Art. XLI.—Supplementary Notes on Wellington Physiography.
[Read before the Wellington Philosophical Society, 22nd October, 1913.]
On the map‡ accompanying my former “Notes on Wellington Physiography” the Wellington fault is indicated as ending south-westward in the valley of the Tinakori Stream. This is the end of the fault-scarp, and therefore the end of the line upon which recent movement took place. There is, however, evidence of extensive earlier faulting in the form of a broad zone of crushed rock at the head of the Tinakori Stream and on the divide between it and the Kaiwarra, and farther on there is such an exact alignment of the Upper Kaiwarra and Silver Stream Valleys with the Tinakori Stream and with the Wellington fault-scarp that I am now convinced that all these features constitute a single lineament.
[Footnote] ‡ Trans. N.Z. Inst., vol. 44 (1912), fig. 2, p. 247.
The head of the broad subsequent strike valley—the Karori—Khandallah or “Long” Valley of the paper referred to above—appears to be at Karori. The branch that is now occupied by the Upper Kaiwarra and the Silver Stream, while also probably subsequent, or perhaps resequent, appears to have been guided by a fault-line. It is remarkably straight, and, where the streams in it have been revived, they develop by erosion a scarp on the north-west side, which continues the line of the Wellington fault-scarp, and closely resembles it. This is apparently an example of a fault-line scarp. It will be seen from the map that this valley makes a decided angle with the longitudinal features, which in the southern part of the Wellington Peninsula are nearly meridional.
It has been shown* that the inbreak along the Wellington fault took place very recently; but, if there is any significance in the fact that the Silver Stream rises close to the southern coast and flows inland, the Silver Stream—Kaiwarra Valley is more ancient. Further, there is no topographic evidence of downthrow to the south - east along the line of the valley; the absence of a scarp is particularly noticeable on the divide between the head of the Kaiwarra and the Silver Stream.
These facts, together with the presence of the zone of faulted and crushed rock beyond the end of the Wellington fault-scarp, seem to indicate that a much earlier fault movement than that which produced the existing scarp had taken place along the same line and its continuation, and that, during the long periods of denudation accelerated by successive uplifts to which the Wellington area has been subjected, topographic evidence of the movement has been obliterated. This may easily have taken place even though the formation of the original fault happened long after the folding of the strata.
The foregoing explanation of the features continuing the line of the Wellington fault appears much more satisfactory than an alternative view that fracturing took place contemporaneously with the inbreak along the shore of Port Nicholson, but along a much longer line, and that for a great part of the length of the line shattering of the rocks took place without resultant differential movement.†
It is easy, on the former hypothesis, to account for the coincidence of the Wellington fault with the older-faulted zone. As the Port Nicholson block sank, the fault forming its north-western boundary followed the old line of weakness.
The descriptions given in my former paper of the captures of the Silver Stream and Kaiwarraare not affected, but the valleys of these streams, while they may still be regarded as subsequent, must be described as faultline valleys‡
The Hutt Valley.
A brief note on the Hutt Valley is here offered, because the Hutt River continues the line of the Wellington fault to the north-east, just as the, fault-line valleys already described continue it to the south-west. I am-
[Footnote] * Loc. cit., p. 258.
[Footnote] † Compare the explanation of fault-line valleys in central Sweden given by Davis (W. M. Davis, Bull. Am. Geogr. Soc., vol. 45 (1913), p. 519; “Die erklarende Beschreibung der Landformen,” Leipzig, 1912, p. 170).
[Footnote] ‡ Davis, loc. cit.; and also “Nomenclature of Surface Forms on Faulted Structures,” Bull. Geol. Soc. Am., vol. 24 (1913), pp. 187–216.
able at present, however, merely to hint at a few points. The physiography of the Hutt Valley is a subject that deserves careful investigation and thorough treatment.
The lower portion of the valley is undoubtedly a part of the Port Nicholson depression, resulting from the inbreak along the Wellington fault, that has been filled by a delta built forward by the Hutt River. This is proved by the continuation of the fault-scarp into the lower valley without break.
There appear to be good reasons for believing that the valley of the Hutt River to its very source is determined by a line of faulting which continues the line of the Wellington fault; but it has yet to be shown how far the valley is a fault valley, and how far a fault-line valley.
The predominant, longitudinal, topographic features, which I. regard as guided by the strike of the strata, trend rather more north easterly just north of Wellington than they do in the south of the peninsula: but within the basin of the Hutt they are again nearly meridional, trending about N. 10° E. Everywhere the valley of the Hutt meets these longitudinal features at an acute angle (seefig. 1). The Hutt River, which flows S. 60° W., receives tributaries flowing on the one side S. 10° W. and on the other N. 10° E. In the field this is most conspicuous, though the map shows it well only in the case of large tributaries like the Mungaroa and Akatarawa.
Eight miles from the mouth the broad plain of the lower Hutt Valley, which is probably all delta, ends, and the valley, narrowing almost to a gorge, is clearly the work of erosion. Farther up, the valley opens out again, and for five miles has a flat floor, in places a mile in width. The valley here is strongly asymmetrical. The south-east side is formed by long, sprawling, gently tapering spurs, the lower ends of which seem to be buried in an aggraded plain. The spur-ends are not truncated, and thus there is no evidence that the valley has been widened by the river swinging against this side. The north-west side is, on the other hand, formed by
high facets, all in line, which strongly suggest a fault-scarp, but may be a fault-line scarp. It is, of course, true that a vigorous stream, in the process of excavation of a broad-floored mature valley, might itself cut back both its valley-sides to this form, but the absence of bluffs on the south-east side of this portion of the Hutt Valley negatives this explanation.
This portion of the valley may perhaps be a Graben, bounded, as the northern end of the Port Nicholson depression is perhaps also bounded, by a fault on one side and a flexure on the remaining sides. Fig. 2 shows two trough subsidences formed in this way along a single fault, and bearing to each other a relation similar to that between the lower and upper plains of the Hutt Valley. The sea may be supposed to have access to the nearer of the two troughs, and to fill it up to the broken line. The broken line in the farther trough represents the level of a lake occupying the trough, and overflowing at the lowest point (the south-west end) along the faultscarp. If a river were already in existence flowing south-westward along the fault-line, and if the subsidence took place rather slowly, the farther trough might be filled with waste as rapidly as it was formed, a [ unclear: ] graded gorge being at the same time cut along the fault-line between the two troughs. When later a delta had been built out into the nearer trough the system would bear a good deal of resemblance to the Hutt Valley.
In the actual case the pre-faulting surface was of strong relief, and unless the Hutt River already existed it is difficult, if not impossible, to account for the present large drainage-basin of the river, for the amount of the fault-movement seems insufficient to effect profound changes in drainage.
Thus we have several reasons for believing that before the Wellington fault movement took place the Hutt River existed as a fault-line river guided by the same old fault as the Upper Kaiwarra.
Another indication of the previous existence of the Hutt is found in the presence of a broad terrace along the north-west side of the river near the mouth, evidently a remnant of the flood-plain of the Tongue Point cycle, the front of which is formed by the fault-scarp.
The continuous scarp that bounds the Hutt Valley on the north-west side seems to be—at least, in part—a fault-scarp. If it is true, however, that the recent break occurred along the floor of an older valley, the fault-
scarp can be of no great height, and the higher facets, like those in the Upper Kaiwarra and Silver Stream Valleys, probably form part of a faultline scarp.
The Old Fault-Line.
It is possible that the old line of fracture, the extent of which has been indicated in these notes, represents, in the Wellington area, the Kaikoura orogenic movement,* which was accompanied in Marlborough by the formation of enormous reversed faults. This view gains some support from the fact that the numerous planes of movement of this old distributed fault at the head of the Tinakori Stream dip north-west, whereas the Wellington fault-scarp faces the south-east.
It is not, however, advisable to attempt a correlation with any particular fracture in the South Island.†
[Footnote] * C. A. Cotton, Geogr. Journal, vol. 42 (1913), p. 227.
[Footnote] † See, however, A. McKay, Geol. Surv. of N.Z., Reports 1890–91 (1892), p. 19.