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Journal of the Geological Society

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Discussion of an interpretation of the Variscan Structures of SW England

Mr J. W. F. EDWARDS writes: I wish to present geophysical evidence which supports the thesis of Shackleton et al. that the Variscan granites of the Cornubian Peninsula originated to the SSE of their present positions, and were injected to the NNW as a sheet, from which diapirs rose to form batholiths. Thrust nappes in the basement of the English Channel Basin and the South-western Approaches Basin (Day & Edwards, in press) provide the anisotropy necessary for non-vertical intrusion. The model presented in fig. 7 of Shackleton et al. proposes two parallel batholiths with the NNW batholith being asymmetric, but the evidence for two batholiths rather than one is not given. The gravity evidence suggests that there is only one batholith under the Cornubian Peninsula, but that offshore under the Cornubian Platform there are two batholiths (Haig Fras and Cornubian) which are separate (Edwards, in press; Smith et al. 1965). A model in which the two batholiths are joined by a deep, gently inclined wedge of granite could be made to be consistent with the gravity data by changing the regional field. Two models of the Haig Fras batholith have been prepared to find whether a batholith with an asymmetric cross-section is consistent with the gravity values. The line of the models runs through the centre of the area of outcrop of granite as mapped by Fletcher (unpublished), and this map has been used to fix the edge of the batholith on the seabed in the models. As shown in

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				R. A. CHADWICK, T. C. PHARAOH, and N. J. P. SMITH Lower crustal heterogeneity beneath Britain from deep seismic reflection data Journal of the Geological Society, August 1, 1989; 146(4): 617 - 630. [Abstract] [PDF]