Migration of a synclinal depocentre from turbidite growth strata: the Annot syncline, SE France
Résumé
The Annot Sandstone turbidites of the Alpine foreland basin in SE France (Eocene-Oligocene: 40-32 Ma), provide an excellent case-study of tectono-sedimentary relations in a deepwater compressional system. The Annot outlier is a synclinal remnant previously interpreted as a static depocentre. A multi-disciplinary approach including geometrical and kinematic analyses and modelling demonstrates instead that this was a tectonically active turbidite depocentre where gentle thrust related folding controlled turbidite architecture. Stratigraphic and new structural field data are integrated with previous sedimentological studies to build a 3D geometric model of the Annot depocentre. Derived thickness maps associated with paleocurrent measurements clearly illustrate three main phases in the evolution of depocentre topography. (1) Early turbidite flows were mainly trapped by oblique intrabasinal inherited structures. (2) Once these structures were buried, the NNW-SSE active syncline constituted the main topographic control. (3) Decreasing activity of this syncline is recorded by filling and flow bypass. The progressive stages of the accepted depositional model (flow ponding, flow stripping and flow bypass), for the Annot depocentre, may therefore have a tectonic origin. The kinematic evolution of the synclinal depocentre was defined at different scales. Stratigraphic architecture records a decrease in bedding dips up through the turbidite succession on the western synclinal limb. Comparison with idealized case studies of the interaction of sedimentation with an active syncline indicates that this geometrical pattern corresponds to progressive westward migration of the synclinal hinge and depocentre. This tends to promote lateral rather than vertical stacking of sand bodies during turbidite sedimentation. Trishear kinematic modelling was used to simulate (in 2D) the rolling synclinal hinge. Stratigraphic surface geometries and turbidite depocentre migration define thrust and fold geometries at depth. The synclinal depocentre developed between two alternating or coeval fault propagation anticlines that exploited two detachment levels (Triassic evaporaites and Cenomanian marls) in the underlying succession.