The role of extensional detachment systems in thinning the crust and exhuming granulites: analogies between the offshore Le Danois High and the onshore Labourd Massif in the Biscay/Pyrenean rifts

Patricia Cadenas,Gabriela Fernández-Viejo,Rodolphe Lescoutre,Gianreto Manatschal,O Lacombe,D Pedreira,S Calassou,S Tavani,A Teixell

doi:10.1051/bsgf/2021045

Abstract

Large uncertainties remain about the architecture, timing and role of the structures responsible for high degrees of crustal thinning and the exhumation of mid-crustal granulites in the Pyrenean and Biscay rift systems. Both, the Le Danois High in the North Iberian margin and the Labourd Massif in the Western Pyrenees preserve evidence of extensional detachment faults and include exhumed granulites, which are locally reworked in syn-rift sediments. In this study, we compare the crustal structure and its link to the overlying sediments at the two sites based on the interpretation of high quality 2D seismic reflection profiles offshore and field observations and published geological cross-sections onshore. New reported seismic and field observations support that extensional detachment systems delineate the top basement in the Le Danois High and the Labourd Massif, advocating for a similar tectonic evolution. We propose that the Le Danois and North Mauléon extensional detachment systems were responsible for high degrees of crustal thinning and the exhumation of the pre-rift brittle-ductile transition and associated mid-crustal granulites during Aptian to Cenomanian extension, leading to the formation of the Le Danois and Labourd crustal tapers. Subsequently tilted and uplifted during the Alpine convergence, the two taper blocks lay at present in the hanging-wall of major Alpine thrusts. Their position at overlapping, en-echelon hyperextended rift segments at the end of rifting, and the occurrence of shortcutting structures at depth linking neighbouring rift segments can explain the preservation of the rift-related detachment systems. This study proposes for the first time analogies between the offshore Le Danois High and the onshore Labourd Massif and demonstrates the importance of extensional detachment systems in thinning the crust and exhuming mid-crustal granulites at the seafloor in the Biscay and Pyrenean rift systems during Aptian to Cenomanian extension.

Highlights

This study proposes for the first time analogies between the offshore Le Danois High and the onshore Labourd Massif and demonstrates the importance of extensional detachment systems in thinning the crust and exhuming mid-crustal granulites at the seafloor in the Biscay and Pyrenean rift systems during Aptian to Cenomanian extension
We address the problem of crustal attenuation and exhumation of pre-rift midcrustal granulites based on the analysis and comparison of the offshore Le Danois High and the onshore Labourd
Between 200 Ma and the deposition of the onlapping mid-Cretaceous sediments onto the granulites. Considering this and the occurrence of greenschist facies deformation along the shear zone, we suggest that the Louhossoa shear zone (LSZ) was-activated as a top-to-the-south ductile shear zone during the Mesozoic extension, and assisted for the exhumation of the granulites

Summary

1) Introduction

Geological and geophysical observations have suggested the occurrence of highly thinned crust and exhumed mantle in the Bay of Biscay and in the Pyrenean domain (e.g., Pinet et al, 1987; Fernández-Viejo et al, 1998; Thinon et al, 2003; Lagabrielle et al, 2010; Tugend et al, 2014; DeFelipe et al, 2017; Ruiz et al., 2017), leading to a general agreement about the relevance of crustal attenuation and mantle exhumation during. The pre-rift units included in these blocks are likely to correspond to the Upper Jurassic to Lower Cretaceous shallow platform carbonates and pelagic sediments dredged at the top and along the northern slope of the Le Danois High (figure 2A) (Boillot et al., 1979, Malod et al, 1982). The northdipping NMD outcrops at the northern boundary of the Ursuya and the Jara massifs (Jammes et al, 2009; Masini et al, 2014) It separates the Paleozoic basement from the Upper Triassic to Early Aptian shallow marine pre-rift carbonates and the thick Late Aptian to Cenomanian syn-rift flysch-type siliciclastic deep marine sediments of the Mauléon Basin (e.g., Masini et al, 2014). This indicates that a brittle, top-tothe-north deformation postdates the ductile deformation associated with the LSZ

6) Discussion

7) Conclusions