RELATIONSHIPS BETWEEN STRUCTURAL UNITS IN THE TORMES GNEISS DOME (NW IBERIAN MASSIF, SPAIN) - GEOMETRY, STRUCTURE AND KINEMATICS OF CONTRACTIONAL AND EXTENSIONAL VARISCAN DEFORMATION

Situated in the inner zone of the Iberian massif, the Tormes gneiss do me is composed of two units with different lithological contents and m etamorphic evolution. The upper unit consists of a thick sequence of l ow- to high-grade metasediments, ranging in age from Late Proterozoic to Silurian. The lower unit is a high-grade metamorphic complex compos ed mostly of granitic orthogneisses and minor amounts of metasediments . Four Variscan deformations are distinguished. At deep structural lev els, the most prominent D1 ductile structures are recumbent anticlines with NE vergence, cored by orthogneisses, and separated by narrow syn clines. These recumbent folds grade upward into less-flattened and NE- vergent steeper structures. The overall structure is that of a large-s cale stacking of orthogneissic slices underlying a shortened and thick ened sedimentary sequence that formed a huge orogenic wedge in this re gion. During the heterogeneous and ductile D2 deformation, the rheolog ical behaviour of the orthogneisses and metasediments became similar. The vertical D2 shortening associated with a strong top-to-the-SE shea ring in a large-scale subhorizontal shear zone folded the prior SW-dip ping structures, developing SW-vergent folds with axes close to NW-SE L2 mineral and stretching lineations. D2 corresponds to post-collision al crustal thinning following D1 crustal thickening. The D3 and D4 lat e structures are much more localized and occurred under retrograde con ditions, but have a significant effect on the final geometry of the me tamorphic complex. This sequence of contractional and extensional defo rmative events permits a tectonic interpretation in the framework of t he dynamic wedge theory based on the evolution in the time of the stre ss configuration applied to a portion of the crust.