Geophysical constraints on the nature of geological domains of continental rifted margins: examples from the West Iberia margin and Ligurian Basin

Abstract

In this PhD work, we carry out a geophysical and geological study of two classical examples of rifted continental margins: the Gulf of Lions (GoL), located in the Western Mediterranean, and the Tagus Abyssal Plain (TAP), located in the West Iberia margin. In spite of numerous previous studies of these regions, there is a debate on the crustal structure and the processes that led to the formation of the basin. Our study aims to 1) determine the nature of rocks forming the main geological domains of the basin, 2) define the tectonic structure of the basement and 3) place constrains and discuss the kinematics and tectonic and magmatic mechanisms involved in basin formation.

To study the GoL, we used a geophysical data set acquired during the SARDINIA-2006 survey by the Ifremer Institute (France). In particular, we used a Multi-Channel Seismic (MCS) line and a coincident Wide-Angle Seismic (WAS) profile. Altogether, these lines cover a seismic transect that runs NW-SE across the GoL until the central part of the Liguro-Provençal basin. The geophysical data used to study the TAP were collected during the FRAME-2018 survey within the framework of the FRAME project. We present spatially coincident MCS and WAS data, along a 350 km-long, E-W trending profile located at 38º N, crossing the basin in the North-West Iberian margin.

We apply joint refraction and reflection travel-time tomography (TT) that combines travel-times from MCS and WAS data to provide new constraints on the structure and petrological nature of basement domains along the margins. The result of this joint WAS- MCS tomography is a P-waves velocity (Vp) model of the margin that is fully consistent with the MCS image along the profile, making the geological interpretation less subjective. The processing of MCS data provides the tectonic structure and geometry of the sedimentary basins.

The results from the GoL support the existence of three geological domains that are: 1) a continental domain formed by normal faults that tilted the continental basement, 2) a ~100 km wide domain bounded by continental crust domains, characterized by a 4-5 km thick layer with high velocity and steep gradients that we interpret as a lens-shaped body of oceanic crust and 3) a thin continental crust (<4 km). This configuration implies that the continent-ocean transition (COT) occurs abruptly (<10 km along profile) at each side of the oceanic domain.

In the case of the TAP, the models show that the crustal structure is more complex, presenting sharp boundaries between five different domains at the base of the continental slope and across the J-anomaly. Thus, the profile across the TAP shows that Domain I and Domain III are made of 4-6 km thick continental crust. Domain III shows a lower crust with comparatively higher velocities possibly due to limited magmatic intrusions. Domain II, previously interpreted as oceanic crust, is shown to constitute a ~70 km wide domain of exhumed and serpentinized mantle. The westernmost 200 km of the profile include Domain IV and Domain V with a basement made of oceanic crust. The new Vp model and seismic images support that the COT is located ~300 km offshore and that occurs abruptly from 10 to 15 km wide. Based on these results, we discuss a new geodynamic scenario characterized by two main phases of crustal extension. According to the presented distribution of the basement, rifting in the TAP would have started with continental crust extension, continued with exhumation of the mantle, followed by the

formation of the oceanic crust of the J-magnetic anomaly, and continued with spreading of oceanic crust of the Cretaceous Magnetic Quiet Zone.

The interpretation of these results differs from current conceptual models of the formation of both examples of rifting systems. Its integration offers the opportunity to review the existing conceptual models related to rifted margins that involve mantle exhumation and indicate that the response of the continental lithosphere to extension processes may be more complex than previously assumed.

En esta tesis doctoral, se ha realizado un estudio geofísico y geológico de dos ejemplos clásicos de márgenes continentales: el margen de la Cuenca de Liguria y el margen de la Llanura Abisal del Tajo, ubicada en el margen Oeste de Iberia. A pesar de los diversos estudios previos de estas regiones, existe un debate abierto tanto sobre su estructura cortical como sobre los procesos que operaban durante su formación.

Este trabajo de tesis ha tenido como objetivos: 1) determinar la naturaleza de las rocas que forman los principales dominios geológicos de ambos márgenes, 2) definir la estructura tectónica del basamento y 3) discutir la cinemática y la interacción de mecanismos tectónicos y magmáticos involucrados en la formación de los márgenes.

Para conseguir estos objetivos, se han analizado e integrado diversos datos geofísicos. Los datos principales son de sísmica de reflexión de “streamer” multicanal y de sísmica de refracción y refracción de gran ángulo marinos. También se han integrado datos batimétricos y gravimétricos. La parte metodológica más novedosa de esta tesis es la utilización de los tiempos de trayecto de fases sísmicas de datos de streamer y gran ángulo en una tomografía conjunta. Esta metodología permite determinar con más precisión que otros métodos las velocidades de las ondas sísmicas (Vp) a través del basamento a lo largo de los perfiles. El modelo resultante permite establecer la naturaleza petrológica con menos incertidumbre que los métodos más comúnmente usados.

El análisis, procesamiento, modelado e interpretación de estos conjuntos de datos permite una interpretación novedosa de los aspectos relacionados con la estructura y naturaleza de la corteza, así como la discusión de nuevas propuestas para los procesos tectónicos que llevaron a la configuración actual de cada uno de los ejemplos de márgenes continentales.

La interpretación desarrollada difiere en gran medida de modelos previos en cuanto a la formación de ambos sistemas de rifting. Por ello, proponemos que su integración ofrece la oportunidad de revisar modelos conceptuales existentes en la literatura. En particular, los resultados muestran que la respuesta de la litosfera continental a los procesos de extensión puede ser más compleja de lo que se suponía hasta ahora.