Deep-Seated Slope Deformations in active tectonics areas: the Mt. Watles sackung (Eastern Central Alps, Italy)

Despite of the large number of published works, the understanding of Deep Seated Slope Deformation kinematics, triggering and controlling factors is still at the beginning. The role of structural settings and active tectonics in the triggering and development of a sackung is particularly debated, as well as the importance of tectonic (fractures, in situ stress) versus climatic (postglacial debuttressing) features. Large Deep Seated Slope Deformations have been recognised all along the Upper Venosta Valley (Eastern-Central Alps, Italy), and particularly in the Mt. Watles area. Along the western side of the valley, rocks consist of paragneiss, mica-schist and ortogneiss belonging to the Austroalpine basement of the Otztal Nappe. They lie on an important Alpine shear zone (known as “Shlinig Line”) active during Upper Cretaceous. This zone separates the Otztal basement from the underlying dolostones belonging to the Campo Nappe. The structural setting developed during the oldest stages of the Alpine orogenesis has been complicated by the development of brittle structures, mainly consisting of normal and strike-slip faults active at least since Late Miocene. Basing on detailed mesoscopic analyses, we recognised a complex recent evolution suggesting the activity of N-S left-lateral strike-slip faults followed in time by the activation of conjugate NNW-SSE and NNE-SSW trending sets. Although no indications of superficial fault activity has been detected in the area, seismological data from this sector of the Central-Eastern Alps suggest neotectonic activity at very low rates of deformation (0.1 mm/y N-S convergence; Westaway, 1992). The whole Mt. Watles is involved in a large sackung, characterised by huge gravitational morphostructures (doubled ridges, scarps and counterscarps), often filled by lacustrine and peat deposits. The sackung activity obliterated glacial forms and deposits, well preserved in surrounding areas. Morphostructures trend NE-SW, suggesting a strong structural control on the sackung. A numerical model of the slope deformation was performed by finite difference method (FDM). The model suggests post-glacial debuttressing as a triggering factor, while structural features (Shlinig Line, NE-SW fractures) play a passive control on the sackung evolution. The role of neotectonic activity is discussed and remains unclear, since the high rate of the gravitational deformation should mask neotectonic effects.