The southern Tuscany - Northern 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 to 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, wich represents one of the most interesting geothermal italian regions. In this paper we present an attempt to combime the stratigraphical, geocronological, volcanological and geophysical data in an unifying picture of the regional picture of the regional structural evolution, and of the relationships between Neogene sedimentary basins, subsidence and uplift phases, and magmatic activity. New biostratigraphic dating of the Pliocene successions have been performed, together with several radiometric, K/Ar and 40Ar-39Ar, dating of Plio- Pleistocene volcanics, 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, have permitted to define the present thicknes of Pliocene sediments, the geometry of the bottom surface of the Pleistocene volcanic cover, and the map of the present altitude of the Plio-Pleistocene marine sediments, reflecting the sum of the total vertical moviments. The chemical composition of Pleisticene volcanic complexes of the Northern Roman Magmatic Province has been investigated, trying to delineate its temporal evolution. The subsurface geology of the area has been finally analysed in the light of drilling results and of gravimetric and thermal data. Since Pliocene, a very intense extensional tectonics affects the invetigated area. Two Pliocene sedimentary cycles can be distinguised, leading to deposition of sedimentary succesions up to two thousands meters thick. Subsidence moviments appear more intense in a relatively narrow, NW-SE trending belt close to the Apennine range, while Tyrrhenian side suffers a less developed subsidence witnessed by deposition of several discontinuous, not very thick sedimentary piles. Pliocene magmatic activity sets up just on the Tyrrhenian border, with the emplacement of several intrusive bodies and extrusion of prevalenntly acidic, crustal anatectic magmas and of minor subcrustal mafic melt. We suggest that the different structure presented by 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 2My b.p.; marine sedimentation resumes during lower Pleistocene, inside more eastern basins. The associated volcanic axis (definited by the Radicofani Torre Alfina, Monti Cimini alignment) assumes a more eastern position than the proceeding one, and is characterized by orenditic transitional to potassic magmas. During Pleistocene, the regional stress field undergoes a strong variation, connected to opening the Marsili basin in Southern Tyrrhenian Sea. Positive vertical movements seem to prevail since Middle-Upper Pleistocene, probably linked to the strong isostatic uplift of the Apennine chain. The present geometry shown by the Neogene sediments mainly reflect this uplift phase. The potassic volcanism of Roman Magmatic province strarts in the whole region (and probably also further South) around 0.6 My b.p., showing a strict relationship with the new prevailing tectonic regime, this magmatism being distribuited 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-My b.p., with a very intense volcanicity affecting all the main volcanic complexes, accompanied by maximum in the compositional spreading of erupted products. The time variations of magma composition and the tectonic evolution of the area, showing a progressive deepening of magmatic sources associated to a general decreasing of extensional tectonics, seem to be strongly realted. In this picture, the present heat flow anomaly would represent the residual of the thermal anomaly associated with Neogene extensional setting.
Plio-Pleistocene geological evolution of the geothermal area of Tuscany and Latium
Laurenzi MA;
1994
Abstract
The southern Tuscany - Northern 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 to 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, wich represents one of the most interesting geothermal italian regions. In this paper we present an attempt to combime the stratigraphical, geocronological, volcanological and geophysical data in an unifying picture of the regional picture of the regional structural evolution, and of the relationships between Neogene sedimentary basins, subsidence and uplift phases, and magmatic activity. New biostratigraphic dating of the Pliocene successions have been performed, together with several radiometric, K/Ar and 40Ar-39Ar, dating of Plio- Pleistocene volcanics, 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, have permitted to define the present thicknes of Pliocene sediments, the geometry of the bottom surface of the Pleistocene volcanic cover, and the map of the present altitude of the Plio-Pleistocene marine sediments, reflecting the sum of the total vertical moviments. The chemical composition of Pleisticene volcanic complexes of the Northern Roman Magmatic Province has been investigated, trying to delineate its temporal evolution. The subsurface geology of the area has been finally analysed in the light of drilling results and of gravimetric and thermal data. Since Pliocene, a very intense extensional tectonics affects the invetigated area. Two Pliocene sedimentary cycles can be distinguised, leading to deposition of sedimentary succesions up to two thousands meters thick. Subsidence moviments appear more intense in a relatively narrow, NW-SE trending belt close to the Apennine range, while Tyrrhenian side suffers a less developed subsidence witnessed by deposition of several discontinuous, not very thick sedimentary piles. Pliocene magmatic activity sets up just on the Tyrrhenian border, with the emplacement of several intrusive bodies and extrusion of prevalenntly acidic, crustal anatectic magmas and of minor subcrustal mafic melt. We suggest that the different structure presented by 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 2My b.p.; marine sedimentation resumes during lower Pleistocene, inside more eastern basins. The associated volcanic axis (definited by the Radicofani Torre Alfina, Monti Cimini alignment) assumes a more eastern position than the proceeding one, and is characterized by orenditic transitional to potassic magmas. During Pleistocene, the regional stress field undergoes a strong variation, connected to opening the Marsili basin in Southern Tyrrhenian Sea. Positive vertical movements seem to prevail since Middle-Upper Pleistocene, probably linked to the strong isostatic uplift of the Apennine chain. The present geometry shown by the Neogene sediments mainly reflect this uplift phase. The potassic volcanism of Roman Magmatic province strarts in the whole region (and probably also further South) around 0.6 My b.p., showing a strict relationship with the new prevailing tectonic regime, this magmatism being distribuited 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-My b.p., with a very intense volcanicity affecting all the main volcanic complexes, accompanied by maximum in the compositional spreading of erupted products. The time variations of magma composition and the tectonic evolution of the area, showing a progressive deepening of magmatic sources associated to a general decreasing of extensional tectonics, seem to be strongly realted. In this picture, the present heat flow anomaly would represent the residual of the thermal anomaly associated with Neogene extensional setting.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
