Correlating deformation in Variscan NW-Iberia using porphyroblasts; implications for the Ibero-Armorican Arc

3-D microstructural analysis of 30 porphyroblastic samples from the Variscan orogen in NW-Iberia has revealed four sets of inclusion trails with distinctive geographic trends and relative timing. The chronological and orientational consistency of these microstructures provides a reference frame for the correlation of macroscopic structural successions. The two younger inclusion trail sets correlate with the main map-scale structures of Middle- to Upper-Carboniferous age, whereas the two older sets correspond to mesoscopic structures preserved locally in low-strain pods, and dating back as far as 365 Ma (Upper Devonian). Inclusion trail orientations appear uninfluenced by a major orocline in the study area (Ibero-Armorican Arc) outlined by late-stage folds and thrusts with variable trends. It is proposed that the orocline developed by WNW–ESE-trending transpression superposed on originally N–S to NE–SW-trending structures during a late stage of the orogeny (Upper-Carboniferous). Domains of low- or zero-strain (e.g. porphyroblasts) are inferred to have maintained relatively constant orientations in the zone of transpression, presumably due to deformation partitioning effects. Consequently, they preserve a succession of pre-orocline structural trends. The collected microstructural data also indicate moderate preferred vertical and horizontal orientations of inclusion trails in the analysed samples as reported earlier in other Variscan regions. This probably witnesses a dynamic equilibrium between tectonic forces and gravity during four periods of differently oriented crustal shortening that generated the four recognised sets of inclusion trails.