Open Session - SWISS GEOSCIENCE MEETINGs

Symposium 1: Structural Geology, Tectonics and Geodynamics
1.33
Structural influence on glacial cirque morphology: the case of Pizzo Arera
(Orobic Alps, Southern Alps, Italy)
Marzorati Andrea*, Ghiselli Alice* & Bini Alfredo*
*Dipartimento di Scienze della Terra „A. Desio”, via Mangiagalli 34, I-20133 Milano (andrea.marzorati@email.it)
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.
REFERENCES
Schönborn G. 1992: Alpine tectonics and kinematic models of the Central Southern Alps. Memorie di Scienze Geologiche,
44, 229-393.
Schumacher M. E., Schönborn G., Bernoulli D. & Laubscher H. P. 1997: Rifting and collision in the Southern Alps. Deep
Structure of the Swiss Alps, 186-204