VARISCAN DEXTRAL TRANSPRESSION IN THE FRENCH PYRENEES - NEW DATA FROMTHE PIC-DES-TROIS-SEIGNEURS GRANODIORITE AND ITS COUNTRY ROCKS

New structural data from both the Trois-Seigneurs granodiorite and its country rock lead to new conclusions about the Variscan tectonic hist ory of this part of the Pyrenees. Sedimentary rocks of the massif, tha t increase in metamorphic grade from north to south, are imprinted by a single major deformation responsible for both the tight, upright, E- W-trending folds with vertical to steeply N-dipping foliations and the gently W-plunging stretching lineations. Syntectonic emplacement of t he Pic des Trois-Seigneurs granodiorite is demonstrated by the paralle lism between structures of the pluton, as determined by magnetic fabri c measurements, and the foliations and lineations imprinted in the cou ntry rocks. Large deformation of the granodiorite magma during emplace ment is indicated by strong anisotropy and ubiquitous magmatic microst ructures. The high thermal gradients of regional metamorphism, which i s also syntectonic, are attributed to a large thermal aureole provided by deep-seated magma reservoirs. Shear-sense criteria in XZ thin sect ions of the metasedimentary country rocks indicate a dextral sense of the non-coaxial component of strain throughout the massif and regardle ss of metamorphic grade. The overall structure of the massif is conclu ded to have derived from a major shearing event that is equated with t he D-2 main phase observed in the entire Variscan Pyrenees. As the E-W -oriented stretch and dextral shear cannot be separated from compressi ve structures generally considered to be characteristic of D-2, this p hase is concluded as representing regional transpression.