Silvia Palandri

Oligocene-Miocene foredeep deposits in the Lake Trasimeno area (Central Italy): insights into the evolution of the Northern Apennines

In the Trasimeno Lake area (Umbria Region), several thrust sheets belonging to the Sansepolcro-Monte Filoncio and Rentella Units, are interposed between the Tuscan Nappe (in the hanging-wall) and the Umbria-Romagna Unit (in the footwall). The thrust sheets succession of the Sansepolcro-Monte Filoncio Unit is made up of Rupelian-Chattian Scaglia toscana Fm. and Chattian-Aquitanian foredeep deposits of Macigno Fm.; the base of the siliciclastic succession becomes gradually younger eastward. The compositional mode of the fine-grained rock fragments in the arenaceous turbidites is comparable with that of the upper part of the Macigno Fm. in the Tuscan Nappe. The succession of the Rentella Unit includes Rupelian-Aquitanian Monte Rentella Fm. and Aquitanian-Burdigalian foredeep siliciclastic turbidites of the Montagnaccia Fm.. In the lower portion of the Montagnaccia Fm. a level of black cherty horizons is present, as detected in all Aquitanian-Burdigalian successions of the Northern Apennines. The compositional mode of the fine-grained rock fragments in the turbidites is comparable with that of Marnoso-Arenacea Fm.. All these data allow an Oligocene-Miocene paleogeographic reconstruction of the Northern Apennines foredeep. Based on the age, the compositional mode and structural position, the Rentella and Sansepolcro-Monte Filoncio Units can be compared to the Carigiola and Acquerino Units cropping out in the Tuscan-Emilian Apennines.

Structural and Thermal Constraints to the Tectonic Evolution of Foredeep Successions in the Northern Apennines, Italy

A multi-method investigation (structural analysis, organic matter thermal maturity and clay mineralogy) was carried out on highly deformed foredeep successions in the Northern Apennine with special regard to shear zones in the Lago Trasimeno area (Rentella Unit) to unravel the main structural mechanisms and the thermal condition of the Miocene accretionary processes of the Northern Apennines belt. The Rentella Unit consists of a foredeep succession deposited in a paleogeographic domain between the Tuscan and the Umbria-Marche Domains. Meso- and microstructural data indicate shear planes compatible with C-planes (N160°, 45°SW). C-planes veins show dilational jogs and a staircase shape coherent to a top-to-the-NE sense of shear. Type II calcite twins in the shear zone associated veins suggest temperatures of 150-200°C. Temperature-dependent clay minerals and vitrinite reflectance indicate lower maximum paleo-temperatures (<100-110°C) in the early diagenesis and in the immature to early-mature stages of hydrocarbon generation. Thermal parameters suggest the development of the shear zone in the foredeep deposits at shallow depths (<3 km). Hot fluids coming from deeper structural levels within the collisional prism caused higher temperatures recorded in calcite twins of the shear zone veins.