Detachment Faulting and Mantle Exhumation Beneath an Allochthonous Carbonate Cover: Insights From the Gouffre Georges (Lherz Massif, Pyrenees, France)

Abstract The Cretaceous North Pyrenean hyperextended rift system recorded HT‐LP metamorphism in the pre‐ and syn‐rift sedimentary cover during lithospheric mantle exhumation. We describe and analyze for the first time the rocks exposed in the Gouffre Georges, a deep cave in the Lherz peridotite massif (Ariège, France). The cave walls expose the lithospheric detachment between the Mesozoic pre‐rift sedimentary cover hanging wall and the mantle rock footwall. Extensional structures have been preserved despite the positive inversion of the North Pyrenean rift system during the Pyrenean Orogeny. The extension direction is downdip WNW, consistent with rifting models of transtensional left‐lateral regime during the final stage of rifting. Field, microstructural, and petrographic data document a continuum of extensional deformation from high‐ to low‐temperature conditions. This detachment fault underwent high‐temperature hydrothermalism, with a possible contribution from evaporite‐born fluids. New observations also strongly suggest the formation of a hot, low‐viscosity material along the detachment interface. This material was then injected in the uppermost part of the peridotite body during the final stage of mantle exhumation. We consider three possible scenarios: (a) partial melting at the base of the pre‐hyperextension carbonates; (b) precipitation from carbonate‐rich hydrothermal fluids; and (c) flow of hyper‐ductile marble due to extreme plasticity of calcite under high temperature. Likewise, the base of the pre‐hyperextension cover recorded intense hydraulic and tectonic brecciation during extension. As indicated by their lack of syn‐rift sedimentary reworking, the Lherz peridotite recorded partial exhumation at the base of the Jurassic sedimentary cover, without exposure to the seafloor.