STRUCTURAL EVOLUTION OF THE WESTERN DRAU RANGE, EASTERN ALPS: A DEXTRAL PRECURSOR OF THE PERIADRIATIC FAULT?

Depending on rheology, strike-slip systems migrate within orogens. However, this fact received surprisingly little attention. We show an example from the Drau Range (DR), which is separated from the Periadriatic (PA) fault in the S by the Gailtal metamorphic complex. The DR comprises an Upper Carboniferous to Cretaceous cover sequence squeezed in between the Gailtal basement in the S and the Kreuzeck basement in the N. The DR is tectonically isolated from other Austroalpine and South Alpine cover units. Triassic facies relationships exist to the western Northern Calcareous Alps, which implies significant sinistral displacement along an unknown ca. E-trending fault. Major strike-slip faults border the DR: the ENE-trending sinistral Drautal fault in the NW, the dextral NW-trending Isel and Möll faults in the N and E.

Within the DR, large tight high-amplitude, ca. E-trending, N-vergent folds prevail and indicate dominant N–S shortening. In the central and western part, the fold structure is overprinted by the DR South Margin (DRSM) fault representing a crustal-scale first-order strike-slip fault subparallel to the PA fault. The DRSM fault juxtaposes Permian-Lower Triassic siliciclastics to Middle Triassic to Jurassic carbonates, and incorporates major shear lenses of metamorphic basement rocks. Dislocation and counter-clockwise rotation of fold structures, including a prominent syncline with Jurassic formations, occurred along NW-trending splay faults. The Drautal fault is younger than the DRSM. A prominent WNW-trending sinistral strike-slip duplex (Pittsberg wedge) with more than 10 km lateral offset has an uncertain age-relationship to the DRSM. A conjugate shear system indicating N-S shortening displaces the DRSM. The youngest visible structures are the NW-trending Isel and Möll Valley faults, which are interpreted as Riedel shears to the PA fault.

We discuss the tectonic significance of the DRSM together with other, hitherto not considered strike-slip faults (e.g. E-trending Bleiberg fault) along the southern margin of the DR and propose a tentative model: The DR is an original extension of the Mesozoic units now exposed 80 km to the W (Penserjoch), which was later disrupted by the Drautal and subsequently by the PA fault. In this case, the DRSM fault would represent a dextral precursor of the PA fault.