Abstract
Abstract. We compare data from three deep-seated gravitational slope deformations (DSGSDs) where palaeoseismological techniques were applied in artificial trenches. At all trenches, located in metamorphic rocks of the Italian Alps, there is evidence of extensional deformation given by normal movements along slip planes dipping downhill or uphill, and/or fissures, as expected in gravitational failure. However, we document and illustrate – with the aid of trenching – evidence of reverse movements. The reverse slips occurred mostly along the same planes along which normal slip occurred, and they produced drag folds in unconsolidated Holocene sediments as well as the superimposition of substrate rocks on Holocene sediments. The studied trenches indicate that reverse slip might occur not only at the toe portions of DSGSDs but also in their central-upper portions. When the age relationships between the two deformation kinematics can be determined, they clearly indicate that reverse slips postdate normal ones. Our data suggest that, during the development of long-lived DSGSDs, inversion kinematics may occur in different sectors of the unstable rock mass. The inversion is interpreted as due either to locking of the frontal blocks of a DSGSD or to the relative decrease in the rate of downward movement in the frontal blocks with respect to the rear blocks.
Highlights
Deep-seated gravitational slope deformations (DSGSDs) consist of 10–100 m thick rock masses, which can involve the whole slope of a mountain and are affected by gravitational instability (Zischinsky, 1966; Nemcok, 1972; Radbruch-Hall et al, 1977; Savage and Varnes, 1987)
The Foscagno site suggests that activation of at least a part of a DSGSD can originate from progressive downslope movements of the unstable rock mass along discrete slip planes
It may be claimed that the presence of different lithotypes at the Bregaglia site is consistent with the more complex structural architecture of this DSGSD, we argue that the amount, orientation and kinematics of the various slip planes of a DSGSD can result from a more complex series of parameters; these may be explained in terms of (i) the presence of slip planes inherited from previous tectonic phases, (ii) the amount of gravity deformation and the degree of development of the DSGSD, (iii) the geometry of the basal main slip plane, (iv) the topography, and (v) the lithology of the involved rock types
Summary
Deep-seated gravitational slope deformations (DSGSDs) consist of 10–100 m thick rock masses, which can involve the whole slope of a mountain and are affected by gravitational instability (Zischinsky, 1966; Nemcok, 1972; Radbruch-Hall et al, 1977; Savage and Varnes, 1987). In order to improve the hazard assessment of DSGSDs, the reconstruction of their kinematics is of paramount importance to gain a better knowledge of their evolution and expected ground deformation This is usually achieved thanks to in situ instrumentation and radar interferometric techniques designed to analyse active structures. It is possible to reveal the presence of shallow deformation structures, measure their geometry and kinematics, and define their spatial and chronological characteristics These data are fundamental to better constrain the structure of a DSGSD and to carry out numerical and analogue modelling with a more robust set of data inputs. Trenching allows better understanding the significance of surface landforms produced by slope deformation This methodology enables assessing the age of the main past deformations, which is paramount to define the long-term behaviour of a DSGSD. Our data and interpretations might help shed light onto the workings of gravitational structures and contribute to understanding how DSGSDs may develop over time
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