Mesozoic and Cenozoic exhumation history of the SW Iberian Variscides inferred from low-temperature thermochrdnology

Abstract

The post-Paleozoic tectonothermal evolution of the SW Iberian Variscides is poorly known mainly due to the scarce low-temperature geochronological data available. We have obtained new apatite fission-tracks and apatite (U-Th)/He ages to constrain the Mesozoic and Cenozoic tectonic evolution of this portion of the Iberian Massif located just north of the Betic-Rif Alpine orogen. We have obtained nine apatite fission-track ages on samples from Variscan and pre-Variscan granitoids. These ages range from 174.4 (+/- 10.8) to 54.1 (+/- 4.9) Ma, with mean track lengths between 10.3 and 13.9 mu m. We have also performed 5 (U-Th)/He datings on some of the same samples, obtaining ages between 74.6 (+/- 1.6) and 18.5 (+/- 1.4) Ma. Time-temperature path modeling of these low-temperature geochronological data leads us to envisage four post-Paleozoic tectonically controlled exhumation episodes in the SW Iberian Variscides. Three of these episodes occurred in Mesozoic times (Middle Triassic to Early Jurassic, Early Cretaceous, and Late Cretaceous) at rates of approximate to 1.1 to 2.5 degrees C Ma(-1), separated by periods with almost no cooling. We relate these Mesozoic cooling events to the formation of important marginal reliefs during the rifting and opening of the central and northern Atlantic realm. The fourth exhumation episode occurred in Cenozoic times at rates of approximate to 3.2 to 3.6 degrees C Ma(-1), being only recorded in samples next to faults with topographic escarpments. These samples cooled below 80 degrees C at approximate to 20 Ma at rates of 3-13 degrees C Ma(-1) due to roughly N-S oriented compressional stresses affecting the whole Iberian plate, which, in the particular case of SW Iberia, reactivated some of the previous Late Paleozoic thrusts.