Some thermal considerations of the alpine metamorphism—past, present and future

Abstract

The thermal budget for the Alpine metamorphism is examined. As a well constrained example, the Southeast Tauern Window is modelled on the basis of a tripartite post-Jurassic history: Phase 1 to 65 m.y.: subduction of an ocean lying between Pennine zone and Austro-Alpine zone continental masses, culminating in overthrusting of the latter on the former. Phase 265-30 m.y.: burial metamorphism of the thrust pile. Phase 3 30 m.y.-future: erosion of the pile and exposure of metamorphic rocks. A Phase-2 regime in which normal continental heat flow was augmented by a high mantle contribution is not likely in view of the low present-day surface heat flow which constrains the amount of heat available for metamorphism. An important consequence of the establishment of a near-equilibrium gradient after overthrusting, followed by a rapid period of erosion, is that temperatures at depth are raised considerably above those for a normal geothermal gradient. The conditions for partial melting of the crust probably exist under much of the Alpine chain today and continuing erosion is likely to expose a metamorphic belt characterized by post-orogenic granites and mineral facies of generally higher T/ P than are now observed in the Alpine chain.