Fold-controlled quartz textures in the Pennine Mischabel backfold near Zermatt, Switzerland

Abstract

Quartz fabric analyses along the basal thrust of the Dent Blanche nappe and in the footfall Pennine units indicate south directed thrusting for the last stage of Austro-alpine/Pennine nappe emplacement. Quartz fabrics—in particular, those from the Pennine Mischabel backfold—imply complex spatial and temporal texture developments with crystallographic preferred orientations (CPO) that are commonly in disagreement with mesoscopic grain shape lineations. In consequence, kinematic directions inferred by quartz textures do not necessarily coincide with those predicted by mesoscopic structural elements such as grain shape lineations. In our particular case, orientation of grain shape lineations is rather complex in the study area, while principal axes inferred from the quartz textures remain geographically constant, and exhibit a simple geometric relationship to the Mischabel backfold. It is suggested that the grain shape lineations correspond to the total accommodated (finite) strain whereas the quartz fabrics reflect late increments of deformation. Quartz CPOs in the hinge of the Mischabel fold are similar to those along the thrust, revealing asymmetric single girdles to c-axes point maxima. Subgrain rotation is the predominant recrystallization mechanism, suggesting steady state conditions in fabric development during progressive rotational deformation. In contrast, asymmetric cross girdles dominate the upper limb and uneven small circle c-axes distributions characterize the lower limb, although recrystallization in both limbs is characterized by grain boundary migration. This clear separation of quartz textures and recrystallization (deformation) mechanism conflicts with the common view on heterogeneous deformation patterns in folds where the major component of rotational shear is expected in the fold limbs. Our study suggests that fabric development in the limbs of the recumbent Mischabel fold was reset during progressive deformation shortly prior to the transition from an active buckling to passive fold amplification, while the hinge suffered continuous bulk deformation provided by the back thrusting of the Dent Blanche nappe.