Abstract
MR J W Pallister asked Dr Hepworth two questions. In his account of the structure and geotectonics of the Precambrian of Tanzania, which has unfortunately not yet been published, Harpum describes the predominant structure of the craton in the Dodoman Belt as linear elements which show a tendency ‘to plunge in opposite directions although along the same strike; no distinctive folding, however, could be identified with certainty'. Harpum emphasizes that structures in the craton are relatively small and often in varying directions over short distances, by comparison with larger, more consistent structural features typical of the, so called, Usagaran. Would Dr Hepworth substantiate this distinction?
Secondly, the concept of lineation produced by widespread stretching is a new one in relation to Tanzania. Is such pervasive stretching brought about at a low tectonic level and are the more normal fold structures parallel to the orogenic front characteristic of a higher tectonic position? Where both types of structure are imposed on the same rocks, is this due possibly to substantial vertical movements during the main tectonic events?
In reply the Author said that he agreed with the distinction made by Harpum between structure in the craton and orogen. This was an expression partly of different lithology and the way in which deformation affected massive, granitoid rocks and layered gneissose rocks respectively, but was also attributable to different degrees of tectonic reworking which rose from a minimum in the stable foreland, through the front where it was probably at a maximum, into the orogenic belt.
