The use of geomorphological markers in the footwall of active faults for kinematic evaluations: examples from the central Apennines

Abstract

The different geomorphological characteristics of the footwall and hanging wall sectors of Apennine active normal faults usually prevent the possibility to correlate synchronous geomorphological features across the fault and, therefore, to define the kinematic parameters. This is particularly evident in case of faults active during the Late Pleistocene – Holocene, evolving along mountain fronts in piedmont areas. Therefore, the use of geomorphological features such as paleolandscapes and chronologically constrained deposits of pre-Late Quaternary age can be useful for the definition of kinematic parameters. Following this approach we have analysed three cases in Central Apennines: 1) the Magnola Mts. normal fault, 2) the Mt. Morrone normal fault system and 3) the Norcia basin fault system. As for the cases at points 1 and 2, isochronous sub-horizontal breccias have been detected both in the hanging wall and in the footwall sectors of the Magnola Mts. fault and of the western fault segment of the Mt. Morrone fault system (made of two parallel fault branches) laying on an almost flat paleolandscape of older age, characterized by relict surfaces gently dipping towards the basin bottom, whose formation occurred close to the local base level. Since BOSI et alii (2003) attributed this kind of breccias to the Early Pleistocene, we can hypothesize that the formation of the fault-related slopes occurred subsequently to the Early Pleistocene. The vertical offset affecting the breccias can be quantified in 650 m for the Magnola Mts. fault and in 350 m for the Mt. Morrone western fault segment. Therefore, assuming that a reliable age for these deposits is 10.2 Ma, a slip rate ranging from 0.54 to 0.81 mm/yr can be estimated for the Magnola Mts. fault and ranging between 0.30 and 0.43 mm/yr for the Mt. Morrone western fault segment. As for the Norcia basin fault system, alluvial fans attributed to the Middle Pleistocene are geomorphologically embedded in almost flat relict surfaces carved into the limestone bedrock and detectable in the highest sectors of the relief representing the eastern basin border. Considering the difference in elevation between these paleosurfaces and the present bottom of the basin (assuming that surfaces or deposits synchronous to those present in the footwall are “contained” in the succession filling the depression), a minimum vertical offset due to the fault activity following the formation of the relict surfaces can be estimated in about 900 m. Moreover, considering that these relict surfaces may have an age ranging between the Middle Pliocene (after the end of the compressive tectonic phase) and the Middle Pleistocene, a minimum vertical fault slip rate ranging between 0.25 and 1.15 mm/yr can be estimated.