The Plio–Pleistocene evolution of extensional tectonics in northern Tuscany, as constrained by new gravimetric data from the Montecarlo Basin (lower Arno Valley, Italy)

Abstract

A detailed gravimetric study has been integrated with the most recent stratigraphic data in the area comprised between the Arno river and the foothills of the Northern Apennines, in northern Tuscany (central Italy). A Plio–Pleistocene basin lies in this area; its sedimentary succession can be subdivided from the bottom, in five allostratigraphic units: (1) Lower–Middle Pliocene shallow marine deposits; (2) Late Pliocene (?)–Early Pleistocene fluvio-lacustrine deposits; (3) late–Early Pleistocene–Middle Pleistocene alluvial to fluvial red conglomerates (Montecarlo Formation); (4) Middle Pleistocene alluvial to fluvial red conglomerates (Cerbaie and Casa Poggio ai Lecci Formations); (5) alluvial to fluvial deposits of Late Pleistocene age. The Bouguer anomaly map displays a strong minimum in the northeastern sector of the basin, and a gentle gradient from west to east. The map of the horizontal gradients permits to recognise three major fault zones, two of which along the southwestern and northeastern margins of the basin, and one along the southeastern edge of the Pisani Mountains. A 2.5D gravimetric modelling along a SW–NE section across the basin displays a thick wedge of sediments of density 2.25g/cm3 (about 1700m in the depocenter) overlying a layer of density 2.55g/cm3, 1000m thick, which rests on a basement of 2.72g/cm3. The most of the sediment wedge is here referred to Upper Pliocene (?)–Lower Pleistocene, because borehole data show Pliocene marine deposits thinning northward close to the southern margin of the area. The layer below is referred to Ligurids and upper Tuscan Nappe units; the densest layer is interpreted as composed of Triassic evaporites, quartzites and Palaeozoic basement. According to Carmignani low-angle extensional tectonics began between Serravallian and early Messinian, thinning the Apennine nappe stack. At the end of Middle Pliocene, syn-rift deposition ceased in the Viareggio Basin (west of the investigated area) as demonstrated by Argnani and co-workers, and high-angle extensional tectonics migrated eastward up to the Monte Albano Ridge. A syn-rift continental sedimentary wedge developed in Late Pliocene–Early Pleistocene, until its hanging wall block was dismembered, during late Early Pleistocene, by NE-dipping faults, causing the uplift of its western portion (the Pisani Mountains). This breakup caused exhumation and erosion of Triassic units whose clastics where shed into the surrounding palaeo-Arno Valley in alluvial–fluvial deposits unconformably overlying the Lower Pleistocene syn-rift deposits. In the late Pleistocene SW–NE-trending fault systems created the steep southeastern edge of the Pisani Mountains and the resulting throw is recorded in Middle Pleistocene deposits across the present Arno Valley. This tectonic phase probably continues at present, offshore Livorno, as evidenced by the epicentres of earthquakes.