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Discussion |
1 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
2 BGS Keyworth, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK
We thank Scourse and Preece for their interest in our paper.
We agree that we should have included a reference to Gale et al. (1999), who used stratigraphic data to quantify the uplift history of the northern limb of the Sandown pericline, Isle of Wight. We did, however, include a reference to Green et al. (2001), who used an apatite fission-track analysis of outcrops of Palaeozoic basement rocks to determine the uplift history of the English Midlands. Together, these studies suggest a mean long-term uplift rate of c. 0.10 m ka–1 for the Mid–Late Eocene over a wide region of southern Britain.
We agree that a long-term rate of c. 0.10 m ka–1 could not have been maintained throughout the Cenozoic because it implies an uplift of >5 km. Recent studies limit the uplift to <2.5 km, attributing it to either magmatic underplating (Jones et al. 2002) or far-field compressional stresses (Hillis et al. 2008). Scourse & Preece suggest that because Lane et al. (2008) argued for Pleistocene uplift rates of 2 m ka–1 and higher, then the uplift must have been pulsed. We agree with this suggestion.
One possibility is uplift as a result of a flexural bulge flanking an ice load. However, elastic plate modelling (Watts 2007) using the elastic thickness structure determined by Pérez-Gussinyé & Watts (2005) suggested that the bulge generated by a load the size of the last Scottish ice sheet (i.e. Devensian) is <12 m and its crest would have been located well to the north of the Cotswolds, in the Humber region. Although earlier ice sheet loads, such as the Anglian, were probably more extensive, the strong, rigid, Avalonian ’core' of central England would have ensured that any flanking bulges were both small and wide. We therefore prefer a model of river excavation and denudational isostasy, as this is capable of producing significant uplift (up to c. 150 m) local to the Cotswolds region. What is interesting is the suggestion of Lane et al. (2008), based on considerations of the elastic thickness structure of the UK, that isostatic compensation is essentially complete within c. 50 ka and that a scarp and vale topography could have been generated within a 100 ka duration glacial–interglacial cycle.
Finally, we agree with Scourse & Preece that a pulsed uplift has important implications for landscape evolution. If it is repeated during successive glacial–interglacial cycles, then it implies that climate and tectonics are acting in concert to re-surface the topography and produce a landscape that is in a state of perpetual change. There are also implications for calculations of the sea-level response to the waxing and waning of ice sheets. For example, a pulsed uplift will lessen the impact of a sea-level rise, even in a region such as the Humber that is in the collapsing bulge of an ice sheet load.
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References |
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Gale, A.S., Jeffery, P.A., Huggett, J.M. & Connolly, P., 1999. The inversion history of the Sandown Pericline, Isle of Wight, southern England, Journal of the Geological Society, London 1999, 156 , 327–339.
Green, P.F., Thomson, K. & Hudson, J.D., 2001. Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, Central England, Journal of the Geological Society, London 2001, 158 , 59–73.
Hillis, R.R., Holford, S.P., Green, P.F., et al., 2008. Cenozoic exhumation of the southern British Isles, Geology 2008, 36 , 371–374.
Jones, S.M., White, N., Clarke, B.J., Rowley, E. & Gallagher, E. 2002. Present and past influence of the Iceland plume on sedimentation. In: Doré, A.G., Cartwright, J., Stoker, M.S., Turner, J.P. & White, N. (eds) Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Hydrocarbon Exploration. Special Publications, Geological Society, London; 13–25.
Lane, N., Watts, A.B. & Farrant, A., 2008. An analysis of Cotswold topography: insights into the landscape response to denudational isostasy, Journal of the Geological Society, London 2008, 165 , 85–103.
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Watts, A.B. 2007. An overview. Watts, A.B. (ed.) Treatise of Geophysics. Volume 6: Crust and Lithosphere Dynamics. Elsevier, Amsterdam; 1–48.
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