Stratigraphic correlation of the Upper Palaezoic-Triassic successions in southern Tuscany, Italy

Recent stratigraphic and structural studies on the Argentario promontory and in the Mid-Tuscan Ridge allow for a revision of the Upper Palaeozoic-Lower Triassic successions of southern Tuscany, and provide the basis for an original hypothesis on the significance of Tuscan Palaeozoic formations within the palaeogeographical framework of the western Mediterranean domain during Carboniferous times. These successions, cropping out in the Argentario Promontory and in the Mid-Tuscan Ridge, from Monti Leoni in the south to Iano in the north, all belong to the Monticiano-Roccastrada stratigraphical-structural unit. These studies show that the Monticiano-Roccastrada Unit, from Iano to Monte Leoni, consists of three distinct thrust-sheets which are, from the geometrically highest to the lowest one: the innermost lano thrust-sheet 3; the Monte QuoioMontagnola Senese thrust-sheet 2; the outermost and the deepest Monte Leoni-Farma thrust-sheet 1. A new interpretation of the palaeotectonic-sedimentary evolution of the Devonian-Carboniferous marine deposits of the Monticiano-Roccastrada Unit is herein provided. These formations, lacking traces of volcanic activity or pre-Alpine metamorphism, would be deposited within the external sectors of the Variscan orogen in an evolving belt-foredeep-foreland system. They belong to two particular successions related respectively to a normal-fault-bounded structural high (Calcari di S. Antonio Formation) and to a foredeep basin (Carpineta and Farma formations). The foredeep represents a post-collisional contract tonal structure. It is posterior to the Variscan collisional phase, whose onset is constrained by the age of the regional metamorphic peak of Northern Sardinia (336-339 Ma). This sector of Sardinia that belongs to the inner zones of the variscan orogen presumably had its along-strike continuation in the present Italian peninsula, to the north of the Torrente Farma. Sedimentary structures within the Farma Formation point to a SW-NE palaeocurrent direction, probably oriented parallel to the basin, for which development was probably related to the Variscan NW-SE shortening direction documented in the Apuane Alps. The northwestern compressional edge of the foredeep basin would represent the thrust sheet front, whose emplacement was caused by gravitational spread of the Variscan erogenic prism, triggering a collapse of the internal areas and movement towards the external ones. The southeastern edge of the foredeep basin is less steep than its northern counterpart. It was affected by a system of north-dipping normal faults responsible for the development of escarpments. It could be interpreted as the northern edge of a peripheral bulge which was uplifted and eroded. The advancing thrust sheet determined the flexure of the basement; the loading and flexures produced an asymmetric depression where clastic sediments eroded from the thrust sheet were deposited. On the basis of the Carboniferous flysch type facies distribution, the foreland basin was rather small (30-40 km), thus the flexure related to the thrust sheet loading cannot have affected the entire lithosphere, but only the less thick, elastic horizon of the upper crust. The flexure would be enhanced by the lower ductile crust. The Farma foreland basin could have had a westward continuation into the Carboniferous foredeep basin of southern Sardinia and the Iberian Peninsula, and a northeastward continuation into the Carnic region.