We explore the timing of deformation and exhumation of the
Siviez-Mischabel 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
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. Primary foliation developed
in cover units by nucleation, growth, and rigid rotation of mica grains
during noncoaxial Alpine deformation. Although some samples show a
crenulation of this primary foliation, mica growth appears to have
occurred only during the development of primary foliation, the main
phase of greenschist facies deformation related to imbrication of the
Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement
exists between independent estimates of the timing of deformation and
reported Ar-40/Ar-39, white mica ages from cover units of the central
and southern Siviez-Mischabel Nappe. In cover units from the central and
southern Siviez-Mischabel regions of the study area, Ar-40/Ar-39 ages
appear to date synkinematic white mica growth. Results suggest that the
Siviez-Mischabel :Nappe was emplaced and developed foliation during a 5
m.y. period from 41 to 36 Ma. In cover units from the eastern
Siviez-Mischabel, however, Ar-40/Ar-39 white mica ages appear to date
postkinematic 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. Variations in the relation
between Alpine age and grain size for cover samples from the central,
eastern, and southern Siviez-Mischabel correlate well with the regional
variations in temperature inferred from quartz microfabrics and the
pattern of regional metamorphism. 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
crystalline nappes in hinterland 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. Copyright 1998 by the
American Geophysical Union.