USING DETRITAL ZIRCON FISSION-TRACK AGES AS A PROXY FOR LONG-TERM EXHUMATION OF MOUNTAIN BELTS

Convergent mountain belts are characterized by large volumes of synorogenic sediments in their fore- and hinterland basins. These sediments are evidence of continuous erosional exhumation. Therefore, detrital zircon fission-track (FT) ages from stratigraphically superimposed samples from such basin sediments can be used as a proxy for long-term exhumation of mountain belts. To study the long-term exhumation of the European Alps over 15 stratigraphic samples were collected from the French and German Alpine foreland basin from Eocene to Upper Miocene sandstone units, and about 20 samples were collected from Oligocene to Pliocene turbidite sequences of known Alpine provenance in the Italian hinterland. Additionally about 20 samples from rivers draining the Alps were collected to test detrital FT analysis against bedrock FT ages in specific drainage areas. All zircon samples were dated using the external detector method for FT analysis. To identify major age components in a detrital grain-age distribution all results were decomposed into main peaks using the binomial peak-fitting method. Peak ages were used to calculate lag times (peak age - depositional age). The youngest age component or peak of each sample (P1), with the shortest lag time, is the most diagnostic for exhumation because grains of this peak are derived from the fastest exhuming areas of the mountain belt. Both, the Alpine fore- and hinterland samples show a similar pattern in lag time evolution. From about 27Ma to the present the lag time for the youngest peak (P1) remained fairly constant around 8 m.y. This means that zircons were exhumed in a steady fashion and delivered to the fore- and hinterland basins since the Oligocene, indicating a relatively constant average long-term exhumation rate of about 0.7km/m.y. for the fastest exhuming areas. The source areas of these young zircons can be found in certain parts of the Central and Western European Alps. Exhumation is driven by a combination of normal faulting and erosion in these areas. Short-term exhumation rates can be highly variable and less steady than the long-term signal derived from zircon FT analysis.