The effects of brittle-plastic transitions in basement-involved foreland belts: the Central Southern Alps case (N Italy)

Abstract

The Central Southern Alps (CSA), a 100-km-wide basement-involved fold-and-thrust belt, formed during the Europe-Africa convergence from the Cretaceous onward. Two main phases of deformation have been recognized in the Central Southern Alps. The first is a pre-Late Oligocene thrusting of pre-Alpine basement units upon Triassic cover sequences which is sealed by the Adamello intrusion (42-30 Ma) in the northern sector of the belt. The folds and thrust faults of this phase are locally overprinted by a post-Adamello deformation phase, possibly corresponding to the Middle-Late Miocene (16-6 Ma) Milano phase, recorded by the Oligo-Miocene sediments buried under the Plio-Pleistocene Padane plain. Several lines of evidence indicate that compressive deformations, originating from the current relative convergence between Eurasian and Adriatic plates, are still occurring within the Southalpine wedge, which can therefore be modelled as a critical wedge. A simple brittle rheology is not adequate to explain the slope and plateau topographic features of the belt which compare more favourably with those predicted by a critical taper model including the effects of temperature-dependent deformations. Low-angle surface slopes, typical of frontal sectors of wedges deforming with a purely brittle behaviour, are interrupted by relatively steeper slopes, where the wedge base crosses the brittle-plastic (B/P) transition and starts creeping while the basal décollement, deforming at higher strain rates, still behaves frictionally. Further to the rear, where both the wedge base and the décollement are plastic, the surface slope flattens (plateau). Calculated brittle-plastic transition depths along the basal décollement fit well with observed topographic changes. The location of slope-to-plateau topographic changes in the CSA is strongly influenced by surface heat-flow distribution, and the occurrence within the wedge and in particular at the wedge base of rock types characterised by different plastic rheological parameters, is responsible for repeated topographic changes, each related to a different B/P transition. The transition from brittle to plastic behaviour at the base of the wedge is responsible for a significant weakening within the basal décollement, which in turn has dramatic consequences on the dynamics and hence on the kinematics of the foreland fold-and-thrust belt. The maintenance of the critical state of a wedge with a weak plastic basal décollement necessitates a lower amount of internal deformation than that required by completely brittle wedges. Moreover, these deformations are strongly localised in the lower weak portions of the wedge and do not pervasively affect the upper brittle portions. These characteristics have been ascertained in the rear part of the CSA. B/P transitions commonly develop not only within wide mountain belts (e.g., the Andes) but also in narrow ones (e.g., CSA). This means that B/P transitions must be taken into account when modelling the mechanics of foreland fold-and-thrust belts at all scales. The occurrence of B/P transitions must be tested, particularly when topographic features suggest their existence, since only an appropriate rheological behaviour can account for the structural evolution and kinematics derived from field studies.