FAULT SLIP ANALYSIS ALONG THE NORTHERN MARGIN OF THE EASTERN ALPS (MOLASSE, HELVETIC NAPPES, NORTH AND SOUTH PENNINIC FLYSCH, AND THE NORTHERN CALCAREOUS ALPS)

Fault-slip data are analyzed and paleostress tensors computed from 84 stations along the northern margin of the Eastern Alps between Liechtenstein and Vienna. Most fault sets resulted from more than one deformation. Heterogenous sets are divided into homogenous subsets by cross-cutting relationships observed in the field, and stress tensors are calculated separately for each subset. The deviatoric stress tensors fall into six distinct groups that are regionally consistent in orientation, stress ratio, and relative age throughout the northern margin of the Eastern Alps. From oldest to youngest the tensor groups are as follows: (1) sigma1 subhorizontal E-W, sigma3 subvertical: thrusting-related E-W compression. This group is only encountered in the South Penninic flysch and the Northern Calcareous Alps; (2) sigma1 subhorizontal N-S, sigma3 subvertical: thrusting-related N-S-directed compression; (3) sigma1 subhorizontal N-S, sigma3 subhorizonal E-W: strike-slip faulting conjugate to N-S compression; (4) sigma1 subvertical, sigma3 subhorizontal E-W: E-W-directed subhorizontal extension; (5) sigma1 subhorizontal E-W, sigma3 subhorizontal N-S: strike-slip faulting during E-W compression; and (6) sigma1 subvertical, sigma3 subhorizontal about N-S: sigma3 trajectories are oriented perpendicular to the general strike of the Eastern Alps and parallel topographic gradients. The six trensor groups correlate with distinct steps in the Cretaceous and the post-Eocene tectonic evolution of the northern units of the Eastern Alps: (1) Thrusting-related E-W compressional structures in the South Penninic flysch and the Northern Calcareous Alps are correlated to Cretaceous top-to-W-directed thrusting in the South Penninic and Austroalpine domains. (2) Thrusting-related N-S compressional structures encompass large-scale folds with E-W-trending axes, rare E-W-oriented thrust branch lines and cut-off lines, and S-dipping ramps in the Rhenodanubian Flysch. Tensors reflect the stresses during northward thrusting in the Late Eocene-Early Miocene. (3, 4) Strike-slip faults and extensional structures depicting N-S compression and E-W extension formed during the eastward lateral extrusion of the Eastern Alps. Prominent reactivations of strike-slip faults during subhorizontal extension in the east and permutation between sigma1 and sigma2 reflecting periodic superposition of strike-slip and normal faulting in the western Eastern Alps during overall E-W extension suggest a close relationship of both tensor groups. Deformation resulted in the eastward lateral motion of the northern units of the Eastern Alps that started during the Miocene. (5) The significance of Late Miocene E-W-directed compression reactivating pre-existing fractures and faults is presently not well understood. We tentatively relate this event to a major change of boundary conditions within the Miocene. (6) The youngest deformation recorded is caused by surface uplift and gravitational collapse of the Eastern Alps.