A Discussion on natural strain and geological structure - Strain patterns in the Pyrenean Chain

The Pyrenean Chain is a deformed part of the crust, fan-shaped in cross section, in which we can define the main characteristics of the major deformation as follows: (a) East—west folds always have their axial planes nearly vertical; the B axes of these folds have gentle plunges; (b) in the domain where schistosity is present (dominent flattening), the direction of maximum apparent elongation on cleavage planes, i.e. the X deformation axis, is nearly parallel to the geometric A axis of the folds. Inside the domain of strong flattening, a very narrow zone is present (less than 2 km wide on some cross sections) bounded by discontinuities, one of the most important is the North Pyrenean fault. This narrow zone is fundamentally different from the rest of the chain: (i) here, the deformation has the highest intensity and the rocks are metamorphosed; (ii) the B axes of the folds are curved and display steep plunges; (iii) the X deformation axis is parallel to the B geometric axis. We imagine that these anomalies have been created by sinistral horizontal displacement on the North Pyrenean fault during the folding. In addition to these facts, a brittle-deformation analysis permits the drawing of deformation trajectories in the flat northern foreland up to 400 km from the chain itself.