Salt tectonics in the Parentis Basin (eastern Bay of Biscay): Origin and kinematics of salt structures in a hyperextended margin affected by subsequent contractional deformation

Abstract

The geological history of the north-Iberian margin and the eastern sector of the Bay of Biscay suggest that these areas are ideal places to study and characterize the tectonic evolution and the geodynamic mechanisms governing the opening of the Atlantic Rift. This fact can be seen very well since several evolutionary stages have been preserved within the rifting system along the North-Iberian Margin. Similarly, the study of these lateral variations also reveals how they conditioned the subsequent development of the Pyrenean orogen. At a more specific level, the Bay of Biscay appears as a good place to understand how the processes of mantle exhumation and subsequent mantle accretion occurred in the Atlantic Ocean, as well as to understand the role played by the resulting structures during the growth of the Pyrenean orogen.

Furthermore, in its eastern sector, the Bay of Biscay allows the observation of the Mesozoic Parentis Basin slightly deformed by the Pyrenean compression. The study of this basin provides a first approach to understand how the Mesozoic basins were currently involved in the Pyrenean building previously to the Pyrenean compression (eg. Organyà Basin, Aulet Basin, etc…). These basins include highly deformed evaporitic materials from the Upper Triassic (Keuper facies) whose role during both the Mesozoic extension and the Cenozoic compression have been poorly studied.

In this context, this thesis initially intended to elucidate how changes in the configuration of the crustal structure at the end of the Bay of Biscay opening have been able to control the change on structural style that took place from the Pyreness s.s. to the Cantabrian Pyrenees, to better understand the processes that governed the evolution of the Pyrenees. Second, at a smaller scale, this thesis aims to know which was the general structure of the Mesozoic intracontinental basins developed during the opening of the Bay of Biscay and which was its initial response during the Pyrenean compression.

To achieve this goal we have analyzed several deep seismic surveys (ESCIN, ECORS and MARCONI) and the seismic data of the Parentis Basin. Apparently, the Mesozoic structure of this basin was slightly modified by the Pyrenean compression. In such asymmetric basin, salt tectonics has an important development and both its general structure and the salt structures have been poorly studied by the scientific community. In this sense, this thesis seeks to fill this gap pointing out the crustal fault structure of the Parentis Basin and establishing the style and the evolution of salt structures preserved in the basin.

1) Understanding the role played by the presence of a pre- or syn-kinematic salt level in the geometry and evolution of a thick-skinned extensional basin. 2) Determining the role played by the geometry of the main basin boundary faults in the formation of salt structures in its hangingwall during extension. 3) Investigating how salt structures developed during an extensional stage evolved with a subsequent contractional deformational stage which applied different degrees of shortening.

The main results of the present thesis can be grouped into three action lines: 1) the characterization of structural variations along the Bay of Biscay-Pyrenean Rift System and its influence in the Pyrenean orogen development; 2) the definition of the Parentis Basin structure, in particular its western parts that were practically unknown; and 3) the analysis of the role played by the Upper Triassic pre-rift salt during both the extensional opening of the Bay of Biscay (specifically in the Parentis Basin) and the subsequent contractional development of the Pyrenean orogen.

Aquesta tesi pretén aclarir com els canvis en la configuració de l’estructura cortical al final de l’obertura del Golf de Biscaia van controlar els canvis en l’estil estructural que tenen lloc des dels Pirineus en sentit estricte fins als Pirineus Cantàbrics per tal d’entendre millor els processos que van governar l’evolució d’aquest orogen. A més, i a una escala més petita, intenta conèixer quina era l’estructura general de les conques intracontinentals mesozoiques que es van desenvolupar durant l’obertura del Golf de Biscaia i quina va ser la seva resposta inicial durant la compressió pirinenca.

Per tal d’assolir aquests objectius s’han analitzat diverses campanyes de sísmica profunda (ESCIN, ECORS i MARCONI) així com de sísmica petroliera disponible a la Conca de Parentis. També s’han realitzat diversos models analògics per tal de recolzar les interpretacions realitzades.

L’estructura mesozoica de la Conca de Parentis va ser lleument modificada per la compressió pirinenca. Cal remarcar que la tectònica salina va tenir un paper molt important en el desenvolupament d’aquesta conca. D’altra banda, tant la seva estructura general com les estructures salines no havien estat objecte d’estudi per la comunitat científica fins a la realització d’aquesta tesi. En aquest sentit doncs, aquesta tesi omple aquest buit posant especial interès en les estructures corticals que controlen el seu desenvolupament i estableix l’estil i l’evolució de les estructures salines preservades en ella.

Els resultats principals de la tesi es poden agrupar en tres línies principals: 1) la caracterització de les variacions estructurals al llarg del sistema de rift Golf de Biscaia - Pirineus i la seva influencia en el desenvolupament de l’orogen pirinenc; 2) la definició de l’estructura de la Conca de Parentis i en particular del seu sector occidental; i 3) l’anàlisi del paper que va jugar la salt pre-rift del Triàsic Superior tant durant l’obertura del Golf de Biscaia (especialment a la Conca de Parentis), com durant el desenvolupament contractiu de l’orogen pirinenc.