Abstract

There is now reasonable agreement on the fact that formation of continental margins is accompanied by thinning of the continental crust as well as the mantle part of the lithosphere. Whether or not the amount of thinning is the same in both portions has been debated. However, an equal amount of thinning is a good first approximation. We use this approximation to explore the thermal and mechanical consequences of the initial rifting phase with a kinematic thermal model.

We show on one hand that lateral conduction is not very significant when computing surface heat flow and subsidence values, but that it becomes highly significant in narrow rifting zones to obtain the evolution of the structure at depth. Actually, partial fusion can be prevented by lateral cooling in rifts which are too narrow, thus leading to an abortion of the rifting. On the other hand, we show that high sedimentation rates play a significant role in obtaining heat flow values at the surface and at the base of the sedimentary piles. However, high sedimentation rates do not affect the thermal evolution at depth.

Using this kinematic thermal mode, we explore some mechanical aspects of rifting. First, following England, we estimate the variation in resistance to stretching of the whole lithosphere as rifting proceeds. However, at the difference of England, we introduce a brittle as well as a ductile zone. Our results show that there is no increase in resistance to stretching as rifting proceeds. We then compute the evolution with time of the brittle ductile zone. This limit migrates upward in the thinnest portion of the margin. Thus, it is not safe to use estimates of final brittle crust thickness to obtain a relative amount of thinning. Finally, we compute the body forces which introduce a significant “rift push force” and in addition may result in decollement of the tilted blocks in the lower part of the margin. In view of these theoretical results, we present a model of evolution of the Bay of Biscay margin.

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