Deciphering the tectonic deformation history in fold and thrust belts using U-Pb dating of fracture-filling carbonates. The Pyrenean and the Andean-Neuquén Basin case studies

During the last decade, the U-Pb geochronology of fracture-filling carbonates has been used to reconstruct the tectonic and diagenetic history of fold and thrust belts, worldwide. These studies unequivocally show the potential of the U-Pb dating method to quantify geological processes in compressional settings such as activity and duration of fluid migration, folding and faulting, duration of thrust sheet emplacement, and calculation of shortening rates. Here, some examples from the SE Pyrenean fold and thrust belt and from the Andean-Neuquén Basin are presented.

U–Pb ages measured in fracture-filling carbonates from the SE Pyrenean fold and thrust belt reveal Late Cretaceous to Oligocene compressional ages ranging from 71.2 to 25.7 Ma and a minimum duration for the emplacement of the thrust sheets of 18.7 Ma (Bóixols–Upper Pedraforca), 11.6 Ma (Lower Pedraforca) and 14.3 Ma (Cadí). These ages also show that piggy-back thrusting occurred coevally with the post-emplacement deformation of the upper thrust sheets above the lower ones during their south-directed tectonic transport. The duration of the thrust sheet emplacement combined with well-balanced cross sections of the SE Pyrenees allow to calculate shortening rates of 0.6, 3.1 and 1.1 mm/yr from the older to younger thrust sheets, which agree with previous estimations based on the magnetostratigraphic and biostratigraphic studies of syn-orogenic deposits.

In the SE Pyrenees, geochronological results also reveal the long-lasting tectonic history of fault zones and folds. As an example, at Bóixols thrust sheet, dating of multiple samples along the Abella de la Conca thrust fault zone at the frontal Sant Corneli anticline reveals multiple reactivations from 66.9 to 36.55 Ma spanning ∼30 Myr of tectonic activity. Furthermore, systematic dating of fracture-filling carbonates along the whole Sant Corneli anticline, combined with the structural analysis of fractures, constrain its evolution for ∼62 Myr: 1) layer-parallel shortening and folding (from 71.2 to 56.9 Ma); 2) fold tightening (from 55.5 to 27.4 Ma); and 3) post-folding extension (from 20.8 to 9 Ma).

In the Agrio, Chos Malal and Malargüe fold and thrust belts in the Neuquén Basin along the front of the Andes in Argentina, the dating of bed-parallel fibrous calcite veins “beef” reveals mild tectonic pulses that triggered fluid overpressures and oil migration from 116.7 to 78.8 Ma, partly coevally with the Late Cretaceous syn-tectonic deposition of the Neuquén Group. U-Pb dates determined in veins cutting calcite beef register Late Cretaceous to Palaeocene period of layer-parallel shortening in the Neuquén Basin from 72.8 to 60.9 Ma and early-middle Eocene and middle-late Miocene stages of folding and thrusting from 52.0 to 42.2 Ma and from 13.9 to 6.2 Ma, respectively.

This research was funded by the DGICYT Spanish Project PID2021-122467NB-C22, the Grups de Recerca reconeguts per la Generalitat de Catalunya “Modelització Geodinàmica de la Litosfera” (2021 SGR 00410) and ”Geologia Sedimentària” (2021 SGR-Cat 00349).