Contrôles structuraux en 3 dimensions de la sédimentation turbiditique dans les chaînes plissées : exemple des Grès d'Annot (SE de la France)

Abstract : The Annot Sandstone, exposed in the southern sub-alpine chains, consists of sand-rich turbidites deposited in the Tertiary Alpine foreland basin (Upper Eocene Lower Oligocene Annot Sandstone basin). While the sedimentology and stratigraphy of this turbidite system have been largely described and discussed, its structural framework remains unconstrained. This thesis presents a study of the tectonics of the Annot Sandstone basin based on field surveys and 3D geometrical (gOcad) and kinematic modelling (Trishear), focusing in particular on the Annot, Grand Coyer and Sanguinière sub-basins. Objectives were to determine the structural evolution of the sub-basins, including the role of inherited features, and to use these data to better constrain the alpine tectonic history of SE France.Infill of the early alpine foreland basin is more complex than a simple foreland migration of sedimentation and deformation. Results reveal complex interactions of tectonics and sedimentation at different scales during deposition of the Annot Sandstone. At the foreland scale, a balanced cross-section illustrates the critical control by the pre-existing salt-based passive margin architecture on the structure of the fold and thrust belt. Triassic evaporates regionally acted as a weak basal detachment, accommodating an early alpine deformation phase of the whole foreland from the Middle to Upper Eocene. Variations in mechanical stratigraphy within the Mesozoic sediments generated a change in structural style. For example, the eastern region is characterized by a thinner, shalier Mesozoic succession, leading to several active detachments during alpine compression. Multiple fold wavelengths and synsedimentary rolling hinges generated by these active detachments led to complex depocentres. The previously under-estimated structural heritage of the foreland basin derived from rifting and spreading of the Alpine Tethys and from an early regional deformation responsible for E-W trending folds, which began in the Late Cretaceous and increased during the Middle Eocene. Preliminary thermochronology results (U-Th/He datings on detrital zircons of medium to coarse sandstone blocks and granodiorite blocks from debris flows of the Sanguiniere depocentre) suggest an exhumation of source areas during this Cretaceous deformation. A detailed study of structures within the Annot and Grand Coyer sub-basins and their Cretaceous substrate revealed connected depocentres evolving in 3D at the basin scale. Paleocurrent directions are mainly parallel to the main alpine fold axes recording structural confinement, while some diverging directions indicate flow interaction with minor or oblique tectonic features. Turbidites were deposited along the axis into several alpine synclines, which were simultaneously or successively active.Concerning the depocentre infill, 3D geometrical modelling of the Annot depocentre (using gOcad) was used to identify inherited structures and to constrain alpine fold activity during turbidite deposition. The onlap migration and the specific stacking and dip pattern within turbidite members (laterally rather than vertically stacked deposits) record depocentre migration linked with a rolling synclinal hinge. Due to its sedimentology and its stratigraphy, the Annot Sandstone is well-known as an analogue for sand-rich reservoirs deposited at delta toes in deep-water passive margins. This study reveals that, structurally, it is also a good example of axially-sourced turbidite depocentres developed on a multi-detachement fold and thrust belt
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Lise Salles. Contrôles structuraux en 3 dimensions de la sédimentation turbiditique dans les chaînes plissées : exemple des Grès d'Annot (SE de la France). Sciences de la Terre. Institut National Polytechnique de Lorraine, 2010. Français. ⟨NNT : 2010INPL024N⟩. ⟨tel-01749551⟩

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