Structural influence on glacial cirque morphology : the case of Pizzo Arera (Orobic Alps, Southern Alps, Italy)

Glacial cirques are typical, common morphologies in high mountain areas. They are represented by a semicircular hollow open downslope and bounded upslope by steep walls. The aim of this study is to point out the relationships between cirque morphology, bedrock structures and lithology and to suggest a genetic model of these landforms. The study area includes six glacial cirques and is located in the Orobic Alps, between Val Brembana and Val Seriana, on the Pizzo Arera (2512 m a.s.l.) massif. The Orobic Alps belong to the Central Southern Alps domain and are characterized by a series of thrust and folds with an E-W trend, a ramp and flat geometry and a southward transport direction (Schumacher et al., 1997). In particular, in the Pizzo Arera area the variable stratigraphy controls the distribution of detachment layers and lateral ramps of three thrust sheets (Schönborn, 1992). These thrust sheets are constituted by Triassic platform/basin limestones and dolostones, and are separated by continuos and thick detachment horizons controlled by interbedded marls. Geological, geomorphologic and structural data has been collected on the field (mapping scale 1:5.000) and then elaborated through a structural analysis (both at meso and micro-scale) and a geomorphologic analysis. The meso-structural analysis led to the identification of three deformation phases of Alpine age. The first one (D1 phase) is the most important: during this phase the emplacement of regional thrust surfaces is accompanied by minor structures such as folds (F1), foliations (S1 and S1bis), boudins, faults and fractures. The following deformation phases (D2 and D3) are characterized only by brittle deformation, producing faults and fracture planes. The micro-structural analysis was conducted on thin sections of rocks belonging to the thrust deformation zones. The thrust sheet limestones resulted affected by brittleductile deformation with the formation of cataclastic and mylonitic rocks in which five deformation stages have been detected. The geomorphological analysis permitted the recognition of macro and meso forms produced by gravitational processes, glacial action and karst dissolution. DEM (Digital Elevation Models) and aerial photos analysis allowed the identification of the distribution and morphological relief of structural elements in an area wider then the mapped one. Afterwards, in each cirque, the results of the different analysis were integrated, in order to find out the relationships between the different factors. In all the six cirques an important role is played by structural discontinuities, but differently in two group of cirques: the northward and westward open cirques and the southward open cirques. The first group shows a strong structural control on the different sectors that make up the cirque morphology: the bottom of three cirques follows the course of the same thrust surface and the lateral walls are always bounded by a fault, a fracture or the stratification surface; the slope processes are mainly represented by rock falls and slidings and the rupture surfaces are often controlled by faults and fractures. The second group exhibits the outcropping of more erodible rocks, therefore the structural surfaces have a less important control on slope processes (mainly debris flow and soil creep). In one of these cirque, though, a lateral wall is internally confined by an important fault and the difference in hight between the cirque floor and the ridge is compatible with the displacement and the kinematic of the fault, showing a possible active role played by tectonics. In conclusion, the genesis and development of the studied cirques was mainly due to gravitational movements directly controlled by the distribution of structures. Faults and fractures often lead the development of weakness zones playing a passive role in the slope shaping, except for one case. The glaciers seem to have played only a secondary role, filling the cirques already existing and removing the debris previously produced; however other studies are in progress to point out the extension of glaciers and better define their role.