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Volume 19 (2) - 1996

D’AMATO AVANZI G. & PUCCINELLI A.

Deep-seated gravitational slope deformations in North-Western Tuscany (Italy): remarks on typology, distribution and tectonic connections

Pages 325-334

Abstract

Some features make north-western Tuscany prone to Deep-seated Gravitational Slope Deformations (Dgsd), such as: high relief energy, high rainfall, intensely fractured and deformed rocks, active or recently active tectonics, strong seismicity. The territory investigated shows many examples of such phenomena, which differ owing to their geological and structural conditions, typology and activity. The case history shows that both tectonics and lithostratigraphic structures have greatly influenced typologies and kinematic mechanisms, whilst neotectonic evolution and climatic conditions have had their influence on all phenomena studied. Among the cases where the tectonic structure is a prevailing factor, deep-seated gravitational slope deformations located on normal faults could be quoted, as those outlining the tectonic depressions of the Serchio and Magra valleys or other regional normal fault systems. These faults release and subdivide the bedrock into very large blocks, making them subject to gravitational adjustment. In addition, a preferential orientation of geomorphic features (scarps, trenches, reverse slopes, etc.) can be verified in accordance with the trends of tectonic displacements and brittle deformation systems. Some examples include the gravitational processes near Canossa, Bagnone and Chioso in Magra Valley, near San Romano in Garfagnana in the Serchio Valley and near the Abetone Pass. Block slide and rock flow are generally common types of movement, often structurally controlled by fault planes. The lithostratigraphic structure may be frequently regarded as the main control feature in the development of deep-seated gravitational slope deformations; in particular, a structure of thick rigid rocks overlying ductile rock types is instrumental. In this case, block slides and lateral spreads are the most common kinds of movement. Some examples are found at Mt. Castri in the Serchio Valley (sandstone over shale) and in the Magra Valley near Bagnone and Chioso (marly limestone overlying shale). The underlying ductile rocks are remarkably deformed by tectonics and sometimes show deformations resulting from the load of overlying brittle rocks, such as bulges and reverse slopes. Along the deep valleys which transversally cut the main tectonic structures, the topographic stress is sometimes considerable, owing to the Pleistocene – Holocene uplift; brittle/ductile deformations may then occur, due to gravitational slope tension rather than tectonic stress. Finally, a considerable amount of the Dgsd here studied are believed to be dormant, because of debris filling up the trenches and no movement evidence in the past decades; in some cases, evidences of activity were recognised.

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