LOOKING AT THE ALPINE RANGES OF EUROPE IN THE PAST: 40AR/39AR DATING OF WHITE MICAS FROM CLASTIC SEDIMENTS TELL A TALE OF PROVENANCE AND EXHUMATION

Clastic sediment sequences represent the record of rock units exposed at the surface in mountain ranges through time. Single crystal ⁴⁰Ar/³⁹Ar dating of white micas can be utilized to shed light on the provenance of various rock units present in the hinterland: isotopic 'fingerprinting'. In actively developing orogens there is potential to gain insights into the tectonic processes of cooling and exhumation as the orogen is developing. Assuming simple boundary conditions, predictions can be made whether cooling: moving isothermal plains, and moving mass: exhumation - erosion, are in equilibrium: steady state is reached.

White micas from 31 - 13 Ma sediments of the North Alpine Foreland Basin, Switzerland, revealed three age clusters indicating events in the Carboniferous, Permian, and Tertiary. Paleozoic mica ages exist in all samples. Tertiary micas from sediments younger than 21 Ma in central and western drainage systems document increasing cooling rates in the Lepontine dome at 14 Ma, which confirms thermal models based on currently exposed rocks.

The Tertiary Piemonte Basin on the internal side of the Western Alps acted as catchment area for the Ligurian Alps and the internal massifs of the Western Alps from 35 - 6 Ma. The age distribution of white micas in the detritus shows that from ~34 Ma until ~20.5 Ma the clastic sediments were fed mainly from a southern source that recorded Eocene HP metamorphism. From the middle Miocene, the main source area moved to a Western Alpine provenance: strong Eoalpine (70 Ma) and early Cretaceous signals (120 Ma) are recorded until the Present.

Thus in terms of white mica cooling the Lepontine Alps reached steady state whereas to the southwest rocks failed to do so.