Sequence of Thrusting Within A Thick‐Skinned Tectonic Wedge: Evidence From40Ar/39Ar and Rb‐Sr Ages From the Austroalpine Nappe Complex of the Eastern Alps

Abstract

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°–450°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 foot‐wall 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.