An Isoclinal Fold on Häckel Peak, Southern Alps, New Zealand (S 79)

Transactions and Proceedings of the Royal Society of New Zealand, Volume 83, 1955-56, Page 345

An Isoclinal Fold on Häckel Peak, Southern Alps, New Zealand (S 79) By J. B. Waterhouse New Zealand Geological Survey [Received by Editor, January 27, 1955.] Abstract A syncline exposed on Hackel Peak is described and discussed. Although the strata of the axial ranges in New Zealand are commonly considered to be tightly and even isoclinally folded (e.g., Cotton, 1945, p. 53), major folds are rarely visible in the greywacke mapped as Permian-Triassic-Jurassic in the Geological Map of New Zealand (Wellman and Brodie, 1954, p. 445). Haast (1879, p. 243; Pl. 4, opp. p. 240) referred to “a succession of huge folds, dipping throughout at high angles” in the Southern Alps, but his sections are obviously diagrammatic. Bell and Frazer (1906, p. 42) assumed that the alpine rock is isoclinally folded to explain the diversity of attitude, but they mentioned no localities where folds can be seen. Brodie (1953) has recorded a large overturned syncline, considered to be inverted, on the south coast of the Wellington Peninsula, and Wellman et al. (1952, pp. 223—4) have mapped several folds with axes up to half a mile apart in the Trent River district. These folds in the Wellington and Trent River districts have been inferred by determining the stratigraphic top and bottom of the beds forming the limbs: the actual folds cannot be seen. Park alone has reported large visible folds, with approximately a mile to half a mile between the axes, in the Southern Alps near the Hooker and Tasman glaciers (1910, pp. 65–6) The accompanying photograph (Plate 11, fig. 1) shows an isoclinal fold exposed on the western face of Hackel Peak (see N. Z. Dept.Lands and Survey map of Mt. Cook Alpine Regions, 1953) 6 to 10 miles to the south-west of those described by Park. The rocks are red and green beds, greywackes, and argillites, classed as “middle sub-schist zone” of Wellman et al. (loc. cit.). The strike of the axis of the syncline illustrated in Fig. 1 was not taken in the field, but judged from the photograph, the trend is between N. N.E. and N.N.W. The folds described by Park and by Wellman et. al. trend N. E. and Morgan (1910, p. 277) gives the average strike in the Southern Alps as 22°. In the greywackes to the north, on the other hand, strikes are variable, “but the average trend appears to be west of north” (Cotton, 1916, p. 245). Morgan (1910, p. 277). Bell and Frazer (1906, p. 42), Speight (1915, p. 139) and others record occasional N.W. strikes in the Southern Alps. A number of photographs lent to the writer by Mr. P. Barcham show that much of the rock of the Malte Brun Range strikes N. W. North-east of the Malte Brun Range, north-west strikes are predominant in highly contorted strata well exposed west of the Grey Glacier between Mt. Moffat and Mt. Loughnan (see,

Haast, loc. cit., p. 24). The strike is shown by an oblique aerial photograph [News Bulletin, Canterbury Mountaineering Club, 1954, 1 (8): 8-9]. and other photographs taken by Mr. J. Harrison during a mountainecring trip with the writer. The tight folds west of the Grey Glacier can be seen in a photograph by Harrison published in the Canterbury Mountaineer, 1953-4, 6 (23), opposite p. 153. The folding in the Southern Alps, and in other axial ranges is complex, but the clear and extensive exposures in high alpine country occur on a scale not visualized by lowland geologists. This will ultimately lead to detailed and exact mapping of structure in the undifferentiated Permian-Triassic-Jurassic of New Zealand. Acknowledgments The writer is grateful to Messrs. P. Barcham and J. Harrison for lending negatives and photographs. Mr. M. Bowie, Chief Guide at “The Hermitage”, kindly confirmed the identification of peaks in the illustration. References Beil, J. M. and Frazer, C, 1906. The Geology of the Hokitika Sheet, North Westland Quadrangle. N.Z. Geol. Surv. Bull., 1 (n.s.). Benson, W. N., 1921. Recent Advances in New Zealand Geology. (Pres. Address) Rep. A'asian Ass. Sci., 15: 1-84. Brodie, J. W. 1953. Stratigraphy and Structure of the Greywackes and Argillites on the South Coast of Wellington Peninsula, N. Z. J. Sci. Tech., B, 34 (4): 205-27. Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand. Geol. Mag., dec. 6, 3: 243-9. —— 1945. Earth Beneath. Whitcombe & Tombs, Christchurch. Haast. J., 1879. Geology of the Provinces of Canterbury and Westland. Christchurch. Morgan, P. G., 1910. On the Structure of the Southern Alps. Trans. N. Z. Inst., 43:. 275-8. Park. J., 1910. “The Geology of New Zealand” Whitcombe & Tombs, Christchurch. Speight, R., 1916. The Orientation of River Valleys in Canterbury. Trans. N.Z. Inst., 48: 137-44. Wellman, H. W., Grindley, G. W. and Munden, F. W., 1952. The Alpine Schists and the Upper Triassic of Harper's Pass (Sheet S 52), South Island, New Zealand. Trans. Roy. Soc. N.Z., 80: 213-27. J. B. Waterhouse N.Z. Geological Survey The Terrace Wellington, C.1

Fig. 1.—A exposed in the Malte Brun Range on the western face of Hackel Peak. The visible face extends vertically for 3.500ft. Part of the Darwin Glaerer lies in the foreground. An to the right (re west) or the illustration can be seen clearly visible from the summit and west ridge of Malte Brun Photographed in March 1954 from 9 000ft on the leading westerly ridge of Mt. Darwn looking N.w.