ORIGIN AND SIGNIFICANCE OF STRUCTURAL MORPHOLOGIES IN MOUNTAIN BELTS: EXAMPLES FROM THE NORTH CASCADES CRYSTALLINE CORE AND OTHER TERRAINS

Structural analysis and kinematic interpretation of mountain belts are commonly based on concepts that assume undeformed and sometimes isotropic rock materials subjected to a single period of continuous load. Such considerations may lead to inconsistent conclusions, particular in such terrains which experienced several deformation episodes. We will introduce examples of repeatedly folded rock units exposed in the North Cascades Crystalline Core (Washington State) and the Tauern Window (Eastern Alps), where structural morphologies and finite strain patterns seem to cumulate over long periods of the deformation path instead of reflecting individual deformation episodes. Mid to deep-crustal metamorphic rocks of the Cascades reveal coaxial fold superposition with stretching lineations predominantly sub-parallel to the fold hinges but sometimes sigmoidally curving over the fold limbs. Similar situations occur in the Tauern Window where the unidirectional lineation is used as an argument to infer crustal-scale flow of the last deformation. Both study areas, as most metamorphic domains in orogens, are dominated by a single foliation that lacks evidence for transposition or later overprint. Correlation with individual fold generations is dubious as foliation mimics the morphologies of last generation folds, giving the impression that fabric is related to earlier folding and has been passively refolded by the last folds. This, however, conflicts with the lineation, thought to be formed in conjunction with the last generation folds, whereas the foliation that carries the lineation should be an older structure. In contrast, extensive strain measurements (690 stations) in the Tauern Window document significant variation in state of finite strain relative with position in the fold interference pattern. Foliation always parallels the flattening plane of finite (=total accumulated) strain and thus seems to be incompatible with individual deformation episodes. In addition, mineral and stretching lineations coincide with the maximum principal axis of total strain and are therefore unsuitable indicators for kinematic directions of individual deformation phases.