Late Pleistocene and Recent Faulting in the Otaki-Porirua and Dalefield-Waipoua Districts of South Wellington, North Island, New Zealand
[Read before Wellington Branch, May 14, 1953; received by Editor, June 5, 1953.]
The geographical position, trend, geomorphic features, and type of hitherto little more than named late Pleistocene and Recent faults situated in the areas specified in the title, receive more detailed and formal treatment. The nomenclature of recognized kinds of fault-trace surface features is reviewed, definitions of acceptably named features are given, and terms suitable for additional newly-described features proposed and defined. The location on the local fault traces of the minor land-forms systematized is given in the text and shown on the accompanying maps. In a few instances corroborative evidence on points at issue is drawn from examples of like structure and topography in adjacent parts of the Wellington district.
At present the active fault lines of the Wellington-Wairarapa region are not satisfactorily recorded. A number of the faults, which furnish useful evidence of the structure of the area, have been named and referred to incidentally in geological papers, but their exact geographical trend and position have remained undefined and their surface features and the type of displacement involved largely undescribed. The position is analogous to the naming and listing of botanical, zoological, or paleontological species without formal detailed description, with statement of locality and affinities—in short, they are nomina nuda. A similar approach to nomenclature and description as that adopted in the sciences cited seems desirable in structural geology.
The purpose of this paper is to place on record observations concerning the position, geomorphic expression, type of movement, and structural relations of fault lines of selected areas of South Wellington. It is also proposed to revise the current nomenclature of fault-trace features and contribute additional terms suitable for precise description of certain of the surface forms observed.
The global areas in which active faulting has occurred in Recent times and is still occurring are of restricted range. New Zealand is a tectonically active region sharing this state with some others, notably, Western United States, Japan, and Alaska; the distribution is circum-Pacific and is evidence of Pleistocene-Recent diastrophic stress in this segment of the globe.
General Features and Classification
The present consideration of the fault lines of the selected areas does not include reference, except incidentally, to fault lines of early orogenies that do not show late movement, though these may appear clearly on aerial mosaics as the result of new selective erosion on a peneplaned surface that had obliterated
the original tectonic topographic relief. Active faults are characterized by tectonic topographic relief on the existing land surface and by a remarkable variety of surface forms, a striking aspect of which is a frequent, consecutive, apparent inconsistency in amount and direction of movement.
An essential factor in the interpretation of fault-trace surface-forms is a recognition of the operation and effects on a line of active faulting of both primary tectonic and secondary gravitational displacement, which can frequently result in apparently differential movement in opposite directions in juxtaposition. The direction of ground slope, the direction of fault dip, the composition and degree of compaction of rock-formations involved, and the local stage of topographic relief have controlling influence, individually and collectively, in the production of contiguous anomalous surface forms and relations that are difficult to explain unless both tectonic displacement and the effects of the force of gravity are taken into account.
In plan, the fault lines of the areas dealt with are (1) sinuous, consisting of ares of wide but varying radius facing in opposite directions alternately, or, (2) a succession of straight lines of varying trend with the angle of change in trend usually rather distinct A combination on a single fault line of these two kinds of pattern as well as lineation of en echelon arrangement may also occur.
In the Otaki-Porirua Harbour area all the known late Pleistocene and Recent faults except one are regarded as being normal faults due to some degree of tensional stress. It should be noted that all known faults of this class in Western Wellington except perhaps Gibbs Fault, are contiguous to and appear to be connected with the postulated Port Nicholson-Pukerua Sunkland strip [Adkin, 1951: 170, 171 (map)], or to its offshoot, the Hutt Valley fault-angle depression. The latter is dominated by the Wellington Fault, and bears a similar relation to the Rimutaka axial belt that the Port Nicholson-Pukerua Sunkland does to the western (Tararua) axial prolongation (see Adkin, Ibid: map Fig. 4).
In the Dalefield-Waipoua area on the eastern side of the axial Tararua Range, the principal active fault is a late-movement expression of the great Wairarapa Reverse Fault, itself the local culminating displacement of the Kaikoura orogeny. The active fault—a subsidiary lineament nowhere actually coincident at the surface with the main primary fracture—is commonly referred to as the “1855 trace,” although it originated earlier than that date; for this feature the name, Wairarapa Piedmont Trace, is proposed (Fig. 1). Apart from surface-trace
land-forms wholly due to upthrust (or perhaps, underthrust) and compression, both reverse and normal types of faulting are seen to have numerous kinds of topographic expression in common.
Nomenclature of Fault-Trace Surface Features
Fault Trace. The term “trace” or “fault trace” is, in New Zealand, the preferred term applicable to the linear mark or linear series of minor landforms that indicate the intersection of the plane of a fault of any type with the landscape surface. This term supersedes the older designations for such a feature—namely, “earthquake rent” or “earth-rent” (McKay, 1888, 1892, 1902; Park, 1910; Cotton, 1922; Waghorn, 1927), “ground rent” (McKay, 1888), “earth-fracture” (McKay, 1888, 1902; Hector, 1889). The local term “trace” is the equivalent of the American usage of “rift”; also, of the less acceptable term “cicatrice” of Cotton (1949).
The term “trace” covers the surface indication of a fault of any age, whether recent and active, or ancient and dormant. It may be considered advisable to distinguish between the two by employing the terms “fault trace” and “faultline trace”, the former for active and recent faults, the latter for topographically obliterated faults that have been renewed as surface lineations by sufficiently closely spaced effects of selective erosion only.
Faceted Spur The downthrow, or upthrust, of the lower section or end of a hill spur at the intersection of a fault plane produces a characteristic steep triangular facet, which may occur singly or in linear series. If beyond the range of fluviatile lateral corrasion and of marine abrasion (according to their respective location) in the present or in the penultimate erosion cycle, such facets form a reliable criterion for the recognition of recent faulting even where supplementary evidence may be wanting.
Notched Spur. The intersection of the crest of a hill spur (or ridge) by a fault plane may produce a break in its profile by transverse fissuring (with or without differential tectonic movement), resulting in a well-defined nick or notch. The term is preferably restricted to the effects of recent and active faulting, though similar forms are produced by selective erosion on spur-crests at “faultline traces”. (See under “Fault Trace” above.)
Scarplet. This term, said to have been first used by W. M. Davis, is applied to one of the commonest of fault surface features. When it occurs, the fault-intersected surface is broken by an abrupt slope or face, generally of considerable linear extent, contrasting with gentler slopes in front and to the rear, indicating displacement of a former continuous surface. The term is restricted to lesser components that may occur on or at the foot of a major fault “scarp”, but the restriction is not precisely definable in units of measurement, and magnitude is entirely relative. An apparently single scarplet may be the sum of more than one of a succession of movements on the one plane, though more often each of such a succession will produce its own individual surface break and displacement to form a scarplet.
Rampart. A term, suggested by the present writer, for a fault feature consisting of a scarplet backed by an opposing slope of lesser steepness, to form a small ridge extending parallel to the trend of the trace.
Compression Ridge. This denotes a buckling of the ground surface at a fault trace in a direction transverse to its trend to form a low ridge by elastic distortion, without actual fracture and the formation of a scarplet.
Trench. An elongated hollow or discontinuous succession of elongated hollows coincident with the line of a fault trace indicating a narrow belt of settlement on the fissure of movement; or, the incomplete infilling of an initially gaping fissure, especially during the passage of earthquake vibrations, by the crumbling of its upper edges. Trench-in-trench forms may occur.
Downwarp. The subsidence by down-bending under gravitational stress of a strip of land-surface along one side (occasionally both sides) of a fault fissure, extending laterally from it back to a distinct hinge-line of elastic distortion, without any parallel secondary fracture of the ground. This term is suggested by the writer for this important secondary surface feature that tends to falsify direct estimates of direction and, especially, of magnitude of tectonic displacement on a fault trace. It can occur on traces of both normal and reverse faults (Figs. 11 and 10). This phenomenon has been recorded by Gilbert (1890; 355, fig.) as a feature of fault-trace morphology.
Bent Stream. A term descriptive of a watercourse that takes a Z-pattern route, where it crosses a fault trace. The stem of the “Z” coincides with the fault trace (or initially did so) and indicates transcurrent movement on the fault, except where it can be shown that the coincident reach is merely consequent on the fault line.
Shutter Ridge. This feature (see Cotton, 1951) is complementary to a bent stream course, the lateral offsetting by transcurrent movement on the fault, of a spur or ridge bounding one side of a fault-distorted stream-gully, shutting off the former direct flow of the stream and causing it to turn aside along the line of trace in order to retain or regain its similarly offset lower gully. A rampart may have the same effect on a drainage-line with or without transcurrent movement on the fault plane.
En-echelon Offset. This term is descriptive of the ends of parts of a fault trace that are laterally off line and either parallel or slightly oblique to the general trend of the fault plane at depth. Such a feature is apparently due to a measure of elastic distortion of the surface layers dependent on local factors at the places where such occurs.
Buckling (or Longitudinal Buckling). A term applicable to contiguous extension and compression of the surface layers, longitudinally along a fault trace, varying in degree on either side of it, so that in one direction topographic margins (such as at intersecting streamgaps) fail to match because of unequal stretching, but show no discrepancy in the opposite direction, there indicating relative compression. In some places this phenomenon may be due to a tendency to transcurrent movement that is retarded at some points by friction on the fault plane.
Bifurcation and Convergence The main fissure at depth below a fault trace may reach the surface as a subparallel assemblage of secondary fissures. If no appreciable subsidence of the intervening wedge or wedges of ground has taken place, the surface manifestation will be two or more diverging and converging trenches. Subsidence of the wedges will result in a similar surface pattern of scarplets facing inwards.
Fault-line Graben. A wedge or contiguous wedges of subsided ground along a recent fault between subparallel fissures meeting at depth, produce inward-facing scarplets bounding miniature graben depressions conveniently termed “fault-line graben”. The form of these may show considerable variation as determined by the number, pattern, and relative heights of the bounding scarplets, by the relief of the initial surface, and by the amount of subsidence of the involved wedges.
Otaki-Porirua Harbour Area
Fault Traces, with Chronology of Discovery
The fault traces of the Otaki-Porirua Harbour area (Fig. 2) that show recent movement or retain topographic relief of tectonic origin, are five in number.—
Kaka Fault, found by Ferrar in 1927, but not recorded until two years later (Ferrar and Grange, 1929: 188).
Mount Wainui Fault. Paraparaumu sector (alternative pattern) suggested by Ferrar, 1928 [Ferrar, 1928: Fig. 1 (map)]; northern end observed from air by Pritchard and Ongley [Ongley, 1944: 78 (table)]; general line recognized, named, and plotted by Adkin, April, 1944; recorded without description, 1947, and 1951 (Adkin, 1947: Fig. 1; Adkin, 1951: Fig. 4); Otaihanga sector mapped by Macpherson, 1949 [Macpherson, 1949: 71 (map), 72 (sect.), 76].
Mount Welcome Fault, found by Adkin, April, 1944; recorded without description, 1947, and 1951 (Adkin, 1947: Fig. 1; Adkin, 1951: Fig. 4).
Gibbs Fault, found by H. S. Gibbs, March, 1947; named and mapped without adequate description by Macpherson, 1949 [Macpherson, 1949: 71 (map), 72 (sect), 80 (fig.)].
Kakaho Fault, found by Adkin, April, 1947; named and recorded without adequate description, 1947, and 1951 (Adkin, 1947: Fig. 1; Adkin, 1951: Fig. 4).
Geographical Position and Trend of Fault Traces
Mount Wainui Fault. From the East Branch of Horokiwi Stream, Mount Wainui Fault swings in an are of wide radius round the western side of Mt. Wainui, 2,360ft., as a linear series of faceted spurs; the fault-line valleys of upper Horokiwi East Branch and Te Puka streams, both heading in a low tectonic saddle, also mark this part of its course. Notched spurs on the northwest side of Mt. Wainui fix its position before it leaves the hills to cross the lowland, where it truncates the fan of Rongo-o-te-wera Stream and scarps in succession the western sides of “Para” and Otaihanga greywacke foothill ridges. At Waikanae, recent alluvium of the lowland shows no trace, but northward the alignment is taken up by the truncation of steep fans coincident with the line of railway. Southward of Te Horo the scarplet is obscured by dune sand, but northward again it reappears, intersecting and downthrowing a segment of the fan of the Otaki River. The scarplet steadily diminishes in height and dies out as a topographic feature close to the south bank of the Otaki.
The general trend of Mount Wainui Fault is N. 25° E. (range, N. 5° E. to N. 40° E.), in two major arcs, the southern convex to the west (except for a median eastward subsidiary arc), and the northern convex to the east. Length, 22 miles.
Mount Welcome Fault. This trace converges with the Mount Wainui trace about 15 chains north of the Horokiwi-Te Puka tectonic saddle, and some late movement on the line of the older Mount Wainui Fault is shown by the notched spurs on it a mile and a half farther north. On the Mount Welcome Fault discontinuous sections of trench occur north of the Horokiwi-Te Puka saddle, but south of it the trench is continuous for over a quarter of a mile. Its line is bent, with distinct angles. Southward, notched spurs and ridge-crest carry the line over into the West Branch of Horokiwi Stream, where its trend becomes NE. by E., swinging to NE. by N. Notched spurs, especially on the eastern slopes of Sheep Hill, exactly define the trace. On the western slope of Mt. Welcome a straight, deep gully (trend NE.) marks it and is confirmed by a notched ridgecrest at the trig station and a length of trench immediately beyond. Farther southward the trace has not been followed. Length, 3 miles.
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Kakaho Fault. The known southern end of this trace commences near the shore of Pauatahanui Arm of Porirua Harbour and bounds the eastern margin of the alluvial flat of Kakaho Stream as a series of faceted spurs, trending N. 33° E. Northward, it intersects the eastern slope of Kakaho valley in a series of 16 notched spurs with 10 bent streams and their accompanying shutter ridges, this sector, 2 1/2 miles in length, trends approx. N. 30° E. After crossing the Kakaho-Horokiwi divide at a notched ridge-crest, a narrow, straight, composite fault and fault-line gully, three-quarters of a mile in length, marks the line. Northward of this the trend swings round to N. 15° E., and the trace is marked, on the eastern slopes of the valley of the headwater stream of Horokiwi West Branch, by a linear series of faceted spurs that diminish in height and die out near the valley head. The length of this fault is 6 miles.
Kaka Fault (Fig. 3). This trace has an identical relation to the Kakaho Fault that the Mount Welcome Fault has to the Mount Wainui. The Kaka converges with the Kakaho at a point 72 chains north-north-east of the intersection of the latter with the shore of the Pauatahanui Arm. Recent movement on the Kaka Fault extends along the line of the older Kakaho as far north as the head of Kakaho valley.
The Kaka Fault, along most of its trace of 34 chains, intersects a high-level terrace of Kakaho Stream and presents remarkable topographic features: it is thrice laterally offset, is marked by scarplets, trenches, and pits, has “one-way” longitudinal overlaps apparently due to transcurrent buckling, and the trends of its straight-line components are successively (from north-east to south-west) N 40° E., N 38° E, N. 42° E; and N. 47° E—all but one oblique to the general trend of N. 40° E, which may be taken as the local trend of the fault-plane at depth. The alluvial flat of the Kakaho Stream downstream from the high-level terrace shows no trace, but the buried base of a line of faceted spurs forming the lower western side of the alluvium-bottomed valley, trending approx. N. 47° E, if projected south-west, would carry the trace just north of the Paremata road and railway bridges and on down the main arm of Porirua Harbour to link up with Quennell's Owhariu Fault (Quennell, unpubl. M.Sc. thesis, 1938, Victoria College).
Gibbs Fault. The trace of this fracture intersects longitudinally the upper basin-valley of the Muaupoko Stream, which lies on the eastern (inner) side of Otaihanga foothill ridge, near Paraparaumu This trace, the very recent inbreak of which is shown by the fact that it has left its mark on the most recent alluvium
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of the valley-bottom, has a length of nearly a mile and a quarter, and its general trend is NE. by E. In detail in plan, the trace consists of three or perhaps four straight lines continuous endwise. At its northern end the first length of about 8 chains trends N. 66 1/2° E.; from a distinct angle it trends N. 54° E. for 60 chains, passing the Hadfield woolshed to a point about 15 chains south of it. After another distinct angle the trend is N. 56° E. for about 10 chains. South of this the mark is very faint and perhaps doubtful, but the trend appears to be N. 61° E. The trace ends a few chains short of the hill ridge forming the southern boundary of the Muaupoko valley.
Gibbs Fault has furnished no decisive evidence of the relative movement on its line, but Macpherson (1949: 76) considered it to be a reverse fault. His determination of an upthrust of 10–20 feet, however, is an over-estimate. Detailed examination has shown that stream erosion has considerably increased the height of the original tectonic scarplet, giving a false impression of greater magnitude of differential movement than has actually taken place. The true tectonic displacement is probably only one to three feet.
Evidence of Normal Faulting
Evidence has been obtained that most, if not all, of the late Pleistocene and Recent faults of south-western Wellington are normal faults.
The Mount Wainui Fault, where it swings round the slopes of Mt. Wainui, is flanked on the west by a downthrown tilted block as demonstrated in Fig. 4. North of Waikanae similar downthrow to the west on the same line of fracture is indicated by a displacement of the order of 75 feet of Otaki Sandstone and underlying greywacke (Adkin, 1951: 175).
The movement on the Kakaho Fault has been largely transcurrent in a dextral (clockwise) direction, but a strong vertical component with downthrow to the west is consistent throughout its length. East-south-east of Mt. Welcome the high mass of that hill is on the downthrow side, with the result that the adjacent
sector of the Kakaho Fault is represented by a deep, narrow, composite fault and fault-line gully; elsewhere westward-facing faceted spurs mark the line of this fault.
Corroborative evidence for normal faulting is furnished by a member of the same fault zone, the Wellington Fault, the eastward-facing scarp of which fronts the downthrow side. The critical locality for the determination of direction of vertical displacement is at Grant Road, in the City of Wellington. There, where no retrogradation of the initial scarp either by fluviatile lateral corrasion or by marine abrasion could have taken place, and where fortuitous coincidence with ancient shatter belts does not occur, quarrying reveals solid unbroken stratified greywacke and argillite formation that extends down to the very toe of the scarp. If this fault were of the reverse type, solid rock could not be present in this situation, which would be occupied by massed blocks and other debris from the initial overhang.
Further evidence of normal faulting on the Wellington Fault is the structure seen in the recently driven Wellington City sewerage tunnel under Thorndon and Wadestown. The Thorndon Flat alluvial sediments, related to the existing topography and apparently late Pleistocene in age, are in fault contact with the greywacke to the west. The greywacke contact dips south-east at 53°. The western margin of the sediments show conspicuous drag at the greywacke contact and have also undergone a gentle anticlinal arching (or serial incipient doming) in an eastward direction—compressional buckling due to the wedging of the strip on the downthrow side of the fault into a lessening space (Fig. 5).
All late Pleistocene and Recent faults and fault traces of western Wellington (except perhaps Gibbs Fault) are geographically, and also appear to be structurally, related to the Port Nicholson-Pukerua Sunkland strip [Adkin, 1951: 170, 171 (map)], in part bounding it or intersecting it obliquely. This strip was, and is, one of relative tension producing normal faults. This view had the support of Macpherson (1949, and personal communication) who regarded the Mount Wainui Fault as a tension or normal fault caused by Kaikoura and post-Kaikoura marginal “collapse” (loc. cit.: 76).
Dalefield-Waipoua River Area
The principal active fault on the eastern border of the axial range is the Wairarapa Piedmont Trace. At Featherstone it diverges from close parallelism with the primary major line of fault, and at the Waiohine River it leaves the eastern margin of the old rocks of the axial range, to trend more to the east and intersect the Quaternary alluvium and late-Tertiary formations of the Wairarapa Geosyncline [see Adkin, 1917: Fig. 1 (map)].
The Dalefield-Waipoua sector (Fig. 6) of the active fault, 13 miles in length, trending approximately N. 40° E., takes a seeming sinuous course, but which when plotted in detail is seen to be made up of a connected succession of straight lines of varying trend, with a maximum divergence from the general orientation given above of about 10°.
Within the area treated the geographical position of the Wairarapa Piedmont Trace is as follows: It intersects the upper end of Dalefield Road about 2 chains south-east of the right-angled bend; it crosses the Hokioi Stream Road where the latter first comes close to the stream; it crosses Hururua Road at the Enaki Stream bridge. Northward, it runs behind the Carterton Bush hill-ridge, ascending its western slope and crossing its crest before meeting Mangatariri Valley Road a quarter of a mile south-east of Hururua Road junction. The trace then crosses Mangatariri River at the junction of Ti Creek, following the valley of that stream before ascending the western side of Boys Trig Ridge to a tectonic saddle at its north-east end. On Norfolk Road the trace intersects at Russell Trig. North of the Waingawa River it coincides with the Burnett Road—Upper Plain Road junction, and further north crosses the Whakamoekau Stream close to the Coote Road—Matahiwi Road junction. At Lydia Trig it lies 15 chains to the north-west, and after crossing the Waipoua River it joins up with the two-storied scarplet at Mikimiki Road—Main Highway junction figured by Ongley (1944: Fig. 16).
Analysis of Land-Forms Associated with Fault Traces Faceted Spurs and Scarped Ridge Flanks
Typical and persistent surface features of tectonic topographic faults are faceted spurs in areas of drainage-lines transverse to the trace, and linearly-extended steep faces bounding more isolated or longitudinal hill-ridges. Both these varieties of fault scarp occur on Mount Wainui Fault—spur facets on the western side of Mt. Wainui itself, and continuous abnormally steep faces on the western sides of “Para” and Otaihanga foot-hill ridges and similar scarps truncating steep fans and an Otaki Sandstone outlier farther to the north.
The Kakaho Fault, also, has faceted spurs marking nearly the whole of its length, in most places distant from major streams and not retrograded or modified by lateral corrasion in the present or the preceding cycle of erosion.
The older primary line of the Wairarapa Fault, between Cross Creek and Featherston, is marked by a linear series of faceted spurs, now modified into rounder form, but these are not present on the Dalefield-Waipoua sector, where the diverging Piedmont Trace traverses late-Tertiary and Pleistocene terrain.
These also are rather durable surface features marking fault traces, being acute when of recent date, but more widely opened and of rounded form at later stages (see also, McKay, 1902: 25).
Along the upper Kakaho Valley sector of the Kakaho Fault, the eastern spurs exhibit a consecutive series of sixteen notched profiles (see Fig. 2). These extend from the intersection of Kaka Fault to the Kakaho-Horokiwi divide.
On the Mount Wainui Fault, also, notched spurs occur, apparently (the Horokiwi-Te Puka saddle excepted) due to an extension along it of late movement on Mount Welcome Fault. The latter fault is also marked by at least eight notched spurs.
The line of the Wairarapa Piedmont Trace is notable for notched spurs that distinctly show recurrent dislocation. A spur between Kaipaitangata and Hokioi streams, spurs south-east and north-east of Swan Trig, a spur on the western side of Carterton Bush Ridge and its crest at its north-east end, and the north-east end of Boys Trig Ridge—all show broadly opened U-shaped notches of an older dislocation cut by later V-shaped elongate nicks (trenches) of more recent diastrophic movement.
Scarplets are a dominant feature of local active and recently active faults. Single, multiple, and two-storied (of two kinds—“visible”, and “obscured”) examples of scarplet occur. On all local traces single scarplets indifferently intersect flat land, hill-slopes, river-terraces, etc., and truncate or step-down alluvial fans and river-terraces. Downstepped fans occur at three localities on the Mount Wainui Fault between Paekakariki and the Otaki River—namely, one mile north of Paekakariki; near Frasers Hill Road, between Waikanae and Te Horo; and at Te Waka Road, between Te Horo and the Otaki River. On the Wairarapa Piedmont Trace, the dormant fan of a small stream, a tributary of the Hokioi, 3 miles north-west of Carterton, is downstepped a maximum height of 18 feet (see Fig. 6).
Dislocated river-terraces may be seen on the Wairarapa Piedmont Trace at the Kaipaitangata, Hokioi, and Whakamoehau streams and at the Waiohine, Mangatariri, Waingawa, and Ruamahanga rivers. On the Mangatariri, its right-bank terrace is dislocated on two parallel lines and its surface is downstepped upstream, the two scarplets facing west, not east, which is their general aspect. The major scarplet (the eastern one) increases in height away from a normal river-cut terrace-face, in a south-westerly direction, the lesser one dies out (Fig. 7, a). The eastern scarplet, for nearly half a mile, forms so complete a barrier to tributary drainage that the extensive displaced terrace-surface has been converted into a swamp, from which the surplus water moving north-east along the trace directly towards the river, forms two elongated sag-ponds discharging at the terrace front (Fig. 7, b).
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The reference to “obscured” two-storied scarplets requires explanation. A good example of a “visible” two-storied scarplet occurs on the Main Highway 7 1/2 miles north of Masterton, and has been figured by Ongley (1944: Fig. 16); the lesser scarplet stands at the base of the major one, a narrow ledge separating the two. An “obscured” two-storied scarplet is one in which the scarps of two separate diastrophic movements have occurred on the same plane at the surface and form a single face of a height equal to the sum of the two-fold displacement.
The locality where this phenomenon is convincingly demonstrated is on the Wairarapa Piedmont Trace near Pigeon Bush railway station, at the intersection
of Owhanga Stream. On the south side of the latter, about 5 chains distant, the scarplet marking the fault trace is 52 feet high and presents a single, even, steep face. Nearer the stream, in the re-entrant eroded by it, the scarplet is double, the lower component being 16 feet high, the upper, 36 feet (Fig. 8). Except for the re-entrant in the earlier scarplet, eroded by the Owhanga during the time interval between the two displacements, thereby revealing the two-fold nature of the uplift, the two-storied character of the 52-foot scarplet would have eluded observation.
The minor land-form termed “rampart” is apparently found on the trace of reverse faults only. On the Wairarapa Piedmont Trace the largest example seen is located on the western slope of Carterton Bush Ridge (Fig. 9). Its scarplet has a height of 16–18 feet, facing east (uphill), and is backed by a broad low ridge a couple of chains in length. A small stream is turned aside by the rampart and takes a Z-shaped course round it; slight local transcurrent movement on the fault plane is probable at this place.
Another Wairarapa example occurs 15 chains south of the intersection of the trace by the Hokioi Stream (Fig. 10).
Gibbs Fault has examples of rampart topography similar to that shown in Fig. 10, but are located entirely on gently sloping ground.
This minor compressional land-form, which may be taken as an incipient stage of a rampart, is less common. An example occurs alongside Matahiwi Road at the junction of Coote Road, on the Wairarapa Piedmont Trace, 5 1/2 miles north-west of Masterton.
Tectonic trenches are characteristic of all active and recently active faults in the areas under review. Typical examples occur on Mount Welcome, Kaka, and Gibbs faults, but the most sharply defined trenches (formerly graphically termed “rents”) are found along the Wairarapa Piedmont Trace. These were formed during the 1855 earthquake (see also, Ongley, 1943) and their present form is a V-shaped elongated cleft, usually 4 feet in width and 4 feet deep. Trench-in-trench forms indicate recurrent fissuring of progressively decreasing magnitude.
This feature of fault-trace topography is considered of great importance in the interpretation of magnitude and direction of tectonic movement on recent lines of fracture; its non-recognition can lead to erroneous or, at least, inaccurate assessment.
The best examples observed occur on the Wairarapa Piedmont Trace, on Gibbs Fault, and, outside the particular areas covered by this paper, on Owhariu Fault; ground surfaces of even slope have sagged down towards the line of trace from a distinct line parallel and adjacent to it. Fig. 10 illustrates this phenomenon on the Wairarapa Piedmont Trace 15 chains south of Hokioi Stream. Fig. 11 shows another example on the Owhariu Fault. In all cases the apparent height of the original scarplet is increased entirely by gravitational sagging, to the extent, as in Fig. 11, of creating a 5-foot scarplet where, locally, probably no differential tectonic displacement really took place.
Fig. 10—Section showing combination of rampart and downwarp on Wairarapa Piedmont Trace.
Fig. 11.—Section showing downwarp on Owhariu Fault.
Fig. 12.—Plan showing effect of buckling at a gully crossing Gibbs Fault, cf. Fig. 3.
Fig. 13.—Plan of stream consequent on a fault line (Wairarapa Piedmont Trace).
In many instances downwarping may be the more probable explanation of an appearance of reversal of earlier faulting movement. Downwarp commonly occurs on the upslope side of the fault fissure, that side having less subsurface support than the other. Where the line of an active fault crosses a hill ridge obliquely, ascending one side and descending the other, as at Carterton Bush Ridge and at Boys Trig Ridge, the more usual phenomenon (but not necessarily excluding variations) is for the fault scarplet to face uphill on both sides of the ridge (though perhaps developed to a lesser degree on one slope than on the other), and where this occurs to frequently face a ground surface that has to all appearance sagged to form what is herein termed “downwarp”. If such a combination of surface forms were taken to indicate “reversal of faulting” it would seem logical to expect dissimilar, not similar, relationships on opposite hill slopes.
Shutter Ridges and Bent Streams
In the areas under review, only one example of these features, but one of impressive extent, indeed the most extended series yet recorded from the North Island, occurs, on the Kakaho Fault on the eastern side of the upper valley of Kakaho Stream (see Fig 2). This series of 10 bent streams, each with its associated shutter ridge, indicates dextral transcurrent movement of the western terrain increasing from 6 chains to 16 chains in a northerly direction.
What appears to be longitudinal buckling of the ground, differing on opposite sides of the fault trace, occurs on the Kaka Fault where it intersects a highlevel terrace in Kakaho valley. The terrace is cut across by three or four small gullies, of low gradient and swampy At the Kaka trace the channels are constricted by the projection, on one side only, of an overlapping segment of terrace; this has resulted in a doubly-bent gully-margin on one side and a nearly straight margin on the other. The morphology is anomalous but is tentatively interpreted as an incipient transcurrent displacement, retarded here and there by friction on the fault plane at depth, causing buckling (compression and expansion) of the superficial layers, with the maximum of transcurrent displacement occurring at the gully-gaps (see Fig. 3). On an adjacent hill-spur, the tip of which is intersected by the trace, a 20-foot-long scarplet with trench at base marks one lateral slope, but an entirely unbroken slope (on line) occurs on the opposite side. This anomaly seems to confirm an erratic buckling of the terrain.
An exactly similar topographic anomaly occurs on Gibbs Fault (Fig. 12).
The Kaka fault trace has three lateral offsets of en-echelon pattern on the sector where it intersects the high-level terrace in the lower Kakaho valley. The northern offset is 10 yards, the intervening one 3 yards, and the southern, about 4 yards (Fig. 3). On all other fault traces in the areas reviewed, any change in trend has shown no perceptible offsetting.
Stream-Courses Consequent on Fault Traces
In addition to those parts of courses of “bent streams” consequent on a fault line by transcurrent movement, at least two examples occur of streams consequent on the line of a fault by diversion by interception.
At the northern end of the Kaka trace close to its convergence with the Kakaho Fault, an eastern lateral tributary makes a right-angled bend and follows the Kaka trace southwards for about 6 chains. By the process of down-cutting, the adjacent scarplet, elsewhere 4 feet to 10 feet in height, has been increased to 25 feet This illustrates a third factor—namely, stream-erosion, in giving a false increase in height to an original tectonic fault scarplet.
A notable fault-guided stream-course is located on the Wairarapa Piedmont Trace three-quarters of a mile north-east of Kaipaitangata Stream, near Dalefield (Fig. 13). There, the toe of three contiguous spurs has been downstepped as one, to the east. On reaching the line of trace, two streams were, at its inbreak, diverted along it southward, to flow out on to the plain at the south end of the downstepped segment. The beheaded former lower gully of the larger of these streams intersects the downstepped spur-segment directly opposite its present upper gully, indicating that locally no transcurrent movement can have taken place.
A branch of the Muaupoko Stream, at Otaihanga, near Paraparaumu, though strictly not originally consequent on the line of Gibbs Fault, now follows its scarplet for some distance. The ground surface on the eastern side of the trace has been lowered thereby from 5 feet to 8 feet or more, increasing the height of the original tectonic scarplet by those amounts.
A similar considerable increase in height of an original tectonic scarplet by stream-erosion on one side of a recent fault is well seen on the left bank of Ti Creek, a left-bank tributary of Mangatariri River, 5 miles due north of Masterton.
Transcurrent movement over a lengthy period of time has undoubtedly occurred on the Kakaho Fault on a scale sufficient to break alignment of the prefaulting topographic features and offset them to a distance of the order of 6 chains increasing to 16 chains over a distance of about 3 miles.
The Wairarapa Piedmont Trace also shows transcurrent movement in a dextral direction but, on the evidence, intermittently spatially, the topography of intervening sectors negativing it.
The fact, however, that many of the lesser watercourses coming in from the western side of this trace, cross it obliquely and almost invariably sinistrally (anti-clockwise), seems to indicate that earlier movements on this trace were dextrally transcurrent plus a vertical component, but that more recent movements were restricted, except locally, to a vertical direction only Support for earlier dextral transcurrent movement on this fault is given by a rather ancient river-cut escarpment bounding Carterton Bush Ridge on its north-east side; a dextral offset of 20–30 yards breaks the continuity of this escarpment almost on the line of the most recent fault features (see Fig. 7, a).
Among the land-forms marking the lines of late Pleistocene and Recent faulting in the areas under review, “fault-line graben” occur on the Wairarapa Piedmont Trace only. Two linear sets were observed, one of two adjacent units (Fig. 14, a, b), on the western slope of Boys Trig Ridge, the other, comprising three adjacent but offset units (Fig. 15, a, b, c), on the hill-ridge between the main Enaki Stream and its west branch.
The graben (Fig. 14, b) shows the topographic effect on its profile of a hillock lying directly on the line of fault; and the graben (Fig. 14, a), the effect of a larger hillock laterally intersected by the fault. In Fig. 15, a, b, c is shown the tectonic topographic effect on graben profile and offsetting by a principal fissure associated with variable subsidiary fissures.
Key to Lettered Features on Maps
bs bent stream.
cr compression ridge.
ds downstepped spur end.
dt dislocated terrace.
fs faceted spur.
ns notched spur (and) tectonic saddle.
sr shutter ridge.
trsc transcurrent shift.
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