Geophysical studies of the continental margin northeast of Newfoundland

Abstract

A seismic and magnetic survey has been made of an area that straddles the continental margin northeast of Newfoundland from the edge of the shelf to the ocean basin of the Labrador Sea. The results bear on the question of the extension of the mobile belt of the Appalachian System in Newfoundland to the margin. Our seismic studies show that the crust is approximately 30 km thick beneath the edge of the shelf northeast of Newfoundland, and thins to approximately 12 km at the foot of the slope. Seismic studies by Lamont Geological Observatory suggest that the mobile belt continues to the upper part of the slope; our studies support this. Large magnetic anomalies run nearly parallel to the edge of the shelf over the slope and rise, at right angles to the strike of the Appalachian System. Their amplitude is 1400 gammas and the belt of anomalies is 180 km wide. It runs continuously for at least 220 km. One interpretation is that the anomalies owe their origin to the juxtaposition of a magnetic continental plate 30 km thick against a thinner magnetic oceanic plate 10 to 12 km thick. We suggest that the continental crust is particularly magnetic, and the 'shelf-edge' anomalies particularly large, because it represents the basic mobile belt of the Appalachian System terminating beneath the lower part of the continental slope. The contrast in magnetization between non-magnetic oceanic mantle and magnetic continental crust is due to this, and perhaps also to higher temperatures in the oceanic mantle. They could be particularly high if the Labrador Sea is the site of a mid-ocean ridge. Some evidence from the magnetic survey suggests that a fault with dextral slip runs on the continent side of the margin in a northeasterly direction. It does not continue into the ocean basin.