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Air gun devices and explosive charges (dynamite) were utilized as energy sources and expendable sonobuoys as detectors to shoot five refraction lines in the southern Labrador Sea off the Southwest Greenland margin, in a direction roughly parallel to the continental shelf edge. The most western of these lines was shot in a region of well developed oceanic magnetic anomaly pattern and thus over true oceanic crust. In the region of the four eastern refraction lines, no correlatable oceanic magnetic anomalies have been identified and the seismic refraction studies revealed the presence of thin crust (less than 5.0 Km thick), of dubious nature. The most eastern of the lines lies at a distance of approximately 43 Km from the continental shelf break and reveals a structure proposed here to represent continental crust.

This study is mainly concerned with the elaboration of the seismic data and the use of the seismic refraction results along with other pieces of geophysical information (gravity, seismic reflection and magnetic observations) as additional constraints, to reveal the crustal structure of this continental margin. A satisfactory model of the crustal structure across the refraction lines was developed, in a direction almost perpendicular to the margin.

Among the characteristic features of the proposed model for the Southwest Greenland margin are a zone of ascended upper mantle material and thin crust, coinciding with a zone of high seismic wave velocities, and the presence of an acoustic basement high, a marginal sedimentary basin and a marginal magnetic quiet zone. The ocean-continent boundary proved to be difficult to locate precisely. The average crustal thickness (about 6.2 Km), and the mean pressure at the bottom of the lithosphere are very close to those resulting from the standard crustal structure, while the water depths are considerably shallower ( 3.0 Km). Mass balance calculations suggest that the model is very close to isostatic equilibrium if simple Airy-type compensation is assumed. A comparison of the proposed model to other relevant structural models from the Labrador side confirms previously conceived ideas about the asymmetry of spreading in Southern Labrador Sea region.

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Supervisor:聽 Patrick J. C. Ryall