Abstract

R. H. Maddock, W. P. Aspinall, E. A. Hailwood, Ting Fung & E. H. Rutter write: In a paper concerning the ages of fault movements in NW Wales, Bevins et al. 1996 cite palaeomagnetic dating by Hailwood et al. 1992 in support of their argument for large sinistral fault displacements of post-Tertiary dyke age. Bevins et al. misinterpret the observations and results published by Hailwood et al. We wish to correct this misinterpretation and to make further comments.

Bevins et al. state To date the only evidence of Cenozoic faulting in North Wales is provided by Hailwood et al. 1992 who, from an investigation of palaeomagnetism and fabric analysis of a clay-rich fault gouge in northern Anglesey demonstrated 180 m of sinistral strike-slip or 215 m dip-slip movement during the last 20 million year period (with the minimum age for last movement as 0.73 Ma)'. In fact, the observations and results reported by Hailwood et al. support the contrary view that the greater part of the movement on the fault in question, the E-W-trending Porth-y-pistyll fault, took place well before the intrusion of a Tertiary dyke which cuts and intrudes the fault. Hailwood et al. noted the following:

(1) The offset of the boundary between the (early Palaeozoic) Gwna and New Harbour Groups across the fault was consistent with end-member (total) displacements of 215 m normal dip-slip or 180 m sinistral strike-slip movement.

(2) A sub-vertical Palaeozoic dyke showed a 10 m dextral strike-slip offset across the fault. Over the

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