Successive Hercynian structures in some areas of the Central Pyrenees

In the central part of the axial zone of the Pyrenees five distinct phases of folding have been distinguished from the study of minor structures. Traces of a very early phase have been found only in the northern and southern part of the region, which appear on the map as oblique fold structures, and are most prominent from the divergency of lineations and fold axes in these parts of the region. The present aspect of the mountain chain is primarily due to the effects of the main phase in which strong compression produced tight but non isoclinal folds with axial plane slaty cleavage. In the Garonne Dome the slaty cleavage was initially flatlying, in general parallel to the bedding, but occasional folds with crosscutting cleavage have been found. Steep slaty cleavage folds of the Devonian overlying the flatlying slaty cleavage folds of the Cambro-Ordovician of this dome form a beautiful example of disharmonie folding. The main phase slaty cleavage has been found to be folded in the greater part of the area investigated, generally by small minor accordion folds. In several areas two phases of refolding have been distinguished, one with a trend diagonal to the orogene, one parallel, E—W. The intensity of the refolding is strongest in the oldest strata of the sequence. The succession of phases is evident from the folding of the planes of reference and twisting of the lineations. The patterns of the stereographic plots of data do not always show clear evidence of the succession. Knick zones accompanied the end of the Hercynian history of the mountain chain which mainly consisted of arching of the orogene, together with faulting and blockwise tilt. This period of deformation shows several characteristics of tensional stress.