Sequence of thrusting within a thick-skinned tectonic wedge: Evidence from 40Ar/39Ar and Rb-Sr ages from the Austroalpine Nappe Complex of the Eastern Alps

The sequence of thrusting within a regional-scale Alpine thick-skinned tectonic wedge-has been constrained by Rb-Sr and 40Ar/39Ar dating of whole-rock constituent white mica and biotite from structural units of the Austroalpine Nappe Complex, Eastern Alps (Austria). Preservation of Variscan and pre-Variscan isotopic signatures in pre-Alpine basement units indicates a non-penetrative metamorphic Alpine overprint. Alpine nappe assembly and accompanying ductile deformation occurred during maintenance of lower greenschist-facies metamorphic conditions (c. 300 degrees-450 degrees C). Analyses from greenschist-facies metamorphic mylonites and penetratively ductile deformed cover sequences display decreasing 40Ar/39Ar plateau ages structurally downward. Thrusting and accompanied ductile deformation commenced at c. 100-90 Ma in the uppermost structural levels. Footwall fault propagation effected internal deformation and out-of-sequence thrusting within intermediate structural levels at c. 95-80 Ma. This was contemporaneous with regional exhumation and cooling of deeper crustal basement units at c. 88-82 Ma and was associated with extension and strike-slip faulting, leading to the formation of synorogenic sedimentary basins. Additional footwall thrust propagation was associated with ductile deformation within lowermost structural units at c. 80-70 Ma. The structural and new geochronologic results presented herein suggest that (1) the Austroalpine Nappe Complex represents a forward propagating thrust complex that formed as a result of footwall propagation of a master fault and associated piggyback transport of hangingwall structural units; (2) most parts of the Austroalpine block occupied a lower-plate tectonic setting during Cretaceous tectonics subsequent to subduction of the Meliata-Hallstatt ocean; and (3) the Austroalpine Nappe Complex had already been formed when subduction of Penninic oceanic domains was active.