Stress Patterns and Crustal Anisotropy in the Eastern Alps: Insights From Seismological and Geological Observations

Abstract

In the Eastern Alps, the indentation of the Adriatic promontory since the Cenozoic affected the kinematics of separate crustal domains bounded by faults that accommodate lateral extrusion processes and differential shortening. Deciphering the pattern of crustal stresses in the orogen interior is challenging, due to the lack of in situ stress measurements at crustal depths. We define stress regimes and the orientations of the most-compressive horizontal stress (S_Hmax) by integrating published results with new data, including stress analysis from fault plane solutions, estimation of crustal anisotropy through the shear wave splitting analysis, paleostress determination from fault slip data, and computation of cumulative seismic displacements. The retrieved regional S_Hmax are generally consistent with N-S convergence. In the northern part of the study area, current stress orientations are almost parallel to paleo-S_Hmax, suggesting a rather uniform compressional regime since the late Cenozoic. Conversely, sharp deflections and divergence with paleo-S_Hmax appear at the western border of the Adriatic promontory across major transpressive and extensional shear zones and in the Southalpine domain, indicating a change in tectonically induced second-order stresses. A current strike-slip regime with subordinate orogen-parallel seismic displacements affects a belt to north of the Periadriatic Lineament and NE-extension characterizes the Ortles-Engandine region. Seismic anisotropy locally exhibits fault-parallel fast axes (Brenner-Giudicarie fault-systems, Dinaric and Southalpine thrusts), whereas stress-induced anisotropy parallel to S_Hmax characterizes the southern part of the orogen. Cumulative seismic displacements are small compared to geodetic ones, and unravel partitioning of deformation into second-order transpressive and extensional belts in response to indentation.