Evidence For 280-Mile Movement Along Fault

Press, Volume CI, Issue 29994, 1 December 1962, Page 19

Evidence For 280-Mile Movement Along Fault

Support for the idea that Nelson and Westland have moved 280 miles north-east compared with Canterbury and Otago over a period relatively short on the geological time-scale has resulted from a new examination of the Nelson section of the Alpine fault by three members of the Geological Survey, Department of Scientific and Industrial Research. They are Mr A. R Mutch, of Dunedin, Mr A. C. Beck 'Christchurch), and Dr. J. B. Waterhouse (Wellington). Mr Beck has been investigating the Nelson section of the fault for some time, and Mr Mutch has been doing corresponding work on the section behind Big Bay in north-west Otago. Dr. Waterhouse is a palaeontologist specialising in fossils of the Permian era. the period of many of the rocks under examination. The three men plan to visit Big Bay later this season.

The Alpine fault crosses the north-west Otago coast at Yates Point, just north of Milford Sound. It runs up the western side of the Alps to the upper Grey river, then across to the headwaters of the Wairau and down the Wairau valley to Cloudy Bay. Increasingly Probable

“While we can never hope to provide incontestable proof of a 280-mile movement along the fault, all the evidence we are collecting makes it increasingly probable," said Mr Beck yesterday. "The new evidence gathered on this trip was mainly in the thickness of the various strata, which tallied well as between Otago and Nelson.” The succession of rocks on the south side of the Alpine fault in the Skipper’s, Red Hill and Olivine ranges on north-west Otago was almost identical with that on the north side of the fault in the Red Hill and Richmond range areas of south-east Nelson, he explained. Hie coincidence of the name “Red, Hill” was significant, he thought Both ranges carried reddish outcrops of dunite and peridotite.

The similarity of the rock series in the Nelson and Otago sections of the fault had first been noticed by Dr H W. Wellman, now of the Victoria University of Wellington. about 20 years ago. The theory of a 280-mile

horizontal movement had been developed through work by Dr. Wellman and Mr R. M. Willett, now director of the Geological Survey. The detailed mapping of difficult countr undertaken in connexion with the new fourmile map series had brought in more information bearing on the subject, and on the basis of this new information the present investigation had been planned. Still Moving There was evidence of continuing movement along the fault, Mr Beck said, in that some West Coast rivers which crossed it had suffered a displacement of as much as two miles. Terraces which could not be more than 10,000 years old were displaced across the fault line by 200 to 300 feet. This would give an average horizontal movement of something like two feet a century. There was also a vertical movement at about the same rate, with Otago and Canterbury going up and Westland and Nelson down. Dr. Wellman had stated that the time since the horizontal movement began might b<. no more than 30 to 40 million years, said Mr Beck. The maximum age for the movement was fixed by the minimum age of the strata on either side of the break, and this was estimated as between 150 and 200 million years. These figures would give a rate of movement of the same order as was thought to be occurring now.

These movements did not necessarily imply large and frequent earthquakes, although displacements of tens of feet might well occur in such disturbances. There had not in fact been any major earthquake along the Alpine fault in history times.