ALPINE DEFORMATION AND AR-40 AR-39 GEOCHRONOLOGY OF SYNKINEMATIC WHITE MICA IN THE SIVIEZ-MISCHABEL NAPPE, WESTERN PENNINE ALPS, SWITZERLAND/

We explore the timing of deformation and exhumation of the Siviez-Misc habel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents Ar-40/Ar-39 ages obtained on both synkinem atic white mica from Permo-Triassic cover sediments and more complex w hite mica populations from basement gneisses of the Siviez-Mischabel a nd middle Pennine Nappes. Primary foliation developed in cover units b y nucleation, growth, and rigid rotation of mica grains during noncoax ial Alpine deformation. Although some samples show a crenulation of th is primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist f acies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between indepe ndent estimates of the timing of deformation and reported Ar-40/Ar-39, white mica ages from cover units of the central and southern Siviez-M ischabel Nappe. In cover units from the central and southern Siviez-Mi schabel regions of the study area, Ar-40/Ar-39 ages appear to date syn kinematic white mica growth. Results suggest that the Siviez-Mischabel :Nappe was emplaced and developed foliation during a 5 m.y. period fr om 41 to 36 Ma. In cover units from the eastern Siviez-Mischabel, howe ver, Ar-40/Ar-39 white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic acti vity in the lower Pennine Nappes or to Miocene development of the Simp lon fault zone. Variations in the relation between Alpine age and grai n size for cover samples from the central, eastern, and southern Sivie z-Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamor phism. When considered in concert with other recent isotopic studies o n 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 crystalline nappes in hinterl and units and exhumation of high-pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale o f a few million years. Copyright 1998 by the American Geophysical Unio n.