Texture and shape preferred orientation in mylonites developed under a complex kinematic frame: the Lalín-Forcarei thrust (NW Iberian Massif, Spain)

Selected mylonites from a major shear zone in NW Iberian Massif, the Lalín-Forcarei thrust (LFT) , have been analyzed with time-of-flight (TOF) neutron diffraction for texture, and synchrotron X-ray microtomography for volume shape preferred orientation. The regional evolution of the structures indicates a complex frame where a general dextral oblique collision resulted into a progressive rotation of mylonitic linear shape fabric and an heterogeneous reworking of the crystallographic preferred orientation (or texture) along the LFT shear zone. As a result several groups of mylonitic lineations coexist across the LFT. Texture analysis of quartz mylonites results in a monoclinic symmetry of quartz and muscovite pole figures, coherent with a top-to-the NE shearing. Mylonitic amphibolites showed orthorhombic symmetry for major phases like albite and hornblende. Shear bands with a top-to-the SE sense of shear have been described in the amphibolites, which point to a stretching component along a NW-SE trend. X-ray microtomography experiments were done in quartz mylonites in order to constrain the kinematic frame (lineation and foliation) of the fabric, and correlate with texture obtained by neutron diffraction. Results support a general mylonitic flow to the NE within the LFT and suggest that rheological factors like mechanical contrast should be considered in the analysis of mylonitic fabric in complex orogenic systems.