Weathering of crystalline-metamorphic rocks in the Allaro and Amusa River basin (Serre Massif, Calabria, Italy): general aspects and effects of thermal-metamorphic contact belts

Abstract

The study of natural slope morphodynamics is an extremely complex subject. Aspects to be investigated range from lithological-structural setting, state and type of activity of landforms and processes, climate and related runoff water and groundwater circulation, to behaviour of the rock mass and/or soils involved, also in response to weathering processes. Numerous studies have been conducted on the interactions between weathering processes and their effects at rock mass and slope scales, in particular on the Calabrian arc massif. These studies were basically aimed at understanding predisposing factors and triggering mechanisms of mass movements. The results reported in the paper are focused on the Allaro and Amusa river basin (Reggio Calabria, South Italy), which is located on the Ionian side of the Serre massif. Diffuse landsliding in the study area arises from the combination of multiple factors and, above all, from intense and deep weathering of the crystalline-metamorphic bedrock. After giving a detailed account of the geoenviromental context of the area under review, the paper discusses the findings from a number of minero-petrographical and chemical analyses. These analyses were expected to: i) assess the different susceptibility of the various lithotypes to weathering; ii) develop a weathering susceptibility scale and iii) elucidate relations between the resulting weathering classes of the rock mass. Detailed analyses were also carried out in the Mt. Granieri-Salincriti slope area, typifying a geological setting which is common at the boundaries of the Sila and Serre massifs of Calabria. The slope, experiencing a deep-seated gravitational deformational process, has a metamorphic aureole which is thought to be responsible for intense geo- chemical decomposition processes. Geomorphological evidence as well as macroscopic and microscopic aspects ascribable to a thermal-metamorphic contact suggest that relations between deep chemical weathering and its effects on the geomechanical and rheological behaviour of local materials (characterised by weak belts) are among the determinants of the observed deep deformations. Hence, the case study was intended to gain further insight into the geochemical processes at play and to estimate their effects at rock mass scale, with a view to identifying the rock volumes exposed to intense chemical alteration. These analyses hinged on wide-ranging studies and on a multi-disciplinary approaches. Their results are expected to provide a significant experimental contribution to the acquisition of quantitative data for building a correct geological-evolutionary model of the slope.