Thick‐skinned versus thin‐skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic ‐ Austria)

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Abstract
The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low‐grade metamorphic conditions, and (3) frontal, thin‐skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick‐skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin‐skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal‐scale ramps during foreland‐directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal‐scale rheological con‐trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (>610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I‐type affinities (∼ 590 Ma).