T12C-01
Continental Margins and the Law of the Sea - an 'Arranged Marriage' with Huge Research Potential
The United Nations Convention on the Law of the Sea (UNCLOS) requires coastal states intending to secure sovereignty over continental shelf territory extending beyond 200 nautical miles to submit geological/geophysical data, along with their analysis and synthesis of the relevant continental margin in support of their claim. These submissions are scrutinised and assessed by a UN Commission of experts who decide if the claim is justified, and thereby ultimately allowing the exploitation of non-living resources into this extended maritime space. The amount of data required to support the case will vary from margin to margin, depending on the local geological evolution, but typically will involve the running of new, dedicated marine surveys, mostly bathymetric and seismic. Key geological/geophysical issues revolve around proof of 'naturalness' of the prolongation of land mass (cue - wide-angle seismics, deep drilling and sampling programmes) and shelf and slope morphology and sediment section thickness (cue - swath bathymetry and multichannel seismics programmes). These surveys, probably primarily funded by government agencies anxious not to lose out on the 'land grab', will generate datasets which will inevitably boost not only the research effort leading to increased understanding of margin evolution in academic terms, but also contribute to wider applied aspects of the work such as those leading to refinement of deepwater hydrocarbon resource potential. It is conservatively estimated that in the region of fifty coastal states world-wide have a significant potential for claiming continental shelf beyond 200 nautical miles, and that the total area available as extended shelf could easily exceed 7 million square kilometres. However, while for the vast majority of these states a UNCLOS deadline of 2009 exists for submitting a claim - to date only four have done so (Russia, Brazil, Australia and Ireland). It is therefore predictable, if not inevitable, that within the next four years an unprecedented phase of surveying and analysis on margins will take place in order to prepare for the deadline. The international scientific community as a whole must recognise the potential for research in this work and ensure the data is made available as soon as practically possible for the scientific community. In conclusion, by way of a reality check, this presentation highlights the likely areas of most intense UNCLOS-driven research activity up to 2009, the type of data acquisition anticipated and their likely location, and speculates on the areas of understanding of margin evolution which will be most advanced by this process.
T12C-02
Continent Ocean Boundaries in the North Atlantic: An Overview
The concept of a distinct continent-ocean boundary has been intensely challenged as a wealth of geological and geophysical data has been collected along passive margins in the last decade. Increasing new evidence points out that a sharp transition between continental and oceanic crust is rather an exception than a rule, and a continental-oceanic transition zone is proposed instead. However, the analysis of multidisciplinary data sets does not always lead to the same answer. Different interpretations of the same datasets led to competing hypotheses for the opening and tectonic evolution of the North Atlantic and Labrador Sea/Baffin Bay oceanic basins. The location of the ocean-continent transition zone is of particular relevance to UNCLOS article 76 on the legal continental shelf because it may be used as evidence to the contrary in positioning the foot of the continental slope, and as such enlarge a legal shelf claim. We re-examine the interpretation of continent-ocean boundaries or continent-ocean transition zones using up-to-date geophysical data and integrate it with a quantitative model of seafloor spreading in the North Atlantic. The opening of oceanic basins in the Arctic regions is also merged with the new kinematic model and various scenarios are examined with respect to their implications on the North Atlantic and Arctic continental margins. This overview of the tectonic evolution of the North Atlantic and Arctic is used to highlight regional problems and suggest data collection in key areas for better understanding of continental margin formation. It may well be that UNCLOS article 76 provides an opportunity to collect such data.
T12C-03
Mapping the Ocean-Continent Transition at Rifted Margins using Satellite Gravity Inversion Incorporating a Lithosphere Thermal Correction
Satellite gravity data and digital bathymetry provide powerful data sets for investigating the transition from continental to oceanic crust at rifted continental margins. The inversion of satellite gravity data at rifted continental margins to map crustal thickness variation and predict the location of the ocean-continent transition (OCT) requires the incorporation of a lithosphere thermal gravity anomaly correction for both oceanic and continental lithosphere. Oceanic lithosphere and stretched continental margin lithosphere produce a large negative residual thermal gravity anomaly (up to -380 mgal), for which a correction must be made in order to determine Moho depth. The gravity inversion using the thermal gravity correction predicts oceanic crustal thicknesses consistent with seismic observations, while that without the thermal correction predicts much too great oceanic crustal thicknesses. The lithosphere thermal model used to predict the gravity anomaly correction may be conditioned using magnetic isochron data to provide the age of oceanic lithosphere. The resulting crustal thickness determination is however sensitive to errors in the magnetic isochron data. A new method of inverting satellite gravity at rifted continental margins to give crustal thickness, incorporating a lithosphere thermal correction, has been developed which does not use a priori information of the location of the OCT using magnetic isochron data and provides an independent prediction of OCT location. Where oceanic isochrons are believed to be reliable the lithosphere thermal gravity correction may be determined for oceanic lithosphere from ocean lithosphere age, and for the thinned continental margin lithosphere using margin rift age and beta stretching estimates iteratively derived from crustal basement thickness determined from the gravity inversion. Where ocean isochrons are unknown or unreliable, all lithosphere is assumed to be initially continental and a uniform lithosphere stretching age is used corresponding to the time of continental breakup. The thinning factor produced by the gravity inversion is used to predict the thickness of oceanic crust using an empirical relationship to predict the thickness of oceanic crust from lithosphere thinning factor parameterised by a critical thinning factor for the start of ocean crust production and maximum oceanic crustal thickness. This new modified form of gravity inversion with embedded thermal correction provides an improved estimate of rifted continental margin crustal thinning and an improved (and isochron independent) prediction of OCT location. The new gravity inversion method of OCT mapping has been successfully applied to several case histories including the northern N. Atlantic. Predicted Moho depth and crustal thinning, using the gravity inversion with embedded thermal correction, compare well with those produced by wide-angle seismology. This work forms part of the NERC Margins iSIMM project.
T12C-04
Location of the Foot of the Continental Slope Based on "Evidence to the Contrary" - Geophysical Techniques
According to Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS) the outer limit of a state's continental shelf can normally be defined on morphological, or sediment thickness considerations. However, there are regions where this may not be possible, or where submarine prolongation of a landmass may be thought to extend to deep water areas some distance beyond the edge of the shallow shelf region. In these cases, where a morphological solution cannot be resolved, the location of the foot of the continental slope can be determined by means of "evidence to the contrary", a term which has been most recently interpreted as being based on crustal type. While the continuation of continental crust (typically 35-45 km thick) under shallow water shelf areas is clear, the determination of crustal type in the deeper water areas of continental margins is less unequivocal. The transition from continental to oceanic crust is found at the outer limits of a continental margin, often in water depths of thousands of metres, although the exact location may be difficult to pinpoint. The deep water areas of a margin can be underlain by thinned and subsided continental crust as a result of stretching at continental break-Up, or by exhumed mantle material. Despite the difficulties in identification, distinguishing between crustal types appears to be a key factor in establishing limits to an extended continental shelf. Recent research on non-volcanic rifted margins, such as the Iberia margin, has repeatedly shown the highly complex nature of these regions. Thinned continental crust and oceanic crust may be the same thickness; but the relationship of velocity against depth is typically very different for the two crustal types. Measurement of the seismic velocity of the crustal rocks, which gives an indication of lithology, is best determined using wide-angle seismic data, recorded to offsets of tens of kilometres using ocean bottom seismographs (OBS). Deep seismic reflection data, although providing an image of crustal reflectivity, do not provide accurate velocity information for the deeper layers, and hence cannot be used alone for determining crustal type. Other geological characteristics of continental margins, such as deformation fronts, toe-thrust complexes, and oceanward limits to salt diapirism could perhaps be considered as useful foci for evidence to the contrary studies. More precise mapping of such features may prove more accurate and more cost effective than the imprecision of resolving the location of the ocean continent-ocean transition.
T12C-05
The Lomonosov, Alpha, and Mendeleev Ridges: Tectonic Scenarios in the Arctic Ocean and the Test of Appurtenance in UNCLOS Article 76
The land masses surrounding the Arctic Ocean feature several submerged prolongations into the deep central basin. These range in size from enormous i.e. the broad continental shelves off northern Russia and Scandinavia, to small i.e. the Yermak Plateau and the Morris Jesup Rise which flank Fram Strait. Lying between these extremes on the size scale are the Chukchi Borderlands and the Lomonosov, Alpha, and Mendeleev Ridges. At either end of the scale, the prolongations appear to satisfy the Test of Appurtenance in UNCLOS Article 76, where they qualify as components of outer continental shelves. In the middle of the scale, it is generally agreed that the Chukchi Borderlands satisfy the Test of Appurtenance, but the Lomonosov, Alpha, and Mendeleev Ridges are perceived by some as problematic. While the continental composition of the Lomonosov Ridge is generally acknowledged, the structural and tectonic relationships between its extremities and the adjacent continental margins are poorly understood. Meanwhile, the origin and history of the Alpha and Mendeleev Ridges remain shrouded because of the difficulty of undertaking systematic mapping and sampling that would reveal their ages and geological makeups, and shed light on their marginal junctions. There can be little doubt that the nature and emplacement of the Lomonosov, Alpha, and Mendeleev Ridges are related to the tectonic processes that accompanied the creation of the Arctic Ocean. An understanding of those processes would help establish a geological and historic framework for interpreting the sparse knowledge that is available for the three Ridges. This presentation will consider tectonic scenarios that could account for early stages in the development of the Lomonosov, Alpha, and Mendeleev Ridges, and in the process seek to dispel uncertainties concerning their linkages to the adjacent continental margins. Until further research leads to broad general agreement concerning the answers to such questions, it would seem premature to apply the Test of Appurtenance to these features.
T12C-06
Geophysical and Geological Study of the Transition Zone Between the Mendeleev Rise and the Adjacent Siberian Shelf: Preliminary Results
In 2005, integrated geological and geophysical investigations were carried out in the transition zone between the Mendeleev Rise and the adjacent shelf of the East Siberian Sea. The operations had two objectives: to understand the geological structures of these features; and to determine the outer limits of the Russian continental shelf in terms of UNCLOS Article 76. Observations included: deep seismic soundings, seismic refraction and seismic reflection observations; on-ice gravity measurements and geological sampling; and aeromagnetic and aerogravity mapping of an area covering approximately 140,300 km2 at a trackline spacing of about 10 km. The length of the main sublongitudinal seismic profile was 600 km, extending continuously along the Mendeleev Rise and across its transition to the Siberian Shelf. Preliminary interpretation of the data indicates that the thickness of the upper crust beneath Mendeleev Rise is in the order of 10-12 km. The thickness of sedimentary cover along the seismic line varies from 12 km in the south (North-Chukchi Flexure) to 3 km in the north (Mendeleev Rise). Potential field data suggest a block structure for the study area, whereas the results of bottom sampling at sites where the seabed was deeply incised and where bedrock was more likely to be exposed point generally to the possibility of local derivation of coarse bottom debris. The geological and geophysical evidence obtained during the Arctic-2005 expedition demonstrates a morphological and structural continuity between the Mendeleev Rise and the adjoining Siberian shelf.
T12C-07
Tectonic evolution of the Resolution Ridge System, New Zealand: insights gained through UNCLOS surveying for natural prolongation
For coastal States, demonstration of submerged natural prolongation of the land mass is a key element in delimiting the extent of the continental margin under the terms of UNCLOS article 76. Straddling an active plate boundary and with continental margins encompassing most major tectonic settings, the New Zealand (NZ) continent presents numerous, varied examples of natural prolongation of the land mass. The mostly submerged NZ continent covers over 5,000,000 km2. The continent grew by the accretion of basement terranes and the Hikurangi Plateau, a large igneous province, along the eastern margin of Gondwana during the Paleozoic and Mesozoic. Fragmentation of Gondwana initially involved thinning and extension of the continental rocks of New Zealand, and ultimately resulted in the separation of the New Zealand continent from Australia and Antarctica. Renewed tectonic activity in the Cenozoic resulted in the formation of the Resolution Ridge System (RRS) southwest of NZ and several volcanic arcs north of NZ. These volcanic arcs extend onto NZ and are a submerged natural prolongation of the land mass. Geological and geophysical surveys undertaken for the NZ Continental Shelf Project established that most of the RRS was not a prolongation of the NZ land mass, and advanced understanding of NZ's tectonic evolution. The RRS is a series of bathymetric highs extending southwest of Fiordland, NZ, from Resolution Ridge itself, adjacent to the northern limit of the Puysegur Trench, to the southeast termination of the fossil spreading centre in the Tasman Sea (TS; 158°40'E, 48°10'S). A 40° bend at 162°E, 46°30'S divides the ridge system into a northeastern segment, comprising large, en echelon, northeast-southwest-trending basement ridges and basins, and a southwest segment composed of longer, more continuous ridges trending northeast-southwest. The ridge system was formed by rapid reorientation of seafloor spreading directions (through c. 90°) in the TS at ~50 Ma. The younger oceanic rift initiated along the Campbell Plateau margin and propagated northward into continental NZ. Seafloor spreading in this orientation was maintained between about 47 and 30 Ma (anomalies 21-11) to form the Southeast Tasman Oceanic Crust (STOC). Integration of swath bathymetry, seismic reflection, potential field, and dredge data confirms the origin of the ridge system as an uplifted rift flank formed in oceanic crust in the southwest, and establishes the probable extent of fragments of continental crust in the northeast. The initial rift exploited existing faults and TS crustal structure, and the inherited nature of these faults along the ridge system may provide a rationale for its rapid formation. A deep basement trough lies between the uplifted rift flank footwall and the younger STOC oceanic basement. This trough follows the trend of the RRS, paralleling the basement ridges of the southwest RRS and truncating the southern extent of the en echelon ridges of the northeast RRS. This implies rapid faulting across pre-existing northeast-southwest structural grain and an apparent delay in onset of magmatic systems associated with newly established sea floor spreading. There is no evidence of significant structural reactivation post-dating formation of the RRS. More precise identification of TS transform structures along the ridge system provides pinning points that may help better constrain plate reconstructions and the processes of rift propagation during STOC sea floor spreading.
T12C-08
The Bay of Bengal and the Statement of Understanding Concerning the Establishment of the Outer Edge of the Continental Margin: Regional Implications for Delimiting the Juridical Continental Shelf
The Bay of Bengal is the site of massive depositions of sediment from the Ganga-Brahmaputra river systems, which discharge an estimated 2300 million tons of material into the Indian Ocean every year. The accumulated material comprises an enormous fan that extends some 4000 km from the Mouths of the Ganges, a delta system which encompasses the entire coast of Bangladesh and a segment of the coast of India. The major tectonic elements of the Bay of Bengal and surrounding areas are: the passive eastern continental margin of India; the 85E Ridge; the Ninetyeast Ridge; the intervening basin buried beneath deep sediment; and the Sunda Arc system with the associated back-arc Andaman Basin. Except for the Nikitin Seamounts which rise above the seabed just south of the Equator, the 85E Ridge is totally covered by thick sediment. The Ninetyeast Ridge, on the other hand, protrudes above the seabed as far north as 10N, where it plunges beneath the thickening sediment and separates the deposits into the Bengal Fan and the smaller Nicobar Fan. The 85E and Ninetyeast Ridges present the most significant relief in the crystalline basement underlying the Bay of Bengal, and should therefore figure substantially in any analysis of sediment thickness pursuant to the delimitation of the outer continental shelf. In this region, the sediment thickness provision of Article 76 has been modified by a Statement of Understanding in Annex II of the Final Act of the Third UN Conference on the Law of the Sea. To avoid a perceived inequity that might arise from the application of the standard one percent sediment thickness formula of Article 76, the Statement introduced a new formula: a qualified State in this region, even if it has a narrow physiographic continental shelf, may establish the outer edge of its continental margin by a line where the thickness of sedimentary rock is not less than one km. This presentation will describe the development of a joint formula line for the States that border the Bay of Bengal, taking into account the provisions of the Statement of Understanding. The input data for this analysis consisted of a public grid of sediment thickness, created from the archives of the US National Geophysical Data Center. The outcome of this investigation indicates that the entire Bay of Bengal can be enclosed by the modified formula line. Following application of the joint 350 nautical mile constraint line for all coastal States in the region, it would appear moreover that the entire floor of the Bay of Bengal comprises a juridical continental shelf.