HR: 11:05h
AN: T12C-04    [Abstracts]
TI: Location of the Foot of the Continental Slope Based on "Evidence to the Contrary" - Geophysical Techniques
AU: * Edwards, R A
EM: rae@noc.soton.ac.uk
AF: National Oceanography Centre, Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH United Kingdom
AU: Parson, L M
EM: lmp@noc.soton.ac.uk
AF: National Oceanography Centre, Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH United Kingdom
AB: 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.
DE: 3025 Marine seismics (0935, 7294)
DE: 8105 Continental margins: divergent (1212, 8124)
SC: Tectonophysics [T]
MN: Fall Meeting 2005