Dear Sylvain, thanks for your question. You’re right, our study reveals the same range of high Tmax values along the whole rift axes (Fig. a). However, our dataset suggests asymmetric horizontal thermal gradients on both side of the rift axis. This, it is well shown within the Basque-Cantabrian Basin where Tmax values gently decrease from the rift axis to the south (550 to 200°C) whereas in the north it abruptly decreases (Fig. b). Recent modeling results by Lescoutre et al. (2019), within the neighbouring Mauléon Basin, suggests that an asymmetric rift system may generate an asymmetric thermal record perpendicular to the rift axis (Fig. c). Additionally, the abrupt lateral thermal gradient indicates an “upper plate” configuration.
Considering these results at the scale of the Pyrenees, the lateral switch of the thermal asymmetry that we document supports a change in the rift polarity across the Pyrenees. From our results, the Western Pyrenees thermal record suggests that Europe is in an Upper Plate setting whereas in the Central and Eastern Pyrenees where the opposite setting applies (Fig. a).
The recent geophysical images obtained by the OROGEN project revealed that the orogenic structure is changing along strike (Chevrot et al., 2018). This may be due to the lateral change in the rift polarity which was shortened differently along strike.

First, we should acknowledge that there are coexisting models for the pre-orogenic rift setting of the Pyrenees and this is certainly the consequence of the scarcity of outcrops and the orogenic overprints. However, our dataset allows us to rank different models.
Tmax observed and measured in this area are similar to that obtained in the Central Pyrenees as well as within the Basque Cantabrian Basin at the western end of the belt. Considering this, we suggest that a similar rift process affected the whole Pyrenean belt. However, the N-S Tmax distribution is symmetrically opposite between the eastern and western part of the Pyrenees. From that, we believe that the rifting process may have been the same along strike but its polarity (i.e. Lower VS Upper plate) may have switched across rift transfer zones (Fig. a).
Different hypothesis exist to explain the disappearance of the Pyrenean slab towards the east (Fig. b). It may result from a diffusive accommodation orogenic strain (i.e. shortening is not localized along a single underthrusting fault plane) or due to a limited shortening rate in this part of the belt (which is in contradiction with surface shortening and plate kinematic estimates). Alternatively, the lateral change in the rift geometry may also explain the shortening style however this is still under research. Another possibility, proposed by Jolivet et al., (2019), is that the “Pyrenean slab” was cylindrical in the eastern part of the Pyrenees but it was eroded/removed by the neighbouring Gulf of Lion back-arc rifting (Oligocene-onwards).