Tertiary foreland sedimentation in the Southern Subalpine Chains, SE France: a geodynamic appraisal
Tertiary foreland sedimentation in SE France occurred along the western sidewall of the Alpine orogen during collision of the Apulian indentor with the European passive margin. A detailed reappraisal of the stratigraphy and structure of the Southern Subalpine Chains (SSC) in SE France shows that Tertiary depocentres of differing character developed progressively toward the foreland during ongoing SW-directed shortening. The geodynamic controls on each of four stages of basin development are evaluated using a flexural isostatic modelling package of thrust sheet emplacement and foreland basin formation. (1) The initial stage (mid to late Eocene) can be explained as a flexural basin that migrated toward the NW, closing off to the SW against the uplifting Maures–Esterel block. This broad, shallow basin can be reproduced in forward modelling by loading a lower lithospheric plate with an effective elastic thickness of 20 km. (2) The end of detectable flexural subsidence in the early Oligocene coincides with the emplacement of the internally derived Embrunais–Ubaye (E-U) nappes, which caused 11 km of SW-directed shortening in the underlying SSC. The lack of Oligocene flexural subsidence dictates that the E-U units were emplaced as gravitational nappes. Within the SSC, Oligocene sedimentation was restricted to small thrust-sheet-top basins recording mainly continental conditions and ongoing folding. Further west, Oligocene to Aquitanian NNW–SSE extension generated the Manosque half-graben as part of the European graben system that affected an area from the Gulf of Lion to the Rhine graben. (3) Following the Burdigalian breakup of the Gulf of Lion rift, a marine transgression migrated northward along the European graben system. Subsequent thermal subsidence allowed 1 km of marine sediments to be deposited across the Valensole and Manosque blocks, west of the active SSC thrust belt. (4) Mio-Pliocene conglomeratic deposits (2 km thick) were trapped within the Valensole basin by the uplifting Vaucluse block to the west and the advancing Alpine thrust sheets to the east. Late Pliocene thrusting of the SSC across the Valensole basin (approx. 10.5 km) can be linked along a Triassic detachment to the hinterland uplift of the Argentera basement massif.