Late-Hercynia granitoid plutons emplaced along a deep crustal shear zone. A case study from the S. Lucia Nappe (Alpine Corsica, France)

The S. Lucia Nappe (Alpine Corsica, France) is located in the Corte area. This tectonic unit recorded a polyphase Alpine evolution developed under lower greenschist facies conditions. In addition, the basement recorded an older (i.e. late-Paleozoic) tectono-metamorphic history, which mainly developed under granulite to amphibolite facies conditions. The S. Lucia basement represents an example of Ivrea-type lower crustal section, including a segment of a Km-scale granulite-facies shear zone made up of sheared Permian gabbros and enclosed metasediments. The occurrence of a thick (i.e. at least 1 Km-wide) high-grade shear zone indicates that the late-Palaeozoic deformation in the lower crust was homogeneously distributed on a large scale. The non-coaxial character of the deformation and the invariable sinistral shear sense, generalized along the whole section, recalls the models of uniform-sense, crustal-scale normal shear zones. Two distinct granitoid plutons were emplaced at the margins of the S. Lucia shear zone. Field relations, microstructures, quartz LPO fabric and mineral chemistry data document the syntectonic emplacement with respect to the shear zone. Indeed, both plutons experienced a nearly continuum down-temperature deformation, during emplacement and subsequent syndeformational cooling. The observed structural style suggests that the granitoids probably accommodated large strain through magmatic to submagatic flow. In both cases, the pattern of magmatic and subsequent solid-state fabric was strongly controlled by the stress field imposed by the S. Lucia shear zone. Moreover, the bulk composition of the two plutons likely played a fundamental role in the tectonic evolution, influencing the structural style and the spatial distribution of the solid-state overprint. Finally, the occurrence of magmas along the shear zone may have promoted strain partitioning within the melt-rich areas, allowing the exhumation and the preservation of granulite-facies rocks. The Mesozoic syn-rift exhumation of the S. Lucia basement is suggested by the occurrence of undeformed MORB-type dolerites, by the Middle Jurassic thermal anomaly and by the Cretaceous sedimentary cover. However, although the P-T-t evolution of the S. Lucia section is only partly constrained, several lines of evidence suggest that the Permian granulites had already been incorporated into the upper crust prior to the Mesozoic. Several paleogeographic reconstructions propose that, in the Alpine region, Permian granulites were intimately associated with the Mesozoic ophiolites at the ocean-continent transition. However, the case study proposed here points out that the occurrence of granulites alone cannot be taken as an indication of a syn-rift exhumation of the lower crust along the continental margin.