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SELECTED FAULTS
The following descriptions are not exhaustive of all faults or any fault but are chosen to illustrate typical faults and typical kinds of evidence used in their identification. Only the most prominent faults have been named.
Liverton Fault
This fault illustrates the effects of vertical displacement of the later-Tertiary Kaukau Surface. In the map (Fig. 3) contours have been reconstructed by ignoring all the gullies cut in the old surface by rejuvenated streams. Such reconstructed contours dispense with the confusing detail of minor-valley dissection.
It is apparent on inspection of the map that the topography (in this greywacke setting) is anomalous.The sudden change in the general altitude of the topographic surface is, however, readily explained by invoking a fault along the line of change.This fault may be extrapolated in both north and south directions to explain many similar landscape anomalies and finds confirmation in trains of crest-line and spur jogs and notches and stream lineations. The throw of the fault is observed from the map as probably of the order of 400ft, but there is no evidence as to hade or horizontal shift.
Faults such as the Liverton Fault, trending sub-parallel to the Wellington Fault, are here termed subsidiary faults. Towards the Wellington Fault these faults rapidly increase in number, but the evidence for them is not unequivocal On the slopes rising to Belmont no less than five major steps are observable in the topography, but downhill and nearer to the fault where dissection has destroyed the old topography, evidence lies largely in aligned spur notches and stream courses. Shear zones and numerous minor faults with appropriate strikes are observable in valleys like the Ngauranga and in the deep cuttings of Haywards Road. which run across the fault zone.
Korokoro Fault
Quennell (1938) recognized and named the Korokoro Fault, and in order to explain an apparent offsetting of the Wellington Fault scarp assumed that trans-
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current movement had occurred along it. It has been shown above that the offset at Korokoro is probably erosional in origin. In the writer's opinion the Korokoro fault curves into alignment with rather than intersects the Wellington Fault and so produces no offset (i.e., it is a splinter fault). This fault causes a slight vertical displacement of the Kaukau Surface. In the stream floor the crush zone can be observed at several points, and the surface is identifiable at close hand by its deep weathering and undulating topography. Adkin (1951, Fig. 4), on the basis of summit heights to either side of Korokoro Stream proposed that this stream form the north-east border of his “Port Nicholson-Pukerua Sunkland”, and there is no question that the Korokoro Fault downthrows to the south-west. Judging from the topography, the maximum throw on the Korokoro Fault is of the order of 400ft to 600ft.
There is neither marked tilting of fault blocks nor sign of horizontal shift on this fault.
Akatarawa Fault.
The position of the Akatarawa River is controlled by the shatter zone of the Akatarawa Fault. The acute angular relation of the Akatarawa Valley to the Wellington fault line is shown by topographic map N.Z.M.S.1, N161 and the Akatarawa Fault may be regarded as a splinter of the Wellington Fault. Recent fault scarplets (G. I. Lensen, pers. comm.) at grid references N161, 638500, 645517 follow the trend of the now degraded fault, and additional support for fault control of the Akatarawa River is provided by dislocation of the Kaukau Surface along the line of the valley. When the broad profile of the Akatarawa Valley is viewed from the eastern side of the Kaitoke Basin it is obvious that the surface forming the north-eastern side of the valley has been tilted towards the river and the ancient surface to the south-west of the river has been tilted away. The river thus lies in the angle of a fault which downthrows to the north-east, and this throw is inferred from the topography to be of the order of 500ft. This inferred throw is a maximum value, the throw decreasing as the fault passes away from the Wellington Fault.
Other Faults
Faults resembling the Akatarawa in respect of fault-line valleys and dislocated surfaces are the Wakatikei, Speedy's Stream and Belmont faults.The Wakatikei River is aligned along the Wakatikei Fault and the north-east branch of Speedy's Stream along the Speedy's Stream Fault.
The Speedy's Stream Fault is a splinter fault running from the Wellington Fault to the Liverton Fault and conspicuous notched spurs and stream lineations are present along its length.
In the case of the Belmont Fault, the Belmont Stream probably follows the fault line for about the first half mile of its course and then leaves it, apparently to return once again about one mile from its source.Variations in the degree of shattering of the bedrock are clearly shown in the stream profiles (Stevens, 1957b).
Quennell (1938) recognized several of the faults associated with the Wellington Fault; the “Normandale fault” (part of the writer's Liverton Fault) and “Quail fault”, which are two of the subsidiary faults, as well as the “Belmont” and “Hayward” faults. Hall (1946: 428, 429, Fig. 1) also considered that the Haywards Valley was probably controlled by an ancient shatter zone.
In the broader picture Quennell envisaged several simple parallel faults continuing for the length of the western Hutt Valley; in contrast, the writer's view is that a wide fault zone occurs within which there is great complexity and an anastomosing pattern of minor faults.