The association of mafic extrusive rocks and serpentinized mantle rocks is a common feature encountered in numerous ultra-distal magma-poor rifted margins as well as in (ultra)slow-spreading oceanic settings where detachment faulting and mantle exhumation occur. Although seismic imaging and high-resolution TOBI radar allow imaging this association, the nature of this interface is poorly understood (e.g. nonconformity vs. tectonic channelized vs. diffuse fluid flow along the interface). The timing between magmatism and active detachment faulting is often poorly constrained and it remains difficult to decipher if mafic extrusives are pre-, syn- or post-exhumation of mantle rocks at the seafloor. In this study, we characterize the nature of this interface from a section of a Jurassic ultra-distal margin preserved in the Platta nappe, SE Swiss Alps. We show that the basalt-serpentinized mantle contact does neither correspond to an Alpine thrust system, nor to a nonconformity or a detachment plane exhuming mantle rocks from underneath already emplaced basalts. Rather, the basalt-serpentinite interface corresponds to a weak decoupling level formed by conjugate low-angle normal faults onto which conjugate high-angle normal faults branch. These structures are diagnostic of co-axial extensional tectonics occurring during and after emplacement of basalts over a formerly exhumed serpentinized mantle. Carbonate extensional and shear veins and foliated ophicalcites formed during this Jurassic extensional event. Their distribution across and along the basalt-serpentinite interface demonstrates that this decoupling interface was a high-permeability layer that channelized high fluid flow responsible for the widespread hydrothermal alteration. A positive feedback between fluid flow and extensional deformation is proposed.