Kinematic and petrologic analysis of Alpine structures in the Axial Zone of the Pyrenees, Spain

Abstract

In the Axial-Zone of the Pyrenees, in the Pallars Sobirà, Northern Pallars Jussà and Alta Ribargorça regions, a field research has been conducted in order to understand the difference in kinematics between the Variscan and Alpine orogenies, respectively. Structural mapping has been complemented with microstructural and petrologic investigations of Paleozoic rocks from the Axial Zone in order to constrain pressure and temperature conditions during deformation. Furthermore, measurements have been taken of faults and foliations in the studied area. This data has been plotted in stereographic projections to compare the two major deformation phases which have affected the Axial Zone of the Pyrenees. From this study it can be concluded that there is a slight difference in the kinematics of the Variscan and Alpine orogeny. There are three deformation event recognized. An old deformation phase resulting in a penetrative S1 foliation which has been crenulated by a younger C1 structure. These structures have been deformed by the youngest deformation phase reactivating faults and creating an antiformal stack in the central Pyrenees. Small difference in the orientation of the shortening direction between the two orogenies to be found in the studied area. The Variscan orogeny shows general shortening in the NNE-SSW direction, whereas the Alpine orogeny is more N-S. From the petrologic analysis of the thin sections, it can be concluded from analyzing the twinning structures in the calcite minerals, that the pressure and temperature conditions during the Variscan orogeny for the currently exposed rocks near Llavorsi lie around 107 MPa and 300°C.