QUARTZ TEXTURE ANALYSIS IN PARASITIC FOLDS OF THE MISCHABEL FOLD, ZERMATT SWITZERLAND

The study area near Zermatt bears a complex history of Alpine deformation involving north directed nappe emplacement, isoclinal folding and peak metamorphism. Late stage refolding of the nappe stack was mainly caused by reverse, southward directed displacement under low-grade metamorphic conditions. The so-called Mischabel fold appears to be part of this late deformation as it forms a large, southvergent, recumbent fold system formed during southward directed shear along the hanging wall Dent Blanche thrust. Quartz textures show very distinct patterns for different domains of the Mischabel fold suggesting that fabric development was controlled by the large-scale fold formation. The hinge area and the Dent Blanche thrust reveal asymmetric single girdles to quartz c-axes point maxima indicating progressive rotational deformation related to southward directed thrusting whereas the limbs indicate complex spatial and temporal developments due to the limb kinematics of the growing fold. Crystallographic preferred orientations are commonly in disagreement with mesoscopic structures such as stretching lineations causing controversial kinematic interpretations. The quartz textures seem to reflect the last stages of back folding and thrusting whereas mesoscopic structures such as stretching lineations are influenced by the polyphase deformation of the rocks. Although quartz preferred orientations seem to be controlled by the development of the Mischabel fold still the question arises how far quartz fabrics are influenced by local structures such as parasitic folds. We analyzed minor folds at the hinge, the inverted limb and the normal limb of the Mischabel fold. Each small-scale fold was sampled at its hinge and both limbs. The analyzed quartz textures are surprisingly homogeneous across individual minor folds. However, each parasitic fold is dominated by a style of quartz texture that corresponds to the relative position in the large-scale Mischabel fold. These results suggest that small-scale folds had no significant mechanical contribution to the final fabric development. A reason for that might be that small-scale folds develop earlier than large-scale folds and their mechanical amplification is already complete while large-scale folding is still active.