QUARTZ TEXTURES VERSUS STRUCTURAL GEOMETRIES IN THE HINGE OF THE MISCHABEL FOLD, ZERMATT SWITZERLAND

The Pennine nappes of western Switzerland comprise marine sequences of metasediments and metavolcanites (ophiolithes) which were overthrusted by basement rocks of the Austroalpine Dent Blanche nappe. 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 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. Crystallographic preferred orientations are commonly in disagreement with mesoscopic structures such as stretching lineations causing controversial kinematic interpretations. The disagreement between quartz textures and structures becomes particularly obvious when analyzing second order folds in the hinge region of the Mischabel fold. This region preserves fold interference patterns of symmetric to asymmetric minor folds on the meter scale. Our study focuses in particular on the geometric relationship of the different generations of minor folds, the quartz crystallographic preferred orientations and the mesoscopic structural elements. Stretching lineations are highly variable in orientation but concentrate near the WSW plunging fold axes. However, stretching lineations are commonly oblique to the axes of individual minor folds. In contrast, quartz textures are more consistent in orientation and closer related to the last generation folds. 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.