The structural and thermal evolution of the Siviez-Mischabel Nappe, western Swiss Alps

The fundamental goal of this project is to expand scientific discourse on two topics: (1) the basic geometry and tectonic significance of deformation in the Pennine basement nappes of the western Swiss Alps, and (2) the possibilities and limitations of dating deformation events by isotopic dating of fabric-forming minerals. The Alpine thermal history of the Siviez-Mischabel Nappe includes rapid burial during the Eocene, late Eocene greenschist facies metamorphism at peak temperatures of 300-450{dollar}spcirc{dollar}C during the Alpine collisional event, and Oligocene and Miocene exhumation and cooling at a rate of 10{dollar}spcirc{dollar}C/m.y. Results of field mapping near Chandolin (Valais, Switzerland) strongly suggest that the Siviez-Mischabel Nappe is not a ductile basement fold-nappe, as has been previously mapped. The frontal part of the Siviez-Mischabel instead consists of a stack of slices of upright basement and cover units, and this geometry is best explained by Alpine imbrication and thrusting during prograde metamorphism. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. This study presents {dollar}rmsp{lcub}40{rcub}Ar/sp{lcub}39{rcub}Ar{dollar} ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Results suggest that the Siviez-Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. {dollar}rmsp{lcub}40{rcub}Ar/sp{lcub}39{rcub}Ar{dollar} white mica ages from the eastern Siviez-Mischabel, however, appear to date post-kinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of basement nappes in Pennine units and exhumation of high-pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years. By comparing age data with results of numerical models, I propose a new test for identifying mineral growth ages. This test explores the relation between age and grain size and assumes that argon loss occurs by volume diffusion. If two mica grains of different size but with the same growth, temperature, and deformation history yield the same age, that age is likely a growth age. If the two grains show a positive correlation between age and grain size, the ages probably record later thermal events because smaller grains are more sensitive to thermally activated argon loss.