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Models of Extensional Basins
Discrepancy Between Lower and Upper Crustal Thinning
The discrepancy between the amount of horizontal extension and subsidence under several continental basins has been noticed for a long time. Recent deep seismic surveys in the U.S.A. and in Europe point out a similar discrepancy between superficial extensional tectonics and crustal thinning. The lower and upper parts of the crust seem to have a different strain history. In this paper, we show that large-scale material displacements in a ductile lower crust can explain many geological features. It allows, for instance, the genesis of an intracontinental basin without superficial extensional features. Even without any noticeable regional stresses, deep processes may induce localized crustal thinning and thus subsidence at the surface. We assume that the only forces in the crust are related to flow in the mantle. Strains are computed using non-Newtonian mechanical behaviour for both the mantle and lower crust. Mantle flow is initiated by thermal perturbation. The results also suggest that a slow crustal readjustment may occur under some basins after the thinning phase. The observed lower crust may represent only a transient state. Data concerning the Bay of Biscay and marginal areas of the Red Sea are compared with results.
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