Abstract

In this paper we present and discuss the result of extensive field mapping (on a scale of 1:10.000) and a detailed macro and mesostructural analysis carried out in the upper Magra Valley, between the towns of Aulla and Pontremoli. The main purpose is the reconstruction of the present structural setting of the area, with special regard for its extensional structures and their plio-quaternary evolution. In the Lunigana region the Appennines are made up of Ligurian Units, tectonic units derived from an oceanic domain, which overrode the Tuscan Unit, belonging to the Adriatic margin (fig. 2). During the late Cretaceous, the closure of the Ligure-Piemontese ocean started and the Ligurian Units were stacked to form an accretionary prism (intraoceanic phase of Principi & Treves, 1984). From the early Miocene, during the collisional phase, the Ligurian Prism thrusted eastward over the foredeep basin of the Tuscan Unit with the interposition of a transitional unit, known as the Canetolo or Sub-Ligurian Unit. In the study area, the piling up of the tectonic units ends in the late Miocene. Afterwards, during the early Pliocene (Bartolini et alii, 1982), extensional tectonics thinned the Tyrrhenian side of the Apenninic Chain and formed the Lunigiana and Vara Valley grabens. The two asymmetric grabens, separated by a horst (Bernini, 1991), formed a thin-skinned extensional structure that detach above the "basement" (figs. 12 and 21) reached in the well Pontremoli 1 at 2900 m depth (Anelli et alii, 1994). The listric faults which characterize the Lunigiana region, present an Appenninic direction (NW-SE) and are clustered along seven main fault systems (fig. 8), over 10 kilometers long and with displacements over 1500 m. The extensional fault systems generate an asymmetric graben with steep faults dipping to the southwest whilst the northeastern dipping faults are less inclined. On the graben eastern side the biggest displacements are concentrated in the more external Groppodalosio system while on the western side they are concentrated in the internal Mulazzo system. On the surface, inside each fault system and between the fault systems, the displacement is transferred by NE-SW short transfer faults or, more commonly, by relay ramps. On relay ramps the strain is brittle-ductile and local deviations of the stress field occur (figs. 18 and 19). In the axial zone of the Lunigiana graben, two fluvio-lacustrine basins were forming during the extensional phase; the Pontremoli basin to the northwestern and the Aulla-Olivola to the southeastern, divided by the Annunziata ridge (Federici, 1978). The continental deposits of the two basins show the same vertical sedimentary evolution (fig. 3): an initial stage with lacustrine and palustrine deposits followed by fluviatile deposits. However, the onset of the continental sedimentation is not synchronous. In the the Aulla-Olivola basin the lacustrine deposits are Ruscinian, while in the Pontremoli basin they are upper Villafranchian in age (Bertoldi, 1995). In order to unravel the Plio-Quaternary evolution of the Lunigiana region it is fundamental to analyse the fluvio-lacustrine successions and how they relate to extensional structures. In fact the deposition and distribution of continental deposits in the two basins are strictly controlled by normal fault activity. The extensional tectonics started in the early. Pliocene and, still active as can be seen from the seismicity (Frepoli & Amato, 1997), was not continuous and constant in time. Two main breaks of extensional activity, toward the end of the middle-late Pliocene and middle Pleistocene, are recorded by two regional unconformities in the fluvio-lacustrine deposits (Bernini et alii, 1990). During the two breaks, the region underwent a generalized uplift to which the extensional tectonics followed enhanced by the uplift. The middle Pleistocene uplift is associated to a weak compressional regime mainly expressed on a meso-structural scale (fig. 15) without forming macro-structures. Considering the geometry of the Magra-Vara extensional system (fig. 21), which overlies a thin-skinned detachment fault dipping towards the Po Plain and considering that the extensional tectonics restarts after two episodes of regional uplift, we conclude that the main extensional phases, lower Pleistocene and middle Pleistocene in age, are controlled by vertical movements of the basement which stretches the Ligurian and Tuscan units on the Tyrrhenian side and favours their tectonic transport towards the Po Plain. The mesostructural data (tab. 1) agree with the proposed model because they show that the extensional tectonics of the Lunigiana graben is produced by radial stress field (fig. 20) in a brittle environment typical of a thin-skinned extension laterally unconfined.

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