Combined structural analysis and oxygen isotope thermometry of
syntectonic quartz-calcite fibrous veins can be used to correlate the
thermal history of deformed rocks,vith specific structural and tectonic
events. Results are presented for the Mercies nappe in the western
Helvetic Alps, Switzerland, where mineral parageneses, illite
`'crystallinity,'' and fluid inclusion chemistry record an apparent peak
metamorphic temperature gradient that increased across the Morcles nappe
from anchizonal conditions in the foreland to epizonal conditions in its
hinterland root zone. Twenty-seven quartz-calcite veins were analyzed in
this study in order to determine the temperatures of veining during
formation and deformation of the nappe, Peak metamorphic temperatures
ranged from approximate to 260 to 290 degrees C in the shallower,
foreland localities and to approximate to 330 to 350 degrees C in the
deeper, more hinterland localities at the end of S1-foliation formation,
related to large-scale folding. Temperatures gradually decreased
throughout the nappe during subsequent development of the S2 foliation
and S3 crenulation cleavage, Uplift and erosion of the overlying nappe
pile resulted in slow cooling of the Morcles nappe during the waning
stages of the Alpine Orogeny. The dominant foliation-forming deformation
of the Morcles nappe occurred at elevated temperatures over the course
of 10 to 15 Ma. Combined structure-oxygen isotope analyses of
quartz-calcite veins yield better temperature and temporal constraints
on the thermal histories of subgreenschist vein-bearing tectonites than
do other geothermometers.