Central Pyrenees Mountain Building: Constraints From New Lt Thermochronological Data From The Axial Zone

Early phases of mountain building are usually poorly constrained although they may provide insights into both the wedge rheology and the role of inheritance. In the central Pyrenean Axial Zone, a main exhumation peak at 35-30 Ma is particularly well constrained but the previous exhumation stages are much less constrained. In this study, we present new low-temperature thermochronological zircon fission-track and (U-Th)/He data and structural observations to constrain the thermal history of the Axial Zone and its shortening sequence during the whole Alpine collision. The dataset collected along the ECORS profile suggests two main collisional cooling phases. An early stage of Alpine shortening (from 70 to 40 Ma) is characterized by low rates of cooling/exhumation in the whole Axial Zone around 5-10 degrees C/Myr and 200 +/- 50 m/Myr. During this phase, deformation was distributed within the whole Axial Zone. This result questions the occurrence of a quiescence phase in the Axial Zone. Between 40 and 20 Ma, a more localized second phase with cooling rates around 30 degrees C/Myr and high exhumation rates >800 m/Myr is only recorded in the Maladeta massif. During this second phase, much lower denudation rates are recorded further north and south, thus suggesting rather symmetrical exhumation in the Axial Zone, controlled by underplating of the youngest crustal ramp, the Rialp thrust. This sequence of shortening, from distributed to localized deformation, appears to be characteristic in other collisional wedges as well.