Deep-seated gravitational slope deformations in western Sicily: Controlling factors, triggering mechanisms, and morphoevolutionary models

• DSGSDs in Sicily affect areas underlain by brittle–ductile or purely brittle rocks.
• In the brittle–ductile rocks, the DSGSDs follow exhumation of a ductile substratum.
• In the purely brittle rocks, the DSGSDs are due to lateral/vertical decompression.
• The causes of the DSGSDs are linked to tectonics, incision by streams, and erosion.
• High relief energy is only needed to cause DSGSDs over purely brittle rocks.

A study of deep-seated gravitational slope deformation (DSGSD) phenomena affecting areas of various geological and geomorphological settings in western Sicily is described. Western Sicily is underlain by a thin-skinned imbricate wedge of Meso–Cenozoic carbonate and siliciclastic rocks that formed by the stacking of several thrust nappes over the Iblean foreland. Locally, the original thrust sheets are folded and cut by high-angle faults. Large areas of western Sicily now display high relief energy due to Plio-Pleistocene block-faulting and uplifting, and the Quaternary morphogenetic phases are characterised by incision, thereby triggering widespread DSGSDs. To identify controlling factors and triggering causes and to develop reliable morphoevolutionary models for the DSGSDs of western Sicily, a geomorphological study based on field surveys and aerial-photography interpretations was performed. Previous geomorphological data relating to well-known examples of DSGSDs were reconsidered, leading to remarkable revisions of the interpretative models in certain cases. New data were subsequently collected, enabling recognition of additional DSGSD phenomena. The whole body of data involves a total of 27 DSGSDs affecting areas in two specific geological settings: (1) areas with flat thrust surfaces, where differential settlements, back-tilting, lateral spreads in competent rocks overlying marls and clays, large topples, and/or block-type slope movements may develop; and (2) areas where deep-rooted carbonate units come into lateral contact with clayey–marly units along high-angle faults, where lateral spreads in brittle homogeneous rocks, sinking, and/or rock flows may occur. These DSGSD phenomena are associated with different evolutionary stages, allowing a morphoevolutionary model to be defined for the two geological conditions. For these two different morphoevolutionary models, the following structural features play an important role in the development of DSGSDs: (1) where carbonate bodies overlie clayey–marly rocks, triggering of the DSGSDs traces back to deformation of a ductile substratum that follows the exhumation of the flat thrust planes and the underlying clayey–marly rocks due to block-faulting and/or stream incision; and (2) where homogeneous carbonate rocks hundreds of metres thick crop out, the DSGSDs are triggered by very high relief energy and tensile stress that follow the combined actions of Quaternary block-faulting and stream deepening associated with differential erosion.

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