Mylonitic deformation in upper mantle peridotites of the North Pyrenean Zone (France): implications for strength and strain localization in the lithosphere

The Turon de Tecouere peridotite in the west of the North Pyrenean Zone contains four types of mylonitic structures: (1) protomylonites, (2) mylonites, (3) platy ultramylonites, each developed in map-scale domains, and (4) cm-scale, vein-like ultramylonites. These mylonites are marked by increasing volume fractions of very fine-grained matrix enclosing mm-to cm-scale porphyroclasts derived from the Iherzolite protolith. Progressive mylonitization was associated with the transformation from spinel-to plagioclase-bearing assemblages, which indicates exhumation of the peridotites and suggests that mylonitization was related to lithosphere extension. The mylonites are usually interpreted to have formed during Albian-Aptian rifting. Final emplacement of the peridotites in the upper crust must certainly have occurred in the Cretaceous, but we argue that the mylonitic deformation and initial crustal emplacement could also have taken place during Variscan late-orogenic extension. For lack of a pre-mylonitic protolith, our study of the Turon de Tecouere mylonites is supplemented by microstructural data from the Moncaup and Etang de Lhers peridotites. The protomylonites of Turon de Tecouere and the selected samples from Moncaup and Etang de Lhers contain microstructures suggesting high-stress dislocation creep and concurrent dynamic recrystallization, but also planar discontinuities with and without extremely fine-grained material suggesting incipient brittle behaviour. Both ductile and brittle mechanisms may thus have induced strain localization. High flow stress estimates (300-1300 MPa) using piezometry and olivine flow laws indicate that the inferred conditions of incipient mylonitization may indeed have allowed semi-brittle flow. Unlike upper mantle shear zones described from other areas, localization in the North Pyrenean peridotites was independent of the presence of hydrated phases, and we envisage that these shear zones represent a case of effectively dry localization.