The Tararua Range As A Unit Of The Geological Structure Of New Zealand
The publication of the masterly and lucid account of the major structural features of the New Zealand area by Mr. E. O. Macpherson, a senior officer of the New Zealand Geological Survey, (1) has prompted the present writer to submit a small quota of supplementary evidence. Since 1909 the present writer has held views concerning the structure of the Tararua Range which fit into the comprehensive picture of New Zealand structure depicted by Macpherson. These views, based on the thesis that the dominant earth-forms of the given region represent in their broader aspects the superficial expression of relatively late diastrophic events, were not acceptable in 1911 and their placing on record at that date was denied. Since those times, however, the value of the major features of topography and physiography in the interpretation of erosional and, more especially, diastrophic history has been recognized and accepted, until at the present time dominant earth-forms, especially within areas of monotonous rock facies, have come to be regarded as important criteria of the nature of internal structure and the events of the late geological history.
In 1911 the present writer submitted the view, based on geomorphological data, that the Tararua Range, in the current geographical cycle, presented an example of the dissection of a late mature, or perhaps near-senile, land-surface deformed by tangential crustal movement operating from west to east, into a superficial asymmetrical anticlinorium broken on its eastern limb by a great overthrust fault. The existing ridge-elevations and valley-depressions were considered to be the highly dissected but only relatively slightly modified tectonic features, the drainage-systems of the area being initiated by and consequent upon the pattern of secondary folding.
A brief summary of this concept of Tararua configuration was placed on record, in a somewhat incidental way, in papers published in 1920(2) and 1921,(3) and it is now proposed, in view of the favourable opportunity created by the publication of Macpherson's comprehensive statement, to submit the evidence of the Tararua segment in more detail.
Geographical Extent and General Configuration.
The Tararua segment of the structural axis of the North Island is comprised within a distance of 60 miles from north to south—from the headwaters of the Hutt River to the Manawatu Gorge—and within a maximum distance of 25 miles from west to east—from the Manawatu lowland on the west to the Wairarapa depression on the east.
The range consists of a series of parallel and subparallel longitudinal ridges so disposed that they form a wide arc convex to the west; thus it is that from north to south their trend changes from N.N.E. to N.E. by N. In addition to the longitudinal ridges there is a less obvious but equally important, though less-closely spaced series of complementary transverse ridges which link the longitudinal components together.
An outstanding feature of the Tararua Range is its asymmetry. The culminating ridge—that extending from The Mitre to Mount Holdsworth and 4,000 to 5,000 ft. in altitude—lies well to the eastern side of the range, so that on that side the altitudes of the longitudinal foothill ridges decrease more
abruptly towards the subjacent lowlands than on the west. In general, towards both east and west, the longitudinal ridges decrease in height in succession as the respective piedmont areas are approached, the outermost foothill ridge on either side being usually the lowest of the respective series. The whole is suggestive of a high-standing tilted earth-block, having a steep eastward-facing scarp and a longer backslope declining towards the west.
In a less specific and more comprehensive survey, the Tararua-Rimutaka highland belt will be found to form a single erogenic unit and should be regarded as an indivisible whole; the deformational development of the Rimutaka segment is similar in degree to that of the northern twenty-five miles of the Tararua Range proper and tectonically is practically a counterpart of the northern Tararua. The total length of this mountain arc is ninety miles, the southern end being at Cape Turakirae on Cook Strait.
With respect to the south-western offshoot of the Tararua Range, i.e., the hill-system extending through Wellington Peninsula from Paekakariki and the Hut Valley, the position is rather different. This territory, though linked to the main highland geographically and geologically, must be regarded as constituting a separate geomorphological province. In this subsidiary area the major topographic features appear to be genetically less related to deformation, and adjustment to structure more strongly developed, although it will probably be found, when more detailed surveys have been carried out, that incipient folding, i.e., warping, has had a greater influence on the molding of the relief of this area than has so far been recognized. On the other hand, adjustment to structure is by no means wholly wanting in the main Tararua area, rather the reverse; indeed, many minor features there must undoubtedly be ascribed to the operation of this process.
The Evidence of the Ridge Pattern.
The crucial point in the determination of the origin of the longitudinal ridges of the Tararua Range is to postulate processes that will fulfil the persistence of the linearity of many of them over distances of the order of scores of miles.
The Tararua Range can safely be regarded as a two-cycle mountain area with the principal periods of folding referable to the post-Hokonui and the Kaikoura deformations. The folding of earlier orogeny or orogenies was extremely intense and resulted in an isoclinal, sheared, and imbricated structure varying in scale from large to quite minute. The strata of the so-called Trias-Jura system to which the rocks of the range are customarily referred, are predominantly thin-bedded greywackes and argillites and no massive thick-bedded strata of extended distribution are known to occur. The strike exhibits almost endless local variation, as might be expected in such an irregularly overthrust and imbricated terrain, though the general direction is usually nearer N.–S. than W.–E.
A current hypothesis. It has become the current fashion to ascribe the major physiographic trends—the ridges and drainage lines—to the process of adjustment to structure on the assumption that there exists alternating resistant and weaker bands or belts on which have been carved and moulded the larger elements of the relief—positive or negative according to the degree of competency of the material involved.
A factor in the supposed adjustment to structure was the assumed presence of fault-zones or shatter-belts of longitudinal trend and considerable linear extent. Broadgate's map(4) shows the inferred shatter-belts or belts of weak rock with most of the main valleys opened along them. The principal ridges, also longitudinal (i.e.,. N.N.E.), are ascribed by Cotton(5) to resistant compact greywacke strata also having continuous linear extension over many miles, or to such shatter-belts that “have had the network of crevices [joints] resealed by a deposit of mineral matter… are now relatively resistant… to stand out boldly as ridges.”
Among the adherents of the adjustment-to-structure hypothesis reasoning in a circle seems to have occurred: coinciding linear portions of adjacent stream systems or noticeably prominent linear ridges have been ascribed to weak belts of fault-shattered rock or bands of weak argillite in the one case, and to competent compact greywacke strata or shatter-belts sealed and reinforced by an injection of quartz-veins in the other. On the other hand, the linearity of the physiographic features is cited as evidence of the presence of belts of weakness or of resistant material. The point to be emphasized is that the hypothesis of
adjustment to structure has been applied not only to the smaller-scale features of the Wellington Peninsula, but also to the greater and lengthier ridges of the Rimutaka Range(6) and of the Tararua Range,(7) thus including the terrain of the main structural axis of the North Island within its scope.
In actual fact, in such a terrain of intensely squeezed, twisted, and imbricated strata, neither occasional patches of compact greywacke nor faultzones (whether weak or reinforced) can have any great linear extent. Hence, an opponent of the adjustment-to-structure hypothesis has the right to insist on more definite proof of the real existence of bands of hard material of such linear extent as to match up with the end-to-end persistence of the main Rimutaka and Tararua ridge-lines.
A glance at the map, Fig. 1, will show that unless the existence of alternate hard and soft bands of great linear persistence can be fully proved, some other hypothesis of the origin of the through longitudinal ridge-pattern must be sought and that of adjustment to structure as an all-over explanation must fall to the ground. Furthermore, how can the adjustment to structure hypothesis explain the major transverse ridges? These arc primary features extending from side to side of the mountain-system and are not fortuitous erosional products of subsequent origin.
An alternative hypothesis. On the hypothesis of relatively late secondary deformation of a land-surface approaching the peneplain stage (although apparently still retaining considerable relief) the problems of Tararua physiography become more amenable for solution.
On the deformation hypothesis the Tararua segment (in its present erogenic cycle) is regarded as a large upfold—one of an extended series as has been shown by Macpherson(a—p. 22)—plunging northward (and with the Rimutaka segment, also plunging southward) and corrugated by a complete pattern of secondary flexures, longitudinal and transverse. At the culmination of the range the longitudinal components reach the maximum number of eight, seven of which persist to the southern end of the range. At its northern end—near the Manawatu Gorge the secondary longitudinals are reduced to one (as the result of converging and by plunging beneath the level of the adjacent lowlands), there giving the mountain-axis the appearance of a simple arch. This simple arched form at the Gorge has been recognized as such by Ongley and Williamson,(8) but these observers were apparently not acquainted with the multi-folded superficies at the mountain culmination thirty-five miles farther south. Their observations and the conclusion arrived at, however, give support to the validity of the deformation hypothesis of the origin of Tararua physiography.
Complementary to the longitudinal folds, the Tararua area has four major transverse folds and several other lesser ones, the latter being less obvious by reason of their having been cut through in places by the master drainage lines. The transverse folding affords unequivocal evidence of the compressional stress the region has undergone during the later orogeny or orogenies. Various writers(9) have pointed out that the curved form of the earth's surface necessitates that any series of folds must be accompanied by a complementary series approximately at right angles to the first. As stated earlier, the transverse ridges of the Tararua. area are not mere erosional features; the principal ones extend across the mountain-system from side to side and the chief transverse flexure (now the chief transverse ridge) extended eastward beyond the eastern margin of the highland block across the longitudinal tectonic depression of the Wairarapa Valley as a basement upwarp to produce the divide in that depression between the Manawatu and the Ruamahanga drainage-basins.
In the Rimutaka portion of the major upfold the secondary longitudinal anticlinal axes were three to four in number, and the transverse axes, two, both components now persisting as through ridges except for some local modification here and there due to incidental excessive erosion and dissection.
Ignoring the effects of the contemporaneous dissection to which the area has been subjected the land-surface visualized by the deformation hypothesis (and for the existence of which perhaps not unsatisfactory evidence is proffered) may be described as a superficial anticlinorium or a second-cycle anticlinorium.
The Evidence of the Hydrography.
The drainage pattern developed upon the Tararua area is distinctive and exhibits important details, both in plan and cross-section, that demand adequate explanation of origin, position, and arrangement.
As might be expected from the pattern of the positive relief, the main drainage lines are orientated longitudinally and transversely. The drainagepattern is markedly rectangular or trellised. A most notable feature is the presence of two hydrographic centres or dispersion centres of drainage. These are located at the intersection of two of the principal transverse ridges with a median longitudinal ridge which, however, is not quite the culminating ridge of the range, but the one immediately west of it. Local peculiarities of the drainage-lines and valley-trends occur in the region of culmination on the main transverse ridge as a result of the partial obliquity there of some of the longitudinal ridges, by which they converge to join adjacent longitudinal ridges (see map, Fig. 1).
A description of each of the master rivers of the Tararua area would show that one and all possess the rectangular course pattern, with many minor variations in the details. A complete description, however, is unnecessary to establish the salient characters, which can be well shown by citing a few outstanding examples. For brevity of description very generalized direction-terms and angle-values will be used, more exact directions and angles being obtainable from the accompanying map, Fig. 1.
The Ohau River from its source to its mountain debouchure has four major bends. The intervening reaches trend N.–S. or S.–N. three times, and E.–W. twice; the four principal tributaries, trending N.–S. or S.–N., enter the trunk at three of its major angles.
The Mangahao is for the most part longitudinal, trending S.–N., but is offset three times by transverse reaches, once E.–W. and twice W.–E.; four longitudinal reaches come in, intervene, or go on, and a large S.–N. tributary enters at a major angle.
The Mangatainoka, by reason of its chance marginal position, is wholly longitudinal, trending S.–N.
The headwater reaches of the Ruamahanga by reason of proximity to the principal transverse anticlinal axis, are unusually oblique in trend, but this local variation provides additional evidence favouring the deformation hypothesis. The river makes three major angles; the three oblique reaches trend N.E. twice and S.E. once, respectively; the other two–transverse—trend W.–E.
The Waiohina river-system has a large tributary, the Park (a watercourse controlled in its trends to a remarkable degree by tectonic influence) with three reaches, one trending N.–S. and two E.–W. and W.–E. respectively. The main river makes three major rectangular bends, the intervening reaches trending N.–S. twice and W.–E. twice; a N.–S. and a S.–N. trending tributary enters the trunk at two of the major angles.
The pattern of the Otaki River resembles that of the Ohau on a larger scale, but has interesting variations, variety of the main pattern theme also heing a characteristic favouring the postulated diastrophic basis on which the deformation hypothesis is founded. From its source the first short reach (length two miles) of the Otaki is E.–W. This reach has the peculiarity of being genetically parallel with the main transverse axis, but transverse with respect to the longitudinal axes. The next reach—a lengthy one of twelve miles—trends N.–S., and halfway along receives a N.–S. tributary which enters the trunk transversely, W.–E. Receiving a S.–N. trending lesser tributary, the Otaki makes a major rectangular turn westwards and continues in this direction for six miles to its mountain debouchure. On this lower transverse reach the main river receives two longitudinal-trending major tributaries, one flowing north, the other south, and joining opposite each other (see map, Fig. 2).
The remaining rivers of the range may be dismissed with the statement that they possess the above cardinal characters in sort, but of less intricate development.
So much for the general character of the main drainage-lines in plan. In cross-section dominant characteristics are very apparent and except for fortuitous local modification conform to a definite rule: the longitudinal reaches (N.–S. or S.–N. approx.) are for the most part more open and at a more advanced stage of topographic development than the transverse (E.–W. or W.–E. approximately) reaches which are usually narrow, rock-bound gorges (Fig. 3). This diverse character is to some extent erosional, but, as will be shown later, is predominantly inherited from elements of the initial suface conformation.
Fig. 3—Contrasting cross-profiles of transverse (a) and longitudinal (b) reaches of Tararua river-valleys. Example from Waingawa River: (a) upper gorge; (b) Mitre Flats.
From the two hydrographic centres all but three of the larger rivers of the range take their rise. The principal centre—located at Arête Peak, a mile and a-half south of Mount Dundas—is particularly notable; there, seven rivers—the Otaki, the Park, the Waiohina, the Ruamahanga, the Mangahao, and the Southern Branch of the Ohau—radiate to all points of the compass. Similarly, from the second centre, at Mount Hector, four rivers—the Tauherenikau, the Hector, the Waiotauru, and the Hutt—flow in as many directions.
Initiated as consequents on an actively flexuring surface the main drainage-lines were all successful in maintaining by down-cutting outlet courses in one direction or another across the rising flexures and the transverse portions of their courses are thus antecedent to the upfolds; the Tararua river-courses are thus alternately consequent and anteconsequent.
The Origin and Development of the Tararua Drainage Systems.
The existence at the present time of the Tararua Range as a series of mountain ridges rising to an altitude of 5,000 ft. above sea-level appears to require the mechanism of orogenic uplift to produce so great a relief feature. Its origin as a highland of circumdenudation as suggested by Ongley(10) appears to be too improbable for ready acceptance. Such an origin would apparently require the
subaerial erosion and removal of thousands of feet of softer “covering strata” to east and west. The diastrophic origin of the mountain-axis in one or more of a succession of post-Cretaceous orogenies as outlined by Macpherson(1) has, in the Tararua area and elsewhere, much geomorphic support. The tectonic origin of the Tararua superficies may therefore be accepted as a well-founded circumstance.
Discussion of origin of the initiation and development of the drainage-lines on the earth-block under consideration may commence with two assumptions: one, that the earlier orogenies had compressed the block into a rather rigid mass; the other, that the surface at the beginning of the later orogenies had advanced towards peneplanation to give it subdued outlines, but that it still retained, viewed broadly, a considerable measure of relief. Relict fragments of such a surface, or areas which are taken to be surviving portions of it, are to be found in several places on the higher parts of the Tararua Range. They occur at Table Ridge, near The Mitre; between West Peak and Mount Hector; south of Ngamaia trigonometrical station; at Tarn Ridge; and at the northern end of the range from near Tarakamuku trigonometrical station to the Manawatu Gorge. Except in the last-mentioned locality, the relict areas of subdued topography lie between the 4,000 ft. and 5,000 ft. contours and are of the order of hundreds of acres in extent. No residuals of younger covering strata, however, are known to occur on any of these presumed relict areas conclusively to prove their surmised nature, but it is by no means certain that the Tararua geanticline was ever submerged by the Tertiary seas.
The first assumption gives grounds for the probability that the renewal of diastrophic activity resulted in considerable upthrust of the Tararua geanticline, but only a moderate secondary flexuring of its superficies. The longitudinal anticlinal elevations and synclinal depressions were thus broad and slight, especially on the western side whence the tangential pressure operated, but probably increasing in amplitude eastward to the contact on that side with the resistant down-folded mass—the Wairarapa Geosyncline.
The effect of the complementary transverse flexures would be twofold. The anticlinal components would divide up the longitudinal synclinal depressions into closed lengths. The synclinal components would, on the other hand, produce intervening sags in the longitudinal anticlinal ridges and downwarps in the longitudinal synclinal troughs. The general effect would be to produce a rather regular multiple series of canoe-shaped valley-depressions; arranged linearly in tiers. Though in general of similar form, there would be a wide range of divergence in such details as direction and amount of pitch, position and relative height of lowest point of lateral bounding rim, and height and degree of valley-end upwarps. Such a flexured surface gives a picture of the type of land-surface, on the deformation hypothesis, on which the Tararua river-systems originated.
Rainfall on the summit ridges would concentrate and flow as a pair of headwater streams, down the axial lines of any one of the highest series of canoe-shaped valleys. These two streams, one rising at each end of the valley, would flow towards each other and towards the lowest part of the valley-bottom which would be located opposite a down-warped (synclinal) sag in the lateral (longitudinal) ridge. The united streams would then pass over the lowest part of the col (at A, Fig. 4) into the canoe-shaped valley of the next (lower) series. There either one or the other of two courses would be followed. The united streams might be joined by two other streams draining in similar fashion the second valley (as at B, Fig. 4) before passing into a valley of the third series; or, as is more often the case in the Tararua area, the streams from one valley after passing into a lower valley, would be joined by a stream draining one of its ends and all turn in the same direction and flow for some distance in the second valley, before being joined by a tributary from its other end and turning into the third valley (as at C, Fig. 4) through the down-warped depression in the intervening low and broad lateral ridge. The course taken by the drainage in any part depended upon (1) the position of the lowest point in a valley bottom; (2) the relative position of the lowest col in the lateral bounding ridge; and (3) the relative position of the lowest point in the valley of the next (lower) series. These factors are diagrammatically depicted in Fig. 4, which gives a close representation of the general relationships of the reaches of the erratic rectangular pattern traced by all the principal Tararua river-courses. It may here be pointed out that the scale of the Tararua drainage-pattern is of far too great a magnitude to represent a structural pattern possible in such a terrain of
Fig. 4—Diagrammatic plan, with imaginary contour lines, of a flexured upland surface with initiation of main drainage lines—as on the Tararua Range.
1—1′. Longitudinal anticlinal axis forming main divide.
2—2′. Final culminating longitudinal anticlinal axis.
3—3′. Culminating transverse anticlinal axis.
4—4′. Other longitudinal anticlinal axes.
5—5′. Lesser transverse anticlinal axis.
6—6′. Highest series of canoe-shaped valleys (longitudinal synclinals).
7—7′. Middle series of canoe-shaped valleys (longitudinal synclinals).
8—8′. Lowest series of canoe-shaped valleys (longitudinal synclinals).
disrupted, thin-bedded strata. If adjustment to structure were indeed a fact, the pattern of necessity would be on a much smaller scale (compare the drainage-lines of the main Tararua area with those of Wellington Peninsula—Fig. 1).
A word may be added concerning the dissection and erosional development of the terrain. The initial wide diaclinal depressions (downwarps) in the longitudinal anticlinal axes have been converted into valleys of erosion by the rivers utilizing them. In these transverse (diaclinal) reaches vertical erosion has been extremely active, the river down-cutting to counter the slow upfolding athwart its course and thus retain an outlet. For this reason and on account of the absence of decomposed and disintegrated rock in them, the diaclinal reaches have not been widely opened, but have remained narrow and gorge-like. In the longitudinal valleys the case is different. Owing to their tectonic origin these were initially broad and open; the orogenic movements had little effect in stimulating vertical erosion within their limits, so that there vertical erosion by the main river and its small lateral tributaries were equally effective. Under the action of countless
high-gradient streams upon their flanks the initially broad tectonic ridges have been reduced to narrow watersheds buttressed by narrow, steeply sloping lateral spurs. By the headward erosion of these streams the original height of the ridge-crests has been diminshed. The erosion of the structural valley-bottoms by the main rivers, however, has greatly exceeded in vertical range the degrading of the structural ridge-tops, so the general superficial features of the deformed surface have not only been retained but also exaggerated. In a word, subaerial erosion has changed what, in its non-operation, would have been an elevated region of low relief into a less elevated one of high relief (see Fig. 5, a).
The Great Wairarapa Fault.
The major fracture, known along its extension in the southern part of the North Island as the Wairarapa Fault, has been an important concomitant in the second cycle of the orogenesis of the Tararua-Rimutaka Range. Its connection with any particular one of the major South Island faults is a matter of some difference of opinion. According to McKay(11) it is the continuation of the Kaikoura Fault of Marlborough. Park,(12) however, connects it with the Clarence Fault, on the grounds that the latter is marked by a surface trace fifty miles in length and by movements of recent date. The Awatere Fault, also, along which rents opened in 1848 and 1855, also intersects the Wellington area; hence exact correlation is rendered difficult. According to McKay's report and maps,(11) the Clarence Fault, if produced northward, would intersect Wellington City and cross the mountain-axis at the Rimutaka road-saddle; the Kaikoura Fault, on the other hand, if produced in the same direction, would run close to the eastern margin of the Rimutaka Range and coincide with the Wairarapa Fault.
We are here mainly concerned with the line and trend of the Wairarapa Fault proper. This fault for the first twenty miles from the shore of Palliser Bay coincides approximately with the eastern margin of the Rimutaka Range. The rents that opened during the earthquake of 1855, while not exactly coinciding with the older trace, occurred along approximately the same line. North of Featherston the 1855 active fault trace continues to skirt the hill-margin to a point a few miles north of Carterton. Beyond this it bears eastward and intersects Tertiary terrain to Mauriceville and beyond (see map, Fig. 1). This trend makes it evident that the 1855 fault trace was not a movement of the older Wairarapa Fault except perhaps in places at its southern end. The present writer here commits himself to the assertion (and also hopes to demonstrate by means of satisfactory evidence) that at or near Featherston the line of the older major fault and that of the 1855 surface rents diverge.
The older major fracture, the true Wairarapa Fault, changes its direction from N.E. by N. to a little north of N.N.E. and from the vicinity of Featherston turns into the Tararua Range and lies within the outer foothills along the eastern face of the Mitre-Holdsworth Ridge. The precipitous eastern face of this the highest ridge of the Tararuas is taken to be the modified scarp of the Wairarapa Fault, which was of the reversed or overthrust type, the superior altitude of the Mitre-Holdsworth Ridge and the asymmetrical form of the range as a whole being due to upthrust in an easterly direction on this fault-plane. This may seem purely topographic evidence, but other evidence is available.
In his report on the Waiohina and Tauherenikau valleys, Alex. McKay(13) states that in the lower gorge of the Waiohina (which lies on the suggested line of the Wairarapa Fault) the strata are three times repeated on a large scale. This indication of distributive faulting is supported by further, but more detailed, topographic evidence from the eastern slope of Mount Holdsworth from the trigonometrical station to the Mangaterera River. From the trigonometrical station to the timber-line, a distance (horizontal measurement) of about a mile, there is a comparatively easy slope of 780 ft. to the mile; from the timber-line to the mountain house, a distance of three-quarters of a mile, the slope is 2,200 ft. to the mile; below the mountain house a gentle slope dipping westward and terminating in a much steeper declivity dipping east, is two or three times repeated on the spur leading down to the Mangaterera (section, Fig. 5, b). The easy slope between the trig, and the timber-line is taken to represent the denuded crest of the fractured fold; the steep face above the mountain house, the modified scarp of the main fault; and below the mountain house, the steep faces separating gentler back-tilted slopes probably indicate the positions of subsidiary faults.
In a progress report on the geological survey of the Eketahuna Subdivision,(14) published in 1935, is included an up-to-date geological map showing the eastern side of the Tararua Range from the Manawatu Gorge to Masterton. South from the Gorge a major fault is shown along the margin of the range to the debouchure of the Mangahao River, and thence to the head of the Mangatainoka, and in the latter section it turns into the range itself.* The line of the fault referred to, if projected southward, would exactly match up with the line of the Wairarapa Fault as placed by the present writer along the eastern face of the Mitre-Holdsworth Ridge and recorded in field notes many years previously.
With this corroborative data it can perhaps reasonably be submitted that a fair case has been made for placing the line of the Wairarapa Fault, north of Featherston, within the range itself and along the eastern face of its loftiest upthrust ridge. Such a postulate would also dispose of the anomalous and puzzling eastward curvature and petering out of the 1855 fault trace—away from the mountain range with the southern portion of which it had an apparent connection.
The progress report referred to in the last paragraph but one gives further details bearing on the subject under discussion. Ongley refers to the Tararua Range as “an anticline rising from the alluvial plains of the west coast at an angle of 5° and dipping eastward to the lowland at 30°.” He also states that the grain of the Tertiary, Cretaceous and Jurassic terrains to the east of the Tararuas is parallel to the trend of that range. This implies a structural unity between basement and covering strata right across the island. Futhermore, at the base of the Tararua Range between Masterton and Eketahuna a “gridiron” of small, back-tilted greywacke blocks faulted longitudinally along their steeper eastern side occur; these fault-blocks as mapped trend parallel to the present writer's postulated line of the main Wairarapa Fault, and presumably represent distributive faulting subsidiary to it. This complex of faults is linearly north of the topographic and stratigraphic evidence of distributive faulting at Mount Holdsworth and the Waiohina Gorge.
No subsidiary faulting is known along the eastern sides of the Tararua ridges lying to the west of the Mitre-Holdsworth Ridge. Notable lines of truncated, faceted spurs occur in the position specified in the Mangahao Valley, the Ohau Valley, and elsewhere, but the presence of a river-cut rock-floor along the bases of such linear series of facets leads the present writer to prefer an
[Footnote] * South of this no detailed mapping has been carried out by the N.Z.G.S.
alternative explanation. Westward tilting of the mountain block as a whole (and this undoubtedly occurred during the later orogenies) would cause the drainage to impinge strongly on the western sides of the western valleys and there cut the conspicuous lines of faceted spurs [see Trans. N.Z. Inst., Vol. 43, Plate XVI (1911) and p. 503]. Linear series of similar truncated spurs also occur, in many diverse situations within the range where lateral corrasion is sufficient to account for their production.
Linear series of faceted spurs not related to the present drainage linés would provide better evidence of actual faulting. Such do occur in the Wellington Peninsula and on the south-western border of the Tararua Range (see map, Fig. 1), but have not been observed within the range itself
On the evidence, the deformation hypothesis herein outlined provides an adequate explanation of the form and origin of the major physiographic features of the Tararua-Rimutaka Range. It satisfactorily accounts for: the asymmetrical form of the range; the extended linearity of the main elements of the relief; the presence of major transverse ridges complementary to the longitudinal components; the strongly marked hydrographic centres: the rectangular or trellised drainage pattern and its order of magnitude; the contrasting diverse physiographic form of longitudinal and transverse river-valleys; the westward tilting of the mountain-block; local peculiarities of drainage-lines and valley-trends at or near the culmination of the range.
The range has the form of a geanticline (a superficial anticlinorium), the most elevated of a related series, the others only slightly emergent, or occurring at depth.
With this origin and structure the Tararua-Rimutaka mountain are takes its place in and gives further support to Macpherson's scheme of the general geological structure of New Zealand.
1. Macpherson, E. O., 1946. An Outline of Late Cretaceous and Tertiary Diastrophism in New Zealand. D.S.I.R. Memoir, No. 6, pp. 1–32.
2. Adkin, G. L., 1920. Examples of Readjustment of Drainage on the Tararua Western Foothills. Trans. N.Z. Inst., vol. 52, pp. 183–84.
3. Adkin, G. L., 1921. Porirua Harbour: a Study of its Shore-line and other Physiographic Features. Ibid., vol. 53, pp. 147–48.
4. Broadgate, F. K., 1916. The “Red Rocks” and Associated Beds of Wellington Peninsula. Ibid., vol. 48, p. 77.
5. Cotton, C. A., 1918. The Geomorphology of the Coastal District of South-western Wellington. Ibid., vol. 50, p. 13; see also vol. 44, p. 246, and vol. 46 p. 296.
6. Cotton, C. A., 1912. Notes on Wellington Physiography. Ibid., vol. 44, p. 246.
7. Mead, A. D., 1936. Notes on the Structure of the Tararua Range near Mangahao. N.Z. Journ. Sci. and Tech., vol. 18, p. 19.
8. Ongley, M., and Williamson, J. H., 1931. A Note on the North End of the Tararua Range. 25th An. Rep. (new series), N.Z.G.S., p. 55.
9. Seeley, H. G., 1895. The Earth in Past Ages, p. 19. Avebury, Lord, 1902. The Beauties of Nature, pp. 153–55 and diagrams pp. 117 and 154.
10. Ongley, M., 1935. Manawatu Gorge. N.Z. Journ. Sci. and Tech., vol. 16, p. 260.
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