Gravity-Accommodated 'Structural Wedges' Along Thrust Ramps: A Kinematic Scheme of Gravitational Evolution

In active geodynamic areas, such as the Italian Southern Apennines, the specific structural conditions have a profound effect on both the superficial and deep gravitational evolution of slopes. Chain sectors commonly exhibit structural complexities due to the superimposition, in space and time, of diverse tectonic stress fields. In this paper, attention is focused on particular structural configurations, connected to the tectonic quaternary phases that have affected the Southern Apen- nines, constituting a peculiar type of lateral spread gravitational deformations - which have not been previously described. The study area is characterized by regional roughly east-verging overthrusts (Holigo-Miocene). Superimposed on these, is a system of transpressive thrust ramps (Quaternary), connected to the activity of strike-slip faults. The latest tectonic phase, still active, has been characterized by an ex- tensional stress field, which has produced differentiated uplifts along normal faults. All these factors have encouraged an intense and widespread processes of erosion, the creation of considerable relief energy and the development of both superficial and deep gravitational phenomena. It has been ascertained that the late extensional phases favoured gravitational reactivations (with normal movements) along those most recent thrust ramps whose attitude was kinematically consis- tent with the main extension direction of the stress field. Such reactivations were 'accommodated' by antithetic neo-formational structures - these are also characterized by normal kinematics. Overall, such mechanisms lead to the individuation of wedge-shaped rock portions delimited by the normally reactivated thrust ramps, on one side, and by the antithetic structures, on the other. Structural wedges represent a particular type of lateral spread phenomena. On the basis of their orientation with respect to the orography, three typical situations have been defined: (1) wedge perpendicular to the ridge; (2) wedge parallel to the ridge; (3) wedge oblique to the ridge. In this paper three Calabrian case studies, exemplifying the above-mentioned situations, are de- scribed. The same framework can be applied to sectors of chain with an analogous structural setting, for a better understanding of the kinematic features of observed gravitational phenomena.