Geology of the Upper Segre and Valira valleys, Central Pyrenees, Andorra/Spain

Sheet 10 of the 1:50,000 geological map series of the Geological Institute, Leiden University, is presented accompanied by a survey of structural, stratigraphic and morphologic features. The region includes part of the southern Axial Zone, the eastern end of the Nogueras Zone and part of the marginal trough adjacent to the south. Formations mapped range in age from Cambro-Ordovician to Pliocene. Detailed lithostratigraphic studies of the Cambro-Ordovician and Devonian rocks enabled correlations to be made with other regions of the Pyrenees. The Hercynian orogeny formed structures of variable shapes and orientation, which, however, have all be ascribed to stress accumulating in a N-S direction causing consecutive deformations. These can, to a certain extent, be correlated with the deformation phases known in other regions of the Axial Zone to the north. A first phase formed the largest folds of an order of 20 km down to less than 1 km wide. The slaty cleavage is generally parallel to the bedding and may possibly be related to this phase, in which case it is to be interpreted as a “concentric cleavage”. Second phase structures, generally characterized by an axial plane crenulation cleavage, indicate further compression. Temporary relief of the main stress after the first and second phases thought to be a result of accelerated strain rates due to folding, gave rise to cross folds and cascade folds. Gravity sliding of unstable structures in Devonian rocks from the Rabassa dome into the Arcabell syncline and Segre area, resulted in thick accumulations of Devonian material in these depressions. Strong deviations from the general E-W strike of first and second phase structures in the eastern part of the Orri dome and in the Segre unit are attributed to the influence of an inferred infrastructure rise thought to exist below the Orri dome. Epirogenic movements along the northern border of the marginal trough, were particularly important during the Permian and Cretaceous. The Devonian massif of the Monsech de Tost, amidst post-Hercynian deposits in the Nogueras Zone, is interpreted as a Hercynian gravity structure upthrusted during the Pyrenean (Middle Alpine) deformation. The Pedraforca structure is also explained as an upthrusted block. Later Alpine movements during the Neogene formed grabens in the Segre valley.