GLACIAL REBOUND OF THE BRITISH-ISLES .2. A HIGH-RESOLUTION, HIGH-PRECISION MODEL

Observations of ice movements across the British Isles and of sea-leve l changes around the shorelines during Late Devensian time (after abou t 25 000 yr BP) have been used to establish a high spatial and tempora l resolution model for the rebound of Great Britain and associated sea -level change. The sea-level observations include sites within the mar gins of the former ice sheet as well as observations outside the glaci ated regions such that it has been possible to separate unknown earth model parameters from some ice-sheet model parameters in the inversion of the glacio-hydro-isostatic equations. The mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere, the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km, the transition zone down to 670 km depth, and the lower mantle. No evidence is found to support a strong layering in viscosity above 670 km other than the high-viscosity lithospheric layer. Models with a low-viscosity zone i n the upper mantle or models with a marked higher viscosity in the tra nsition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary. I n contrast, a marked increase in viscosity is required across this lat ter boundary. The optimum effective parameters for the mantle beneath Great Britain are: a lithospheric thickness of about 65 km, a mantle v iscosity above 670 km of about (4-5) 10(20) Pa s, and a viscosity belo w 670 km greater than 4 x 10(21) Pa s.