Abstract

The Fiamignano fault represents the north-eastern border of the central Salto River Valley, in the Central Apennines, where a mainly a Meso-Cenozoic carbonate platform sedimentary succession (Lazio-Abruzzi domain) outcrops (fig. 1). This structure has long been studied by many workers who have proposed different interpretations: for some authors this is just a recent (Holocene) and active normal fault, linked to the actual seismicity of the area (Bosi, 1976; Bertini & Bosi, 1976; Bosi et alii, 1989; Morewood & Roberts, 2000); others have proposed a polyphase activity, with reverse kinematics during the compressive Neogene stage, and normal reactivation during the Plio-Pleistocene, post-orogenic stage (Mariotti & Capotorti, 1988; Capotorti & Mariotti, 1992; Bosi et alii, 1994; Faccenna et alii, 1993). They have also described mesoscale flower structures linked to the Pleistocene extension. Further authors have recognized evidences of a strike slip activity during the Pleistocene extensional phase (Capotorti & Mariotti, 1992; Faccenna et alii, 1993). The Fiamignano fault has a NW-SE trend and shows an evident normal offset which rises to several hundred meters (600 to 1000 m). It marks the boundaries between the Salto Valley (hanging wall) in the SW and the Nuria group (footwall) to the NE, both consisting of a Mesozoic-Cenozoic carbonate platform and Miocene ramp deposits. The sedimentary successions closed upward with early Messinian turbidites, that show different facies and thickness in the hanging wall and footwall of the fault. Due to these stratigraphic reasons as well as new data derived from a more detailed structural analysis carried out along the structure, an almost early Messinian activity for the Fiamignano fault has been proposed in this paper. Evidence that the normal fault existed prior to the compressive Neogene phases has been observed along the fault. The primary normal fault is offset and cut by pure shear, conjugate fault systems, compatible with the Neogene stress regime. These systems have been interpreted as buttressing features, which developed on the pre-thrusting normal fault during the compressive phase, as well as the occurrence of thrust and backthrust in the hanging wall and in the footwall of the Fiamignano normal fault. Locally, flexural slip on the footwall bedding, due to the subsequent folding and thrusting reduces the dip of the preexisting normal fault down to approx. 35 degrees -40 degrees . The angle of dip of the fault plane varies along strike: in spite of this, the footwall cut-off angle is quite constant and always larger than 90 degrees . Structural and stratigraphic data have been used to reconstruct a balanced section in this sector of the Central Apennines where the Fiamignano fault has been interpreted as a pre-thrusting normal fault. In this paper we propose an interpretation of the control exerted by the pre-thrusting normal fault on the foredeep geometry, the shortening value and the structural style in the ancient carbonate platform domain involved in the Neogene phase of chain building.

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