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French Western Alps |
Geologisches Institut, ETH-Zentrum, 8092 Zürich, Switzerland
The Tertiary foreland basin of the southern Subalpine chains preserves a stratigraphic record of late Alpine deformation, both ahead of the thrust front in the Valensole basin, and in a series of thrust-sheet-top basin remnants. Stratigraphy and growth structures in these basin remnants have been used to identify the location and timing of deformation and hence to constrain the sequential restoration of a cross section through the region.
The thrust belt developed as a single large thrust sheet riding on a weak Triassic evaporite layer. Minor breaching thrusts occur, in particular at Mesozoic normal faults. Kinematic studies of the Digne thrust sheet show a dominantly SW direction of tectonic transport. Where reactivated structures lie at a slightly oblique angle to the transport direction there has been a partitioning of deformation into SW-directed thrusting and a component of dextral strike-slip taken up on minor faults. The total shortening across the fold and thrust belt is 21.5 km, a much lower value than previously estimated but more in line with regional tectonics.
The foreland basin stratigraphy demonstrates that deformation within the Digne sheet occurred in three stages. Firstly, during the late Eocene, Alpine collision in the hinterland caused flexure of the foreland plate to generate a simple broad marine foredeep. Slight detachment above the Triassic evaporites allowed gentle buckling of the floor of the foredeep. Secondly, in the Early to Mid-Oligocene overthrust shear associated with the SW emplacement of the internally derived Embrunais-Ubaye nappes into the foredeep caused substantial deformation in the underlying Digne sheet. A component of flattening strain was associated with overthrust shear beneath the nappes, but ahead of the nappe front gentle, upright folding and thrusting continued. Sedimentation stepped westward, became dominated by continental facies and confined to small thrust sheet top basins (e.g. Barrême). The Mio-Pliocene stage of deformation involved a change from thin-skinned to a thick-skinned out-of-sequence deformation when basement blocks were uplifted on deepseated structures across the region (e.g. dôme de Chateauredon). Shortening associated with the uplift of the main Argentera basement massif at the internal boundary of the external fold and thrust belt was transferred onto the Triassic detachment from below, and transferred forward to reactivate the frontal edge of the Digne sheet, which was thrust out over the Valensole basin.
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