CENOZOIC TECTONICS OF THE ALPINE-CARPATHIAN-MEDITERRANEAN SYSTEM: HOW MANY PLATES, HOW MANY SUBDUCTION ZONES?

The southward subduction of the European margin beneath the Adriatic Plate resulted in nappe assembly and high-pressure metamorphism of Subpenninic nappes within the Eastern Alps at 33-31 Ma. Although accompanied by massive crustal thickening, this evolution within the lower plate is hardly reflected in the upper plate. Intramontane basin formation and distinct signatures of faulting activity indicate orogen-parallel faulting either prior or subsequent to Subpenninic nappe assembly. Late-orogenic normal faulting subsequent to the juxtaposition of European and Adriatic continental margins is documented along the entire length of the Alps and there is a broad consensus that much of the exhumation history of deep seated rocks is related to slip along these normal faults. The structure of the Eastern Alps is characterized by a system of fault zones that developed during late Oligocene to Miocene times, related to orogen-parallel escape of Austroalpine units towards east, a process also termed lateral extrusion. Apatite fission track ages and sedimentary ages from intramontane basins show that the general process of extrusion can be resolved into distinct tectonic phases. This is used to constrain the mechanics of orogen-parallel tectonics and the configuration of plate boundaries in the Alpine-Carpathian-Mediterranean realm.

During Oligocene to Middle Miocene times extrusion-related faulting continuously propagated from the western towards the central eastern part of the Eastern Alps. Subpenninic high-pressure assemblages of the lower European Plate were exhumed contemporaneously. This is confined by oblique convergence between the Adriatic and European plates. Within this time span the Eastern Alps experienced oblique shortening due to the quickly retreating Apennine subduction zone. Slab retreat along the Carpathian subduction zone did not substantially contribute to lateral extrusion until Middle Miocene times.

During Middle Miocene times extrusion became not only lateral but was characterized by a displacement vector at a high angle to the strike of the orogen. Extrusion was controlled by overall extension and the influence of north-directed compression triggered by the northward moving Adriatic Plate diminished. Instead, the influence of the retreating Carpathian subduction zone increased.