Understanding the processes that controlled rifting of the Tyrrhenian basin

Abstract

This thesis is focused in the processes that took place in the Tyrrhenian Sea, which is a Neogene basin belonging to the Western Mediterranean realm. Its formation is related with the subduction system of the African plate below the European plate. The slab retreat causes a stretching of the overriding plate and triggered the opening of the Tyrrhenian basin.

The sedimentary record described in the Tyrrhenian Basin spans from early-Miocene to the most recent Pleistocene deposits. The distribution of these sediments, their geometry and their relations with the tectonic structures reflects the basin evolution throughout the time.

Oldest sediments consist in pre-Trotonian materials deposited at the Corsica and Sardinian Basins during their formation. Thus this region may have been opened previously to the Tortonian times. In fact, these basins formed as episutural basins during the early Miocene.

The Tortonian unit has only been identified at the continental areas. The rifting onset took place during the Tortonian, as suggested by the sedimentary discontinuity described within this unit. Such discontinuity marks the base of the syn-rift wedges, and it is known at the literature as the "L" discontinuity.

The Messinian deposit have been described at the continental and back-arc areas. Its presence at the Campania and Cornaglia Terraces (back-arc areas), where no Tortonian has been found suggest that they were formed in some moment during the early Messinian. But, in fact, Messinian strata show a clear post-rift character in these areas. Thus both terraces are provably formed during the Tortonian, concurrently to the rifting processes of the North Tyrrhenian.

Thus, in the North Tyrrhenian extension occurred through rifting processes, leading to rotated blocks that can be well appreciated at the bathymetry. North-south trend of these blocks suggest that extension direction was mainly east-west.

While at the same time, at the Cornagla and Campania Terraces, extensional processes become more complex. Continental extension took place in the southern Cornaglia, while at the center, at the north and at the Campania Terrace back-arc magmatic crustal accretion occurred. Instead, in the southern Cornaglia Terrace a limited amount of magmatic crustal accretion occurred.

Finally, according to sedimentary syn-rift geometries, extension continued in all these areas during the Messinian, although extensional processes become attenuated.

The Pliocene deposits are the first unit that can be found everywhere, including the deepest areas of the Vavilov and Magnaghi Basins. It has been divided into two sub-units and the discontinuity between them is known as the "X" discontinuity, and marks the end of this extensional stage. The Magnaghi Basin may have opened during the uppermost Messinian, as suggested by the pre-rift Messinian evaporites observed at the western half

of this basin. Then, in a short time lapse, extensional locus continues its migration towards the east and the Vavilov basin opened at the lower Pliocene. Opening of these two basisn may have occurred during a short period as suggested by the absence of synrift deposits in the area. At the same time, extensional processes at the continental and back-arc regions ceased completely, as suggested by the intra-Pliocene unconformity. It implies that the former magmatic extensional regime changed to an essentially amagmatic extension. Finally, at the boundary between the Pliocene-Pleistocene a new jump in the extensional locus occurred. As in the former case, a new basin opened: the Marsili Basin. Concurrently, in a certain moment during the upper Pliocene, transpression- transtensional tectonics started along the Italian Margin region related with this trench migration towards the south-east, and reactivated the former extensional faults.

Esta tesis está centrada en los procesos que tuvieron lugar en el mar Tirreno, el cual es una cuenca Neógena perteneciente al Dominio del Mediterraneo Occidental. Su formación está relacionada con el sistema de subducción de la placa Africana por debajo de la placa Europea. El retroceso del slab da lugar a un estiramiento en la placa superior provocando la apertura de la cuenca del Tirreno.

El registro sedimentario descrito en el Tirreno abarca desde el Mioceno inferior hasta el Pleistoceno mas reciente. La distribución de estos sedimentos, su geometría y su relación con las estructuras tectónicas reflejan la evolución de formación de la cuenca a lo largo del tiempo.

De acuerdo con la interpretación tectónica y la distribución de los depósitos sedimentarios, las sub-cuencas de Córcega y Cerdeña se formaron durante el Mioceno inferior como cuencas episuturales relacionada con el frente de subducción.

Durante el Tortoniense la extensión migró hacia el este y los procesos extensivos empezaron en lo que sería el actual región del Tirreno Norte dando lugar a extensión continental. Mientras que en el sur los processos extensivos tomaron un carácter mas complejo que en el norte. En las zonas de Cornaglia Terrace y Campania Terrace tuvo lugar la formación de corteza oceánica nueva por acreción magmática, excepto en la parte mas meridional de la Cornaglia Terrace, donde tuvo lugar extensión continental con un pequeño aporte magmático.

Durante el Messiniense, los procesos extensionales se atenuaron y en el Plioceno inferior cesaron casi por completo. Mientras que al mismo tiempo la extensión migró hacia la actual cuenca de Magnaghi y la cuenca de Vavilov, donde se dio ruptura cortical y exhumación del manto.

Finalmente, en el límite Plioceno-Pleistoceno la extensión cesa en esta área y vuelve a migrar hacia la actual cuenca de Marsili.