New geoelectrical characterisation of a continental collision zone in the West-Central Pyrenees: Constraints from long period and broadband magnetotellurics

Continental collision dominates the development of a large number of mountain ranges on Earth. Although the geological evolution of these regions is highly studied, lithospheric-scale physical processes are less well characterised and their interpretations frequently differ. A new magnetotelluric profile crossing the West-Central Pyrenees with broadband and long period magnetotelluric data constrains the geoelectrical features of this continental collision zone confining the geological and physical processes that take place at a lithospheric scale. Three geoelectrical structures associated with partial melting and its melt migration are imaged for the first time in the West-Central Pyrenees suggesting an Iberian subducted lower crust reaching the depth of 70 km. This result, supported by an analysis of the thermal evolution of the Iberian subducted lower crust, reinforces the hypothesis of partial melting in this region. From the long period magnetotelluric data, the asthenosphere has been imaged as a geoelectrical structure with electrical resistivity values between 20 Ω m and 70 Ω m. The lithosphere–asthenosphere boundary close to the collision zone has been constrained between 90 km and 110 km depth below the Iberian plate and between 120 km and 160 km depth below the transition zone between plates and the European plate, in agreement with the previous geophysical data. Additionally, other geoelectrical anomalies imaged have been associated with an unexpected major Variscan boundary located below the Ebro basin, Silurian sediments and free fluids in the Gavarnie thrust, and with a thick sequence of Jurassic and Early Cretaceous sediments situated below the North Pyrenean Thrust Sheet.