Recent structural studies of Variscan plutons in the Central Pyrenees have suggested that the Axial Zone of the Pyrenees acted as a large scale transpressional ductile dextral shear zone during the main (D2) Variscan deformation phase. Several of these plutons which intruded early during D2 exhibit an asymmetrical geometry similar to sigma-type porphyroclasts. As such they were used as kinematic indicators. In order to test this hypothesis a detailed regional structural analysis of the Palaeozoic metasediments which comprise the “matrix” of the postulated shear zone is being undertaken. First preliminary results from field studies are presented.
The study area is located in the French and Andorran Pyrenees, extending 70 km along the E-W strike of the Axial zone and 25 km across strike, covering the area west of the Bassiès Pluton to the east of the Hospitalet Dome. Near the contact with the Bassiès Pluton and along the southern margin of the Hospitalet Dome the metasediments have experienced sub-greenschist to greenschist grade regional metamorphism. Foliation generally dips steeply to the north and northwest and parallels the exposed contact of the plutons. Several kilometers southeast of the Bassiès pluton regional metamorphism reached amphibolite grade. In the higher grade rocks orientation of schistosity is less homogeneous but generally dips moderately northerly. Mineral stretching lineation is prominent and plunges shallowly to the WNW and ESE. Two younger generations of nearly vertical plunging crenulation fold axes can be distinguished.
Macroscopic kinematic indicators are scarce. In the low grade slates these are predominantly boudinaged crosscutting quartz veins and C’-type shear bands. Higher grade schists have experienced higher strain, evident from the mylonitic microfabric and delta-shaped quartz boudins. Prominent feature of the schists are centimeter long andalusite porphyroclasts. The andalusite crystals appear to have grown in the early stages of D2 deformation and are not aligned with their length axis parallel to the mineral stretching lineation. In addition to rotation within the plane of schistosity there appears to be a rotational component perpendicular to it. The kinematic behaviour of these clasts is the subject of current studies. First results from kinematic analysis indicate a top to the southeast sense of shear. This is in concordance with transpressive ductile shear with a dextral strike-slip component as postulated by workers studying the Variscan plutons. More detailed results of the ongoing study are necessary though to substantiate the existence of the proposed Variscan mega-shear zone in the Axial Zone of the Pyrenees.
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