Tectonic structure and formation kinematics of the Western Mediterranean basins

Abstract

In this work we present a geophysical study of three Western Mediterranean basins: Valencia Trough, Gulf of Lions and Algero-Balearic Basin. The work aims to define the tectonic structure, nature of the basement, and to discuss the age, kinematics and mechanisms involved in basin formation. To study the basins we have gathered all deep penetrating seismic data available in the region consisting on several academic and industry data reprocessed and interpreted for this work.

To study the Valencia Trough Basin structure, we reprocessed from field data the 400 km-long multichannel seismic line ESCI-Valencia acquired in 1992 by Geco-Prakla’s survey vessel M.V. Bin Hai supported by the Spanish “Plan Nacional de I+D” with reference projects GEO89-0858 and GEO90-0733. We reprocessed the stack of the 170 km-long VALSIS-819 multichannel seismic line acquired in 1988 by the Lamont- Doherty Earth Observatory Research Vessel (R/V) Robert D. Conrad. We have also interpreted an industry seismic grid SGV01 acquired in 2001 by the Fugro-Geoteam vessel R.V. Geo Baltic.

To study the Gulf of Lions Basin formation we interpreted an industry dataset SPBAL01 acquired in 2001 by Spectrum Energy with the Polar Princes vessel. Some amount of post-migration cosmetic processing was done on some of the lines of this data set for presentation purposes.

To study the tectonic structure of the Algero-Balearic Basin we processed a 97 km- long multichannel seismic line TOPOMED-26 acquired by our group and with my participation in 2011 on-board the Spanish R/V Sarmiento de Gamboa. In order to discuss the nature of the basement of the Algero-Balearic Basin, we also used the Vp model of the wide-angle seismic profile P03 from the WESTMED project. Moreover we have done “mirror imaging” with Ocean Bottom Seismometers along line P04 of WESTMED, and integrated the results of line ESCI-Valencia across the south-Balearic margin.

The analysis, processing and interpretation of all this dataset allows us to a novel interpretation of a series of aspects regarding the structure and nature of the crust, and to discuss some new proposals for the tectonic processes that have led to the current configuration of the main Western Mediterranean basins.

Seismic interpretation of the Valencia Trough reveals that this basin is possibly floored by continental crust and it is possibly the oldest Western Mediterranean basin. Contrary

to what it is typically interpreted; we propose that the present day configuration of the Valencia Trough is strongly dominated by the tectonic activity of Mesozoic structures. During Mesozoic times occurred the greatest thinning of the crust yielding a minimum crustal thickness of ~4.5 km under the Columbretes Basin (SW Valencia Trough). Multichannel seismic sections show that the main tectonic structures are Mesozoic in age and reveal that the Neogene extensional event is very limited comparatively and has had a minor influence in the formation of the current crustal structure of the Valencia Trough. We also found no evidence of any relevant crustal thickening during Paleogene times. Thus our interpretation supports that the extension leading to the crustal configuration of the Valencia Trough was largely formed during Mesozoic times.

Interpretation of a grid of multichannel seismic profiles in the Gulf of Lions supports the existence of three geological domains that are: 1) a continental domain formed by normal faults that tilted the continental basement with syn-rift sediment infill. 2) a ~100 km wide transition domain between continental and clear oceanic domain, characterized by a thin anomalous layer with high velocity (7.5 km s-1) that has been debated in the literature and that we interpret as exhumed and serpentinized mantle peridotite and 3) an oceanic domain characterized by thin oceanic crust (~5 km) with little evidence of important faulting.

Multichannel seismic results from Algero-Balearic Basin have been complemented with information provided by a Vp wide-angle tomographic model coincident with one of the seismic reflection lines. A comparison of depth-velocity profiles of the tomographic model with existing 1D velocity-depth compilations of continental and oceanic crust (e.g. White et al., 1992; Christensen and Mooney, 1995) gives clues on the nature of the basement. The results reveal the existence of three main geological and geophysical domains: 1) a continental domain, with little evidence of faulted and tilted blocks (which are common features on extensional margins), although the vertical velocity structure matches well the Vp compilation for continental crust (Christensen and Mooney, 1995). The Vp data, seismic images and previous works on the region available in literature support abundant magmatism (with volcanism) during extension of soon after. 2) A narrow continent-ocean domain with vertical velocities neither typically continental crust nor oceanic crust and 3) a clear oceanic domain characterized by 6 km thick oceanic crust, with a vertical velocity gradient of oceanic layer 2 and 3, even though in this case formed in a back-arc context.

A new kinematic model for the opening of the Western Mediterranean basins is beyond the scope of our work, and requires integration with other datasets from

Tyrrhenian and Alboran Basins, and an extended study of the West Sardinia and Corsica margins. Also, to advance in such model, it requires a detailed reconstruction of submarine volcanic activity, imaged in the seismic data in all three basins but inadequately studied in available literature. Nonetheless, the results of this work should be considered in the future for new reconstructions that we propose need to be reviewed.

Aquesta tesi té com a objectiu descriure l’estructura tectònica i la naturalesa del basament de les principals conques de la Mediterrània occidental: Solc de València, Golf de Lleó i conca algerina-balear i discutir l’edat i els mecanismes de formació que han donat lloc a la configuració actual. Per això, s’han utilitzat tot un seguit de dades geofísiques tant d’indústria com acadèmiques. S’ha dut a terme el processat i la interpretació de diversos perfils de sísmica multicanal situats a l’àrea d’estudi i la interpretació de dos perfils de sísmica de gran angle de la conca algerina-balear. A més a més, en aquest cas s’ha utilitzat una nova tècnica coneguda com “Mirror Imaging” per a processar el senyal dels múltiples en sísmica de gran angle i que dona uns resultats equiparables als de sísmica multicanal.

La interpretació sísmica dels diferents perfils mostra com el Solc de València presenta un basament de naturalesa continental al llarg de tota la conca i com la seva configuració està lligada als processos i les estructures que van tenir lloc durant el Mesozoic. Per altre banda, la interpretació sísmica dels perfils situats al Golf de Lleó han permès definir el límit entre 3 dominis amb un tipus de naturalesa del basament diferent: 1) Domini continental, 2) domini de transició entre escorça continental i oceànica i 3) Domini oceànic. Finalment, la interpretació dels resultats de sísmica multicanal de la conca algerina-balear juntament amb un estudi exhaustiu de les velocitats obtingudes a través de la tomografia de gran angle a permès comparar el model de velocitats obtingut per aquets zona amb els models de referència per escorça continental (Christensen and Mooney, 1995) i escorça oceànica (White et al., 1992), definint així 3 dominis diferents: 1) Domini continental, 2) domini de transició entre escorça continental i escorça oceànica i 3) domini oceànic.

Els resultats obtinguts en aquesta tesi intenten donar resposta a diverses qüestions encara obertes avui en dia, com son l’edat d’obertura de les diferents conques i la cinemàtica que ha donat lloc a llur formació, així com discutir la validesa dels diferents models establerts per a la formació de les conques de rere arc a la Mediterrània occidental.