Revistes Catalanes amb Accés Obert (RACO)

Evolución tectónica de las unidades sudpirenaicas al W y al E de la línea de accidentes del Segre (Pirineo catalán, España)

J. Ullastre, A. Masriera


The analysis of structural and sedimentary units of the South-Pyrenean region to W and E of the Segre river shows that before the Middle Eocene the South-Pyrenean Central Unit and the Pedraforca Unit, aligned from W to E, formed a body with the western part of the Cadi Unit and constituted a single unit.
Two main E-W faults with several essentially vertical movements affected the tectonic and sedimentary evolution of this unit: the Coll de Nargó - Pedraforca fault (during the Senonian and Early Paleocene) and the Montsec - Catllaràs fault (during the Paleocene - Lower and Middle Eocene).
Sedimentation of the foreland basin, after the Lower Cuisian, is described and divided into units. Then, the relationships of these units with the South-Pyrenean tectonics resulting from the intra and post Middle-Upper Eocene phases are analysed. This analysis shows that between the South-Pyrenean tectonic units and the foreland basin sedimentary units there is no geometrically or chronologically homogeneous contact. In fact, the contact between the Cadí Unit and the Pedraforca Unit is an intra-Lutetian strike-slip fault with moderate thrusting; to the E and S of the Pedraforca Unit in the light of local evidence of stratigraphic continuity between this South-Pyrenean unit and the foreland basin (from the Upper Cretaceous - Paleocene to the Upper Eocene - Oligocene), no quantitatively significant southwards thrusting can be accepted. The contact of the SE margin of the South-Pyrenean Central Unit to W of the Segre constituted a true intra and post-Priabonian frontal thrust.
The interpretation most widely accepted at present sees the South-Pyrenean Units constituting a thrust system developed by a piggy-back thrust sequence between Maastrichtian and post-Priabonian times, in a NNE-SSW direction. However, the numerous objections to this model, based on obvious facts, make it untenable.
Finally, we attempt to explain the development of the present tectonic structures from the pre-Lutetian 'pan-unit', through the concurrence of displacements in two main directions: one from NW to SE (tens of kilometres) and the other from SW to NE (quantitatively shorter). In both cases we hold the immediate causes to have been major basement movements.

Key words: Catalonian Pyrenees. Spain. Mesozoic-Cenozoic tectonics. Vertical faults. Strike-slip faults. Thrusts. Allochthonous South-Pyrenean Units. Segre fault.

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