined by a large body of past and continuing research. Achieving MARGINS research objectives to solve these problems will generally require new experimental approaches, including:
Developing Multidisciplinary Case Studies
One goal of the MARGINS Program is to coordinate a community consensus on a small number of in-depth, multidisciplinary case studies carefully designed to address one or more scientific objectives. We expect these experiments to be three-dimensional in nature, with the size of the study region, the duration of the experiment, and the suite of instrumentation employed to be determined by the processes to be studied. The study areas may be on land, under sea, or crossing the boundary. These case studies require commitments from investigators and funding agencies for extended and diverse experimentation. Each case study will involve field experiments well-integrated with appropriate laboratory studies and theoretical modeling exercises, to relate new observations to relevant physical-chemical-biological processes.
Focusing on Active Systems
One corollary of the process-based, systems approach is that it is most useful to study active systems, as opposed to their fossil counterparts. Once a system is no longer active, it becomes more difficult to completely characterize the boundary conditions and the in situ states of the materials in the system. Furthermore, one or more of its characteristics have usually changed, and paleoconditions may be difficult to infer from the rock record. Investigations of inactive systems will be undertaken as a route to understanding processes in currently inaccessible parts of active systems; for example, the deep roots of a fault zone or volcanic complex.
Studying Whole Systems
An important aspect will be to adopt a whole system approach, rather than targeting a particular physical or chemical component in isolation. In designing the in-depth case studies, it will be important to define a priori the dimensions, boundary conditions, principal rock and fluid components, physical and chemical states of the system. Field, lab and modeling studies should be interpreted jointly by teams of geologists, geophysicists, and geochemists to characterize the coupled dynamic components as an integrated system.
Establishing the Scaling Relations
It is extremely difficult for any study, whether it be field-, laboratory-, or computer-based, to cover simultaneously more than 3 orders of magnitude in length or time scales. However, finding solutions to some of the major scientific questions posed above requires understanding the operation of processes over many orders of magnitude. Therefore the MARGINS Program will include nested experiments that cross both length and time scales.
Including Comparative Global Studies
While the emphasis in MARGINS will be on a few in-depth case studies, more global and satellite studies will be necessary for the purpose of making informed choices on the sites for some case studies. Furthermore, the additional sites will permit testing the generality of quantitative and conceptual models derived from the case studies in regions where some of the system parameters are different.
Establishing Event Response Strategies
Having made a commitment to studying active systems, it will be necessary to develop a strategy of event response, since even "active" systems may be only intermittently active.
Science Initiatives
The MARGINS Steering Committee presented an implementation plan to NSF in January and October 1997 that focuses on four initiatives. Further refinement of this plan is anticipated as the science evolves.
Rupturing Continental Lithosphere
The mechanisms that allow continental lithosphere to be deformed by weak tectonic forces are not understood, nor is the manner in which strain is partitioned and magma is distributed during that deformation. These processes control the fundamental margin architecture and hence the location and magnitude of resources and geologic hazards. One way to solve these problems is to focus a comprehensive investigation on faulting, strain partitioning and magma emplacement at sites of active continental rifting where there is a lateral transition to initial seafloor spreading (Fig. 1) that will provide a spatial proxy for temporal variability. The effects of, and consequences for, hydrous fluids and sediments will be included in these integrated observational, laboratory and modeling experiments. The objectives are to: 1. Determine the local and regional states of stress, the distribution and rate of strain, the pressures and temperatures, and the physical and chemical properties of rocks and fluids associated with a well-imaged and seismically active low-angle normal detachment (the extreme case of the weak fault paradox). Measurements of these in situ parameters made by drilling, instrumenting and long-term monitoring will be used to determine how such faults move at resolved shear stresses far smaller than those expected based on laboratory observations and Coulomb rheologies. 2. Determine the spatial and temporal distribution of strain by (i) mapping the geometry and offset of faults, (ii) inverting and modeling the stratigraphic and structural record to resolve the history of strain variation and its control on topography/erosion/deposition, (iii) using seismic, gravity/geoid and geothermal methods to obtain an integrated sum of the deformation and a measure of the ductile thinning of the lower crust, and (iv) evaluating the heterogeneity of the continental lithosphere prior to rifting. 3. Determine the pattern of mantle flow, the extent of melt generation, and the style of melt migration and emplacement during continental rifting and the early stages of seafloor spreading by imaging with seismic and electromagnetic methods an active rift-spreading transition, by measuring the heat flow distribution, and by analyzing the chemistry of magmas emplaced in these regions.
Seismogenic Zone Experiment
Subduction zone megathrusts produce the largest and potentially the most destructive earthquakes and tsunamis on our planet by shear along converging plate boundaries. Despite the societal and economic importance of great earthquakes, little is known about the seismogenic zone that produces them. A shallowly dipping subduction zone thrust provides a large fault surface, partly seismic and partly aseismic, that is accessible to study by a combination of selective drilling and extensive ongoing monitoring using passive and active seismology and geodesy. It lies within a forearc where sediments undergo compaction, lithification, and dehydration reactions as they underthrust at a low angle. Therefore, the processes and products that control the partitioning of strain, the flow of water and other volatiles, the formation and behavior of faults, and the onset of seismic slip are all relatively accessible (often at depths less than 8-10 km) to geophysical imaging and direct sampling/monitoring. This experiment represents an opportunity to address primary MARGINS objectives related to the mechanics of seismic and aseismic faulting in a new, comprehensive, and aggressive manner.The objectives of the Seismogenic Zone Experiment (SEIZE) (Fig. 2) are: 1) to measure stress and strain across a seismogenic subduction margin, 2) to determine the nature and fluxes of the fluids and solids throughout the forearc, 3) to establish the relationships between earthquakes and the geometry and mechanical state of faults, and 4) to determine the inter-relations between the above and thermal structure, lithification and intrinsic rock strength. In concert with the data acquisition, SEIZE investigators will conduct laboratory experiments and formulate testable quantitative models of how the subduction cycle works, including the complex interactions among the various chemical and mechanical processes. Testing these models through deep riser drilling into the seismogenic zone is a prime goal of SEIZE. This experiment is also the first priority defined by the July 1997 international Conference on Cooperative Ocean Riser Drilling (CONCORD). The investigators will need to observe active tectonic, seismic, and geochemical processes that occur from milliseconds to decades, and document their accumulated geologic record. These characterizations will help define the conditions and materials that control earthquake cycles and improve evaluation of natural hazards.
Subduction Factory
At convergent margins (Fig. 3), raw materials (sediments, oceanic crust and upper mantle) are fed into the "subduction factory" where many processes (including dewatering, metamorphism, melting) under changing physical and chemical conditions shape the final products (magma, volatiles, ore deposits, new continental crust, recycled materials) with some environmental consequences (hazardous seismicity, explosive volcanism, noxious and greenhouse gases). In practice, it has been difficult to investigate processes and estimate fluxes through the "factory" owing to poor constraints on the volumes of magmas, fluids, and volatiles produced. The approach here is to implement an interdisciplinary study of this problem at a margin having characteristics that optimize study of volatile cycling and crustal growth, and where geological and geophysical measurements will constrain ongoing processes in real time.Some major questions to be answered include:
1. What fraction of subducted volatiles (H2O, CO2) are returned to the oceans and atmosphere, stored in crustal rocks, and subducted to the deep mantle? Does subduction of carbonate lead to enhanced volcanic CO2 fluxes to the atmosphere?
2. What is the rate and mechanism of continental growth at convergent margins? How do forcing functions such as convergence rate, volatile input and upper plate structure control magma production rates and composition?
3. How much continental material is recycled to the deep mantle? After processing through the subduction factory, are residual slab compositions suitable to serve as food for mantle plumes?
Sediment Dynamics and Strata Formation
Understanding the processes that form and modify continental margin stratification at all scales, and the events that trigger those processes, is key to unlocking the encrypted record of earth-ocean-atmosphere system history produced by the convolution of lithospheric deformation, sea-level variations, climate fluctuations, and fluid fluxing.
The traditional approach to stratigraphic characterization along continental margins has been primarily to describe and to classify. This initiative, instead, seeks to identify a small number of natural laboratories where large-scale, interdisciplinary experiments, involving field, laboratory and theoretical components, can be designed and implemented (Fig. 4). These experiments will achieve the following:
1. Determine the timing, spatial distribution, and causes of stratal discontinuities in the context of depositional and erosional processes, which will allow us to evaluate the interrelations among tectonic subsidence/uplift, landscape erosion, sediment supply/compaction, physiography, climate, oceanography, and eustasy in the formation of stratigraphy.
2. Clarify the scaling relationships among physical processes operative on various spatial and temporal scales, their control on the formation of sedimentary signatures (e.g., "event" strata), and how such strata are preserved in the longer-term stratigraphic record.
3. Investigate the fluid flux through the integrated stratigraphic-structural-thermal-chemical margin system, in order to understand how fluids act as the primary coupling agent of the physical and chemical processes controlling sediment transport, deposition, burial, and diagenesis.
Implementation
MARGINS has initiated a series of workshops at which site selection and experiment development occur in a forum open to the interested scientific community. The first of these open workshops was held in June 1997 by the SEIZE community and convened by Greg Moore, Tom Shipley, Casey Moore, and Miriam Kastner. A follow-up meeting organized by Eli Silver and Marino Protti and specific to Costa Rica - Nicaragua was held in December in San Francisco. Julie Morris will convene a Subduction Factory workshop next June. All three meetings are sponsored by NSF and JOI/USSSP. The MARGINS Steering Committee is planning future workshops and theoretical institutes to further the sedimentology, rheology and deformation objectives. Reports on these meetings, the Initial Science Plan, and updates on MARGINS activities are available at http://www.soest.hawaii.edu/margins.
We envision that the four science initiatives outlined above could be carried out in five or six geographic areas of focused, interdisciplinary investigations. Recent and nascent advances in global positioning, seismic data acquisition, scientific drilling, seafloor observatories, remote sensing and sampling, computational simulations, and laboratory measurements of rock physical and biogeochemical properties provide exciting prospects for major scientific advances from coordinated, interdisciplinary MARGINS research programs in the next decade.
Why has MARGINS elected to focus on only a few sites?
Interesting and productive science is now being done on continental margins without benefit of the MARGINS program so, "Why MARGINS?" and "If MARGINS, then why focus on only a few areas?" MARGINS came into existence because of a community perception that continental margin science can benefit from increased communication and cooperation of scientists making different types of observations, doing different types of experiments, and explaining observations with different models and simulations. MARGINS proposes to foster much larger, more interdisciplinary experiments, albeit in relatively few areas due to resource limitations. Our science is currently advanced by small groups of investigators making observations in the best place in the world for their particular type of observation. This approach diffuses effort and the opportunity to compare diverse data types in a single locale.
MARGINS does not seek to eliminate or replace single investigator proposals that fall outside MARGINS sites. MARGINS instead wants to see a mix of smaller globally distributed experiments and a few larger, carefully chosen, multidisciplinary experiments. MARGINS is committed to involving a broad cross-section of investigators in any focused study area and to rapidly distribute the resulting data. This will provide a vehicle for individual researchers to undertake laboratory and theoretical studies, or piggy-back field experiments, that are reinforced by a greater range of samples and observations than would otherwise be possible.
Acknowledgments
This article was contributed by the members of the MARGINS Steering Committee: M. Coffin, W. Dietrich, T. Dixon, N. Driscoll, G. Karner, S. Klemperer, D. Kohlstedt, C. Moore, C. Nittrouer, T. Plank, D. Sawyer, R. Stern, E. Stolper, B. Taylor (chair), M. Underwood, and D. Wiens. Funding for the MARGINS Program is provided by the NSF Divisions of Ocean and Earth Sciences.
For more information, contact Brian Taylor, SOEST, University of Hawaii, 1680 East-West Rd, Honolulu HI 96822.