The Nappe des Marbres Unit of the Basque‐Cantabrian Basin: The Tectono‐thermal Evolution of a Fossil Hyperextended Rift Basin

Abstract

The architecture of the Pyrenean‐Cantabrian belt results from the inversion of a series of former Cretaceous rift basins. A HT‐LP metamorphic event dated at 105 to 85 Ma ago is commonly associated with an Albo‐Cenomanian episode of hyperextension of the continental crust. This metamorphism is well known in the eastern Basque‐Cantabrian Basin within the Nappe des Marbres Unit (NMU) that is preserved from intense compressional deformation during the Pyrenean orogeny. Based on a structural study at the scale of the eastern Basque‐Cantabrian Basin and on a dense sampling for TRSCM estimates with the Raman spectroscopy of carbonaceous material (RSCM) method (Raman Spectrometry on Carbonaceous Material), we show following results: (1) the NMU has recorded two major phases of deformation, related to a localized extensional ductile foliation (S1) during the Cretaceous rifting and later by the regional N‐S shortening, recorded by a regional cleavage (S2) observed only in shale sediments. (2) The NMU is affected by early salt tectonics related to several diapirs and salt walls fed by the Upper Triassic evaporitic layer. (3) The NMU recorded maximum temperatures exceeding 550°C, which represent some of the highest temperatures along the Pyrenees. These new data demonstrate that there is no E‐W lateral metamorphic gradient across the belt and that hyperextension rifting occurred consistently within a high‐thermal regime; (4) metamorphic isograds are oblique in respect of the main structures, suggesting that the metamorphic event postdates most of the early salt‐related deformations prior to or coeval with early stages of rifting. As it represents one of the best preserved example of preorogenic hyperextended basin, the eastern Basque‐Cantabrian Basin record may be used regionally to better understand the rift to orogen evolution of the Pyrenean Internal Metamorphic Zone but also more generally as an analog of salt‐bearing hyperextended rift record.

Preorogenic Folds and Syn‐Orogenic Basement Tilts in an Inverted Hyperextended Margin: The Northern Pyrenees Case Study

“…In the study area, peak temperatures vary between~330°C and~480°C, being similar to the RSCM peak temperatures defined in the Baronnies-Barouse area (Clerc et al, 2015), located about 30 km to the east and slightly higher than maximum RSCM temperatures in the Mauléon domain, 30-40 km to the west (300-400°C; Clerc et al, 2015). Our new temperatures fit the westwards temperature decrease along the North Pyrenean Zone defined by previous studies (Clerc et al, 2015), although this trend is abruptly interrupted further west (in the Basque Pyrenees) where peak temperatures exceed 550°C (Ducoux et al, 2019). Apart from the E-W variation in Cretaceous peak temperatures, previous data in the Eastern Pyrenees also indicate an N-S zonation, with the highest temperatures being limited to the southernmost area of the North Pyrenean Zone and the northern domains being characterized by a rapid decrease of the metamorphic imprint (Clerc et al, 2015).…”

“…This crosscutting relationship between isotherms and folds can be considered as an indication of the relative age between them, the isotherm imprint postdating the folding. Similar relative time relationships between isotherms and cover folds have been reported in the Basque-Cantabrian Pyrenees (Ducoux et al, 2019). Based on the absolute age of high-temperature metamorphism (i.e., 85-110 Ma, mostly obtained by 40 Ar-39 Ar and U-Pb dating methods on metamorphic and magmatic minerals; see Clerc et al, 2015 and references therein), we can propose that these folds were mostly formed during syn-rift stages in the Northern Pyrenees (i.e., in Jurassic to Early to mid-Cretaceous), before the onset of contraction.…”

“…This metamorphic event lasted almost 25 Ma between 110 and 85 Ma (late Aptian-Coniacian, Golberg & Maluski, 1988; see review by Clerc et al, 2015). It was characterized by maximum temperatures that ranged between~600°C and <400°C (Clerc, 2012;Clerc et al, 2015;Corre, 2017;Ducoux, 2017;Ducoux et al, 2019;Espurt et al, 2019). Peak metamorphism predated the onset of the convergence between the Iberian and European plates whose first evidence in the western North Pyrenean Zone dates of Campanian-Maastrichtian (Poitevin et al, 2014;Teixell, 1996).…”

“…Peak temperatures in the Northern Pyrenees are related to extreme continental thinning during the Cretaceous and indicate that this episode was characterized by high geothermal gradients (~60 to 80°C/km, Chelalou et al, 2016;Ducoux et al, 2019;Hart et al, 2017;Vacherat et al, 2014). In the study area, peak temperatures vary between~330°C and~480°C, being similar to the RSCM peak temperatures defined in the Baronnies-Barouse area (Clerc et al, 2015), located about 30 km to the east and slightly higher than maximum RSCM temperatures in the Mauléon domain, 30-40 km to the west (300-400°C; Clerc et al, 2015).…”

“…To complete the restorations, fault displacements were restored by joining isotherms in their side blocks. An inherent uncertainty of 150 to 300 m in fault throws has to be considered because of the 10-20°C temperature uncertainty of the reconstructed isotherms (maximum of 425 m, considering the~30°C RSCM calibration error and an average geothermal gradient of 70°C/km, Chelalou et al, 2016;Ducoux et al, 2019;Hart et al, 2017;Vacherat et al, 2014). The present-day topography line (dotted black line in Figure 12) was also passively restored using remagnetization and peak temperature isotherms.…”