Abstract
The eastern Beausset Syncline and Toulon Belt, in the southern France, represents the easternmost remnant of the Pyrenean Apto-Albian Rift System and of the Pyrenean-Provençal orogen (Late Cretaceous-Eocene). Detailed structural and stratigraphic field mapping as well as the integration of published structural and stratigraphic data, are used to reconstruct the Jurassic to Late Cretaceous tectonic evolution of this area. A layered evaporite sequence, composed of a succession of evaporitic units interbedded with more competent lithologies, behaved as the main decoupling horizon and source of diapiric bodies. Structural and lithostratigraphic observations in the Mont Caumes area are interpreted as halokinetic in origin (wedges, flaps, welds, thrust welds, highly localized depocentes). These were controlled by the sinuous Mont Caumes salt wall that grew along the southern flank of the eastern Beausset Syncline and interacted with regional tectonic stresses from Early Jurassic to latest Santonian times. Jurassic and Lower Cretaceous carbonate units thin toward the Mont Caumes salt wall, recording deposition in salt controlled broad synclinal depocentres controlled by early salt mobilization. Inverted relics of Apto-Albian rift depocenters are aligned along the northern margin of the Toulon Belt and the adjacent Bandol Belt to the west. In the Turonian-Coniacian Revest depocenter, stratal thickness variations, progressive unconformities lateral depocentre, and the westward increase in stratal, overturning of a flap on the basin’s southern margin all record localized strong asymmetrical growth of the 3D Mont Caumes salt wall. During Pyrenean-Provençal N-S convergence starting in Early Campanian, the salt wall was squeezed and reactivated as a thrust weld. The upper part of the flap was sheared and thrust north over the Beausset Syncline (Mont Caumes imbricate). Further to the west, the Saint-Cyr salt extrusion associated with the Santonian-Muschelkalk unconformity, indicates that the Bandol salt wall extruded at the same time as the main diapiric activity of the Mont Caumes salt wall. Compressional reactivation further extruded the salt body leading to emplacement of the Beausset Klippe onto the Beausset Syncline. The Toulon salt structures can be correlated with other examples of contractional salt structures in the external Alps and Pyrenees.
Highlights
In salt-rich external orogenic systems, it can be difficult to distinguish deformation related to pre-collisional rifting, halokinetic deformation and compressional deformation, often leading to conflicting interpretations
Added difficulties include: (1) evaporitic lithologies have often been removed through dissolution, deformation or erosion, leaving little or no trace of what once may have been a considerable volume of mobile material, (2) the superposition of several phases of noncylindrical salt-influenced deformation can produce extremely complex stratal and structural records that (3) may be mistakenly interpreted as recording deviatoric strain
The main scientific questions addressed in this paper are: (1) what was the role of salt in the evolution of the Inner Coastal Units of southern Provence?; (2) what are the implications for alpine-cycle convergence in this area?; (3) what is the regional significance of these inverted basins in the context of the Pyrenean orogeny? First, we describe the complex structures and stratal architectures of the eastern Toulon Fault Zone
Summary
In salt-rich external orogenic systems, it can be difficult to distinguish deformation related to pre-collisional rifting, halokinetic deformation and compressional deformation, often leading to conflicting interpretations. Recent regional scale studies have demonstrated the critical role of Triassic salt in the evolution of Pyrenean and Iberian fold belts, the Provence fold and thrust belt, and the southern Subalpine chains from the onset of Triassic rifting to latest Cenozoic shortening (e.g., Canérot et al, 2005; Graham et al, 2012; Saura et al, 2014; Saura et al, 2016; Bestani et al, 2016; Espurt et al, 2019; Vergés et al, 2020; Labaume and Teixell 2020; Ford and Vergés 2020) These new insights have stimulated this investigation into complex structural and stratigraphic geometries in the Toulon area, where allochthonous nappes associated with Triassic evaporites were first recognized by Bertrand in 1887. We discuss the regional significance of our findings for Cretaceous to Cenozoic paleogeographic and tectonic reconstructions
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