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.

Transcurrent Nature

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.

Throw

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.

Erosional Features

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.