Thermal and isotopc regimes during thrusting and extension within the Graz Thrust Complex. A stable isotope and fission track study

Pressure-Temperature-Time paths provide important information on tectonic processes during orogeny. Usually the cooling path within these P-T-t loops may be well constraint by standard petrological and geochronological techniques and it is interpreted in terms of rock exhumation and/or erosion. Two major contrasting tectonic processes may be envisaged, both of them may cause cooling and decompression. These scenarios include (1) thrusting of rocks from deeper structural levels coeval with denudation; and (2) exhumation of rocks by extensional tectonics. However, the two processes arise in different thermal regimes within the lithosphere. Whereas thrust tectonics generally "stretches" regional isotherms, exhumation by extension "condenses isotherms". In a first approximation thrusting is a process that initially cools whereas extension arises in enhanced local geotherms. Besides these processes that involve heat advection and conduction elevated temperatures within thrust zones and normal faults may arise from fluid circulation from deeper crustal levels. Combined stable isotope and fission track studies may provide inforination on both, fluid transfer within fault zones and information on thermal regimes during thrusting and extension. Within the Graz Thrust Complex thrust tectonics as well as extension tectonics has been reported for the Alpine cycle. Although there are numerous data on the metamorphic evolution and timing of events in this area, only a rough estimate on thermal budgets during thrusting and/or extensional tectonics may be done. This is mainly due to the fact that correlation of thermal with tectonic regimes is a non-trivial task in these low grade terrains which lack suitable mineral assemblages that may be correlated with deformation events. We intend to overcome this problem by the combination of stable isotope and fission track technology within and around well defined deforrnation zones. These include a detachment horizon where highly metamorphosed rocks of the Koralm / Gleinalm Crystalline complex have been juxtaposed against low grade rocks within the Graz Thrust Complex. The second study area is an Alpine thrust within the Graz Thrust Complex.