Abstract

Mr. A. M. Quennell suggested an explanation for the absence of recorded microseismic activity along the straight section of the Alpine fault south of Maruia. Many years ago the late E. O. Macpherson observed that known faulting in the Otago Schist belt was aseismic and he attributed this to the schistose nature of these rocks. This belt extends northward and northeastward along the Alpine fault and could provide the condition for aseismic movement on it.

Regarding the nature of the Alpine fault considered as a transform, in the southwest it was initiated within what was a continental plate, since fragmented, remnants of which are now seen in western Southland and northwest Nelson. Northeastwards, the fault traverses and offsets the eastern margin of one remnant of this plate and then continues within the New Zealand Geosyncline which was no doubt marginal to the former continent. This Mesozoic belt is neither continental nor oceanic in nature, in plate tectonic terms, although it must have had an oceanic floor now probably cratonized at depth. Significantly the sigmoidal bend in the Alpine fault is located at the transition where it finally departs from the continental margin. It is suggested that a transform fault of this nature, passing from one kind of plate to another, is a special case and should be distinguished from those separating plates of the same crustal nature along their whole length.

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