Fall and Rise of the Tarentaise Zone, Western Alps

The Tarentaise Zone is an intensely deformed thrust-bounded meta-sedimentary package which represents a poorly understood belt of late Cretaceous subsidence on the northern passive continental margin of the Tethyan ocean. This contribution presents new field data which place tighter constraints on models of basin evolution. Structurally, the Tarentaise units have been stripped from their basement and imbricated to form a WNW-verging fold-thrust stack with complex internal strains. Demonstration that tectonic displacements were uniquely foreland-directed allows a preliminary qualitative reconstruction of basin architecture and hence provides a framework for stratigraphie analysis. The major sequence boundary recording the onset of late Cretaceous subsidence of the Tarentaise Basin is a subaerial erosion surface which truncates Jurassic rift structures of the precursory continental margin. This unconformity is directly overlain by sub- wave-base clastic sediments without any intervening shallow marine facies, indicating that subsidence occurred rapidly relative to sedimentation. The early basin-fill sediments are dominated by coarse, immature mass-flow conglomerates, deposited on a deep water fan-delta adjacent to a rapidly eroding area of high relief. Syn-extensional tholeiitic magmatism (the so-called Versoyen 'ophiolite') is consistent with intense localised crustal stretching: geochemical analyses of these metabasites revealing evidence of hydrothermal alteration but preservation of MORB-like trace element ratios. The implication of active subsidence contradicts those models for the development of the north Tethyan continental margin which predict strong attenuation and deep submergence by the end of the initial subsidence episode: the Tarentaise segment of the passive margin must have been capable both of being elevated above sea level and of being subsequently rifted a second time