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Tectonic constraints from 40Ar/39Ar ages of detrital mica from flysch and molasse basins of the Eastern Alps

NEUBAUER, F., SCHNEIDER, D., HANDLER, R., MADER, D., FRIEDL, G. & GENSER, J.H.

Institute of Geology and Paleontology, University of Salzburg, Hellbrunner Str. 34, 5020 Salzburg, Austria

The filling histories of Variscan and Alpine flysch and molasse basins of Eastern Alps and Bohemian Massif have been studied by 40Ar/39Ar dating of single and multi-grain samples (5-15 grains per sample) of detrital white mica in order to reveal exhumation processes in the hinterland. The main emphasis was to study the time lag between cooling of white mica through the Ar retention temperature (ca. 400 oC corresponding to ca. 12 - 16 kilometer overburden) during exhumation in the hinterland and the deposition. A second aim was to compare dating results of Variscan and Alpine orogens with previous studies carried out in the Himalayas. These authors reported a ca. 70-80 percent removal of the crust above the Ar isotopic retention temperature, and therefore, far-reaching rejuvenation of crust.

The studied Variscan basins include Carnic Alps, Greywacke zone, Gurktal nappe complex and the Graz Paleozoic of the Eastern Alps: These units comprise similar Ordovician to Devonian passive continental margin successions, Early Variscan syn-collisional flysch and Late Carboniferous to Permian Molasse sequences. As an important result we note that molasse sequences distinguish from flysch and passive continental margin successions by means of their high proportion of detrital mica within framework constituents. Furthermore, all three units display consistent 40Ar/39Ar dating results including:

Cadomian sources (ca. 580 - 640 Ma) in the Ordovician-Devonian passive continental margin sequences;

Devonian ages (390-373 Ma) within syncollisional flysch sequences;

and exclusively Carboniferous ages (333-309) in molasse sequences. This means that 100 percent of the upper continental crust (12- 16 km overburden) was removed prior to molasse deposition. This implies, therefore,e extremely high and rapid exhumation rates during Variscan orogenesis similar to the Cenozoic evolution of the Himalayas.

The data of Early Carboniferous flysch basins in the Eastern Alps and similar data from the eastern Bohemian Massif record a predominant Devonian, the Bohemian Massif also Early Carboniferous metamorphic sources where the time lag is ca. 40 to 10 Ma. The subsequent molasse basins are dominated by exclusively Variscan ages with a time lag of ca. 2 to 15 Ma. These relationships constrain rapid regional removal of upper crustal levels prior to molasse deposition and very rapid rock exhumation from deep crustal levels.

These results from Variscan sedimentary basins are contrasted by results from Alpine flysch and molasse basins. Here we report results from two basins:

1. The Rhenodanubian Flysch basin (Cretaceous - Eocene depositional ages, ca. 110-40 Ma) mainly records Variscan ages (340-320 Ma), subordinate ages from the extension stages between ca. 200-180 Ma, some ages between ca. 155-135 Ma of uncertain significance (formation of an accretionary wedge?) and a very few (less than 5 percent) Alpine ages due to initial plate collisional (ca. 110-100 Ma). The time lag between age of cooling and deposition is minimum ca. 30 Ma.

2. The Alpine Molasse basin (Late Eocene to Neogene, ca. 35-12 Ma), a peripheral flexural basin due to continent-continent collision, show a similar record: Nearly no Late Alpine ages, a broad scatter of subordinate Triassic to Cretaceous ages due to various extensional and initial compressional events and predominant Variscan ages. These data indicate that exhumation of metamorphic crust was minor during Alpine orogeny.

These relationships show that regional rejuvenation of continental crust within collisional orogens is essentially driven by large-scale tectonic processes like crustal underplating and growth of plateaus where upper plate upper crust is largely removed. The data also reveal that Variscan and Alpine orogenic processes were dissimilar. Compared to Variscan orogeny, Alpine tectonic processes led to insignificant crustal rejuvenation. The data also reveal that the Variscan orogeny contributed to formation and rejuvenation of the continental crust in Central and Southern Europe.