The Tuscan Nappe front in the Trasimeno area (Western Umbria): geological interpretation of a thin-skinned, Deep Water Fold-and-Thrust Belt

Modern deep water fold and thrust belts (DWFTBs) develop at both continental passive and active margins, driven by gravity and tectonic forces respectively. Despite ancient (i.e. fossil, exhumed) DWFTBs have been intensely studied for about 200 years, a detailed comparison of modern vs. exhumed case-histories is still lacking. Here we present a multidisciplinary geological study of the Outer Tuscan Nappe (OTN), an exhumed deep water imbricate thrust system. The OTN is the object of a wide scientific literature, including recent and advanced contributes, but many dilemmas are still present, i.e i) the presence and the influence of the Ligurian units behind and above the allochtonous OTN; ii) the actual amount of shortening accommodated by the thrust system; iii) the mechanical and kinematic relationships between the basal decollement and the internal imbrication of the OTN; iv) the possible role of gravity in the OTN emplacement. For our study we integrated surface geology data and the interpretation of a set of 2D seismic reflection profiles, crossing transversally and longitudinally the study area, from the Pratomagno1 well (also used for calibrating seismic reflectors) to the Trasimeno Lake. Using these data, we obtained a set of transversal (i.e. WSW-ENE trending) geological cross sections, about 30 km long, showing the internal geometry of the imbricate thrust system, down to the main basal detachment, corresponding to the Late Cretaceous-Early Tertiary Scaglia Toscana Fm.. Subsequently, we performed the 2D restoration of the geological sections, using the software MOVE (Midland Valley). As a parallel activity, we also performed selected controls of key outcrops, mainly aimed at reconstructing: i) the actual transport direction during the OTN emplacement; ii) the position and offset of the subsequent, NNW-SSE trending, extensional faults dissecting the tectonic wedge; iii) the role of transversal faults, segmenting longitudinally the thrust system. Our preliminary results show that the OTN consists of a series of thin-skinned imbricate thrusts, involving the Scaglia Toscana Fm. and the overlying Macigno Fm.. The thrusts splay out from a major basal decollement, superposing the Tuscan succession (detached from its original Mesozoic substratum) over the Mt. Rentella and Marnoso-Arenacea turbidite successions. The basal thrust becomes progressively shallower from W to E, from a depth of about 4 km to 1 km. Correspondingly, the reconstructed tectonic wedge is up to 5 km thick in its western part, and tapers progressively eastward: these values are consistent with previous estimates, based on thermal burial data. The total shortening of the imbricate thrust system is in the order of 50 km, including, at least, 30 km of horizontal ENE-ward transport along the basal decollement.