Plio-Pleistocene geological evolution of the geothermal area of Tuscany and Latium

Abstract

The Southern Tuscanv-Norrhern Latium area has been affected by a complex geological evolution during Neogene times. Starting from Miocene, several subsidence episodes, alternated with periods of differential uplifting, brought
\nto the development and closure of some marine and continental basins. Since Pliocene, an intense and widespread magmatic activity .has also interested this region, characterized by a space-time eastward migration of the magmatic centers and by a strong variation in time of the magma composition. Very high heat flow values are observed over the whole area, which represents one of the most interesting geothermal Italian regions.
\nIn this paper we present an attempt to combine the stratigraphical, geochronological, volcanological and geophyslcal
\ndata in an unifying picture of the regional structural evolution, and of the relationships between Neogene sedimentary
\nbasins, subsidence and uplift phases, and rnagmatic activity. New biostratigraphic datings of the Pliocene successions have been performed, together with several radiometric, K/Ar and 39Ar/40Ar, datings of Plio-Pleistocene volcanic, from samples of outcropping and subsurface sequences. The geothermal wells drilled by ENEL, and by J.V. AGIP-ENEL, integrated by results from surface survey’s, have permitted to define the present thickness of Pliocene sediments, the geometry of the bottom surface of the Plistocene volcanic cover, and the map of the present altitude of Plio-Pleistocene marine sediments reflecting the sum of the total vertical movements. The chemical composition of Pleistocene volcanic complexes of the Northern Roman Magmatic Province has been investigated, tryng to delineate its temporal evolution. The subsurface geology of the area has been finaly analyzed in the light of drilling results and of gravimetric and thermal data. Since Pliocene, a very intense extensional tectonics affects The investigated area. Two Pliocene sedimentary cycles can be distinguished, leading to the deposition of sedimentary successions up to two thousands meters thick. Subsidence movements appear more intense in a relatively narrow, NW-SE trending belt close to the Apennine range, while the Tyrrhenian side suffers a less developed subsidence witnessed by the deposition of several discontinuous, not very thick sedimentary piles.
\nPliocene magmatic activity sets up just on the Tyrrhenian border, with the emplacement of several intrusive bodies and
\nthe extrusion of prevalently acid crustal anatectic magmas and of minor subcrustal mafic melts. We suggest taht the
\ndifferent structure presented by the western side of the investigated area could reflect a primary role of a diffuse intrusive process in this area. Pliocene sedimentation stops in the whole area around 2 Ma b.p.; marine sedimentation resumes during lower Pleistocene, inside more eastern basins. The associated volcanic axis (defined by the Radicofani, Torre Alfina, Monti Cimini alignment) assumes a more eastern position than the preceding one, and is characterized by orenditic transitional to potassic magmas. During Pleistocene, the regional stress field undergoes a strong variation, connected to the opening of the Marsili basin in the Southern Tyrrhenian Sea. Positive vertical movements seem to prevail since Middle-Upper Pleistocene, probably linked to the strong isostatic uplift of the Apennine chain.
\nThe present geometry shown by the Neogene sediments mainly reflects this uplift phase. The potassic vo1canism of Roman Magmatic Province starts in the wholele region (and probably also further South) around 0.6 Ma b.p., showing a strict relationship with the new prevailing tectonic regime, this magmatism being distributed along the disengaging zone between the uplifting chain and the subsiding Tyrrhenian basins. A further pulse of magmatic activity occurs around 0.4-0.3 Ma b.p., wlth a very intense volcanicity affecting all the main volcanic complexes, accompanied by a maximum in the compositional spreading o f the erupted products. The time variations of magma composition and the tectonic evolution of the area, shoving a progressive deepening of magmatic sources associated to a general decreasing of extensional tectonics, seem to be strongly related. In this picture, the present heat flow anomaly would represent the residual of the thermal anomaly associated with the Neogene extensional setting.