Morning clouds covering the Arties valley, south of Vicdessos. View from the Refuge de Fourcat.
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| Location of the Variscan Pyrenees. | Geologic elements of Axial Zone. |
The Pyrenees have experienced two orogenic events, the Paleozoic Variscan and the Mesozoic Alpine orogeny. The Variscan rocks underlie the central and highest portion of the Pyrenees, the so-called Axial Zone. The Axial Zone forms a natural border and marks the international boundary between France and Spain, and also contains the principality of Andorra. The highest peak is the Aneto with 3404 m. The northern and southern Pyrenees, outlined in grey on the overview map, are rocks effected by Alpine thrusting.
The Axial Zone is comprised of three major geologic elements: gneiss domes cored by pre-Variscan orthogneiss, Cambro-Carboniferous sediments and metasediments, and late-Variscan granitoids. The major structural elements are a steeply dipping E-W striking foliation, the so-called suprastructure, and a flat-lying foliation, the so-called infrastructure. The latter occurs predominantly near gneiss domes. There is ongoing debate on the relative age of supra- and infrastructure and the modes of emplacement of the gneiss domes. Tectonic models proposed include diapirism (Soula 1982, J.Struct.Geol. 4), extensional shear zone (van den Eeckhout & Zwart 1988, Geology 16), extension following compression (Vissers 1992, Tectonics 11), and orogen-parallel simple shear zone (Carreras & Capella 1994, J.Struct.Geol. 16). Most recently, anisotropy of magnetic susceptibility studies (AMS) of late-Variscan plutons (Bassies, Mont Louis-Andorra, etc.) indicate that these plutons intruded syntectonically during the late stage of the main D2 deformation phase of the Pyrenees (Gleizes et al. 1997, Terra Nova 9). In other words, according to these authors, the Axial Zone was a major dextral shear zone during D2.
For a good summary on the current tectonic models of the Axial Zone look up Carreras and Capella's 1994 paper "Tectonic levels in the Paleozoic basement of the Pyrenees: a review and a new interpretation" (Journal of Structural Geology 16, 1509-1524).
This study, a collaboration with Cees Passchier, focusses on the emplacement of gneiss domes, which is key to understanding the tectonic evolution of the Axial Zone. The project started in summer of 1998 as a post-doctoral research in the Graduiertenkolleg "Stoffbestand und Entwicklung von Kruste und Mantel", and is now a DFG-funded research project. The study comprises:
Field work has shown that an unambigous distinction between steeply dipping suprastructure and flat-lying infrastructure is not possible. Rather a gradual change is observed. Microstructural analysis recorded shear-sense indicators with predominantly sinstral sense of shear, and also a strong dip-slip component. A simple dextral shear zone model does not seem to reflect the reality. These results and more have been presented at the EUG X in 1999, the GSA 32 in November 2000, and the EUG XI in April of 2001. A paper on the structural and metamorphic evolution of the Bossòst dome has been submitted to Geological Magazine in July 2002.