Thermal evolution of onshore West Iberia: A better understanding of the ages of breakup and rift-to-drift in the Iberia-Newfoundland Rift

Abstract

The age of breakup which formed the Central-North Atlantic has been debated for many decades and is still subject to debate: from ca. 150 Ma to 110 Ma. To address this issue, we have carried out a thermochronological study of the eastern margin of the rifted Iberia-Newfoundland sector. New apatite fission-track (AFT) data acquired on samples from the footwall (Variscan basement) and one sample (Triassic of the Lusitanian Basin) from the hanging wall of the principal normal fault bounding the basin. Thermal history of western Iberia can be then reconstructed since ca. 250 Ma. Fission-track ages of Variscan granitoids (whose crystallization age is >275 Ma) range between 191 ± 8 Ma and 75 ± 5 Ma, indicating that significant thermal events affected the study area during that period. Thermal inversion supports two main cooling events that we attribute to a major uplift and denudation of the Variscan basement, consistent with widespread basement-derived siliciclastic rocks of similar ages: Late Jurassic/Early Cretaceous (ca. 150–145 Ma) and late Early Cretaceous (ca. 110 Ma). From the sedimentary record in the Lusitanian Basin and the new AFT data, we deduce that: (1) temperatures >70–110 °C affected west Iberia between ca. 190 and 150 Ma, which can be explained partly by a subsiding basement in both foot and hanging walls; (2) the main cooling event at ca. 150–145 Ma is interpreted as major rift flank uplift; (3) the ca. 110 Ma cooling event may be linked to the final evolution of the margin and onset of oceanic spreading. We infer that the AFT main cooling event at ca. 150–145 Ma reflects the break of the elastic core of the lithosphere (whole lithosphere failure = breakup) with significant rift shoulder uplift (end of Rift 1), which was followed by hyper-extension and mantle exhumation (Rift 2) and finally, by oceanic spreading (rift-to-drift) at ca. 110 Ma (onset of Rift 3).