Relationships between structural units in the Tormes gneiss dome (NW Iberian massif, Spain): geometry, structure and kinematics of contractional and extensional Variscan deformation

Abstract

Situated in the inner zone of the Iberian massif, the Tormes gneiss dome is composed of two units with different lithological contents and metamorphic evolution. The upper unit consists of a thick sequence of low- to high-grade metasediments, ranging in age from Late Proterozoic to Silurian. The lower unit is a high-grade metamorphic complex composed mostly of granitic orthogneisses and minor amounts of metasediments. Four Variscan deformations are distinguished. At deep structural levels, the most prominent D1 ductile structures are recumbent anticlines with NE vergence, cored by orthogneisses, and separated by narrow synclines. These recumbent folds grade upward into less-flattened and NE-vergent steeper structures. The overall structure is that of a large-scale stacking of orthogneissic slices underlying a shortened and thickened sedimentary sequence that formed a huge orogenic wedge in this region. During the heterogeneous and ductile D2 deformation, the rheological behaviour of the orthogneisses and metasediments became similar. The vertical D2 shortening associated with a strong top-to-the-SE shearing in a large-scale subhorizontal shear zone folded the prior SW-dipping structures, developing SW-vergent folds with axes close to NW–SE L2 mineral and stretching lineations. D2 corresponds to post-collisional crustal thinning following D1 crustal thickening. The D3 and D4 late structures are much more localized and occurred under retrograde conditions, but have a significant effect on the final geometry of the metamorphic complex. This sequence of contractional and extensional deformative events permits a tectonic interpretation in the framework of the dynamic wedge theory based on the evolution in the time of the stress configuration applied to a portion of the crust.