EXHUMING THE ALPS THROUGH TIME: CLUES FROM DETRITAL FISSION-TRACK AGES

Fission track grain-ages (FTGA) of detrtial zircons from 16 major river systems, which carry sediment from the European Alps to the fore- and hinterland indicate symmetric erosional exhumation. Comparable FTGA distributions were found in detrital samples on both sides of the orogen. Using detrital samples from siliciclastic sediments of stratigraphic sections in adjacent basins allows to reconstruct the exhumational history of this convergent mountain belt. Recent work (Bernet al. 2001) on Miocene sandstone units from the Alpine hinterland suggests that FTGA from detrital zircons long since eroded from the Alps have maintained a relatively steady lag-time distribution since at least 15 Ma. Lag time refers to the difference between the time of fission-track (FT) closure at ~240oC, and the time of deposition in the basin. Calculated lag times can be used to estimate long-term average exhumation rates, which are between 0.7 and 0.4 km/m.y. for the fastest exhuming source regions in the Central and Western Alps. This steady condition suggests that the orogen overall was close to a general long-term steady state, including a balance between the accretionary flux of material into the orogenic system by convergence and the erosional flux of material out of the system. Our ongoing work is focused on: 1) extending the range of dated samples from the hinterland back into the Oligocene with samples from the Cervarola and Macigno Formations of the northern Apennines; and 2) examining foreland basin sediments from Eocene to Upper Miocene sandstone units of France and Germany. The purpose of this new work is to: 1) determine the onset of the zircon FT exhumational steady state, 2) resolve when symmetric exhumation was established and 3) judge the extend and significance of possible contamination of the exhumational signal by surface-cooled zircons associated with Periadriatic volcanism (28-32 Ma).