Abstract

15–18 September 1975 Engineering Group Meeting held in the Department of Environmental Sciences, University of East Anglia, Norwich.

Foundations on Quaternary Deposits

17 September 1975 Geological Information Group Meeting held at Nottingham University

Current research in mathematical geology

1 October 1975 Ordinary General Meeting

A palaeomagnetlc study of part of the. Lewisian Complex, N.W. Scotland

G. E. J Beckmann

(Journal 132:45–59)

(Discussion)

G. E. Morgan writes: In his recent paper Dr Beckmann suggested that the magnetization of nine Scourie dykes and a granulite from the Central Zone of the Lewisian was probably produced by shocks in the late Laxfordian, these shocks either (a) inducing dislocations into multidomain grains and hence increasing coercivity by pinning the domain walls, or (b) producing local high temperatures which resulted in the acquisition of a thermo-remanent magnetization. Dr Beekmann also suggested that the temperature of these rocks at the time might have been 400–500°C, and in the discussion following his reading of the paper Dr. Beckmann indicated that he regarded the shocks as being seismic in origin.

This area of crust almost certainly experienced very slow cooling after the Laxfordian metamorphism, probably on a time-scale of at least tens of millions of years, and so temperatures of the order of 400–500°C would have persisted for a considerable time after the postulated magnetization events. This and other considerations appear to pose certain problems regarding the proposed mechanisms of magnetization. In the case of mechanism (a) it would seem that temperatures of this order maintained for

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