Transition from autochthonous to parautochthonous deformation regimes in Murça-Marão sector (Central-Iberian Zone, northern Portugal)

J. Rodriguez, C. Coke, R. Dias, E. Pereira, and A. Ribeiro

A comparison is made between the structures induced by the main variscan deformation tectonic event (D1) in the Autochthon of Central-Iberian Zone (northern Portugal) and in the overlying lower unit of the nappe pile complex of NW Iberia; due to the close lithostratigraphic similarities between both units, the lower allochthonous complex is considered as a Parautochthonous unit

In the Autochthon the D1 tectonic event have produced folds in a transpressive regime where a strong heterogeneous partitioning between the pure and the simple shear components could be emphasize. The amount of partitioning was influenced by the proximity, either to deep structures in the pre-variscan basement or to major sin-kinematic faults.

Concerning the Parautochthonous, several features have controlled the geometry and kinematic of the D1 variscan structures:

the existence of stratigraphic levels with specific rheologies (e.g. the Silurian black shales);

the obliquity between the transport direction of the thrust nappes and the autochthonous structures;

the steepness of the D1 macrofolds in the Autochthon.

The discussion of the distinct behavior of D1 variscan structures in both units allows a better understanding of the geodynamical evolution of the NW Iberian nappe complex. In the proposed model, the upper part of the Parautochthon (with a fold nappe style) was emplaced by a spreading-gliding mechanism, while in the lower part (consisting of an imbricated thrust system) a push-from-the rear mechanism predominates.