CUMULATIVE DEFORMATION IN MULTIPLY DEFORMED ROCKS AND ITS SIGNIFICANCE FOR REGIONAL TECTONIC INTERPRETATION

Kinematic interpretation of tectonic terrains rely to theoretical and experimental models of deformation processes that predict the relationships between former boundary conditions and present strain patterns or corresponding structural morphologies ranging from regional to outcrop scale. The underlying concept of these, rather scale invariant approaches commonly considers unspoilt and sometimes isotropic rock materials experiencing a period of continuous load. Such approaches may lead to inconsistent conclusions on terrains, particular in orogens, which suffered multiple deformation cycles. We will present complexly deformed rock units from the Cascades Crystalline Core (WA), the Tauern Window (Eastern Alps) and the Variscans (Sardinia) where we believe structural morphologies and strain patterns cumulated over a large period of the deformation path instead of reflecting individual deformation episode. Results of numerical modeling are used to show that the strain patterns in each region can be reproduced by superposition of individual fold generations. In low-grade metamorphic Ordovician sediments of Sardinia excellent strain markers are preserved indicating variation in state of finite strain with location in the type 2 fold interference pattern. The metamorphic rocks of the Cascades Crystalline Core exhibit coaxial type 3 fold superposition forming stretching lineations predominantly parallel to the fold’s hinge lines but sometimes sigmoidally curving over the fold limbs. Similar situation occurs in the Tauern Window where extensive strain measurements indicate significant variation in finite strain between fold hinges and limbs. All examples are dominated by a single foliation that lacks evidences for transposition or later overprint. Correlation with individual fold generations is dubious as foliation sometimes mimics the morphology of the last generation folds (Cascades, Tauern) or transects folds of all generations (Sardinia). In contrast, foliation always parallels the flattening plane of finite (total, accumulated) strain and thus seems to be incompatible with individual deformation episodes. The same applies to stretching lineations that coincide with principal axis of the total strain. Lineations are therefore unsuitable indicator of kinematic directions.