FISSION-TRACK AGES OF DETRITAL ZIRCON FROM THE EUROPEAN ALPS

First attempts to solve the question of how the European Alps were formed and have evolved through time date back to Suess (1875) and Heim (1878). Over the last 160 years the European Alps have been intensively studied so that meanwhile their geologic history is well established. Nevertheless, new techniques and approaches provide new insights, and help to shape our understanding of the long-term evolution of mountain belts. To describe the Alpine orogenic evolution, it is necessary to date the timing and rate of exhumation of rocks in the orogenic system. Crustal rocks in convergent mountain belts are subject to tectonic and erosional exhumation and will eventually be exposed at the earth's surface. On their way to the surface they pass through the closure temperatures of various isotopic systems (Rb-Sr, K-Ar, Ar-Ar, zircon fission-track, zircon U-Th/He, apatite fission-track, apatite U-Th/He), which can be used to determine the rock's cooling history.

In the past, the majority of thermochronological studies in the Alps were very successful in determining the rate and timing of local exhumation. However, these studies were focused on currently exposed bedrock. This approach gives information about the exhumation of today's surface rocks but does not provide insight in the overall long-term exhumation in the geologic past on a regional scale. Yet, this information is available in the stratigraphic record of synorogenic sediments in adjacent foreland and hinterland basins. These sediments are the remnants of previously exposed bedrock. Fission-track analysis of detrital zircon from such synorogenic sediments allows the detailed reconstruction of the overall long-term exhumational history of the Alps.

Zircon is a common accessory mineral in many crustal rocks, and because there is no active volcanism in the Alps since the Oligocene, zircon FT ages are cooling ages related to exhumational cooling of crustal rock. Using the lag time concept (cooling age - depositional age), on suites of stratigraphically controlled samples, it is possible to estimate long-term (over tens of millions of years) exhumation rates, and to identify potential sediment source areas within the Alps.