Variscan structure in the eastern part of the Pallaresa massif, Axial Zone of the Pyrenees (NW Andorra). Tectonic implications

Abstract

AbstractThe Variscan structures of the Axial Zone of the Pyrenees are traditionally separated into the “infrastructure” and the “suprastructure” domains. Three deformation events have been recognized within Cambro-Ordovician rocks in the eastern part of the Pallaresa massif. D1 consists of a slaty cleavage (S1), and no folds have been found associated with it. The D2 event is characterized by E-W trending and north-verging recumbent folds with an associated foliation (S2) that can be observed at all scales. The S2 foliation has a horizontal orientation, and is the main foliation in the northern and central parts of the study area. The D3 deformation event is distinguished by development of E-W trending, upright folds, of centimetric to metric scale. These folds are associated with a steep and rough crenulation cleavage (S3) in the southern part of the study area. Subvertical faults trending E-W are associated with D3 folds. Most of the structures recognized in the eastern part of the Pallaresa massif have developed under low metamorphic grade conditions, although close to the Aston and Hospitalet domes, in the vicinity of some igneous intrusions, the metamorphism is coeval with S2 foliation and may reach high grade conditions. A gradual transition can be observed from zones characterized by structures of type D2 to zones exhibiting a better development of D3 structures. This transition can be explained by a décollement level located in the Cambro-Ordovician succession. This level can be related to faults and D3 folds observed in the eastern part of the Pallaresa massif. Most of the structures can be explained in a contractional geological setting. In addition the relationships between the different Variscan structures of the eastern part of the Pallaresa massif are better described. The different models recently presented by some authors about the Variscan structure of the Pyrenees are discussed in the light of these new results.