Prof. Anthony B. Watts
Professor of Marine Geology and Geophysics
EUROMARGINS:
a new European-led programme to study passive continental margins
1. Introduction
Passive continental margins are features on a global scale that mark the transition between continents and oceans. They develop as the consequence of the break-up of continents and the formation of new ocean basins, are the sites of some of the world’s largest accumulations of sediments and, are among the best indicators that we have of past changes in climatic, sediment flux and sea-level. Passive margins are prone to major natural hazards since a majority of the world's population lives within a short distance of the coastal zone.
The European scientific community has already identified the deep structure and rifting processes, the sedimentary processes and products, and fluid flow, seeps and deep-water biota at passive margins as high priority targets that need to be addressed in the immediate future. Furthermore, the hydrocarbons industry regards the slope and rise regions of passive margins as one of the few remaining frontiers for the production of oil and gas.
In May 1999, the Executive Council of the European Science Foundation (ESF) approved the establishment of a 3-year Network in "Ocean Margins". One of the main aims of the Network, which currently includes representatives from 10 countries in Europe, together with two industry representatives and observers from ESF and the USA National Science Foundation (NSF), is to develop a new European-led programme in Ocean Margins.
In February 2000, the ESF Network hosted a workshop in Sitges (Spain) to which key members of the European scientific and industrial community interested in Ocean Margins were invited. The workshop was attended by 40 scientists from 16 countries within Europe. In addition, 21 letters were received from 30 additional scientists who were interested in Ocean Margins, but were unable to attend the workshop. The main outcome of the workshop was to develop a new, European-led, programme in Ocean Margins called EUROMARGINS. The main aim of the new programme is to address fundamental scientific questions concerning the origin, structure and evolution of passive margins. It was proposed that particular emphasis be given to questions such as rifting processes; the structure, rheology and tectonic and magmatic styles; slope instability and turbidite systems and the role of factors such as tectonic setting, climate, sea-level and magmatism in controlling stratigraphic sequences. Also to be addressed would be the societal importance of passive margins, especially as they relate to hydrocarbon exploration and production, the economic potential of gas hydrates, deep-sea biological communities and the occurrence of natural hazards.
We summarise here some of the main scientific objectives of an EUROMARGINS programme. We discuss the European "added value" of the programme, its relationship to existing national programmes in Ocean Margins and, suggest a strategy that outlines how a EUROMARGINS programme might be implemented in the future
2. Scientific objectives
EUROMARGINS will focus on 3 main research themes: rifting processes, sedimentary processes and products, and fluid flow.
- Rifting processes
Recent studies have improved our knowledge of the physical and chemical processes that operate at passive margins. However, we still lack a complete understanding of how they evolve through space and time. Key questions concern the dynamics of rifting; the mechanisms of magma generation and emplacement; the interaction of rheology, strain rate, crustal flexure and faulting; the nature of the ocean-continent transition; and the thermal and mechanical properties of rifted continental crust.
To address these problems, we envisage several large scale experiments to better image a conjugate pair of rifted margins on a variety of spatial scales. In particular, we require a large scale seismic program that involves a large array of hundreds of Ocean Bottom Seismometers (OBS’s) to construct a 3-D image of the velocity structure of the crust combined with a 3-D multi-channel seismic reflection profile experiment.
Of particular to interest to scientists in Europe is the interplay between extension and magmatism. The problem is that the amount of magmatism along many volcanic margins is not known. In cases where the volumes are well constrained, the timing is not well enough known to determine magma generation rates. Often rifted margins are well studied on one side but, the conjugate is unknown. Thus, the total extensional strain and the true total volume of magmatic products remain unconstrained. Some data suggest that there may be significant assymetry in the distribution in magmatism during volcanic margins formation. The determination of the composition of the intrusives that are emplaced during margin formation requires precise measurements of seismic velocities and sampling for geochemical analysis of their extrusive counterparts. In addition, whether or not there is significant along-strike variation in magmatic production is unknown.
The Northwest European margins are considered to be the type examples of volcanic margins and are sufficiently well studied to allow highly focused new studies to advance our understanding of magmatism and rifting. For example, the Vøring Margin offshore Norway has an extensive existing grid of seismic and well data to determine optimal experimental design. Complementary 2-D studies on the East Greenland conjugate margin could be carried out in order to fully constrain the volumes of magma produced.
The passive margins of the Mediterranean Sea also offer a unique natural laboratory for the study of rifting processes. Not only are the margins relatively young, and therefore their syn-rift geometry is well preserved, but they have developed in a compressional setting in the region of the diffuse boundary that now separates the Eurasian and African plates. The mechanics of how extension, which eventually leads to continental break-up and the formation of new oceanic crust, can occur contemporaneously with compression in surrounding thrust and fold belts, is one of the most fundamental questions in the Earth sciences that needs to be addressed in the future. A large scale marine seismic acquisition program that involves a mix of refraction and reflection techniques, together with land recording stations, on the passive margins of, for example, the Alboran, Tyrrhenian and Ligurian Seas in the western Mediterranean would help address this question in the future.
A final point concerns the topography that is generated as a consequence of rifting. There is evidence, for example, that mechanical loading and unloading during extension, lateral heat flow and, magmatic underplating can generate rift flank uplifts of a few hundred m over widths of a few hundred km. The EUROMARGINS programme will therefore make a special effort to integrate land- (e.g. fission track, exposure dating) and sea-based (e.g. subsidence and uplift history) data in combined models for the thermal and mechanical evolution of the entire passive margin system.
Northwest Europe and the Mediterranean Sea are not the only margins of interest for rifting process studies. By studying other margins, one or more ancillary problems to be tackled in addition to the principal problems.
The Gulf of Aden offers the opportunity to study not only along margin transition from the volcanic in the western gulf to the non-volcanic margins in the eastern gulf, but also the nature and origin of the strong segmentation reflected in numerous fracture zones offsetting the coast line. At the same time, within a relatively short distance, we can acquire coast to coast seismic transects along well defined flow lines. Both sides of the gulf have extensive geological outcrops that have enable comprehensive onshore mapping, as well as industry data sets that constrain the evolution of rifting.
Several other margins also offer the opportunity for studying segmentation of various types. In the South Atlantic there is a clear but enigmatic segmentation that is observed in the free air gravity anomaly at a scale that can be investigated by a small number of seismic transects across the conjugate non-volcanic margins of both North Angola/Gabon and Brazil. These areas have the special distinction of being sites of intense current hydrocarbon industry activity. Unfortunately, there are still many questions that need to be addressed along these margins concerning the nature and origin of the ocean-continent transition and the spatial distribution of extended continental crust. One problem is that these margins are associated with a large thickness of salt (up to 5 km thick off Angola) which makes it difficult to use seismic reflection techniques to image the top of the rifted basement. We believe, however, that by working closely with industry, the European academic community should be able to address this and related questions in the future.
Off Western Iberia it is possible by acquiring a few seismic transects from unequivocal oceanic crust to the continental slope and shelf to complement data that has already been acquired on the northern two-thirds of this margin and enable a segmentation study of the whole margin to be made. This includes the effects of south to north rift propagation, which crossed one or more transverse geological and tectonic boundaries that today are exposed onshore. The additional acquisition of deep-towed magnetometer profiles will, together with the seismic lines, enable the width and origin of the ocean-continent transition (exhumed upper mantle?) to be investigated.
The Labrador Sea offers the opportunity to study conjugate pairs as well as margin segmentation and the transition from non-volcanic to volcanic margins. Well studied and accessible onshore mapping indicates that some of these transitions may be related to an along-strike change from Archean continental crust in the south to Proterozoic crust in the north. 3 major conjugate transects would provide the basis for more detailed future studies, including scientific drilling. The pre-, syn-, and post-rift sedimentary sequences are all theoretically accessible through drilling. In addition the highly controversial region of transitional crust from continental to oceanic along the non-volcanic part of the margin could be examined and provide a comparison study to the work that has already been carried out along transects of the Iberia/Newfoundland Basin.
- Sedimentary processes and products
Sediments from rivers and other sources reach the passive margin system where they are redistributed by a variety of processes including turbidity currents, mass sliding and along-slope currents. It is essential to study entire systems (the "source to sink" concept) because there is a continuum from sediment sources onshore to coastline evolution, to shelf deposition and to deep-water sedimentation. Fundamental questions concern controls (i.e. tectonics, sea-level fluctuation, earthquake triggering, fluid flow) of large-scale, down-slope, and along-slope sediment transport. The distribution of the various sediment types allows us to identify the processes responsible for their accumulation. Sedimentary processes on passive margins are of economical and societal importance for reasons as diverse as hydrocarbon resources and natural hazards.
The European academic and the oil industry scientific communities have already identified turbidite and slope instabilities at passive margins as high priority targets that need to be addressed in the future. Most hydrocarbon reservoirs occur in sand-rich, turbidite systems for which the best analogues are found in present-day continental slope and base-of-slope settings. Slope instability constitutes a potential hazard for deep-sea hydrocarbon exploitation that could have both strong environmental and economical implications. In order to address these problems, a combination of the techniques that are available within academia and industry will be required. For example, high resolution imaging of the sea-floor (i.e. swath bathymetry and deep-towed side scan sonar) has primarily been developed by academia. Analysis of the deeper structure and the mechanical behaviour of sediments (e.g. 3D seismics and geotechnical boreholes) is a major contribution by the oil industry.
Sedimentary processes need to be studied in the context of the entire margin system, including tectonic influences and fluid flow circulation. An integrated study requires that we adopt a process-oriented approach. Such an approach is essential to take account of the wide variety of factors which are known to influence margin sedimentation. These include sediment input, tectonics, climatic variability, sea-level fluctuations, oceanographic changes and interactions with the biosphere. The factors above operate at time scales from hours to millions of years. Individual sedimentary systems may range in size from basin-wide features to a few tens of metres.
A good understanding of the processes requires a focussing of effort on active areas. We recognise, however, that some comparison between active and less active margin systems is also essential. For example, areas affected by slope failure have to be contrasted with nearby stable areas. It also recognises that some processes, like the glaciogenic ones, have been more active in the past. Whereas more detail can be obtained through the study of the present-day seabed, it is important to go back in time. This gives an understanding of the development of the margin systems through time and allows modern systems to be used as analogues for the interpretation of the subsurface.
The Northwest European margin offers the opportunity to understand the mechanics and kinematics of slope failures. For that it is required to address the possible controlling factors which may include sea-level and climatic fluctuations, isostatic rebound and associated tectonics, variations in sedimentation rates along the margin, and the behaviour of fluids and gas hydrates. We need to document the failure’s history, to understand the geotechnical properties of sediments which are prone to failure, and the architecture of both failed and unfailed segments of the margin. The overall aim is to lead towards a better predictive capability, for use by society and industry.
The Mediterranean Sea offers the opportunity to study small and large turbidite system. Our aim is to determine the dynamic response of large turbidite systems to external forcings. We recognise the main external forcing as the huge sediment input from a large latitudinal range in the African continent. Sediment accumulation in the Nile system, for example, is both strongly influenced by climatic variation and contains a record of that variation. Important topics are the transport and distribution of sands within the general evolution of the system. The processes driving channel variability and internal geometry are of primary interest since substantial amounts of sand are funnelled through them. Causes of slope failure within this environment will also be investigated as well as the underlying triggering factors, such as fluid circulation and salt tectonics.
Other margins, such as offshore Iberia (e.g. Alboran Sea and Gulf of Cadiz) are ideally suited to investigate the variability of small turbidite systems across a relatively short climatic and latitudinal range. They are also ideal sites to examine the entire system, which allows tracing sediment development from catchments onshore to the deep basin. We plan to look primarily at the Quaternary record as a route to understanding the variability of subsurface systems and the rock record. The reasons for wide morpho-sedimentary variability between systems will be examined in the light of variable sediment input, slope gradients, climatic forcing and tectonic setting.
Finally, the Hellenic margin offshore Greece is probably the best place in Europe to examine the interplay between sedimentation and seismicity. Here, there are numerous historic examples of slope failures triggered by earthquakes. Because of the active tectonics in the area high rates of erosion provide sediment which is transferred downslope through shelf-edge spill over, cascading and along canyon transport. High pore pressures and fluid venting also influence slope stability.
The work offshore Greece would provide a link with a developing interest in Europe in the dynamics of subduction and collision processes. In particular, the EUROMARGINS work described here would complement plans that were discussed at a recent EUROPROBE meeting in Thessaloniki concerning an exploratory workshop on "Neotectonics and mantle dynamics in the Aegean and surrounding regions".
The planned work in sedimentary processes and products will require access to existing academic and industry data sets such as swath bathymetry, side-scan sonar images and long (> 20 m) piston cores. It will also require the innovative use of existing technology and the development of new instrumentation. The Northwest Europe margin, for example, will require access to geotechnical measurements (particularly pore pressures) making use of boreholes that have been acquired by the hydrocarbons industry. The Mediterranean Sea will require access to industry standard 3-D seismic data which will be used to expand classical 2-D mapping of turbidite systems into the subsurface and OBS and pore pressure instruments in order to monitor the long term response of sediments to seismic events.
- Sub-sea floor fluid flow systems
A major result of recent studies in active (i.e. subduction related) margin settings has been the discovery of significant sub-seafloor fluid flow. In contrast, there have been relatively few equivalent investigations of the fluid flow systems in passive margins. Fluid flow facilitates and mediates a variety of geochemical processes such as the oxidation and diagenesis of organic material. There is evidence that the volume of fluids re-supplied to the ocean–atmosphere system in passive margins might exceed the total volume of fluids circulated through the global mid-ocean ridge system. Fluids may also be a significant contributor of methane and other compounds which may produce local or regional anomalies in the non–anthropogenic trace components of sea water. In shallow areas, these compounds may enter the atmosphere which, in turn, influence the pattern of global climate. Gas hydrate accumulations in the sedimentary column, considering their potentially wide distributions, represent huge natural concentrations of methane and in addition can act as traps for upward migrating oil and gas. Fluid flow is therefore a first order problem that has important implications for the architecture, evolution, and exploitation of passive margins.
A strategy to identify those areas of most rapid sub-sea floor fluid flow and to sample these fluids already exists in active margin settings where the strong outpouring of pore fluids supports exotic deep water biological communities. Although not as well explored as active margins, there are indications of seeps and their associated biological communities on the passive margins of Northwest Europe and the Mediterranean Sea. Little is known, however, about the fluxes, plumbing geometry and temporal and spatial scales of the fluid migration.
We believe that tectonics and sedimentary processes play an important role in controlling fluid flow in passive margins. For example, the accumulation, migration and preservation of fluids in passive margins are likely to be controlled by the basement architecture produced by rifting, faulting, changes in regional stress patterns and, slope stability. The mechanics of the coupling between these processes and fluid flow is, however, poorly understood. On short time scales, the episodic nature of de-watering events by earthquake activity and faulting has become evident from monitoring studies of seismically active areas. To understand the interplay of basement geometry, faulting, stresses and fluid flow on longer time scales, however, we will need to adopt a 4-D approach (i.e. one that includes spatial and temporal variations) . Such an approach should include numerical modelling, together with the monitoring of the fluid fluxes and, the reconstruction of paleo-fluid flow patterns. The structural controls on overpressures in deforming continental margins also needs to be evaluated, focusing on the structural controls on pathways for fluid flow and the temporal variations in fluid flow as a result of changes in surface gradients induced by tectonic processes.
In order to address these questions, we propose to focus on the margins of Northwest Europe and the Mediterranean Sea, where there is already abundant evidence for fluid flow in the form of seafloor seeps and deep-sea biota. The Northwest Europe margins, the site of simultaneously occurring post-glacial uplift and tectonic reactivation of major margin fault systems, is a key area to address these objectives, using a focus on dynamic topography and slope (in)stability. 3-D seismics will be available to constrain the models. The margins of the Mediterranean Sea, in particular of the Alboran and Tyrrhenian Seas, are intrinsically characterized by strong active tectonism in a regime of overall extension in a convergent plate setting. These basins therefore offer excellent sites to validate fluid flow models by an integration of marine data and field data collected on land.
Gas hydrates in passive margins represent a huge, largely untapped, organic carbon reservoir. Current estimates of the size of this reservoir exceeds the amount of organic carbon in conventional fossil fuels (e.g. coal, oil, natural gas). Gas hydrates are a source of interest from three viewpoints; (1) as a resource; (2) as a geohazard (e.g. gas hydrate dissociation weakens slope stability); and (3) as a factor in current or ancient climatic changes. A challenge for future research is to develop models of formation and dissociation of gas hydrates in passive margins that account for such processes as accumulation of gas hydrates in marine sediments, slope failure related to gas hydrate dissociation, and methane release to the atmosphere during post-glacial warming.
Fluid flow and gas hydrate formation or dissociation may be strongly linked phenomena. Gas hydrate dissociation releases fluids (water and gases) previously trapped in the solid hydrate. The released fluids may flow through the sediments overlying gas hydrates and produce intense fluid seepage through the seafloor. This may occur in response to a change in state of the gas hydrate stability zone, as is expected to follow events of sea bottom warming or sea level fall. Conversely, gas hydrates can accumulate in vast amounts in the gas hydrate stability zone of margins, in response to upward migration of deep fluids along faults. Thus, gas hydrate systems are not steady-state but evolve with time and this evolution strongly interacts with the pattern of fluid flow.
In order to quantify the amount and properties of gas hydrates that have accumulated in continental margins sediments, we propose to observe and model gas hydrate systems at selected sites in passive margin systems. The focus will be on areas where existing data already indicate fast evolving gas hydrate systems with strong interactions to the pattern of fluid flow. Specific objectives will be to determine heat and fluid fluxes, heat and chemical budgets and, the geotechnical and fluid transport properties of the sediments that host gas hydrates. Geometries and properties of sedimentary models will be constructed from 3-D seismic data. Deep-tow and high resolution sea-bed geophysics, submersibles for sea floor observations and shallow drilling will be required to make repeated measurements of parameters such as the seepage rate through the seafloor. In addition, numerical and analogue modelling and analytical facilities (e.g. isotope hydrology) will be required in order to fully understand the spatial and temporal variability of fluid flow.
- Programme implementation
Although the EUROMARGINS programme is focussed on 3 main scientific themes, we do not view them as independent topics of research. On the contrary, there are strong scientific linkages between the themes (e.g. fluid flow, rifting processes and, large-scale slope failures) which will require a multi-disciplinary, international approach in order to address them. In particular, they will require the European-wide academic community working in partnership with the hydrocarbon and other industries (e.g. telecommunications, insurance) and international efforts such as Inter-MARGINS and the new Integrated Ocean Drilling Programme (IODP, post 2003).
We recognise that EUROMARGINS is an ambitious programme that poses new scientific, technological and funding challenges for the European scientific community. These challenges are difficult, if not impossible, to address at the national level. However, together we believe that they can be addressed.
In terms of implementation, for example, we propose that the programme is carried out in two main phases. Phase I will focus on two "target" areas which are of immediate priority for Europe - one on the Northwest European margin and the other in the Mediterranean Sea. Here, generic problems related to the interplay between magmatism and the regional extensional and compressional stress field will be addressed. Phase I will include the first 3-D seismic tomography and imaging of the crust and upper mantle in the transitional region between the continent and ocean. In addition, sonar studies of the mechanics and kinematics of slope instabilities and the dynamics and variability of turbidite systems and chemical, biological and physical studies of fluid flow, gas hydrates and deep-water seeps and biota will be carried out. A key element of Phase I will beintegrated 4-D numerical and analytical modelling of the coupling between tectonic (e.g. rifting) and surface processes. Phase II will extend the work off Europe to other margin systems. This will lead to comparative studies and the development of new generic models for passive margin evolution. Particular target areas, which were discussed at the Sitges workshop, include conjugate margins pairs in the North Atlantic (e.g. Labrador, Greenland and W.Iberia), the young margins of the Gulf of Aden and, the highly segmented margins of the South Atlantic.
The new ESF initiative, EUROCORES, provides a mechanism by which a new European-led programme in passive margins might be supported in the future. In particular, we are willing to work jointly with ESF to mobilise national funding as well as seek funding from other sources such as the European Union and industry in order to support a new EUROMARGINS programme.
4. European added value
The nations of Europe share one of the longest passive margin systems in the world, yet no one country has access to all the large-scale facilities (e.g. ships, submersibles, deep-tow devices, seismic and sonar) that will be needed in the future to study these features. Furthermore, there is a large skill-base of scientists in Europe in passive margin research, yet collaborative research to date has been limited to one or two countries and to partners in the USA and Canada. A new programme that involves the sharing of facilities, the training of a new generation of young process-oriented geoscientists and, engages the skill-base within Europe is an exciting "value added" prospect which has far-reaching implications, not only for passive margin research but, also for important societal issues.
5. Participating countries
The Network in Ocean Margins is currently made up of scientists from the following nation states of Europe :
Belgium
Denmark
France
Germany
Italy
Norway
Portugal
Spain
The Netherlands
UK
In addition, it emerged from the ESF workshop in Sitges that scientists from Ireland, Israel, and Greece also have a strong interest in a EUROMARGINS programme. In particular, scientists from Ireland have formally expressed an interest in collaborating with EUROMARGINS and we anticipate that other countries may do so in the future.
6. Existing Programmes
During the past few decades, there has been considerable progress made in our understanding of the deep structure and evolution of volcanic and non-volcanic passive margins. Scientists in Europe have been at the forefront of this activity. In particular, European scientists constructed some of the first images of the shallow and deep structure of volcanic and non-volcanic passive continental margins and some of the first models for their thermal and mechanical evolution. The increased societal interest in passive margins, particularly by the hydrocarbons industry, has led some countries to build on this expertise base and organise their own national programmes in Ocean Margins. In contrast to the USA, which has focussed mainly on active margins, scientists in Europe have had passive margins as their principal focus.
The main funded programmes in Europe are GDR MARGES (France) and NERC/LINK Ocean Margins (UK). Both programmes have been developed in close collaboration with the hydrocarbons industry. MARGES has an annual budget of about FFR 1.2 m (excluding salaries) and has as its main focus the structure, thermal history, organic matter and fluids in passive margins. Funding for specific projects is not included in the budget and is decided each year by the executive committee. NERC/LINK Ocean Margins will focus on rifting processes, sedimentary processes and products, and fluid flow. NERC and the Department of Trade and Industry will contribute up to £ 4.5 m to the programme over 4 years. The research council and government department will provide up to 50% of eligible costs, with the remaining balance to be funded by industry.
Other funded programmes exist in The Netherlands and Belgium. Government sources in these countries have allocated funds to scientists for research in specific aspects of passive margin research. In addition, Germany, Denmark, Spain and Norway are currently supporting activities in passive margin research at various levels. Some of these countries are contemplating the development of national programmes. Norway, for example, has already appointed an interim committee charged with funding a national infrastructure in Ocean Margins. Furthermore, Germany is currently hosting the Inter-MARGINS office that has been set up to promote research in Ocean Margins globally.
These considerations suggest that national efforts in Ocean Margins vary considerably among the countries of Europe. The two largest programmes in France and the UK have both been developed in close collaboration with industry. While both programmes are global in their focus, it is inevitable that a significant component of such programmes will be carried out according to national rather than European-wide interests.
We believe that a EUROMARGINS programme, which engages the entire skill base within Europe, is an exciting prospect that will lead not only to new generic models for the evolution of passive margins but, also a better understanding of their management, exploitation and significance to society. While a number of scientists in Europe already have considerable experience in both the complexities of acquiring seismic, sonar and sampling data at sea and in the development of new predictive analytical and numerical models for passive margin evolution, the collective experience in these activities varies widely between countries. Some countries have access to "state of the art" seismic data source, recording and processing facilities. Others do not. Some have access to "state of the art" submersibles, sonar imaging and deep-towed devices. Others do not. Therefore, no one country is able to mount the kinds of experiments that will need to be carried out in the future if we are to better understand the passive margin system and the linkages that exist between rifting, sedimentary and fluid flow processes.