The stress field in the frontal part of the Eastern Alps (Austria) from borehole image log data

Abstract

Stress orientations in the frontal part of the Eastern Alps are inferred from borehole breakouts and drilling-induced tensile fractures depicted in image logs from 62 vertical or slightly deviated wellbores with up to 5km depth. Borehole breakouts (2463 individual features with 1673.9m cumulative length) are observed in all the wells except one, whereas drilling induced fractures only occur in four wells.Microresistivity image log data from the Alpine foreland, the external thrust sheets (allochthonous Molasse, Helvetic- and Flysch nappes) and the autochthonous units in the footwall of the Alpine units reveal consistent in-situ stress orientations with approximately N-S-directed maximum horizontal stresses (SHmax). Indications for stress partitioning across thrusts including the Alpine floor thrust are not observed. In the Western Sector of the study area the mean SHmax is oriented 002° with a standard deviation (SD) of 11.1°. In the central and eastern sector, the mean SHmax is oriented 177° (SD 14°) and 173° (SD 14.3°), respectively, indicating a counterclockwise rotation of SHmax of about 10° from west to east. The observed stress indicators and the analysis of deviated wells suggest a strike-slip regime for the western and Central Sector, whereas a thrust regime is probable in the east of the study area. The thrust regime there coincides with the occurrence of Middle Pleistocene conglomerates which were previously described to be displaced by Alpine thrust faults.The orientations of SHmax fit into the general observation that SHmax directions at the Alpine front are oriented about perpendicular to the strike of the mountain belt and rotate along the Alpine arc. Our stress orientations - including those derived from the autochthonous basement - are, however, markedly different from the NW-SE-directed maximum horizontal stresses reported from the Variscan units in the European foreland north of the study area.