Quartz c-axis texture mapping of a Variscan regional foliation (Malpica–Tui Unit, NW Spain)

Mineral assemblages developed under high pressure and low to intermediate temperature conditions (HP–LIT) are found in eclogites, orthogneisses and metasediments of the Malpica–Tui Unit (MTU), either defining or surrounded by a regional foliation that was mostly re-equilibrated under amphibolite to greenschist facies conditions. This regional tectonic foliation, tracking the exhumation path of the MTU in the context of the Variscan collision in SW Europe, has been studied using the microstructural record in quartz from the different rock types. Most of the quartz c-axis patterns show two distinct asymmetries, contained in perpendicular planes, which are related to two different non-coaxial components of the finite fabric (i.e. the regional foliation and the lineation). The asymmetry of deformation inferred from quartz c-axis textures suggests that fabric accumulation during exhumation occurred under 3-D general flow, departing strongly from the simple shear type. This has significant tectonic implications for the interpretation of regional stretching lineations, which in the present geodynamic setting of irregular collisional tectonics, are not parallel to the direction of general tectonic transport and, ultimately, to the plate convergence direction.