Abstract

Dr K. P. Isaac writes: Papers by Barnes (1984) and Rattey & Sanderson (1984) discussed the igneous rocks and Upper Devonian meta-sediments of SW Cornwall in terms of the Gramscatho Basin originally being floored, at least in part, by oceanic crust (Barnes 1984, pp. 83–4; Rattey & Sanderson 1984, p. 94). They were following a long line of authors who have argued that the Rhenohercynian geosyncline of SW England (or W Europe) is best explained as a back-arc marginal basin (e.g. Reading 1973; Leeder 1982; Anderton et al. 1979; Dewey 1982; Floyd 1982). Whilst Floyd (1982 Whilst Floyd (1984) would prefer that the Rhenohercynian trough represents intra-plate continental rifts, others such as Leeder (1982), Barnes (1984), and Rattey & Sanderson (1984) infer the presence of bona fide oceanic crust underlying the basin floor. There is now little doubt that the Lizard Complex is an ophiolite (papers in Journal141, part l), although the relationships between this ophiolite and possible (now absent) ocean crust underlying the Gramscatho Basin is still not clear (Barnes 1984).

The purpose of this discussion is to draw attention to geological evidence for the persistence of an oceanic marginal basin in SW England, at least into lower Viséan times. Isaac et al. 1982, 1983 recognized five demonstrably allochthonous tectonic units in the central SW England area, the lowest of which is the Greystone Nappe. This nappe is composed of upwards of 20 small thrust sheets, each being generally less than 100m thick and having an area of the

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