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 Zon e contains four types of mylonitic structures: (1) protomylonites, (2) mylonites, (3) platy ultramylonites, each developed in map-scale doma ins, and (4) cm-scale, vein-like ultramylonites. These mylonites are m arked by increasing volume fractions of very fine-grained matrix enclo sing mm-to cm-scale porphyroclasts derived from the Iherzolite protoli th. Progressive mylonitization was associated with the transformation from spinel-to plagioclase-bearing assemblages, which indicates exhuma tion of the peridotites and suggests that mylonitization was related t o lithosphere extension. The mylonites are usually interpreted to have formed during Albian-Aptian rifting. Final emplacement of the peridot ites in the upper crust must certainly have occurred in the Cretaceous , but we argue that the mylonitic deformation and initial crustal empl acement could also have taken place during Variscan late-orogenic exte nsion. For lack of a pre-mylonitic protolith, our study of the Turon d e 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 con tain microstructures suggesting high-stress dislocation creep and conc urrent dynamic recrystallization, but also planar discontinuities with and without extremely fine-grained material suggesting incipient brit tle behaviour. Both ductile and brittle mechanisms may thus have induc ed strain localization. High flow stress estimates (300-1300 MPa) usin g piezometry and olivine flow laws indicate that the inferred conditio ns of incipient mylonitization may indeed have allowed semi-brittle fl ow. Unlike upper mantle shear zones described from other areas, locali zation in the North Pyrenean peridotites was independent of the presen ce of hydrated phases, and we envisage that these shear zones represen t a case of effectively dry localization.