The structure and formation of the Tyrrhenian basin in the Western Mediterranean back-arc setting = Formación y estructura de la cuenca del Tirreno en el contexto de retrarco del Mediterráneo Occidental

Abstract

In this thesis I present a geophysical study that aims to define the structure and petrological nature of the main geological domains in the Central Tyrrhenian basin, and to investigate the mechanisms involved in their formation.

The geophysical data used in this thesis was acquired during the MEDOC (2010) survey within the framework of the MEDOC project, which was designed to improve our understanding of the origin and evolution of rifted margins. The present work is based on the analysis, processing, modeling, and interpretation of coincident Wide-Angle Seismic (WAS), Multichannel Seismic (MCS), and gravity data corresponding to the two longest transects acquired during the MEDOC survey in the Central Tyrrhenian basin, the southern Line GH/MEDOC-6 (~450 km) and the northern Line EF/MEDOC-4 (~400 km). Both lines run across the Central Tyrrhenian basin from Sardinia to the Campania margin.

The processing of MCS data provides the tectonic structure and geometry of the sedimentary basins, whereas the modeling of WAS data from travel-time tomography provides 2D seismic velocity models from which the velocity distribution of the crust and uppermost mantle, and the geometry of the crust-mantle boundary are inferred. The WAS models are then converted to density models using existing empirical relationships for different lithologies in order to test which of the different hypothesis concerning the petrological nature of the basement (e.g. continental/oceanic crust or exhumed mantle) explain better the observed gravity data. The results obtained together with the integration of geological data from rock sampling of the seabed reveals the existence of three geological domains in the Central Tyrrhenian, that is: continental crust, magmatic crust, and exhumed mantle.

The comparison between the results of Line EF/MEDOC-4 (northern line) with those of the Line GH/MEDOC-6 (southern line) reveals that the velocity and tectonic structure of the three geological domains differ in some regions from north to south. These differences are most likely attributed to the southward increase of extension that characterizes the Tyrrhenian basin.

The basement configuration presented in this thesis led to a completely new definition of geological domains in the Central Tyrrhenian. According to the presented distribution of the basement, rifting in the Central Tyrrhenian basin would have started with continental crust extension, continued with back-arc spreading leading to generation of magmatic back-arc crust, and followed by mantle exhumation intruded by later magmatic episodes. The interpretation of these results differ from current conceptual models of the formation of rifting systems involving mantle exhumation and indicate that the response of the continental lithosphere to extension processes may be more complex than previously assumed.

Finally, to explain the mechanism involved in the formation of these domains, I examine the modes of back-arc basin formation proposed to explain the formation of the western Pacific basins [Martinez et al., 2007; Dunn and Martinez, 2011], as well as the causes that may have led to mantle exhumation [Pérez-Gussinyé, et al., 2006]. In summary, the proposed conceptual model is based on a slab rollback and depleted mantle setting, in which production of extension-related melting is limited, thus, crustal accretion is attributed to hydrous flux melting. The model presents 5 stages of opening that includes: (I) a normal subduction scenario followed by (II) development of the back-arc rift, (III) initiation of back-arc spreading, (IV) mantle exhumation, and finally (V) emplacement of large volcanic edifices in the central parts of the basin.

La tesis que presento se centra en el estudio de la estructura cortical y de los procesos de formación de la cuenca del Tirreno, con el principal propósito de ampliar el conocimiento sobre los mecanismos involucrados en la evolución de márgenes divergentes. Para ello, en esta tesis se realiza el procesado, análisis, modelización, e interpretación geológica de dos perfiles geofísicos adquiridos durante la campaña de sísmica marina MEDOC (2010) en el mar Tirreno.

Ambos perfiles geofísicos incluyen datos de sísmica de gran ángulo (SGA), de reflexión multicanal (SRM) y de gravimetría, a partir de los cuales he obtenido los resultados siguientes:

1.Dos modelos de velocidad de propagación de ondas P (Vp) y de la geometría del Moho. 2.Análisis estadístico de incertidumbre de los parámetros de cada modelo de velocidad. 3.Modelado gravimétrico a partir de la conversión a densidad de los modelos de Vp. Con este método se pretende constreñir la petrología de los diferentes dominios geológicos a partir del empleo de diferentes relaciones empíricas de velocidad-densidad para diferentes tipos de roca. 4.Dos perfiles de SRM stackados y migrados en tiempo, de los cuales procesé uno de ellos (MEDOC 4).

La interpretación conjunta de estos resultados junto con un análisis exhaustivo de las velocidades de los modelos, e información geológica de las rocas del lecho marino, han permitido caracterizar la naturaleza cortical (continental/oceánico) de los principales dominios geológicos en la cuenca del Tirreno central, los cuales incluyen: corteza continental, corteza de retroarco de carácter magmático y manto exhumado. Estos resultados junto con observaciones de la geomorfología de la batimetría, han dado paso a la discusión de la distribución en planta de dichos dominios geológicos en toda el área de estudio.

Finalmente, propongo un modelo de apertura de cuenca en el cual se especula sobre la formación de los diferentes dominios geológicos identificados anteriormente. Dicho modelo se basa en los modos de apertura de cuencas de retroarco propuestos para las cuencas del Pacífico occidental, y en estudios de modelización numérica realizados para explicar la exhumación del manto en los márgenes pasivos del Atlantico Norte.