The Tauern Window is the largest tectonic window of the Alps. It contains relics of the subduction channel that formed during Cenozoic subduction of the Alpine Tethyan Ocean and the European margin below the Adriatic plate. This thesis documents the structural, kinematic and metamorphic evolution of a segment of this subduction channel exposed in the central Tauern Window.
The thesis presents new structural data that document a tens-of-kilometers-scale recumbent sheath fold in the center of the Tauern Window. The fold comprises an isoclinally folded thrust that transported relicts of the former Alpine Tethys (Glockner Nappe s. str.) onto a distal part of the former European continental margin (Rote Wand Nappe). New petrologic data indicate that the fold formed during early stages of exhumation from maximum burial depth at high-pressure conditions (ca. 2 GPa, 500 ◦C). Exhumation of the fold to mid-crustal levels is evidenced by near-isothermal decompression to roughly 1 GPa. The fold ascended in the subduction channel between two contemporaneous opposite-sense shear zones; normal-sense (i. e., top-hinterland) at the top of the fold and thrust-sense (i. e., top-foreland) below.
The subduction-related thermal structure of the sheath fold was constrained by Raman spectroscopy on carbonaceous matter (RSCM) thermometry. The greatest peak-temperature conditions are located in the center of the fold at the folded ocean-on-continent thrust. The peak-temperature contours are oriented roughly parallel to the folded nappe contact so that they display a sheath-like pattern that mimics the geometry of the sheath fold itself. This pattern indicates that finite strain during sheath-fold formation decreased laterally from the fold’s center. Thus, together with the contemporaneous vertical strain gradients, the fold was shaped at least initially by diapir-like kinematics during exhumation.
Lithostratigraphic correlation of the tectonic units in the central Tauern Window that derive from the European margin documents intense, rift-related segmentation of the margin. The Rote Wand Nappe, which is part of the sheath fold, probably originates from an extensional allochthon that was separated from the main margin by an extensive rift basin above strongly thinned continental basement. Here it is proposed that this extensional allochthon, when subducted, caused a perturbation of the flow field in the subduction channel. In line with sheath-fold theory, this caused strain localisation at the extensional allochthon, facilitating its initial diapiric ascent to form a proto-sheath fold. While being further exhumed by the normal-sense shear zone at its top, the rest of the fold was amplified in overall, thrust-sense simple-shear to a pronounced sheath-fold geometry. This model and above observations are compatible with forced channel flow as driving force of the upward-directed flow.
This study shows how large structures inherited from rifting — e. g., extensional allochthons — could potentially induce perturbation of flow in subduction channels during the subduction of distal continental margins. Such perturbations can lead to complex kinematics in subduction channels and result in highly non-cylindrical nappes. Further investigation of similar scenarios could contribute to a better understanding of subduction zone dynamics during the transition from oceanic subduction to continent collision.
