Strain geometry in Concepenido eclogites during widespread HP deformation (Cabo Ortegal Complex, NW Spain)

Abstract

The Concepenido eclogites are part of an ordered rock sequence in the upper tectonic unit of the Cabo Ortegal Complex (NW Spain). They form a continuous and about 200 m thick layer separating other mafic rocks from quartzo-feldspathic gneisses. The whole sequence has been deformed and metamorphosed at high pressure and high temperature during the development of an early Paleozoic subduction zone. In this study, microfabrics of these eclogites are quantitatively analysed, in particular crystallographic preferred orientations (CPO) in omphacite and garnet measured by electron back-scatter diffraction (EBSD). From field observations and from detailed microstructural analysis we estimate that the high pressure deformation, which produced the crystallographic and shape fabrics in omphacite, has occurred under a strain regime close to bulk coaxial flattening. Both omphacite lineations and omphacite c[001] concentrations scatter in geographical coordinates and do not show a single preferred orientation that would indicate a predominant direction of extension. We suggest that the high pressure and high temperature environment during deformation, the coaxial nature of strain and the significant thinning of the layer and rock sequence may relate to flattening deformation of a subducting slab with extension slightly preferred in a down-dip direction.