Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England

Abstract

An improved method of U–Pb dating of vein calcite formed during deformation is used to determine the age and cause of folding along the south coast of England. Fractures arising from folding of Late Cretaceous Chalk of southern England occurred 34.7 ± 1.7 myr ago. Underlying Jurassic strata have veins within fractures with ages of 55, 48–42, 39–37, 34–31 and 25 Ma, with 34–31 Ma being the tectonic culmination. Folding was slightly younger than the age of the youngest strata in the overlying Solent Group, suggesting that folding terminated basin sedimentation. This age of north–south shortening is inconsistent with attribution to intraplate forces from the mainly younger Alps, but is plausibly a result of the Pyrenean Orogen, which evolved from 50 to 28 myr ago with a late Eocene culmination. A modified method for carbonate laser ablation inductively coupled plasma mass spectrometry U–Pb dating is presented that uses measured 232 Th and 208 Pb as a better monitor of common Pb, and it has distinct advantages over existing methods. Initial common lead determined on samples conforms closely to model compositions calculated by earlier workers, with free regressions giving the most robust dates. The modified method is applicable to structural geology, carbonate diagenesis and to dating of carbonate relevant to palaeo-environmental and archaeological studies. Supplementary material : Details of sample locations and characteristics, photographs of sample localities, microscopic images of veins, details of the experimental procedure and U–Pb data for all samples and reference materials are available at https://doi.org/10.6084/m9.figshare.c.3997086