Features of the Wellington Fault Zone
Recent movement as shown by scarplets
Between Wellington City and Upper Hutt the Wellington Fault lies beneath the waters of Port Nicholson and the recent alluvium of the Hutt Valley, and in this area no recent fault traces have been found To the north-east of Upper Hutt a recent fault trace, the Kaitoke Fault (Hall, 1946: 428) has dislocated terraces of the Hutt River.This terrace dislocation has been mapped by H. E. Fyfe and M. T. Te Punga (pers. comm.) at N161. 645456—to the north of Emerald Hill and at N161 626445—to the south of Emerald Hill (Fig. 2). These two workers have found that the amounts of vertical and lateral displacement increase with the age of the displaced terraces and indicate that two lateral movements of 33 feet and 18 feet have occurred with a 1ft to 2ft vertical movement accompanying the former.
Considering the Wellington Fault as a zone, movement may be regarded as occurring on different fault planes at different times so that at any one time a single fault within the fault zone may become active.The Kaitoke Fault is a case in point, for this branch has recently been more active than other faults of the system, and its scarp presents an important feature in the topography.
The north-eastern continuation of the Wellington Fault zone has been traced by McKenzie (1929) and G. J. Lensen and G. W. Grindley (pers. comm), the latter having found fault traces passing through the low divide between the Pakuratahi and Tauwharenikau Valleys.
Breccias of the fault zone
At several places along the Wellington Fault Zone brecciation of the greywacke country rock is extreme.In such places it is exceedingly difficult to find an unsheared rock fragment even six inches across within 400 yards of the centre of the zone.
At Haywards Road (N160, 520390), half a mile from the Wellington Fault scarp, the alternating sandstone and argillite beds of the greywacke succession, though still retaining their individuality, have been intensely shattered (Pl. 45, Fig. 5). The angle of shattering varies in each bed, and it may be inferred that a large though indeterminate amount of bedding-plane slip has occurred.
On the other hand, the greywacke beds exposed at Raroa Road (N164, 318217) astride the Wellington Fault, have been so completely broken that individual strata are unrecognizable. Plate 45, Figs. 1–4, constitutes a series of photographs taken along Raroa Road to typify the degrees of crushing which occur for 400 yards either side of the Wellington Fault. Fig. 1 illustrates a stage of shattering next after that in the Haywards area. The greywacke is intensely shattered and stratification has been completely destroyed. In Fig. 2 the shattered greywacke is overlain by
Fig.1—Fault-line valley and notched spur developed to the south-west of Hill Road.Belmont(foreground)
Fig. 2.—A fault-line valley developed at grid ref.N160. 458327 (west from Pomare Road) along a probable tensional fault.A spur jog maiking a subsidiary fault is also shown.
Fig. 3.—Fault-line valley north-east from Hill Road, Belmont (grid ref.N160, 460369). Note the dislocation of the Kaukau Surface, downfaulted to the left (north-west)
Figs. 1–4—Raroa Road Wellington Shattering of gicvwackc close to the Wellington Fault.Note the soliflual debus overlying the shattred greywacke in Fig. 2 The soliflual debris/ greywacke contact is indicated by the arrows Figs. 1.2 width of photo about 4ft Fig. 3 width of photo about 10ft Fig. 4 width of photo about 15ft
Fig. 5—Shattered greywacke of the Wellington Fault zone exposed at Haywards Lower Hutt A gorse bush (upper centre) and a cycle (lower centre) indicate scale.
All the photograph on the plate are taken looking south—ie. with west to the right.
soliflual debris, with a “shaved surface” developed at the contact. There is little difference between the angular soliflual debris and the fault-shattered rock.
The next stage in the shattering of the greywacke is the development in the shattered rock of anastomosing zones of even more intense shattering (Fig. 3). In these zones, usually between 3 and 6 inches wide, extreme comminution of the greywacke occurs and fault pug appears—the greywacke having been pulverized to rock flour.
Often the small shatter zones shown in Fig. 3 coalesce into a large shatter zone, some 8ft wide, as shown in Fig. 4. The broken pieces of material towards the base of the shatter zone in Fig. 4 are not of solid rock, but are shattered pieces of lithified fault breccia and fault pug. The attenuation of the pieces should be noted.
Extent of the Wellington Fault
Cotton (1914; 296) was of the opinion that the Hutt River is fault-controlled to its source probably by a continuation of the Wellington Fault. This view is shared by the writer, for the Eastern Hutt River presents a marked lineation (N.S.M.S. 1. N161), and it seems probable that the river flows along a crush zone directly aligned with the Wellington Fault south of Upper Hutt.
This does not mean that the Eastern Hutt River Fault is the only continuation of the Wellington Fault, for it is eminently possible for there to be several branches of continuations each at different times. As the fault south of Upper Hutt lies in a broad crush zone it is necessarily difficult to say where individual branches part from the main fault, and because of the complexity of the faulting it must be difficult to single out any branch as the most important continuation.
North of Upper Hutt the Fault is poorly defined, but a number of faults can be traced to the north-east by recent fault scarplets. Since the southern part of the Wellington Fault is known to have been active recently these scarplets are taken as evidence of the main continuation of the Wellington Fault.
Using this criterion of recency Lensen (Lensen et. al., 1956, p. 131) has traced a continuation of the Wellington Fault into the Tauwharenikau Valley; from there has mapped fault line features as far as Lake Waikaremoana, 180 miles to the north-east, and the whole length of this he names the Eastern Tuara Fault.
Cotton (1951) described a fault cicatrice associated with shutter ridges on the south-west portion of the line of the Wellington Fault and estimated a recent dextral transcurrent movement amounting to approximately 200ft.Following up this work, C. M. Laing and G. J. Lensen (N.Z. Geological Survey—pers. comm.) have determined the horizontal displacement of the two shutter ridges present at N164, 268154 as amounting to 210ft and 270ft, and concluded that the Wellington Fault is transcurrent with a relatively recent dextral movement of approximately 250ft and is perhaps reverse.
Warping on axes normal to the fault has produced along the eastern side of the Wellington Fault zone a series of basins (see Stevens, 1956) so that vertical displacements must vary along the length of the fault (see Cotton, 1914: 297, Fig. 2; cf. Cotton 1947: 371, Fig. 3). The observed vertical shift across the fault zone is of the order of 2,000ft, but this is distributed over many faults and the maximum throw in the centre of the zone and across the major fault scarp is only of the order of 500–700ft. Along the eastern side of the fault bore holes in Recent sediments in the Lower Hutt—Port Nicholson Basin indicate the depth of the down-faulted greywacke on that side of the fault scarp.
At Taita Gorge, the northern boundary of the basin, greywacke lies at only 18ft below the surface, and the throw may be as low as 100–150ft. Farther south in the basin, the Wellington City Council test bore at Wilford (N164/3, 444300)
encountered weathered basement at a depth of 396ft (Stevens, 1956) and as basement greywackes are exposed across the fault at + 100ft, there must be at least 450ft of throw. As the test bore was sited east of the deepest part of the fault-angle depression, 600ft or 750ft of throw probably occurs at Petone and evidence of this kind (Stevens, 1956) extrapolated southwards suggests that throw is at least 1,000ft in the deepest part of the basin (i e., in Port Nicholson).
Direction of Hade
It is difficult for various reasons to determine the direction of hade of the Wellington Fault. Recent fault movements in basement rocks have a superficial and not a direct expression in the soft alluvial and detrital cover and faces of scarplets and fault traces must rarely represent the attitudes, positions or character of deep seated fault planes. Even where erosion is deep and one might normally expect to detect the inclination of fault planes this is hardly possible because of the intense shattering. Fault lines are frequently obscured by scree from fault or fault-line scarps and even where fault planes are seen their attitudes can have little significance. It may not, for example, be assumed that the normal character of minor faults in the scree or sedimentary cover over a major fault or even in a broad shatter zone of greywacke means that the fault is normal in depth. All such expressions of the fault are in such soft rock that they could well be gravity faults along the fault scarp.
Subject to these qualifications, the Raroa Road section, which provides one of the few exposures across the line of Wellington Fault, may be examined. Here the fault zone consists of bands, 1ft to 5ft wide, of fault breccia and the greywacke is very shattered for a considerable distance on either side (Pl. 45). The dips of bands of fault breccia usually approach the vertical but give slight indications that the fault is reverse. Many small faults are exposed in road cuttings from Hutt Road to Korokoro and Belmont, and over all the same reverse tendency of shear planes is shown.
The lack of sinuosity of the trace of the Wellington Fault as it intersects the rugged terrain might be taken to indicate that the fault plane is vertical. This need not be so, for the crush zone of the fault is so wide that minor features are obscured and the straightness of the fault can only be observed as a general and large scale phenomenon. There is some suggestion (Anderson, 1942: 55, and Wellman, pers. comm.) that transcurrent faults are straight and usually approach the vertical, and the evidence from the Wellington Fault is not inconsistent with these views.
Adkin (1954), on the basis of sections exposed in a tunnel under Thorndon and Wadestown and in a quarry at Grant Road, concluded that the Wellington Fault is normal with a hade of 53° to the south-east (Adkin, 1954, Fig. 5). The evidence for this may, however, have an alternative interpretation, for it is probable that the tunnel section is cut in an old marine-cut cliff and that the “sediment”, “upwarped” against the “fault”, might be soliflual debris and the whole structure a congelifractate slope developed from the marine cliff. Such congelifractate slopes must have developed to a sea level appreciably lower than that of the present day and so could appear at the tunnel level.Congelifractate debris may readily be taken to be dragged strata and the shaved surface below often resembles a fault plane (Stevens, 1957a). This view is supported by the fact that congelifractate material was encountered in test bores for the Wellington Anglican cathedral (N164, 337233), in the Thorndon area, about 400 yards from the base of the cliffs. This material was a stratified deposit and evidently had been transported by small streams from its original position at the base of the marine cliffs.
Previous workers in the Hutt Valley—Wellington area, e.g; Cotton (1949, Fig. 176) and Hall (1946) seem to have regarded the Wellington Fault scarp as a primary feature and to consider that the spur facets developed along the scarp represent the actual fault plane This would imply that the fault dips to the downthrow side
and is therefore normal. Cotton (1950, p. 737) now considers, and the writer agrees, that these facets bear little relation to the original declivity of the fault scarp. The trace of the principal fault almost certainly lies from 200 to 300 yards in front of the present-day scarp and is covered by the waters of Port Nicholson and the alluvium of the Lower Hutt and Upper Hutt Basins.
Acceptance of the thesis that the scarp bounding the western side of Port Nicholson and the Lower Hutt and Upper Hutt Basins is a retrograded fault scarp explains many apparent geomorphic anomalies, such as the development of “jogs” (off-sets in plan) along the scarp line. These features may be interpreted as being of several origins:
(i) Offsetting of the principal fault by faults normal to it.
(ii) Side stepping—i.e., the offsets may be the expression of an en-echelon series of faults comprising the Wellington Fault Zone.
(iii) Differential retreat of the fault scarp.
The third or erosional hypothesis is here preferred.
An example of differential retreat is displayed on either side of the junction of the Wakatikei River with the Hutt River (Pl. 46). South of this point, the scarp is well developed and presents the fresh sharp-cut aspect of a recent fault scarp; this is deceptive, for in the Taita Gorge the continuation of this scarp is visibly erosional. North of the confluence the character of the scarp is quite different, for it has been dissected into numerous small spurs and in plan a prominent jog separates the two types of scarp. It appears that the scarp south of the confluence is largely erosional; the fault scarp to the north has undergone comparatively little retreat from the original fault-line and has been subjected only to light dissection by small local streams.
The reason for this differential retreat is that to the north of the Wakatikei River the Hutt River is deflected towards the eastern side of the Upper Hutt Basin by the Akatarawa River* and does not impinge on the base of the fault scarp. The tendency is for this deflection to become permanent as the river entrenches, and it is only after its first great meander that the river swings round to join the Wakatikei and cut away the Wellington Fault scarp.
A similar prominent “jog” at the mouth of the Korokoro Stream (Pl. 43) led Quennell (1938) to propose that a “Korokoro fault” had transcurrent movement normal to the Wellington Fault and that this had offset the main scarp along the harbour. Although the Korokoro fault undoubtedly exists, the jog is more readily attributable to normal erosive processes, and the hypothesis of transcurrent movement seems unnecessary.During the earlier stages of the Lower Hutt-Port Nicholson Basin it is probable that marine erosion was effective along the length of the Wellington Fault scarp, but with the formation of the Hutt delta, the northernmost portion of the scarp (from Korokoro north) was protected from all but river and subaerial erosion. As the tectonic depression of the Hutt Valley widens and deepens towards the harbour, forward growth of the Hutt delta must have fallen off abruptly and at some point the shoreline must have remained relatively stable. Other factors such as tidal sweep in the more open part of the harbour would tend to stabilize the shoreline in its present position. The fault scarp south of the delta (and of the mouth of the Korokoro Stream) must thus have been subject to differential marine erosion for a long period which only ended as a result of tectonic uplifts beginning about 1500 A.D. Since the 1855 uplift the declivity of the marine cliffs has been reduced by subaerial denudation, but the rapidity of marine erosion on the shattered rocks of the fault zone prior to 1855 is still indicated by the presence of numerous “hanging” valleys.
[Footnote] * This river is gorged in the Akatarawa shatter zone, one of the splinter faults of the Wellington Fault, and it thus deflects the larger river (Hutt River) rather than being deflected itself.
Examination of an isobath map of Wellington Harbour (Oceanographic Institute, 1955. by permission of the Superintendent) reveals that the asymmetric profile of the fault angle depression continues below the harbour, the position of the fault trace being indicated by the closing together of the isobaths along the western side of the harbour. On the same map the position of the fault trace beneath the harbour may also be deduced from the intense contortions of the 1 to 8 fathom isobaths, which are taken to mean that there has been slumping of unconsolidated sediments across a low scarp.
Bore-hole data provide further indications of the position and retrograded nature of the fault scarp. At the Fletcher Factory bore hole (N164/3, 423/300), 200 yards east of the scarp, solid greywacke was encountered at a depth of only 99ft. Similarly, at Iron Reconditioning Ltd. (N160, 435303), half a mile up-valley and 250 yards from the scarp base, a yellow clay, probably weathered basement, was encountered at a depth of 105ft. These depths to basement may be compared with the 420ft depth recorded from the W.C.C. test bore at Wilford, about 1,000 yards east of the scarp. A similar situation occurs at a number of other points. Grey-wacke outcrops in the river bed to the north of Liverton Road (N160, 502361), and at Moonshine Bridge (N161, 574421) greywacke was encountered 15ft to 20ft below the river bed.Thus along the western margin of the Hutt Valley the alluvial deposits (Hutt Formation, Stevens, 1956) form a thin veneer over the basement greywacke, which has been cut back 200–300 yards.