Time constraints and plate tectonic controls for lateral extrusion in the Eastern Alps

Late-orogenic normal faulting subsequent to the juxtaposition of European and Adriatic continental margins has been 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. This fault system is related to orogen-parallel escape of Austroalpine units towards east, a process also termed lateral extrusion. Lateral extrusion encompasses tectonic escape (plane strain horizontal motion of tectonic wedges driven by forces applied to their boundaries) and extensional collapse (gravitational spreading away from a topographic high in an orogenic belt). Northward oblique indentation by a rigid crustal block (the so-called Adriatic indenter represented by the Southern Alps) caused thickening in front of the indenter and east- directed tectonic escape. This study comprises a review and discussion of the classical model of lateral extrusion in the Eastern Alps, including the evolution of the confining fault systems in space and time. Distinct phases of extrusion are discerned by thermochronological data from the area of the Eastern Alps east of the Tauern Window. During Mid Miocene times the extrusion of the Central Australpine orogenic lid was not only lateral in terms of parallel to the trend of the orogen, but was characterized by a displacement vector at high angle to the strike of the orogen. This resulted in the exhumation of the so called Schladming block to the east of the Tauern Window and detachment of the Gurktal Block along the Katschberg - Niedere Tauern Southern Fault System. The eastern termination of the Gurktal block is defined by the Pöls-Lavanttal Fault System. Simultaneously the Pohorje Pluton intruded an extensional bridge at the southern termination of the Pöls-Lavanttal Fault System. The Dinaric trench holds a prominent position with respect to the East Alpine extrusion corridor because it separates the wedge into two domains with distinctly different evolution. The domain to the west of the trench, i.e., to the west of the Pöls-Lavanttal fault system, was continuously under compression, the area to the east was continuously under extension. The early phase of the Adriatic or Southalpine plate motion (30-15 Ma) resolved in SW-NE compressive stresses and NW-SE tension. This released initial sinistral shear along the Oligocene Periadriatic lineament, which is in concordance to kinematic studies, and sinistral shear along the Inntal fault. Simultaneously Oligocene plutons, that are exclusively found to the west of the Pöls-Lavanttal fault system, intruded along the Periadriatic fault. From Mid-Miocene times onwards stresses released by the Adriatic plate became N-S compressive leading to shear reversal along the Periadriatic fault system that now became dextral. Direction of compressive stresses during this period was fairly orthogonal to the Periadriatic fault. Thus we suggest that dextral displacement is to a lesser extend stress induced but much more controlled by eastward motion (extrusion) of Austroalpine units that experienced enhanced extension between 15 and 12 Ma. During this phase the Periadriatic fault may therefore be visualized as a southern boundary fault (i.e., stretching fault) of the extruding East Alpine wedge that accommodated extrusion. Interestingly, deposition of intramontaneous basins commenced at this time (ca. 15 Ma) suggesting onset of enhanced extrusion induced exhumation within eastern sectors of the central Austroalpine realm. By contrast, the domain to the east of the Adriatic – Pannonic plate boundary (east of the Pöls-Lavanttal system) remained under extension throughout time.