The Tertiary dynamics of the northern Eastern Alps (Austria): changing palaeostresses in a collisional plate boundary

Abstract

Six Tertiary deviatoric palaeostress tensor groups from 165 stations in the Calcareous Alps describe the upper crustal dynamics of the leading edge of the Adriatic plate during protracted continental collision with the European lower plate. Palaeostress changes are correlated to the kinematic evolution of the plate boundary and to independently derived plate kinematic data of the Alpine-Carpathian-Pannonian area. Each palaeostress direction is defined by 32 to 89 individual tensors which constrain: (1) Late Eocene to ?Oligocene NW-directed compression during thrusting and dextral shearing on WNW-striking faults; NW-directed compression along the northern margin of the Adriatic plate is correlated to its N-directed translation combined with a counterclockwise rotation. (2a) ?Late Eocene to Early Miocene N-directed compression during N-directed thrusting of Penninic and Helvetic units and the Molasse; (2b) Early to Middle Miocene N-directed strike-slip compression during the onset of eastward lateral extrusion; the change from NW- to N-directed compression is interpreted to result from strain partitioning at the Periadriatic fault which has formed the new Adriatic plate boundary since the Oligocene. Dextral shearing on this fault accommodated continued anticlockwise rotation and decoupled the northern Eastern Alps from the rotational component. N-directed compression in the Calcareous Alps paralleled the N-directed translation of the Adriatic plate. (3) Middle Miocene NE-directed compression of thrust- and strike-slip type during lateral extrusion; NE-directed compression in the Calcareous Alps resulted from the drag of the eastward extruding central Eastern Alps. Sinistral shear stress was transmitted across the ENE-trending Salzach-Ennstal fault. NE-oriented σ 1-axes include 45° with the E-W-striking zone of distributed sinistral shear in the Calcareous Alps. (4) Middle Miocene E-directed extension associated with orogen-parallel normal faulting of the central Eastern Alps; E-directed extension paralleled the direction of mass transfer towards the Pannonian Basin during lateral extrusion. Normal faulting was induced by reduced lateral confinement due to removal of material east of the Alps. On the large scale, eastward motion was enabled by the E-directed retreat of the Eastern Carpathian subduction zone. (5) Late Miocene E-directed compression; orogen parallel compression obstructed lateral extrusion. The stress change from extension to E-directed compression was caused by the end of subduction retreat in the Eastern Carpathians due to dynamic changes in the subduction zone. (6) N-directed extension; Late- to post-Miocene extension paralleled recent topographic slopes in the foothill of the Eastern Alps. Palaeostresses changes correspond to distinct phases in the kinematic evolution of the northern Eastern Alps and to the systematic formation and reactivation of major fault zones. WNW-striking faults changed deformation styles from dextral shear to dextral transpression, reverse faulting and finally to sinistral shear. The Early to Middle Miocene Salzach-Ennstal fault which formed the northern boundary of the extruded central Eastern Alps changed from sinistral transpression to sinistral shear, sinistral transtension and finally to dextral shear.