Migration of a synclinal depocentre from turbidite growth strata: the Annot syncline, SE France

Abstract

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 evaporites and Cenomanian marls) in the underlying succession.

Basement shear zones development and shortening kinematics in the Ecrins Massif, Western Alps

“…During this time, south of the Ecrins Massif, in the Grès d'Annot Basin, Apps [, 2004] and Salles et al . [] have shown that SW directed thrusting occurred during the deposition of the Nummulitic series.…”

“…Finally, after this main E‐W deformation phase, a gently N‐S shortening probably affected the European crust around the Ecrins Massif area [ Gratier et al ., ; Gidon and Pairis , ; Salles et al ., ]. The last observed deformations are characterized by the reactivation of the Penninic Frontal Thrust as a normal fault [ Ceriani et al ., ; Tricart et al ., ].…”

“…However, the respective contributions of inherited fault reactivation [ Gillcrist et al ., ; Hayward and Graham , ; Coward et al ., ], Variscan foliation reactivation [ Dumont et al ., ] or basement shearing along newly formed shear zones [ Bartoli et al ., ; Wibberley , , ; Marquer et al ., ; Bellahsen et al ., ] on the kinematics of shortening at crustal scale are still debated and the overall kinematic evolution through space and times remains unclear. For example, in the south of the Ecrins Massif, the direction of thrusting of the Embrunais‐Ubaye nappes is either northwestward [ Kerckhove et al ., ; Dumont et al ., , ] or southwestward [ Fry , , ; Lickorish and Ford , ; Apps et al ., ; Salles et al ., ]. There are also similar uncertainties about the timing of deformation in the east of the Ecrins Massif, near the Grangettes slice [ Bravard and Gidon , ; Lami , ; Butler , ; Pêcher et al ., ] or in the NE, around the La Meije and Combeynot thrusts [ Bravard and Gidon , ; Gamond , ; Beach , ; Ford , ; Ceriani et al ., ; Dumont et al ., , ].…”

Petro-structural map of the Money Unit (Gran Paradiso Massif, Valnontey valley, Western Alps)

“…This method avoids the use of cross sections as an intermediate step that can mask the real 3D geometry of geological structures (Fernández et al, 2004). The contact surface has been interpolated in 3D with the DSI methods, which it is based on the outcrop curve, foliation and axis orientation/plunge data, according to the procedure described in Le Carlier de , Salles et al (2011), andCaumon et al (2009).…”

Understanding the evolution of syn-depositional folds: Coupling decompaction and 3D sequential restoration

“…The 3D model was built from available field data (Albussaïdi and Laval, 1984;Callec, 2001;Puigdefàbregas et al, 2004;Salles et al, 2011), taking into account three faults, including the Rouaine fault, in a domain of interest limited by the Ourgues fault towards the East. Two scalar fields were interpolated to model this Tertiary depocentre: one for members AeG, and another one for the Nummulitic Limestone and the Globigerina Marls (Fig.…”