The « Nappe des Corbières Orientales » (Eastern Pyrenees, France) revisited: role of pre-existing salt structures and importance of gravity gliding

Abstract

In the external zones of mountains belts, it is now recognize that tectonic inheritance and early salt activity (halokinesis) prefigure and localize posterior tectonic structures. Taking this heritage into account leads to reevaluate some mechanisms, classically invoked to explain the emplacement of fold-thrust structures. The Corbières region, in the eastern Pyrenees, is characterized by the presence of a large thrust-sheet: the so-called “Nappe des Corbières Orientales” (NCO) located along the Corbières Languedoc Transfer Zone (CLTZ), an oblique structure joining the Pyrenees and the Provence Chain. This study aims at reconsidering its mechanisms of emplacement by revisiting the geological evidence identified in the 1960s such as olistoliths and unconformities. Geological sections from the footwall of the NCO to the CLTZ show salt walls underlining the major faults array. Along these salt walls, two successive wedges (halokinetic sequences) initiated during Mesozoic rifting episodes (Middle-Late Jurassic and Albian-Cenomanian, respectively) grew. We suggest that the NCO was initiated on a salt wall, along the Cévennes Fault Zone, and then propagated NW-ward over a distance of a few kilometers, during the Pyrenean main phase (Middle-Late Eocene). However, the displacement of the NCO is 15 km much more than the 3km observed at Bugarach in a frontal position. To explain this extra-translation, we suggest that the gliding of the NCO is accentuated by two mechanisms: (1) the uplift of the CLTZ during the Oligocene and (2) the tilting (up to the horizontal) of the initial ramp acting as a rift shoulder during the Miocene extensional deformation. These hypotheses are finally placed in a historical perspective.