Cenozoic uplift of southwest England

Abstract

The uplift history of southwest England is inferred using a composite dataset including marine and fluvial terraces and karstic data. The estimated post-Mid-Pliocene uplift increases eastward from $130 m in west Cornwall and $135 m in south Devon to $150 m in the Hampshire Basin. The post-Early-Pleistocene uplift likewise increases eastward, from $55 m in west Cornwall to $60 m in south Devon and $80 m in Hampshire. Landscape and thermochronological evidence also indicates Eocene uplift, caused by the British Tertiary Igneous Province magmatism; this component tapers eastward from $300 m in west Cornwall to $50 m in south Devon, with subsidence in east Devon. This uplift accompanied magmatic underplating; the mafic layer added to the basal crust thins eastward from $6 km in west Cornwall to $2 km in south Devon, evidently tapering to zero farther east. The laterally variable crustal properties caused by this variation in underplating have affected the post-MidPliocene uplift; the study region is thus intermediate, in terms of crustal strength and landscape evolution, between central-southern England, with no underplating, and Ireland, where $10 km thick underplating has resulted in extreme Late Cenozoic landscape stability. The Eocene mantle-plumerelated uplift is distinct from the post-Mid-Pliocene phase which, the modelling indicates, has been driven by surface processes and, thus, by climate change.

Quaternary fluvial archives and landscape evolution: a global synthesis

“…13). Similar records of post-LGM incision have been reported from the Wharfe (Howard et al, 2000b) and, on the western side of the Pennines, from the Mosedale Beck, NE Lake District (Boardman, 1994(Boardman, , 1997(Boardman, , 2002 and from the River Ribble, in Lancashire (Chiverrell et al, 2007(Chiverrell et al, , 2009 underplating at the base of the crust (e.g., Westaway, 2010;Green et al, 2012). SW Ireland is known to be underlain by a thick layer of underplating, its top at a minimum depth of ~22-23 km (e.g., Masson et al, 1998).…”

“…That interpretation requires the landscape of Britain to have been essentially unchanging over tens of millions of years, if not longer (e.g., Murray, 1992). The recognition of >100 m of Quaternary uplift, based on studies of river terraces (as described in this paper), from the dating of karstic systems in progressively deepening valleys (e.g., Westaway, 2010;Bridgland et al, 2014a), as well as the indication from thermochronology that there has been many hundreds of metres of Cenozoic denudation across much of Britain (e.g., Green, 2002;Green et al, 2012), have largely superseded this type of interpretation.…”

“…Uranium-series dating of carbonate inclusions, interbeds, or speleothems can, under optimal circumstances, also provide numerical ages that are both precise and accurate (e.g., Schwarcz et al, 1988;Murton et al, 2001;Mallick and Frank, 2002;Candy et al, 2004). Dating of speleothems can be relevant to the interpretation of fluvial sequences, because the chronology of the former can reflect the history of adjacent valley entrenchment (e.g., Westaway, 2009aWestaway, , 2010Bridgland et al, 2014a). Where molluscan fossils are preserved their biostratigraphical value as age indicators can be reinforced by amino-acid dating, which is a measure of protein degradation since death (Bowen et al, 1989;Penkman et al, 2011Penkman et al, , 2013.…”

“…A westward increase in crustal stability has been recognized in the British Isles, thought to relate to the westward constriction and eventual disappearance of the mobile lower-crustal layer that gives rise to crust of high stability in Ireland (e.g., Westaway, 2010;Green et al, 2012).…”

“…SW Ireland is known to be underlain by a thick layer of underplating, its top at a minimum depth of ~22-23 km (e.g., Masson et al, 1998). Preliminary geothermal calculations by Westaway (2010) indicate that, at most, only a thin layer of mobile lower crust can be expected here, although estimation of its thickness on this basis is difficult due to uncertainties in geothermal data Beyond the MIS 2 ice limit, latest Devensian and Holocene fluvial deposits are restricted to the floodplain and any buried channel deposits that underlie the floodplain, although sometimes the latter continue above modern river level to form the lowest terrace; older terraces, dating back to the Middle Pleistocene and earlier, will typically form the majority of the record in these areas (Bridgland et al, 2010; see examples in Fig. 1).…”