Theoretical investigation of the ocean-coast effect at a passive continental margin

Abstract

The idea that oceanic lithosphere is thinner than continental lithosphere is widely accepted even though one would like to see clearer evidence to support it. In fact, the very concept of lithosphere is still a matter of some debate. If there is indeed a variation in the thickness of the lithosphere at continental margins and if this change is associated with a lateral variation in electrical conductivity one may envisage detecting it with electromagnetic soundings methods. A model of a passive continental margin has therefore been investigated to test whether this would be feasible. It has been found that the well-known but strong ocean-coast effect masks the minor lithospheric effect in magnetotelluric soundings performed on the shore. Inductive soundings, on the other hand, are highly sensitive to lateral variations in electrical conductivity. An analysis in terms of the induction arrow has shown that such soundings carried out on land would be perfectly suitable to reveal a changing lithospheric thickness, if the continents merely extended to the oceanic coast. However, the presence of only a narrow continental shelf of 100 km width under 250 m of sea-water produces an overriding coast effect ahead of the margin, and thus renders electromagnetic methods unsuited to reveal a changing lithospheric thickness.