EXHUMATION OF THE PYRENEAN INTRA-CONTINENTAL COLLISIONAL OROGEN: NEW THERMOCHRONOLOGIC CONSTRAINTS FROM THE CENTRAL PYRENEES

The Pyrenees are a doubly vergent orogen formed since the Late Cretaceous as a result of convergence between the European and Iberian plates. The orogen comprises a central axial zone (AZ) of Hercynian basement, flanked north and south by fold and thrust belts, in turn flanked by the Aquitane and Ebro foreland basins. The AZ is a complex south-vergent duplex that culminates in an antiformal stack of three upper crustal basement thrust sheets, bounded to the north by the North Pyrenean Fault (NPF), regarded as the Iberian-European plate suture.

Apatite fission track (AFT) data from seven vertical profiles along the ECORS transect indicate an asymmetric multi-phase denudation history. Eocene denudation (ca. 200m/my) initiated at ~50 Ma was followed by rapid denudation (km/my) in the Oligocene (initiated at ~35 Ma) that slowed considerably at ~30 Ma. Apatite (U-Th)/He data suggest denudation continued to ~20 Ma. K-feldspar thermochronology on AZ samples yield ⁴⁰Ar/³⁹Ar age gradients characterized by Late Paleozoic (Hercynian) maximum ages and Late Cretaceous-Early Eocene minimum ages. Variable argon loss can be correlated to sample location relative to the NPF and structural position. AFT ages from granitic cobbles within syn-tectonic conglomerates from the Sis paleovalley on the southern flank of the AZ young upwards, reflecting progressive unroofing of the AZ and recording an inverted ECORS-AFT profile. Some samples with AFT ages 50-40 Ma and mean lengths of 12-13 µm (indicative of slower cooling) suggest a change in local denudation rate or diachronous denudation from north to south. Cobbles with AFT ages >60 Ma have shorter track lengths consistent with slower denudation prior to onset of Eocene rapid denudation. Thermochronologic data combined with geological constraints and geodynamic modeling indicate a tectonic control on the changing rates and patterns of erosional denudation associated with creation of relief dependent on how convergence was accommodated within the AZ. Denudation rates of ca. 200 m/my (~50-35 Ma) reflect accommodation of convergence along relict extensional features, whereas rates of denudation of km/my (~35-30 Ma) reflect internal deformation in the upper crust of the Iberian plate and wedging of the European crust. Denudation rates then slow as the Ebro Basin fills before re-excavation in the Late Miocene.