Abstract

Fission track (FT) analyses on unannealed detrital minerals provide a powerful tool both for refining provenance models derived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies consistently identify the uplifting western Central Alps as the main source region for the Macigno Formation.

Most zircon grains fall into a young age cluster (∼ 40-30 Ma), derived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distinguished at 30 and 40 Ma. In the younger subgroup, the zircon morphology supports the presence of two main populations: (1) from igneous rocks (S-type euhedral zircons), which appear to be partly derived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission-track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and migmatitic rocks affected by an Eocene high-temperature phase.

A Late Cretaceous age cluster (∼ 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively.

Finally, an ill-defined Jurassic age cluster, with a mean in Late Jurassic times, is related to rift-shoulder heating of the Austroalpine/South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neogene erosion is suggested.

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