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 of folding along the south Coast of England, and determine its cause. Fractures arising from folding of Late Cretaceous Chalk of southern England occurred 34.7±1.7 Ma 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 folding terminated basin sedimentation. This age of N-S shortening is inconsistent with attribution from intraplate forces from the mainly younger Alps, but is plausibly a result of the Pyrenean orogen that evolved from 50-28 Ma ago with a late Eocene culmination. A modified methodology for carbonate LA-ICP-MS 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 Stacey-Kramers (1975) model compositions, 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: Appendices 1, 2, 3, 4 and 5 are available at https://doi.org/xxxx' The landscape of the southern coastal region of England from Dorset to Kent is notable historically and for its place in literature, the arts, natural history and tourism. The coastal morphology arises from differential erosion of tilted and folded strata that form escarpments and dramatic coastal cliffs. Folds have been recognised since publication of William Smith's 1815 map more than 200 years ago, for example in early accounts by Webster (1816),

An Image Mapping Approach to U‐Pb LA‐ICP‐MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation

“…A linear regression of the ECDF data in Tera‐Wasserburg space is then undertaken to obtain the initial 207 Pb/ 206 Pb ratio, the lower intercept date, and their associated uncertainties and statistics using Isoplot. To test for consistency of the data, two additional dates are calculated from the same data set: an isochron age from 206 Pb total / 208 Pb common versus 238 U/ 208 Pb common (e.g., Getty et al, ) and the 86TW age ( sensu Parrish et al, ) employing 208 Pb common / 206 Pb total versus 238 U/ 206 Pb total . These two additional approaches to age calculation require correction for radiogenic 208 Pb ingrowth in Th‐bearing samples.…”

“…After identification and isolation of potentially different genetic domains, several tests can be undertaken to check the accuracy and consistency of the U-Pb data derived from Monocle-based selection and pooling of pixels and to avoid inaccurate age determinations resulting from data processing artifacts. Coherent behavior of the data points on Tera-Wasserburg concordia, 206 Pb/ 208 Pb common versus 238 U/ 208 Pb common isochron, and 208 Pb common / 206 Pb total versus 238 U/ 206 Pb total (86TW in the sense of Parrish et al, 2018) plots is essential and must yield consistent (i.e., within analytical uncertainty) U-Pb dates and initial Pb compositions. These results should also be compared to those derived from conventional (i.e., manual) selections made from the time-resolved signal.…”

Direct U-Pb dating of carbonates from micron scale fsLA-ICPMS images using robust regression

“…Since the seminal publication of Li et al (2014), the U-Pb dating of carbonates by LA-ICPMS has progressed significantly with applications as varied as the dating of tectonic fractures and veins (e.g., Roberts & Walker, 2016;Beaudoin et al, 2018;Parrish et al, 2018;Nuriel et al, 2019), carbonate deposition (Drost et al, 2018), speleothems (Woodhead et al, 2019) or https://doi.org/10.5194/gchron-2020-10 Preprint. Discussion started: 14 April 2020 c Author(s) 2020.…”

“…The methodology of age determination proposed by these authors is based on the sorting of pixel ratio values used in dating ( 238 U/ 206 Pb, 207 Pb/ 206 Pb for a Tera-Wasserburg (TW) diagram) based on 238 U/ 208 Pb and 235 U/ 207 Pb pixel ratios and their pooling (i.e., discretization) to subsets ('pseudo-analyses') of similar number of pixels for which the mean values and associated standard errors (i.e., standard deviations of the mean) are calculated. These values are then plotted in TW, 86TW ( 208 Pbcommon/ 206 Pb versus 238 U/ 206 Pb; Parrish et al, 2018) and isochron diagrams ( 206 Pb/ 208 Pbcommon versus 238 U/ 208 Pbcommon) to calculate the age using York-type regressions as implemented in Isoplot (Ludwig, 2012) or IsoplotR (Vermeesch, 2018). Additional examples of the interest of this new approach are provided in Roberts et al (2019).…”

Exploring the advantages and limitations of in situ U–Pb carbonate geochronology using speleothems

“…Recent years, however, have seen a revolution in the field with the emergence of in situ analysis techniques employing laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and offering the prospect of direct determination of U-Pb ages on the scale of a few hundred microns. Although still in its infancy, this method has already been applied to the chronology of marine cements (Li et al, 2014), vein calcites associated with faulting (Roberts and Walker, 2016;Hansman et al, 2018;Parrish et al, 2018), and the alteration of oceanic crust (Coogan et al, 2016).…”