The Alpine Fault of the South Island of New Zealand from the Air.
[Read before Wellington Branch, September 12, 1946; received by the Editor, September 23, 1946.]
We are indebted to Wellman and Willett1 for an indication of the great length, continuity, and remarkably straight course of the Alpine fault. This line of dislocation—the most remarkable lineament in the geology of New Zealand—appears to have had its inception in the post-Hokonui (early Cretaceous) orogeny, during which a thrust developed, making a clean-cut line of separation—as Morgan2 has observed—between the schist zone of the growing welt of the Southern Alps and its foreland. Morgan remarks: “The rocks of the western foothills [foreland] possess a radically different folding in a direction almost at right angles to that of the main range…. The discrepancy of strike in itself indicates a great fault.” He comments on “the absence of any visible contact” and on “marked depressions along the line where contact might be expected.”
When another orogeny affected early and middle Tertiary strata as well as the undermass, it is possible that posthumous faulting redeveloped a more or less continuous scarp on this line; and towards the south-western end of it Wellman and Willett record evidence of some continuance of posthumous activity to the present day. Along the base of the high range of the Southern Alps, however, it appears that scarp-making movements have been unimportant, and the lineament is revealed mainly by erosion.
A view from the air south-westward along the Alpine fault in Westland and western Nelson shows the collinear fault-line valleys—Morgan's “ancient” Gregory Valley—that have been excavated by erosion along the ancient fault zone; it reveals also a surprising accordance of summit levels in the Southern Alps and even considerable survival in nearly flat-topped Alpine ridges of remnants of the up-arched peneplain—Wainihinihi peneplain of Bell and Fraser3—which Wellman and Willett have attributed to late Tertiary erosion and have restored in sketched contours of the main range (Fig. 1).
[Footnote] 1 II. W. Wellman and R. W. Willett: “The Geology of the West Coast from Abut Head to Milford Sound—Part I.” Trans. Roy. Soc. N.Z., 71, pp. 282–306, Part 2: “Glaciation,” ibid., 72, pp. 199–219, 1942.
[Footnote] 2 P. G. Morgan: “The Geology of the Mikonui Subdivision, North Westland.” N.Z. Geol. Surv. Bull., 6, 1908.
[Footnote] 3 J. M. Bell and C. Fraser: “The Geology of the Hokitika Sheet, North Westland Quadrangle.” N.Z. Geol. Surv. Bull., 1, 1906.
Fig. 1.—Key to Plate 32.
As shown in Plate 32, a photograph taken by Mr. Allan Prichard from a position in the air north of the Grey River, the peneplain not only truncates any relief related to the original deformation and upthrusting of the Alpine welt that may have survived pre-Tertiary peneplanation, but also has eliminated any features that may have been developed by posthumous movement of the welt in early and middle Tertiary times. The late-Tertiary peneplain was evidently widely developed prior to end-Tertiary upheaval of the Southern Alps, and during this latest mountain-making episode—one characterized by much differential movement in some other districts—there has been no general renewal of movement on the Alpine fault, at least on any appreciable scale. This may be inferred from the apparent continuity of the peneplain across the fault zone and over outcrops of the Palaeozoic Greenland series in southern Westland, which the photograph shows. It is recognized, however, that there has been some differential movement of late date in the foreland belt,1 and it is perhaps significant that the undeformed peneplain has not been traced across the foreland farther to the north-east; though the discontinuity of the peneplain must be attributed in part to the susceptibility of the granites in the foreland to weathering and erosion.
The recognition of the fault-line erosional character of the Alpine fault lineament in this part of Westland is not inconsistent with some recent development of faulting movement that has produced scarps in the districts farther north-east and south-west, nor does it disprove slow and long-continued horizontal (strike-slip) movement, of which Wellman and Willett have found some indication in the offsetting of river courses. Lateral pressure from the west may be finding relief both in wedge-like upthrusting of blocks, such as Henderson2 has postulated, perhaps accompanied by some underthrusting at the Alpine fault, and in transcurrent movement that is carrying the foreland strip north-eastward.
Strike-slip displacement on the Alpine fault may have been accompanied by some vertical movement of amount varying from point to point. If it be assumed, as seems probable, that it is the strip
[Footnote] 1 M. Gage: “The Tertiary and Quaternary Geology of Ross, Westland.” Trans. Roy. Soc. N.Z., 75, pp. 138–159, 1945.
[Footnote] 2 J. Henderson: “The West Nelson Earthquakes of 1929.” N.Z. Jour. Sci. and Tech, 19; pp. 65–144, 1937.
Fig. 2.—The Alpine fault and foreland of the Southern Alps from Cook River to Big Bay. After Wellman and Willett. Inset: Wellman and Willett's restoration of the early postglacial shoreline. Moraines in black.
on the seaward side of the fault in South Westland that has been recently active (Fig. 2), and that the coastal strip of Alpine foreland has moved north-eastward, then this strip might be expected to buckle in places, producing local upheaval and depression (Fig. 3). Such buckling might take up the horizontal shift in a short distance, thus localizing it. It is stated by Eaton1 that such local vertical movement is recorded on parts of the San Andreas strike-slip fault, in California, the amplitude varying from zero to as much as five miles.
[Footnote] 1 J. E. Eaton: Bull. Am. Assoc. Petrol. Geol., 16, 1932, p. 25.
Parallel with the Alpine fault, along a rocky strip of coast northeast of the Haast River, there is a very fresh marine-cut rock platform which indicates perhaps the most recent upheaval of any considerable stretch of shoreline on the West Coast. Wellman and Willett have recorded its altitude as 50 feet between the Paringa and Moeraki Rivers; but some distance farther south-west they could not identify it. This can be explained by its dipping below sea-level. As I have observed from an aircraft flying low along the shoreline, the bench is about 100 feet (perhaps more) above the sea at the north-eastern end of this rocky stretch of coast, but it descends, apparently uniformly, to sea-level south-westward in about 15 miles. The tilt here observed may be part of a broad buckle, however, that affects a laterally-moving strip seaward of the Alpine fault.
The low stretch of coast at the mouth of the Haast River and that at Cook River could be accounted for by down-buckling. Wellman and Willett describe the former as for the most part a “low-lying area of alluvial flats” four miles wide “between the sea and the Alpine fault.” The area seems to have been drowned and then prograded, and this fits in with the hypothesis of buckling. It also brings the South Westland coastal features into the category of shorelines of transverse deformation, whatever may be the cause of the undulatory deformation of the coastal terrain.
Elevated beach deposits, which have been worked for gold, are present on the coastal salient of bedrock north-east of the Haast River and again at an altitude of 400 feet, according to Wellman and Willett, at Cement Hill, where they are in close association with the next mapped buttress of Palaeozoic rocks in the foreland strip northeast of the filled embayment at Cook River. These beach deposits are described as covered in part by glacial debris, and are evidently considerably older than the marine terrace at Paringa River; but it is highly probable that the mechanism of their upheaval has been similar, and also the uplift of marine varves, which Wellman and Willett have observed at several places.
It may be that the Greenland series, being already folded with north-westerly strike, has yielded fairly readily to renewed crumpling on the old lines; but the salients and embayments of the foreland are not necessarily simple anticlines and synclines. Quite probably a transversely warped coastal strip is broken into blocks by transverse faults. Quite probably, also, the tilt that is indicated by the uplifted marine platform at Paringa River is only the latest of a long scries of differential movements.