Abstract
Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedTmaxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with RoandTmaxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of Rovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.
Highlights
One of the key characteristics of continental rifting is the development of hot geothermal gradients
We investigate the spatial distribution of rift-related HighTemperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt
Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously published Tmax data, we propose a new map depicting the spatial distribution of the HT/LP metamorphism of the PyreneanCantabrian belt
Summary
One of the key characteristics of continental rifting is the development of hot geothermal gradients. Boreholes used for these studies penetrated only basement highs and young syn-rift sedimentary units that are not documenting the syn-rift thermal structure within deeper and thicker basins It has been shown on both onshore and offshore records that hyperextended domains of rifts usually display much higher and spatially variable syn-rift peak temperatures (Tmax) in close association with indications for hydrothermal and/or magmatic processes As measurements were performed on carbonaceous material coming from both surface and drillholes (Vitrinite Reflectance values and Raman Spectroscopy of Carbonaceous Material [RSCM] data), this unique dataset of a sedimentary rift system further enables to contextualize the so-called “Pyrenean HT/LP event” as well as to investigate the role of sediment burial in both time and space Considering these results, we compare the PyreneanCantabrian record to the Alps that show a generalized intense serpentinisation and brecciation of peridotites without significant HT/LP metamorphism of the syn-rift sediments. These various examples are perfect sites to study the thermal evolution of the ocean-continent transition (OCT) during hyperextension, a domain rarely attainable on present-day continental margins
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