Thermochronology of a convergent orogen: Constraints on the timing of thrust faulting and subsequent exhumation of the Maladeta Pluton in the Central Pyrenean Axial Zone

Abstract

The Pyrenees Mountains of Spain and France are a collisional orogen cored by a south-vergent antiformal stack of Paleozoic basement thrust sheets (the Axial Zone). Thrusting accommodated shortening due to convergence between Iberia and Europe. Granitic plutons, intruded during the Hercynian orogeny, form an important component of the basement units. Thermochronology of granodioritic samples collected from a vertical [age–elevation] profile from the south-central Maladeta pluton provide new constraints on the timing of thrust faulting and rock exhumation of the Pyrenean orogen. The Maladeta massif lies within the Orri thrust sheet, presently occupying the immediate footwall of the Gavarnie thrust, a major Axial Zone Alpine-age thrust fault. New biotite and K-feldspar 40Ar/ 39Ar and apatite (U–Th)/He (AHe) data combined with previous apatite fission track (AFT) thermochronology and structural studies of the Maladeta pluton constrain the thermal evolution of this massif. Biotite weighted mean plateau and K-feldspar maximum 40Ar/ 39Ar ages from the highest elevations of the Maladeta pluton (2850 m) are ∼ 280 Ma, close to the age of intrusion, and are interpreted to date the timing of rapid cooling in the Hercynian. Subsequently the Maladeta massif remained close to the surface as indicated by the presence of unconformable Triassic continental sediments. K-feldspar 40Ar/ 39Ar multiple diffusion domain thermal models from two samples at lower elevations (1780 m and 1400 m) indicate heating initiated at ∼ 65 Ma, reaching maximum temperatures of 270 to 280 °C. These data are interpreted to record thrust burial of the Maladeta pluton to depths of ∼ 6–10 km in the footwall of the Gavarnie thrust. Burial and heating were followed by a period of relatively rapid cooling beginning at ∼ 50 Ma, recorded in three K-feldspar thermal models from samples at different elevations (2850 m, 1780 m, and 1400 m). AFT thermal models, in conjunction with the AFT and apatite AHe age–elevation profile interpretation reveal a lower temperature portion of the thermal history. These data indicate the onset of rapid cooling that began at 30–35 Ma, slowed dramatically at 25–30 Ma, and continued at a reduced rate until ∼ 15 Ma. Finally, AFT thermal models from the lowest elevation samples record rapid cooling in the Late Miocene–Pliocene, most likely due to re-excavation of the southern flank of the Pyrenean orogen, following burial by syn-tectonic conglomerates. The integrated thermochronologic data set from the age–elevation profile within the Maladeta pluton illustrates the potential for obtaining age constraints related to processes operating within the interior portions of convergent orogenic systems (e.g., the onset and/or duration of thrust fault activity) that are otherwise unobtainable.