Introduction The scarcity of relic permafrost features in southwest England, except on the high ground of Dartmoor, was noted by Williams (1965, 1969). Following Manley (1951, 1953) he concluded that this was due primarily to a moderation of the Pleistocene climate by invasions of milder Atlantic air. Subsequent work through the CLIMAP project ( McIntyre et al 1976 ), however, suggests that at 18000 years BP, the North Atlantic polar front was at about the latitude of Portugal. Comparison of Williams' map of the areal extent of permafrost in southern Britain with the detailed heat flow map of the UK ( Wheildon & Rollin 1986 ) indicates that regions where evidence of former permafrost is largely absent tend to coincide with areas of high heat flow. The technical note discusses possible relationships between heat flow and permafrost phenomena in southern England as it is clearly important that engineers making site assessments are aware of the likely causes and extent of the disturbance associated with former frozen ground. Extent and depth of former permafrost in southern Britain Figure la is a reproduction of Williams' (1969) map which was based on the deeper periglacial features, particularly ice-wedge casts. It shows that there are far fewer permafrost features in the southwest than in most of Britain. Since then more ice-wedge casts have been found in southwest England, but it is nevertheless generally accepted that in this area the frequency and depth of these is significantly less than
Two notable features of the mineralization in SW England are the asymmetrical pattern of mineral lodes and the mineral zoning within this distribution. Despite the fact that many theories have been advanced to explain the mechanisms of mineralization qualitatively no numerical models have been produced. In this paper, the finite element technique is used to construct a very simple model which helps to explain general features of the pattern of mineralization. The model describes the steady state convection of fluid in the environment of the Cornubian batholith. Convection is driven by the radiothermal contrast between the granite and the host rocks. Results show that the locations of upward flowing, hot fluids are determined by the three-dimensional shape of the batholith. Initially, upward flow was concentrated along the axis of the batholith and these regions correlate with high temperature mineralization. Erosion of the cover rock above the granites altered the fluid flow pattern such that the upward flow became concentrated outside the outcropping granites. These regions correlate with low temperature mineralization. A correlation also exists between kaolinization and areas of downward flowing fluid for present day levels of erosion.
The effect of 3-D geology on conductive heat flow was investigated using the finite element technique on two models from SW England, one a regional model centred on the Comubian batholith as a whole and the other a more detailed model centred on the Cammenellis pluton. Only two rock types, granite and country rock, were used to represent the geology of the region and yet good agreement between modelled and observed data was obtained provided that the granite was assumed to be highly homogeneous, with surface heat production values persisting throughout its depth, and the base of the batholith was assumed to be flat at a depth of 15 km.The results show that the surface heat flow pattern is dominated by the 3-D shape of the batholith which controls the relative importance of the two opposing effects that operate at its margins: heat refraction due to conductivity contrasts enhancing heat flow in the granite, and lateral flow of heat caused by heat production contrasts diminishing it. The highest heat flows occur at the boundaries of outcrops facing the axis of the batholith, where there is significant heat refraction but little lateral loss. A high degree of correlation between mineralized zones and areas of very high heat flow, in both granite and country rock, is also noticeable. The temperature field at any depth displays wide-ranging lateral variations over relatively short distances, the highest values occurring in areas of granite which are covered by insulating country rock and which lie towards the centre of the batholith. Regions of highest heat flow are not necessarily the regions where the highest sub-surface temperatures are likely to be found and it is shown that the hot dry rock geothermal test project at Rosemanowes is not optimally sited from temperature considerations. When the constraints on the physical parameters of the model are sufficiently reliable, as they are in SW England, the heat flow and temperature predictions from 3-D numerical models can be of enormous value in the assessment of the geothermal potential of a region.
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