Structural constraints on deep-seated slope deformation kinematics

A significant sackung-type deep-seated slope gravitational deformation (DSG SD) was recognised for the first time by the authors in the middle part of Valfurva, east of Bormio (Rhaetian Alps, Italy). The reconstruction of its kinematics, age and state of activity is presented, through a detailed desc ription of its morphological, geomechanical and structural features. An int egrated multi-disciplinary approach was performed to achieve a clear compre hension of the phenomenon. Field surveys and aero-photo interpretation were carried out in order to clarify the structural, geological and geomorpholo gical setting in which the DSGSD developed. A kinematic conceptual model of the slope deformation was developed through the analysis of morphostructur es, of their significance and relationships with lithological markers and Q uaternary deposits. After a geomechanical characterisation of the rock mass involved in the slope deformation, numerical modelling was performed to ve rify the hypotheses made on kinematics and driving factors of the phenomeno n. The sackung affects pre-Permian metapelites, metabasites and marbles bel onging to the Upper Austroalpine basement of the Campo Nappe, as well as La te Pleistocene and Holocene glacial and rock glacier deposits. The deformat ion started after the Late-Wurmian age (15,000-11,000 years B.P.), and cont inued until few centuries ago, not excluding a present-day low-rate activit y. Deformation consists in a large oblique "sageing" along a deep confined sliding surface, associated with gravitational reactivation of pre-existing (late-Alpine and recent) tectonic brittle structures, lending to the forma tion of N-S and WNW-ESE trending gravitational morpho-structures. The evolu tion of the WNW-ESE trending system, resulting in asymmetric trenches, led to progressive failure of the lower part of the slope during the last 10,00 0 years, as testified by large paleo landslide accumulations, and it is sti ll in progress. Numerical modelling indicates post-glacial unloading as the main triggering factor of the slope deformation. The importance of this de ep-seated slope deformation is enhanced by the occurrence of the 30 Mm(3) a ctive "Ruinon" landslide in the lower part of the slope. Such landslide is subjected to rapid evolution and threatens the valley floor, establishing a n important risk factor connected to human lives and socio-economic activit ies. (C) 2001 Elsevier Science B.V. All rights reserved.