Crustal structure of the south-western termination of the Alpine Pyrenean-Cantabrian Orogen (NW Iberian Peninsula)

- Crustal-scale section through the southwestern Alpine Pyrenean-Cantabrian Orogen
- Magnetotelluric and gravity data help to interpret deep geological structures.
- Different modes of intraplate deformation occur along the crustal section.
- To the South, a “pop-up” is limited by reverse faults that reach the lower crust.
- To the North, a thrust system is linked to a mid-crustal detachment.

The structure of the western termination of the Alpine Pyrenean-Cantabrian Orogen (NW Iberian Peninsula) has been under debate during the last years. We present new results derived from a new magnetotelluric profile that runs through the western Cantabrian Mountains and the León Mountains, together with gravity data, structural field data and petrological modeling that allows the crustal structure of the sector to be clarified. In the Cantabrian Mountains, the overall Alpine structure is a regional monoclinal flexure that corresponds to the back-limb of E-W trending and north-dipping thrust system. It is formed by a connected thrust-fault splay that overturns the Cenozoic sediments of the Bierzo basin, probably rooted in a detachment at 15 km depth. Further to the South, in the León Mountains, we propose a “pop-up” geometry characterized by high-angle dipping reverse faults that extend toward the lower crust probably causing uplift and the erosion of the early foreland basin. Apatite fission-track and paleontological data suggests that uplift and exhumation of the León Mountains occurred later than in the Cantabrian Mountains. Uplift started in the Middle and Upper Miocene in the León Mountains linked to the activity of the highlighted “pop-up” structure whereas it took place during the Oligocene in the Cantabrian Mountains.