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Volume 43, 1910
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Art. XXXII.—A Note on the Structure of the Southern Alps.

By P. G. Morgan, M.A.

[Read before the Wellington Philosophical Society, 5th October, 1910.]

IN Hochstetter's “New Zealand” (English translation), on page 487, two geological sections from east to west across the South Island will be found. One of these, after Hector, is drawn through Otago; the other, drawn through Canterbury and Westland, was supplied by Von Haast, and illustrates with a considerable degree of correctness the structure of the Southern Alps. In discussing the latter feature, Hochstetter says, “A simple glance at the above sections shows, farther more, that only the eastern half of a complete mountain-chain has been preserved, while the western half is buried in the depth of the main” (1,* p. 489). As a matter of fact, however, the sections do not prove the truth of Hochstetter's statement, which rests rather upon a hypothetical basis. One object of this note is to show that, on the contrary, the Southern Alps are not, as Hochstetter supposed, the remnant of a vastly larger range that once extended far to the westward of their present limits, but retain the same, or almost the same, dimensions as at any past period of their history.

Hochstetter's opinion, however, has been adopted by most New Zealand geologists. In 1879 Von Haast writes, “This remarkable chain, of which the geological structure is generally uniform throughout, is only the eastern wing of a huge anticlinal arrangement, of which the western portion has either been destroyed or submerged below the Pacific Ocean. It has thus the same one-sided arrangement, so conspicuous in almost every alpine chain of which the geological structure is known. The axis of this anticline consists of granite and other plutonic rocks” (2, p. 242).

At a later date Hutton, repeating part of Von Haast's statement almost word for word, says that “the mountain-range is only the eastern half of a huge geanticlinal arrangement of contorted rocks, the western half having

[Footnote] * This and other numbers similarly enclosed in brackets refer to list of literature at end.

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been washed away by the heavy rains which fall upon that side, and which must have fallen for a very long time to have produced so great an effect" (5, p. 161).

In recent years Marshall (7, p. 98; but see also 11, p. 445), Park (12, p. 16), and others appear to have accepted Hochstetter's explanation of the structure of the Southern Alps without question. In 1908, however, the writer pointed out that a series of ancient rocks designated by him the Greenland series* occurs along the western margin of the Southern Alps. These rocks, if Hochstetter's hypothesis were correct, should show the same folding as the rocks of the Arahura series, which form almost the whole of the Southern Alps; but, instead of that being the case, it is found that they are folded almost at right angles to the trend of the Alps (10, pp. 31, 36, 97). This fact is completely opposed to the idea that the present alpine chain is “only the eastern half of a huge geanticlinal” of which the western wing has disappeared.

The great difference in lithological character between the schists east of the granitic mountains that have been supposed to represent the core of the alpine anticlinal and the much less metamorphic argillites and grey-wackes of the Greenland series in a corresponding position on the western side of the granitic mountains is in itself strong evidence against what may be called the Hochstetterian view.

Von Haast was undoubtedly to some extent aware of the structure of the Greenland rocks, and it was probably in order to avoid the difficulty of reconciling this with the structure of the Arahura series, as well as to obviate the difficulty caused by the difference in lithological character, that he supposed his Westland formation to be (in part at least) younger than the alpine folding (2, p. 244). There appears, however, to be no field evidence of any kind in favour of this view.

What, then, is the general structure of the Southern Alps? As has been elsewhere pointed out by the writer (10, p. 43), an exposition of the principles underlying the answer to this question may be found in Eduard Suess's great work, “The Face of the Earth,” where it is maintained that folded mountain-chains of the alpine type are due to overthrusts along lines where more yielding strata are pushed against buttresses of immovable rocks. In the case of the Southern Alps, Suess's main criteria are satisfied. There is a gentle rise from one side—the east—and a steep descent on the other. The strata on the western side are overturned schists. Along the western margin are great faults, believed to be of the overthrust type (10, pp. 43, 71). These faults are associated with a line of granitic mountains,† which correspond to Suess's cicatrices that mark a wound in the earth's crust. To the west of these, beyond the main overthrust, comes a buttress of Greenland sedimentaries, folded in most places almost at right angles to the alpine strike.

Some imperfections in the field evidence that occur to the writer may here be mentioned. North of the Waitaha River granite is well developed along the base of the Alps, but for a hundred miles or more southward no granite except a small outcrop in Mount Bonar (10, p. 132) has been observed. Again, the Greenland buttress is apparently by no means continuous along the western base of the Alps. It is, however, well developed from Bell

[Footnote] * Equivalent to part of Von Haast's Westland series (2, p. 256), and to Bell and Fraser's Kanieri series (8, p. 19).

[Footnote] †In places gneiss partly covers the granite.

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Hill northward,* is seen at Lake Kanieri, and forms a considerable area east and south-east of Ross. Greenland rocks appear near Lake Mapourika, and probably have some development farther south. The breaks in the buttress are attributed by the writer to down-faulting, which also is thought since Miocene times to have caused the disappearance of an ancient landmass to the seaward of the present coast-line (3, pp. 26–28).

Suess distinguishes two types of overthrust mountain-ranges—the Atlantic, with its outer or overthrust face directed away from the nearest ocean; and the Pacific type, with its outer face directed towards the nearest ocean. As judged by these definitions, the Southern Alps, though of the Pacific type with respect to the Tasman Sea, are of the Atlantic type with respect to the South Pacific Ocean. This has already been indicated by Marshall (11, p. 445).

If, as has been assumed throughout this note, the folding of the Greenland rocks from north-west to south-east is older than the folding of the Arahura or alpine rocks from north-east to south-west, then in the alpine region the former folding has been superimposed on the latter. Confirmation of this view is afforded by field evidence. According to observations made by Mr. E. Dobson many years ago, the average strike of the strata forming the Southern Alps is N. 22° E., whilst the trend of the range itself is N. 55° E. (1, p. 485). More recently the New Zealand Geological Survey has noted considerable irregularities in the strikes of the alpine rocks in North Westland (8, p. 42; 10, p. 78). These irregularities increase from the intensely folded western schists in which the supposed older folding has been almost obliterated towards the main divide, and are very marked on the eastern, or Canterbury, side. Practically all the irregular strikes, except one or two that are nearly east-and-west, fall in the N.E.–N.W. quadrant, and are therefore such as might be expected to result from a folding-force acting from the south-east on strata with an original north-west to south-east strike.

Similar irregularities in strike are very apparent in north-west Nelson in rocks of the Aorere series (9, p. 34). It is here suggested that a similar cause to that indicated above may be assigned for the strike-irregularities in this area and in other parts of New Zealand—for example, the Whangaroa Subdivision (Bull. No. 8, N.Z. Geol. Survey, p. 42).

This note refers mainly to that part of the Southern Alps with which the writer is acquainted—namely, from the Otira district for about a hundred miles southward. Thus the Southern Alps as they pass north-eastward through Nelson to the shores of Cook Strait, the interesting central knot of Mount Cook, and the comparatively unknown southern portion of the range are not discussed. Before an authoritative opinion concerning the structure of the whole range can be given, a detailed geological survey of practically its entire length, and more especially of the district south of Mount Aspiring, where the Alps begin to lose their identity in the mountain complex of western Otago, must be undertaken.

In conclusion, it may be said that a full and correct statement of the structure and history of the Southern Alps will be of great importance in aiding the solution of several problems in New Zealand geology, and possibly of still greater value in connection with the elucidation of the wonderful but baffling geophysical problems presented by the Pacific Ocean.

[Footnote] * This district, however, is outside the scope of the present note. See last paragraph but one.

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Literature.
1.

Hochstetter, F. von. “New Zealand” (English translation of “Neuseeland,” 1863). Stuttgart, 1867.
2.

Von Haast, Julius. “The Geology of Canterbury and Westland.” Christchurch, 1879.
3.

McKay, Alexander. “On the Geology of Marlborough and South-east Nelson.” Part II. Reports N.Z. Geol. Surv. during 1890–91, vol. 21, 1892, pp. 1–28.
4.

McKay, Alexander. “Geological Explorations of the Northern Part of Westland.” Mines Report, 1893, C.–3, pp. 132–86.
5.

Hutton, F. W. “The Geological History of New Zealand.” Trans. N.Z. Inst., vol. 32, pp. 159–83. 1900.
6.

Suess, Eduard. “The Face of the Earth” (English translation of “Das Antlitz der Erde”), vols. 1–3. Oxford, 1904–8.
7.

Marshall, P. “The Geography of New Zealand.” 1905.
8.

Bell, J. M., and Fraser, C. “The Geology of the Hokitika Sheet, North Westland.” Bull. No. 1 (New Series), N.Z. Geol. Surv. 1906.
9.

Bell, J. M.; Webb, E. J. H.; and Clarke, E. de C. “The Geology of the Parapara Subdivision, Karamea, Nelson.” Bull. No. 3 (New Series), N.Z. Geol. Surv. 1907.
10.

Morgan, P. G. “The Geology of the Mikonui Subdivision, North Westland.” Bull. No. 6 (New Series), N.Z. Geol. Surv. 1908.
11.

Marshall, P. “Ocean Contours and Earth Movements in the South-west Pacific.” Rep. Aust. Ass. Adv. Sci., vol. 12, pp. 432–50. Brisbane, 1909.
12.

Park, James. “The Geology of New Zealand.” 1910.

The literature listed at the end of 11 may also be consulted with advantage.

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