The role of pre-existing thrust faults and topography on the styles of extension in the Gran Sasso range (central Italy)

Structural analysis and field mapping together with simple geometrical and flexural elastic models, document that two styles of Quaternary extensional tectonics characterized the Gran Sasso range (central Apennines, Italy). In the western part of the range, extension took place on 10-15-km-long range-front normal faults with associated 600-1000-m-high escarpments showing evidence of Late Glacial-Holocene activity. This topography has been reproduced with a thin elastic plate subjected to the isostatic forces induced by the movement along high-angle (55 degrees-65 degrees) planar normal faults. In the eastern part of the belt extension occurred on shallow-dipping normal faults (30 degrees-35 degrees) which reactivated progressively deeper pre-existing thrusts. In this area antithetic 'domino' faults formed to accommodate the mechanical adjustment of the hanging-wall over a variably dipping major fault surface. The eastward increase in shortening, due to the earlier compressional phase, documented in the Gran Sasso belt by previous authors, accounts for the more developed zones of weakness and high topographic relief in the eastern sector. This setting could explain the different styles of extension and the more advanced northeastern limit of normal faulting in the eastern sector. This work suggests that normal faults can originate either with low- or high-angle geometry in the upper crust according to the pre-existing tectonic setting and that topography could be important in controlling the geometry and pattern of migrating normal faulting. (C) 1998 Elsevier Science B.V. All rights reserved.