Strain analysis in the Axial Zone of the Variscan basement of the Pyrenees

Abstract

The structural zonation in the Axial Zone of the Pyrenees, in which an infrastructure and a suprastructure are distinguished, can be correlated with a variation in the orientation of the principal strain axes. The deformation associated with the main foliation in the infrastructure is accommodated heterogeneously. Computed strain ellipsoids in gneisses show an apparent flattening strain (k= 0.29), which is in contrast to the apparent constrictional forms of pebbles in adjacent layers. Strain values in the suprastructure indicate an apparent flattening strain (k = 0.36) of low magnitude (Rxy = 1.36 and Ryz = 2.01), which can be taken as representative of the bulk finite strain throughout that domain. The trend of the X-axes is roughly E-W parallel to the fold axes in the infrastructure, whereas it is NW to N-plunging and at a wide angle to the ESE-WNW trending folds in the suprastructure. The Z-axes are steeply plunging in the infrastructure and almost horizontal in the suprastructure. Strain patterns in each of the two structural domains are suggested to be associated with different tectonic events. Between the infrastructure and the suprastructure, a transition zone exists where both domains grade into one another.