CRYSTALLOGRAPHIC PREFERRED ORIENTATION AND MICROSTRUCTURAL ANALYSES ACROSS A STRAIN GRADIENT, MAGGIA NAPPE, SWITZERLAND

Understanding the microstructural progression of shear zones is important for reconstructing the tectonic evolution of the continental crust. Herein, we investigate the development of crustal shear zones in the Maggia Nappe, Switzerland. These rocks form part of the lower Pennine crystalline nappes of the southern Swiss Alps. The core of the Maggia Nappe exposes relatively undeformed granitic and dioritic rocks that locally display a gneissic foliation. These rocks are dissected by meter scale ductile shear zones that reach displacements of γ>5 in their core. The strain gradient across these shear zones provides an opportunity to investigate the microstructural development of these shear zones. We analyze a suite of 26 samples from undeformed, moderately deformed, and highly deformed sections of the strain gradient. Optical microscopy reveals a weak fabric in undeformed rocks, with 2-3 mm thick domains generally composed of quartz or oligoclase, with subordinate K-spar, anorthite, and biotite. As deformation intensity increases, the compositional bands become elongated and thinned to less than 1 mm. At very high strains, these layers become disaggregated and mixing occurs between the quartz and feldspar. We have characterized sample geochemical composition through SEM-EDS and crystallographic fabric through SEM-EBSD. Both quartz and feldspar crystals preserve little evidence of internal deformation, such as subgrains or undulose extinction, and generally contain straight or smoothly curved grain boundaries, suggesting static recrystallization post-deformation. Preliminary application of the quartz opening angle thermometer suggests crystal plastic deformation occurred at about 650-675 °C, consistent with published estimates for peak metamorphic conditions.