Notes on the Geological Structure of New Zealand
[Read before the Wellington Branch, June 8, 1950; received by the Editor, June 17, 1950.]
Late Cretaceous and Tertiary Fold Movements
The Kaikoura Orogeny
The Importance of Faulting in Tertiary Earth Movements
Relations of Tertiary to Older Trends
The New Zealand Recurved Arc
The Post-Hokonui Orogeny
Similarity between South-west and North-west Parts oF South Island
Original Extent of Tertiary Strata Flanking the Alpine Fault
More Detailed Discussion of Selected Regions
Further Comments on the Relations of Newer to Older Trends
Geomorphology in Relation to Structural Studies
A general north-east trend imparted in a late Tertiary orogeny is dominant both in structural plan and in physiography. Another important trend with a north-west orientation is most clearly seen in Mesozoic strata, this trend being generally attributed to an early Cretaceous orogeny, although locally there has also been renewed movement along this trend during the Tertiary. Both structural trends were at one time attributed to an early Cretaceous age and on this attribution is based Suess's original concept of syntaxes in the North and South Islands.
It has been suggested that the two trends do not run together, but that there is a distinct swing from north-east to north-west trending folds giving an arc. However, Suess's idea of syntaxis (schaarung) describes the Tertiary fold plan quite as well as the concept of arcuate structure, for the latter appears to be largely a composite feature obtained by joining the north-east and north-west trends: the key to many arcuate plans may lie in “posthumous” folding. The validity of both concepts in tectonics depends on precise definition in terms of time. A certain parallelism of trends for consecutive fold movements, particularly during the Tertiary, is evident in many parts of New Zealand, but some emphasis is placed here on folds and faults which cut at an abrupt angle across the trend of a preceding movement. Such later folds appear often to follow the axes of much more ancient folding.
The late Tertiary faults, usually with remarkably straight traces, are of great physiographic and structural importance: many show throws exceeding 3,000 feet and a throw of 10,000 feet has recently been suggested for the great Alpine Fault. Transcurrent movement
has recently been recognised in New Zealand by Wellman and Cotton, and the former has advanced in a paper as yet unpublished the hypothesis of great transcurrent movement along the Alpine Fault.
It is probably true that the strata of New Zealand are more difficult to decipher than those of western Europe and many other classic regions. The Lower Palaeozoic strata have been examined only in part and are difficult of access. Professor Benson has recently found Cambrian strata, and, with co-workers, has described a good graptolitic Ordovician succession, but no Silurian has yet been positively identified. The Devonian rocks are fossiliferous, but the relations to over- and underlying rocks are quite obscure. The Upper Palaeozoic and Mesozoic strata of New Zealand include thousands of feet of argillites and especially arkoses, commonly of greywacke facies, and their subdivision has proved difficult. Of these, the Carboniferous (if any) and Permian strata have not yet been separated, and, indeed, in some places are difficult to separate from the Trias. The Triassic and Jurassic rocks form a comprehensive and thick group, in places at least 28,000 feet thick, and sometimes grouped as the Hokonui System. All these earlier strata are distinctly more indurated, sheared, and folded than later beds, so that it is commonly possible to map the older beds as “undermass” and the later beds as “cover.” Indeed, of recent years much of the country, where Late Cretaceous and Tertiary stratigraphy and structures have presented immediate problems, has been so mapped, with consequent dearth of information concerning the undermass. Certain rocks, attributed to the Aptian and containing poor faunas, would seem, in degree of induration and diastrophic position relative to major unconformities, also to belong to the same comprehensive basement, including Triassic and Jurassic strata, and they can rarely be clearly separated from the earlier Mesozoic strata.
In many places there is passage from the upper Cretaceous to Tertiary and these two systems, in places of very monotonous lithology, and reaching locally total thicknesses of 30,000 feet, show considerable facies variations and local discordances.
In New Zealand any geologist who attempted on purely local evidence a division of strata into Primary, Secondary and Tertiary would be inclined to define his Primary as extending from the Devonian downwards, his Secondary as including the Permian and perhaps the lower Cretaceous, and his new Tertiary system would perforce include some upper Cretaceous rocks and perhaps even some Pleistocene. Moreover, the stratigraphical limits between the groups belonging to each era would be occupied by question marks, representing the times of our major orogenies which might vary slightly in age from place to place. There are no known strata bridging these time gaps within the country.
Although the lower Palaeozoic strata and their relations to some of the metamorphic rocks remain obscure, two problems have engrossed the New Zealand stratigrapher because of the great thicknesses of the formations involved. These problems are the subdivision
of the “greywackes”, particularly those of Permian* and Triassic age, and the relations of the Cretaceous and Tertiary to each other and to the older basement rocks The separation of Permian and Triassic greywackes advanced by McKay and other members of the Geological Survey has stood the test of Trechmann's and Arber's critical examinations and their units are being gradually extended by field workers at the moment. The separation of Triassic and Jurassic greywackes first attempted by Cox and McKay (1878), is also being continued, but with more difficulty.
The Cretaceous and Tertiary sequences have been shown in recent years to include strata of thick continuous geosynclinal sedimentation as well as beds with non-sequences, minor unconformities, and erosion breaks. This record for Cretaceous and Tertiary time has only been made possible by the patient palaeontological researches of Finlay and Marwick, based on thousands of specimens from many field collections, and they have so far described only in summary form the stages which they have separated and the formations corresponding to these stages (1947). The work of these two authors is the outstanding contribution to New Zealand stratigraphy during the last forty years. Without close palaeontological, and particularly foraminiferal zoning, the Cretaceous and Tertiary lithological record in New Zealand is bewildering and even misleading, for there are many beds of very different ages and similar lithology and many bands which mark diachronic boundaries. Since the dominant structural plan of New Zealand is late Tertiary in age, this palaeontological work is all-important. An examination of the recently published Outline of New Zealand Geology together with the earlier papers of Finlay and Marwick reveals a scheme of Tertiary stages probably as closely spaced as any attempted elsewhere. It is not surprising that there seem to be many minor fold movements throughout the Tertiary—a history similar to that of other Tertiary basins when studied with closely spaced stages. These movements on the whole can be regarded as reaching in the late Pliocene a marked climax generally known as the Kaikoura Orogeny.
For orogenies earlier than Tertiary, information is more meagre and as yet only two other major orogenies are separable. The great fold movement, probably of early Cretaceous age, known as the post-Hokonui Orogeny, has already been mentioned, and an important orogeny must have occurred in Devonian or Early Carboniferous times imparting a meridional strike to the lower Palaeozoic rocks. The table of orogenies, which includes other inferred movements in addition to these must be regarded as more tentative, and detailed stratigraphic evidence will not be cited here.
A lag in the publication of stratigraphical and structural studies seriously handicaps an attempt to generalize and to fill in details of fold movements. Much of the Geological Survey work (and all the work of oil geologists) remains unpublished, and although some of
[Footnote] * The recent discovery and identification of Permian fusulinids in the North Auckland district by officers of the Geological Survey is an important event. These organisms have not previously been recorded in New Zealand and Permian strata had not been mapped in the North Island.
this material is current knowledge among members of the Survey, other geologists find difficulty in placing the shorter papers of the last few years in a general structural framework. The Survey workers, engrossed in regional mapping, revealing more and more details, have hesitated to write general statements on structure likely to be soon out of date and thus would seem not to have done themselves full justice. The appearance of the new geological map represents a milestone (the useful black and white maps of Morgan  had already become out of date), and the Outline of the Geology of New Zealand, which accompanies the new map, is the most useful existing synopsis of New Zealand stratigraphy. Professor Benson, in 1921, 1922, produced admirable summaries of New Zealand geology which will long remain the stand-by of all those unfamiliar with the detailed literature. Cotton's papers of 1916 and 1925 shed further light on structure, particularly as revealed by land forms. Henderson summarised the fault pattern of New Zealand and gave a useful synopsis of the late Cretaceous and Tertiary strata of New Zealand (before Finlay-and-Marwick stages). Benson's papers of 1923 and 1924 relate the stratigraphy and structure of New Zealand to those of Australasia as a whole. Apart from these few papers hardly a single general statement on New Zealand geology appeared between Benson's papers and Macpherson's last memoir, which is a lively discussion of the author's views based on wide field experience and data collected by himself and other geologists. Confined to the Cretaceous and Tertiary movements, his work presents a synthesis of structure and covers in brief outline the relations of Cretaceous and Tertiary sedimentation to structural plan. Indeed, it renders much of this paper redundant, except that Macpherson has been more concerned with synthesizing than with describing general knowledge already familiar to the New Zealand geologist. His memoir is quoted extensively in this paper because it serves as a convenient starting point on which to examine theories concerning the structure of the country.
The next few pages comprise a running commentary on the chief views concerning structure that have been recently advanced, and omit many important references already quoted by Benson and Macpherson. A later section gives some further details for parts of the country and introduces some of the writer's own views, which can claim little originality, but maintain that further theories on the nature of late Cretaceous and Tertiary fold patterns require a closer attention to the structure formed in older orogenies.
Acknowledgments and Note on Sections
These notes were first written as a statement on recent work on the structure of New Zealand intended to help students and visitors to the country. Most of the text was completed when the new coloured geological map of New Zealand appeared (late 1948). With the object of making the paper more useful to the New Zealand student the writer has added sections based on published and unpublished information and the new geological maps formed the basis of large parts of the sections. In devising these sections the author is grateful to the following students who assisted him as follows:—Section DE,
FG, JK—Mr. P. Vella; Section NO—Mr. T. Grant-Taylor; some of Section PQ—Mr. D. McBeath. The information of unpublished theses by R. A. Couper and P. Vella is contained in Section JK.
New Zealand stage names have been retained on these sections, since several do not correspond exactly to the European divisions. The approximate correspondence is given in the legend and text.
The help of colleagues in the Geological Survey who generously offered oral information on details is acknowledged in the text.
Parts of all the sections are likely to be wrong even at the time of publication, especially those portions which were entirely compiled from the small scale geological map without access to unpublished reports. Nevertheless, it seems desirable to present them with their imperfections, for detailed studies within New Zealand have advanced far ahead of simple general statements, leaving a difficult gap for the student or stranger to bridge. To aim at great precision on various points would involve much more inquiry with consequent delay. The inspiration for these sketches lies partly in the early sections of workers on the old Geological Survey, particularly Hector. The table of orogenies is an attempt to outline in sweeping fashion the stratigraphic evidence bearing on fold movements. Professors Cotton and Kuenen have read the text critically and the writer is indebted to them for useful suggestions. Generous help, with pertinent criticism, has been proffered by Professor Benson, to whom the writer is deeply grateful. A number of his suggested improvements and additions to the bibliography have been incorporated in the text.
Whilst the information of most writers quoted in the text has contributed to the cross-sections, the most useful papers are those including sections by the following authors: Bell, Benson, Couper, Cox, Ferrar, Grange, Hector. Henderson, Hutton, McKay, Marwick, Macpherson, Ongley, Park, Vella, Williams, Willett. (Some of these await publication.)
The main structural outline of New Zealand shown on the early maps of Hector and Hutton has been little altered by subsequent work, and the two coloured geological maps now published by the Survey bring out even more forcibly the dominant trends established by the pioneers. The impression of a general north-east trend shown by the coast line and mountain chains is accentuated by the great axial range of Mesozoic strata, largely Triassic and Jurassic and including some Permian (the primary fold, p. 8, of Macpherson, 1946) that extends in the North Island from the Bay of Plenty to Cook Strait. In the South Island an axial chain which also consists of Jurassic, Triassic and probably late Palaeozoic rocks, continues from the south of Nelson Province to Mount Aspiring. This north-east trend is indeed further accentuated on the new map by the interpolation of the great Alpine Fault along the western slope of the Southern Alps, a fault which has long been known in part but whose full extent has been best demonstrated by Wellman and Willett. East of the Alpine Fault the ranges consist only of greywacke and crystalline schists, flanked farther east by Cretaceous and Tertiary sediments. West of the fault lie granitic batholiths, schists which have suffered contact as well as regional metamorphism (Benson, 1928), rocks of
age as early as Cambrian, as well as Mesozoic and Tertiary rocks—a much more varied and complicated group of rocks. This fault, then, is a radical line in the New Zealand structural plan which requires fuller discussion later. It recalls the Median Line of Japan in many respects (Yehara).
The structure shown by the strata within the axial chain, both in the South and North Islands, is little known, but the chain can be accepted as constituting a much-faulted complex anticline in the dominant late Tertiary structural pattern. An ancient erosion surface is carved on the greywacke at many localities, and at the Manawatu Gorge (North Island) this surface and its covering of Pliocene beds form a broad anticline, striking north-east and broken by a great fault on its eastern edge (Ongley, 1935). Elsewhere in the North Island the structure of this axial chain presumably showns complications based on a similar pattern. There is marked axial pitch at the Manawatu Gorge and evidence of similar axial pitch on all the erosion surfaces of the greywacke “highs” which appear below the Tertiary in the country flanking the axial range to the east. Macpherson's map shows for the North Island several anticlinal ridges with greywacke cores whose general structure can be considered as rather similar to that of the axial chain.
Late Cretaceous and Tertiary Fold-movements
The Kaikoura Orogeny
The structural interpretations generally accepted to-day differ in many essentials from those of Hector and Hutton, although the older and most recent maps appear to be similar. Hector and Hutton considered the main structural outline to be formed by pre-Tertiary fold and fault movements, but later workers have followed McKay (1884, 1892) in recognising clear evidence that the structural and topographic pattern is dominated by important late Tertiary movements. Cotton (1916, 1925), who has concentrated on the physiographic pattern resulting from these movements, appears to have been the first to realise fully the correctness of McKay's interpretations and grouped the late Tertiary movements as the Kaikoura Orogeny, giving its age as very roughly late Tertiary or post-Tertiary. Furthermore, in calling attention (1916) to stripped fossil erosion surfaces at many places separating Mesozoic and Tertiary strata, he provided a useful clue to the structural interpretation of late Tertiary folds as well as stratigraphy. The late age of many of the faults and folds having been generally conceded by all field workers, attention in recent years has been focused on the stratigraphical evidence in the shape of unconformities, erosion breaks, and local thinnings of sections, all indicative of differential movements throughout Cretaceous and Tertiary times. Macpherson sketches for the Cretaceous and Tertiary an eastern and a western geosyncline, separated by a median land mass occupying roughly the position of the present axial chain. These geosynclines are drawn as continuing along the edges of both North and South Islands. He describes the younger beds as generally tending to overlap the older ones along the geosynclinal margins on the site of the axial chain with a regression of the sea in the late Cretaceous. The evidence is
insufficient to indicate when and to what degree the axial ridge was overstepped by sediments, but for the North Island at any rate this reconstruction seems to be acceptable. It also seems likely that there was intermittent uprise of this axial chain. Such uprise may have commenced very early—probably in the Cretaceous, but certainly not later than Pliocene time, for at the Manawatu Gorge a condensed sequence of Pliocene clastics only 2,000 feet thick rests on greywacke, whilst only 10 miles to the east the Pliocene cover, representing the same time interval, has a thickness of approximately 7,000 feet.* Macpherson extends the idea of intermittent movement of anticlinal ridges to folds in the adjoining geosynclinal belts and to the marginal zones of deformation in the geosynclines. Thus he insists on the development of structural ridges which “did not attain their present amplitude (possibly 7,000–8,000 feet above adjoining synclinal troughs) in one or two folding movements, but grew by recurrent orogenic impulses.” He cites later rocks flanking the greywacke cores of such ridges as commonly marked by discordances, etc., usually absent in the adjoining troughs, where the Cretaceous and Tertiary sediments are of much greater thickness. Whilst admitting that certain anticlines may show such a continuous development, one may query whether this conception has not been extended too indiscriminately. There is evidence to suggest that certain troughs of deep sedimentation were later reversed to become conspicuous anticlines, and this must be a subject for future discussion, as must also the concept of migrating troughs. Did certain nodal axes persist as structural “highs” whilst earth waves fluctuated over neighbouring regions?
Present views, therefore, return in a minor degree to the early ideas of Hector and Hutton, but visualise a continuity in tectonic development, a blending between the views of these two authors and those of McKay, in which compromise the latter's emphasis on post-Pliocene movement is accepted. This is shown in Macpherson's sketches of a diastrophic cycle which from late Cretaceous time was marked by orogenies of lesser moment and mounting intensity culminating in the Kaikoura movements of late Pliocene (post-Castlecliffian) age. The latter, for Macpherson, “may be only an arbitrary end point, for the tilted, warped and stepped terraces at various levels and also the regional seismicity indicate that the New Zealand recurved arc still grows.” Compared with the precursor orogenies, however, the Kaikoura movements do seem to indicate great dislocations concentrated in a short time, and their importance is not entirely dependent on the inference that they mark roughly the beginning of the “Anthropozoic.” The throws on faults breaking late Pliocene strata in many places exceed 5,000 feet, and we must regard the Kaikoura as an earth-storm greater than any of the earlier movements during the Tertiary period. But the principal earth movements called Kaikoura in different localities are not necessarily contemporaneous, for it is known that the most marked movements of this earth-storm vary in date by the magnitude of a stage or two from place to place (Marwick, 1946, p. 11).
Recently in a posthumous paper Macpherson (1948) cited the evidence for an upper Senonian transgresion following post-Albian
[Footnote] * Only Opoitian, Waitotaran and Nukumaruan Stages are included in these figures. The rocks of Castlecliff age have been largely eroded recently.
movements. Locally the base of the upper Senonian (Mangatu formation) is marked by giant boulder beds. In other places it is difficult to separate upper and lower Senonian. The evidence is too complicated to be cited here.
The Importance of Faulting in Tertiary Earth-movements
Early workers in New Zealand were quick to recognise great faults, and Cotton has stressed their existence in elaborating his views on geomorphology, but he has also described folds accompanying the faults (p. 60, 1916b, p. 91, 1925). From the structural as well as the physiographic aspect, the faults are certainly remarkable, but the geologist new to the country may find the nomenclature of some local geological papers between the years, say, 1920 and 1940, a little misleading. The structure of a tract of country may be entirely described in terms of “blocks”, more or less “tilted”, but the stranger, perhaps with a bias to deciphering structure in terms of the growing anticline, comes back from the same field with a mental picture of anticlines more or less pitching or closing, broken by immense faults along one limb. Also, many of the mapped “faults” can, on closer examination, be replaced by flexures (e.g. Wellman, pages 193–194, 1946). The term “block” may be convenient, and may even be correct (although unhappy in its implications): the term “tilt”, however, is actually misleading since it cannot be construed to cover the observed swing in strike of Tertiary strata sympathetic with the swing in strike of the underlying fossil erosion plane near the extremities of a greywacke “high”. The Saxonian nature of the folding and, indeed, the general significance of folds in the New Zealand tectonic pattern was first stressed by Benson (1930) and is well illustrated in his maps of Eastern Otago (1941). To-day the field worker tends to see his faults in most places as part of a general fold pattern, and an elongated dome or anticline can be visualised as one of the dominant “ancestral” forms for many of the fault-bounded structural “highs”. The descriptions of to-day are more akin to those of the earlier geological surveyors in containing a liberal sprinkling of anticlines and synclines. The faults are usually arranged in an echelon pattern and some care is necessary in reading small-scale maps like the Geological Survey map of 1948 because the trends of faults are usually slightly oblique to the general trend of the great axial chain of greywackes shown on this map.
Relations of Tertiary to Older Trends
It is now generally agreed that the trend between north-east and north-north-east results from the late Tertiary Kaikoura Orogeny and its precursor movements, for it is the dominant strike in most Tertiary strata. Moreover, it many fields where this fold trend is dominant the belts of facies in Cretaceous and Tertiary strata are roughly parallel, suggesting sedimentation in a framework of similar trend. In certain fields the evidence even points to a parallel strike in the greywacke strata, so that hypotheses can be advanced postulating Mesozoic and Tertiary geosynclines geographically coincident and with facies aligned parallel to the succeeding Kaikoura folds. Such a concept underlies parts of Macpherson's maps. But it would be wrong to extend this picture involving continuity in strike to the whole country,
for in certain places the strikes of Triassic and Jurassic greywacke are different from those of the Cretaceous and Tertiary strata. In such places these more ancient rocks show strong isoclinal folds much broken by faults and thrusts. The strikes of the actual fold axes are difficult to determine, for pitch is often considerable, but it is clear that a regional north-west strike of axes is in many of these localities common in the Mesozoic and sometimes older greywacke, although nearby Tertiary beds may show a marked north-east strike. Thus, it has been observed occasionally that where Mesozoic greywacke abuts against faults bounding Tertiary strata, the highly sheared greywacke beds strike parallel to the fault lines, but median parts of the same masses of greywacke may show regular strikes with a roughly northwestern trend. Further data on strikes within the pre-Cretaceous rocks will be presented later.
At a first glance one is apt to cavil at Macpherson's map of late Cretaceous and Tertiary structures, because, while in some places the trend lines are based only on the Kaikoura folds and faults bounding the greywacke cores, in other places the strikes seem to be based on measurements within the more ancient strata. Such criticism is not entirely just, however, for in many places he has backed up his acceptance of the latter strikes by indicating similar trends in the neighbouring Tertiary or Cretaceous strata. Macpherson was fully aware of complications. He wrote (p. 9):
“They are late Cretaceous and Tertiary basement folds developed on a subdued mature-land (new term: Willis, 1928) of lower and middle Mesozoic rocks. Their axial trends may in some cases be inherited from an earlier diastrophic cycle, but we have so little knowledge of the internal structure of the basement folds that it is preferable to regard the surface cut on the basement rocks as a datum of reference when studying the late Cretaceous and Tertiary folds of the covering beds that envelop them. Observations show that the foldings of the diastrophic cycle concerned here arched the basement rocks along trends that diverged from the trends of an older diastrophic cycle.”
And yet, in the structure of the South Island, and particularly Southland (p. 10) the strikes in ancient rocks seem to form the most important part of his argument in favour of an arcuate structure.
The New Zealand Recurved Arc
Indeed, the main thesis of Macpherson's paper is that New Zealand constitutes a recurved arc formed in the course of the late Cretaceous and particularly late Tertiary diastrophism. He rejects the idea of Marshall (1911) that the axial chain continues on a north-east trend towards the Tonga and Kermadec Islands, and in doing so he renders unnecessary Suess's idea of a syntaxis. By means of this syntaxis Suess joined the north-east trending fold system and the north-west trending chain of the North Auckland Peninsula somewhere in the Rotorua volcanic region, whose origin could perhaps be regarded as due to this crucial junction. Macpherson swings the trend of the axial range to assume a northerly direction near the Bay of Plenty, and considers that this is coincidental with a gradual regional swing to a north-west trend. Such gradual swing in strike is well seen on the West Coast of the North Island between Mokau and the Waikato River, and the north-north-west trend of faults bounding the Hauraki graben also fits the same picture. Macpherson cites also a marked north-west
strike of Cretaceous and Tertiary strata at the north-east end of the North Island: this we must examine later. He also describes the curious Taitai formation of Aptian age as klippen, the remnants of a nappe which, in places, he interprets as thrust forty miles eastward in Senonian time, its roots being inferred to lie in the axial range. Within this arc are the centres of Tertiary and Recent volcanicity, and the pattern is in agreement with that suggested by Hobbs.
At the south end of the South Island Macpherson sees a similar arc, now bent to the south-east, and with the areas of Tertiary volcanicity concentrated again on the inner margin, namely at Dunedin and Banks Peninsula, near Christchurch. Ingeniously he visualises the boundary between the north-west striking Permian of the Hokonui syncline and the Otago schists as a great reversed fault (the Permian and Trias beds being slightly overturned near Clinton) and continues towards the north-west this great fault, on or near which peridotites and serpentinites are aligned. “It is likely that movements along this thrust have been recurrent from late Palaeozoic, possibly before, and probably up to Pliocene times” (p. 11). The trace of the thrust swings round to run into the great Alpine Fault, and we are led to infer from its curvature that the thrust may in places lie quite flat. Thus, he imagines “the ‘rolling up’ of 13,000 feet of Jurassic and Triassic and a considerable thickness of late Palaeozoic sediment along the western and south-western schist contact”. He traces sympathetic swings in strike of the rocks (mostly Mesozoic) to the south and southwest of the Hokonui syncline largely formed by these movements, and also sympathetic swings in the trend of the Cretaceous and Tertiary strata to the north-east of the Otago schists. These schists form a great dome-like mass, regarded by some as a simple anticline, but by Benson as possibly a packet of recumbent sheet folds.
The Otago schists are considered by many geologists who know them well to be quite distinct from the schists which lie west of the great Alpine Fault: there is general agreement that the strike of the Otago schists swings in the neighbourhood of Mount Aspiring from a north-west orientation to adopt a north-east orientation, the Arahura series which forms the Southern Alps being a continuation of the Otago schists. Turner and Hutton consider the Otago schists to be largely formed by the metamorphism of arkoses, often greywackes, and passage into these rock types has been observed (C. O. Hutton, 1940). The chlorite zone has been subdivided into four sub-zones by these two authors and the highest grade of metamorphism observed in the Otago schists proper does not pass the biotite zone. Turner (1935) has noted, too, that they pass into greywacke belonging to the comprehensive Te Anau series, whose precise age is as yet unknown, but is in part Permian. That the Otago schists are certainly in part Palaeozoic is indicated by Ongley's discovery at Clinton (1939), twenty miles west of Kaitangata, of fossiliferous conglomerate bearing pebbles derived from the Tuapeka series (part of the Otago schist mass) along with Zaphrentids, and a Permian age is at the moment assigned to these later beds. It is conceivable, however, that the Otago schists farther west and their formational continuation, the Arahura series, represent a much more comprehensive group, perhaps including much of the Lower Palaeozoic. Any further evidence bearing on the
total age of these metamorphosed sediments would be important. At the north-east corner of the South Island the rather similar Marlborough schists, whose petrology has been little described (Turner, in Henderson, 1935), also lie near the Alpine Fault and east of Trias-Jura rocks which Wellman has in discussion described as being remarkably similar to those of the Hokonui syncline. In tectonic position the Marlborough schists are perhaps analogous to the Otago schists.
After reading Macpherson's interpretation of the tectonics of the South Island, a comparison immediately springs to mind: the Otago schists and the Arahura series are the “schistes lustrées” of New Zealand, as Wilckens has already (1917) suggested. Macpherson condenses in a footnote concerning the Alpine Fault (p. 11) the following:
“This frontal upthrust trends north-east along the western flank of the Southern Alps. Morgan (1908, 1910) described its significant role in the growth of the Alps. Gregory (1908) suggested that an old north-west-trending range (the Palaeozoic geosyncline of this account) had been overwhelmed by the younger north-east-trending Alps. Henderson (1937) named this fault the “Alpine.” Hobbs (1944) points out that the Alpine chain has overridden an ancient coign situated to the westward. In the present account, the Alpine Fault is identified with the southern tectonic contact of the Otago schists (Maps 1 and 2), and is here regarded as a later structural feature of the major tectonic. It defines a great arcuate thrust plane that truncates late Tertiary folds with Palaeozoic cores (lower Ordovician and upper Silurian) along its north-east extension that shows much late overriding. On this extension late Palaeozoic and middle Mesozoic groups are on the upthrow side. Along what is considered its south-east extension in South Otago, late Palaeozoic, lower and middle Mesozoic (Hokonui Series), late Cretaceous terrestrial sediments, and middle Tertiary marine beds are involved on the downthrow side. A similar frontal fault defines the eastern limit of the primary or borderland fold in the North Island segment. The directions of upthrusting, however, are opposed, indicating opposed tangential forces originating within the concave regions of the recurved arc. The significant fault movements on these frontal upthrusts are late Pliocene and probably post-Castlecliff.”
We have now a picture of great thrusts, presented by Hobbs, and even a foreland obstacle. But in this connection there is one irreconcilable feature: the Alpine Fault has been mapped in many places, often with high country situated on each side of it, and its trace appears to be remarkably straight. There are none of the sinuosities we might expect on the margin of a thrust plane and no klippes. All the available evidence suggests that the Alpine Fault in so far as concerns its present surface trace is not a thrust according to the Alpine worker's usage; it cannot be compared with the Moine Thrust. The great thrust of Otago, postulated by Macpherson, seems to have passed into a reverse fault, in places remarkably near vertical, and of prodigious throw. The possibility of the Southern Alps being formed by a great overthrust cannot be ruled out entirely if such were imagined to have occurred in early or middle Tertiary time. One can conceive of later vertical faulting superimposed over an old thrust pattern and followed by erosion, but evidence to support such a view is insufficient. Indeed, the available evidence is against direct comparisons with Alpine tectonic patterns.
Henderson (1929b) develops Bailey Willis's idea of great ramp-like shear planes, and traces throughout New Zealand a pattern of faults with very arcuate plan. Whilst few dispute the existence of
great late-Tertiary reverse faults with, in places, minor true “thrusts”, the symmetrical arcuate traces which he ascribes to most of the faults are not likely to be acceptable to many field workers. His statement that “many of the blocks are highest near the central part of the fault” can be explained by the concept of the elongated dome broken on one limb by a large fault. The Alpine Fault does not appear to be resolvable into a set of arcs as his map suggests, and many of the faults in Marlborough and the North Island also appear to be remarkably straight. Professor Cotton, who has viewed some of these traces from the air, confirms this view, as does also Mr. A. Prichard, of the Public Works Department, an airman with an observant eye for earth features.
The post-Hokonui Orogeny
We have now summarised the chief points in Macpherson's paper, drawing particular attention to the curving of the arcs which, according to Macpherson, brings the strike of late Tertiary folds into approximate coincidence with the strikes shown in many pre-Cretaceous rocks. A brief resumé of what is known of earlier fold trends is now necessary, but the assignment of particular structures to specific earlier orogenies must be accompanied by reservations, for the Kaikoura movements profoundly affected the whole country, and it is difficult to assess how much of this late Tertiary folding or rotational component must be subtracted from trends visible in older rocks. It was early realised that the greywackes, whether of Palaeozoic or Mesozoic age, are usually more folded and sheared than the overlying upper Cretaceous and Tertiary sediments, and a very late Jurassic or early Cretaceous age is usually assigned to the fold movements affecting these “greywackes”. In the Ruahine Range the isoclinal folding and shearing of the greywacke, probably Mesozoic sediments and perhaps in part Palaeozoic, are in marked contrast with the smoother folds of greater wavelength broken by great faults which alone affect the Tertiary cover. The strike of the former folds, too, is quite commonly different from that of the later Tertiary folds. Morgan (Bulletin 6, pp. 36–37) recognised a dominant north-west trend in certain greywacke beds west of the Alpine Fault. These beds belong to the Greenland series, considered to be late Palaeozoic [Carboniferous (?)] by Morgan, and consist of greywacke and argillite with minor hornfelses and some schists, rocks which show the effect of contact rather than regional metamorphism. The north-west strike oblique to the strike of the great Alpine Fault and that of the Arahura strata immediately east of the fault aroused Morgan's interest.
“The rocks of the Greenland series are thrown into a number of well-developed folds. The strike direction, a few exceptional cases omitted, varies from somewhat west of north (349°) to north of west (280°) within a few degrees of a bearing of 300°. The dip is usually about 60° to 70°, but may be as low as 30°, and sometimes reaches 90°. Except that they are somewhat disturbed by the granite intrusions, and considerably affected by faulting in certain areas, the folds, compared with those of the Arahura, are wonderfully regular and free from complication.” (Morgan, 1908, p. 97).*
[Footnote] * See also p. 36, Bull. 6, where Morgan discussed the possibility of these rocks having had an original N.–S. strike, since rotated by later folding along N.E. (?) trend.
The map of Totara Survey District in the Bulletin referred to is particularly instructive in showing the abrupt change of strike at the Alpine Fault. The Greenland series extends as far as Milford Sound, according to Wellman and Willett, who also insist that the Alpine Fault separates two distinct formations, although admitting that the Greenland formation is schistose in places. The general north-west strike of the Greenland rocks is also noted by these authors. Morgan thought that this north-west trend was visible even in part of the Arahura series on going eastward from the north-east-striking Alpine Fault (p. 34, 78, loc. cit.) into strata less disturbed by the late Cretaceous and Tertiary movements. Morgan also considered the Alpine Fault to be formed during an immediately post-Hokonui (early Cretaceous) orogeny, but if active then, it has certainly been much resuscitated by late Tertiary movement. East of the Alpine Fault a north-west trend is, of course, dominant in the Otago schists and the Trias-Jura greywackes of Southland. Macpherson's interpretation suggests that this fold trend is largely Tertiary and, although he does in passing note that this late Tertiary north-west trend is partly inherited from an older trend (Macpherson, 1946, p. 9), he nowhere discusses the idea of renewed folding as particularly pertinent to his recurved arc structure. Park (1921) uses the name Hokonui for these early fold movements, affecting Permian, Triassic and Jurassic greywacke, for this trend is most clearly seen in the rocks of the Hokonui syncline. Others have applied the better name post-Hokonui since the comprehensive Trias-Jura has been called the Hokonui System.
The surface separating Mesozoic greywacke and Cretaceous-Tertiary is commonly smooth and cuts across the folds of the post-Hokonui Orogeny. This smooth erosion plane, like the post-Hokonui fold movements, must also be of early Cretaceous age and is regarded as a fossil peneplain, discussed by Cotton at some length. This surface is overlain in different places by rocks of age varying from Upper Cretaceous to Tertiary according to the local diastrophic history, and warping and dislocations of this surface are of outstanding importance in deducing the palaeogeography and tectonics of the Cretaceous and Tertiary periods. By the relation of overlying rocks to this peneplain, we can often determine how much of the north-west strike shown in the older strata is really ancient.
Unfortunately, strikes are very imperfectly known for large areas of Mesozoic greywacke and the full extent of strata showing Morgan's dominant north-west trend is not yet known, but the Otago schists and the Trias-Jura in parts of Southland show this trend very markedly. Morgan's observation that the north-west strike did appear in the Arahura Series in places some distance east from the Alpine Fault, and Park's definition of the Hokonui Orogeny, both imply that the Greenland Series and the beds with a dominant north-west strike east of the Alpine Fault, such as the Otago schists and the Trias-Jura greywackes of the Hokonui syncline, may have been folded in the same post-Hokonui Orogeny. The north-west folding of the Greenland Series in this paper is very tentatively regarded as formed by the post-Hokonui Orogeny, so that the writer considers the folds of this early orogeny to have been later cut by faults and folds of the great late
Tertiary Kaikoura Orogeny with north-east trend, and particularly by the Alpine Fault. This interpretation has little good evidence to support it, except that of all the older pre-Tertiary rocks west of the Alpine Fault only the Greenland Series shows a dominant north-west trend. Alternatively, the Greenland Series may have been folded at a different earlier period.
It may be well here to anticipate by giving a view developed further in the present paper. The likelihood of renewal of movement along old trends has been discussed by several New Zealand writers, particularly Benson (1941). Such renewed folding is here regarded as the key to the north-west striking portions of Macpherson's recurved arc ascribed largely to late Cretaceous and Tertiary diastrophism. These are likely to be resuscitated trends first stamped on the rocks in the post-Hokonui Orogeny or even earlier. Although not all the folds of one period of orogeny will show a particular trend exclusively, it is nevertheless likely that the trend will be dominant. It is probably significant that in those areas where the greywacke does not strike north-west, the regional strike in these older rocks is north or north-east and commonly parallel to that of the adjacent Cretaceous and/or Tertiary strata. The Nelson region and the west coast of the North Island require further discussion later in so far as the more ancient and Tertiary strikes are there parallel.
An attempt to disentangle orogenies even earlier than the Hokonui was made by Hutton (1900, p. 166), who postulated a Mid-Devonian folding with a north-east trend. Park (1921) revived this conception in a list of orogenies of doubtful value.
There is a great gap in our knowledge between strata containing Devonian fossils (found at Baton River and Reefton) and the Permo-Carboniferous fossiliferous strata of Nelson. The Devonian age and the north-east strike inferred for the folding were probably based on McKay's observation (1879, pp. 125–126) that a north-east strike appears in the Baton River beds, since shown by Shirley to be of Lower Devonian age.* It is possible that most of the north-east strikes recorded in these beds are due to Tertiary faults and folding. The position of these Devonian beds may be similar to the Lower Devonian beds at Reefton (Allan, 1935) described by Henderson (1917, pp. 74–77) as being intensely faulted and sheared with accompanying Miocene beds. Discarding strikes in the obscure Devonian outcrops, some more consistent results can be obtained in the north-west Nelson district from the Lower Palaeozoic strata, in places fossiliferous. In the Mount Arthur beds, in part Ordovician, and certainly all Lower Palaeozoic, a general northerly trend was recorded by McKay (1879, p. 125), and the Ordovician rocks of the original Aorere Series in Collingwood Subdivision, N.W. Nelson (Ongley and Macpherson, 1923) show a similar strike. In the so-called “Aorere” Series of Reefton, for which Macpherson and Gage suggest a Devonian age
[Footnote] * Presumably the ancient nature of this north-east strike is inferred from a section by McKay (1879) from Mount Peel to the Baton River showing the Devonian, very much folded, overlain by Tertiary also striking north-east but dipping less steeply.
(1937), Henderson considers that the original trend of plications was north-north-east, and in those parts of the Motueka subdivision (N.W. Nelson) furthest from Tertiary infaulted blocks a meridional trend appears to be prominent in folds affecting the lower Palaeozoic rocks. (Henderson and Grange, Motueka Subdivision, 1926, p. 4.) Again, in Murchison Subdivision, south of Motueka Subdivision, Fyfe (1928) finds that a meridional strike is common in the lower Palaeozoic and in Parapara, N.W. Nelson, Bell's (1907) mapping also indicates a marked northerly and north-north-westerly strike. The divergence of this, presumably the most ancient known trend, from the north-east trend resulting from Tertiary movements comes out clearly in the new Survey map of the South Island. We can temporarily assign this approximately northerly trend to some Palaeozoic orogeny, perhaps Devonian, as Hutton has suggested. Of fold movements earlier than this we know nothing.
The name Tuhuan attached to the hypothetical Devonian orogeny by Park (Bull. 23) is not a happy one, derived as it is from the many granite batholiths which Park assumed to have been emplaced in this period. Morgan described the Tuhuan granites near Hokitika (1908, p. 130) as intruded both into the Greenland schists, west of the Alpine Fault, and into the lower more gneissic members of the Arahura schists, which latter lie east of the fault and form the bulk of the Southern Alps. In that part of the field “all the main outcrops occur along or close to the great thrust-plane which separates the Arahura series from the Greenland rocks. They may be regarded, to use one of Suess's phrases, as cicatrices marking the healing of a wound in the earth's crust”. (Morgan, 1908, p. 130.) Morgan regarded the granites as batholiths emplaced in late Cretaceous or early Tertiary time. The emplacement of the Tuhuan granites may be much later than Devonian, but the writer is inclined to regard it as pre-Triassic. Quite provisionally, we can regard all the granites and associated dyke rocks north of Hokitika as being of similar age to the Tuhuan granites. These granites remain a barely touched field of research in New Zealand, and if the theory of granitization is invoked, boundless possibilities are presented concerning the age of the granitization and of the hypothetical sediments that have suffered metasomatism.
Associated with the Ordovician in the north-west and south-west of the South Island are greywackes presumably of Lower Palaeozoic age of which little is known. They have in places been confused with the Greenland Series, but is is probable that most of them have suffered older fold-movements than the Greenland rocks.
At the south-west end of the South Island lies a tract of country whose structure is largely unexplored, but apparently consisting mostly of metamorphic rocks. The oldest known strata are richly fossiliferous Ordovician argillites and greywackes which are found at Preservation Inlet (Benson and Keble, 1936; Benson, 1933). They strike approximately slightly west of north with assymetrical folds, their steepest limbs dipping to the east. Northwards the rocks seem to pass along the strike into spotted phyllites, mica-schists and para-
gneisses with which are associated calc-silicate hornfelses (Benson, p. 401). Marbles and hornblende-gneisses of various types have been found in the country between the Ordovician of Preservation Inlet and Milford Sound (Marshall, 1907; Turner, 1939), and it is likely that they are contained in a vast suite of paragneisses of unknown but possibly Palaeozoic age. The Ordovician rocks are described by Benson as cut by granitic batholiths elongated in an approximately northwest direction. To the east and south-east the metamorphic rocks closely associated with the Ordovician show numerous lit-par-lit injections associated with granodiorites. It seems likely that these “grano-diorites” stretch uninterruptedly to the shores of Lake Te Anau and the neighbouring Lake Manapouri, where Park (1921) in rapid reconnaissance mapped them as the Clinton River intrusives. This group seems to include a great variety of vaguely defined rocks such as diorites, granodiorites, hyperites, trondhjemites, paragneisses and smaller granitic masses, on which the only adequate petrographic work is that of Turner (1937). Though his study in detail was confined to Lake Manapouri, Turner came to the conclusion that certain of the rocks were definitely paragneisses of basic composition, whilst others were plutonic rocks. At Lake Te Anau many of Park's “diorites” show extremely regular banding, recalling that of sediments, and Dr. Turner, in conversation with the writer, has expressed the opinion that they may also prove to be paragneisses. The whole of this territory is, therefore, worth examination in the light of the granitization hypothesis. Park (1921, p. 42) was of the opinion that the Clinton River rocks were intrusive into the Permian, but the evidence for this has not been clearly described. Benson (1921, p. 10, footnote) accepted the evidence of Park and Moir that part of the “diorites” invades annelid-bearing greywacke (Triassic or Permian?) in the Darran Mountains and the Hollyford Valley, north of Lake Te Anau. The age of the Clinton River complex may then be either Palaeozoic or (more unlikely) as late as post-Triassic, and the complex may be of very different age in different places. In general, it would appear that all the oldest known rocks show a nearly meridional strike varying to north-north-west or north-north-east locally, and the Clinton River complex appears to be very closely associated with these older rocks. The writer is inclined to regard all this complex as Lower Palaeozoic and not later than Upper Palaeozoic.
Similarity between South-west and North-west Parts of the South Island
Whether or not the Clinton River complex is partly of Mesozoic age, all the country lying between Lake Te Anau and the south-west corner of the South Island recalls roughly in its structural and stratigraphical ensemble the north-west corner of the island. If the south-west corner represents a coign—a foreland—for Hobbs, so also must the mass of ancient rocks at the north-west extremity. Ignoring detail and theorising sweepingly, a similar pattern can be seen in the south-west and north-west parts of the island, thus:
|Ancient mass of Fiordland embracing fossiliferous Ordovician, schists, marbles, granite batholiths, and the Clinton River complex.||Ancient rocks of north-west Nelson (including fossiliferous Cambrian, Ordovician, Devonian, marbles, schists, granite batholiths.|
|Waiau Tertiary syncline.||Tertiary syncline south-west of Nelson.|
|Hokonui syncline of Permian, Triassic and Jurassic rocks.||Syncline of Permian and Triassic rocks at Nelson.|
|The great “Otago thrust.”||The Marlborough schists.|
|The Otago schists.||The Alpine Fault.|
The positions of the two thrusts do not fit into the pattern nicely.
The writer is aware that this parallelism is already envisaged in part or in whole by colleagues in the Geological Survey, notably Mr. H. Wellman, who first insisted in conversation on a close similarity between the stratigraphy and structure of the Hokonui and Nelson synclines. The position here assigned to the Alpine Fault in the Nelson district, namely as continuing along the Wairan Fault east of the Marlborough schists, is that drawn on the Survey map,* and may be queried by those who know this field more intimately. One merit in the scheme is that folded Tertiary beds fit into part of the picture, so that we are certainly dealing to some extent with late Tertiary diastrophism. On such parallels are based Hobbs’ suggestion of Alpine analogies and Wellman's interesting but tentative and unpublished discussions of possible immense transcurrent displacements. Until more detailed supporting evidence is forthcoming, the suggested parallels must be treated with great reserve, for they are partly dependent on comparing metamorphic rocks on which the petrographic information is very incomplete.
Original Extent of Tertiary Strata Flanking the Alpine Fault: Movement on the Fault
Park believed the Clinton River complex to continue south-eastwards into the Longwood Mountains, east of the Waiau River, and reappear on the coast at Riverton and Bluff; Macpherson (p. 10 and map) believed the outcrop of the complex to swing in a northwesterly directed curve through these points, and he deleted for purposes of clarity the Tertiary strata known to be of some thickness at the mouth of the Waiau Valley. In this manner he partially obliterated the southern extremity of the Waiau syncline, which has a general northerly orientation. Presumably geophysical evidence obtained at Orepuki, at the mouth of the Waiau River, partly justifies this procedure, but one is still inclined to consider that the general orientation of the Waiau Valley is more significant of late Tertiary diastrophic trends than is the strike of the Clinton River complex. The rocks of the Clinton River complex cannot be younger than early Mesozoic (and may well be much older), and Macpherson's view seems to accept the strike within this formation besides the trend of its margins, as indicative of late Tertiary trend lines. It is likely that Macpherson is referring to the Waiau syncline when he writes (p. 11): “In a minor fold of the major syncline late Cretaceous and early Tertiary terrestrial sediments and post-Eocene marine beds are involved. However, the Tertiary syncline although it curves with the regional arcuate structure, does not generally follow the major syncline or axes in the Mesozoic sediment, except where the major syncline peters out to the north-west.”
[Footnote] * Macpherson places the continuation of the Alpine Fault west of the Marlborough schists, but the Geological Survey map places it east of the schists.
Following the Waiau syncline to the north, the Tertiary strata are seen to be folded between the Permian beds prolonging the southern limb of the Hokonui syncline and the rocks of the Clinton River complex. Tertiary strata can also be followed, deeply involved in the older rocks, over the Homer Saddle and Lake Fergus, immediately north of Lake Te Anau (Wellman and Willett, p. 299; Benson, 1935, p. 9). We can tentatively regard these mid-Tertiary beds as formerly continuous with those mapped along the coast west of the Alpine Fault in South Westland.
If we are to accept Macpherson's view concerning the great “Otago thrust”, it would have to be regarded as tectonically a much more significant branch than the seaward prolongation of the Alpine Fault, although Wellman and Willett insist on the dominance of the latter in their field mapping. The writer, however, would prefer to see any southern prolongation of this fault as following the western side of the Waiau Valley; for the displacement would then be defined in terms of deformed Tertiary strata. There has been great movement on the Alpine Fault in late or post-Tertiary time, although it may have been initiated in earlier times, but the latest movement on Macpherson's Otago overthrust, if such really exists, cannot be ascribed to late-Tertiary diastrophism: a glance at the geological map indicates that near the Mataura River, some fifty miles north-north-east of Bluff, Oligocene strata spread over any possible trace of the Otago thrust.
Wellman and Willett in their instructive paper also discuss the Tertiary history of the Southern Alps, pointing out that little is known of the surface on which the Tertiary strata were deposited, and that land on the site of the Southern Alps may well have existed before the Tertiary period. The distribution of “mid-Tertiary beds and the absence of lower Tertiary beds suggests that in mid-Tertiary time the lower Tertiary sea transgressed both along the southern part of the West Coast and eastward over western Otago and parts of adjoining Southland. The sea may not have extended over what is now the higher part of the Southern Alps,… the Alps have nevertheless since been elevated along a pre-Tertiary and possibly Mesozoic trend line.” (The Alpine Fault was regarded by Morgan as chiefly early Cretaceous in origin.) “The schist belt which lies along this line may represent a positive unit which has been intermittently uplifted in a similar manner at other times in the past.” The same authors point out that the Tertiary beds are most intensely deformed along the line of the Southern Alps (p. 303), and they postulate an uplift of about 10,000 feet along the Alpine Fault in later Tertiary time (approximately mid-Pliocene, but believe like most writers, particularly Benson and Cotton, that after this uplift the surface was reduced to a late mature surface of low relief. The present physiography of the Alps is ascribed to erosion of this peneplain (Wellman and Willett, p. 304), but the throw of 10,000 feet need not be necessarily concentrated into later Tertiary time, for the estimate of Wellman and Willett is based on the deformation of a peneplain surface alone, there being no Tertiary outliers immediately east of the Alpine Fault. The same authors also point out that the Tertiary beds where deeply involved in faults, as at the outcrops intervening between the Waiau Valley and the Tertiary of Jackson Bay, are so indurated as to be easily confused with the rocks of the older undermass (p. 303).
Greenstones of various types, including serpentinite, peridotite, talc schist, tremolite schist, etc., are aligned parallel to but not along the trace of the Alpine Fault, and their occurrences have been summarised by Benson (1926, pp. 42–44). C. O. Hutton has also described serpentinites and gabbros in the Livingstone Range, in western Otago (1937); and Turner, peridotites and serpentinites in the Cascade Valley (1930), South Westland.
Many of the rocks occur as intrusions, generally of sill-like character, commonly in schists belonging to the Otago schists or Arahura Series, or sediments of Mesozoic age belonging to the comprehensive Hokonui System, so that they have been provisionally correlated with the post-Hokonui fold movements, probably early Cretaceous.
Occurrences of greenstone that must have been emplaced at other periods are also known. At Auckland, Turner and Bartrum (1928, pp. 871–873) describe serpentinites and other ultrabasics as intruded in an orogeny later than the Cretaceous beds and preceding the deposition of the first Tertiary, presumably an early Tertiary orogeny (Bartrum, 1934; Benson, 1924, p. 130). They also describe (1928) pillow lavas, serpentinites, etc., of early Tertiary emplacement in the North Cape area. Fleming (1947) describes a serpentinite in contact with Oligocene and Miocene sediments near the Mokau River and gives good evidence for regarding its present position as due to a diapir thrust.
There are many other occurrences which cannot be mentioned here, and a complete restatement concerning all the greenstones of New Zealand and their relations to structure is highly desirable, for the concept of diapirism has been insufficiently considered.
More Detailed Discussion of Selected Regions
Suess's Syntaxis in the North Island: Waiapu
Suess, following the information of previous writers, attempted to explain by syntaxis the existence of north-west-striking strata near strata with a north-east strike, but Morgan has demonstrated that for the South Island any conception of simple syntaxis is untenable, the two fold trends being sharply separated by the Alpine Fault.
Macpherson demolishes Suess's syntaxis in the northern part of the North Island by suggesting that the strikes of all the Cretaceous and Tertiary strata swing to follow a north-west trend. He substitutes a curved arc and discards “faulty structural concepts such as branching fold trends”.
Curiously enough, Macpherson, although he derives much of his evidence from the Waiapu subdivision, north of Poverty Bay, has made little mention of some interesting structural features which he and Ongley had already described in the Waiapu Bulletin. The accompanying reduction from the maps of these two authors shows the structure very clearly. Their description is as follows (p. 19): “Over a wide area in Hikurangi and west Mata survey districts the Raukumara beds (Albian?-Cenomanian), the oldest rocks of the subdivision, are compressed into steep, narrow and vertical folds, trending north. Southwest in north Arowhana S.D. and north-east in Mangaporo S.D., the
same beds are folded less intensely and regularly, trending nearly east”. More precisely, the strike in Arowhana S.D. is usually nearer to west-north-west and in Mangaoporo generally east but often east-north-east.
“The Tapuwaeroa beds (Senonian) are extremely contorted, but their distribution as mapped clearly shows that they have been compressed into broad folds trending east-south-east. A well-marked syncline extending along the Tapuwaeroa valley and reaching the coast,… crosses the middle of the subdivision.” “An anticline on the south flank of the Tapuwaeroa syncline crosses the middle of Mata S.D.…” “The Mangaoporo anticline to the north is not so clearly marked…” “The Tertiary beds in the south-eastern part of the subdivision are broadly folded almost at right angles to the east-west folds of the Cretaceous rocks.” (Italics added by the present writer.) “A syncline twenty miles long and five to seven miles wide strikes diagonally north-east through Tutamoe S.D. into the south-west corner of Mata S.D. Along its south-east flank the beds rise into a well-marked anticline extending through south-east Tutamoe and north-west Tokomaru.” “The structure of the Tertiary beds in the north-east of the subdivision is dominated by faults rather than by folds.” “Innumerable faults traverse the Cretaceous beds… most of them are subparallel with, and no doubt were formed during, the east-west folding of the older rocks. In the northern part of the subdivision the faults that break the Tertiary beds have nearly the same trend. Farther south the later faults are mostly parallel with the folds in the Tertiary strata.”
A glance at the map confirms that the principal folds affecting only Cretaceous rocks strike approximately west-north-west and show axial pitch east-south-east. Considerable erosion also followed the deposition and folding of the beds which Ongley and Macpherson mapped as “Mangatu”. The Miocene beds were laid on all the earlier formations with unconformity strikingly shown in the south-west part of the subdivision, where a great thickness of Mangatu beds (Senonian plus early Tertiary) is preserved in a syncline and cut out on an anticlinal crest. Since the lower Miocene beds belong to the Southland Series (Finlay and Marwick, 1947) and the Mangatu beds of Ongley and Macpherson include several lower Tertiary as well as upper Cretaceous stages, this great unconformity sheds light on the curious absence of the Upper Oligocene (Pareora Series of Finlay and Marwick) at many localities along the east coast of the North Island. Major earth movements with considerable planation must be postulated immediately before or during this time.
Ongley and Macpherson clearly realised that the folds affecting Tertiary beds ran athwart those earlier folds which affected only lower beds. And yet the strike in Tertiary strata swings markedly to an approximate easterly orientation on approaching the axis of the Mata anticline. It seems highly probable that near this anticline we have evidence of either three superimposed trends or of a cross-trend formed contemporaneous with dominant north-east folding over most of the region. The east-south-easterly pitch of folds in the Cretaceous beds may have preceded the Pareora planation, but can be
more conveniently regarded as an early expression of the second folding along an approximately north-east trend.
Now, evidently in a pattern showing such a diversity of trends formed within a small area and in a comparatively short lapse of time it is difficult to aver that trends definitely curve, for one may be joining the fold trends formed at different periods in a pattern which is essentially criss-cross. It may be legitimate to group the dominant structural outlines produced by several fold movements within a certain lapse of time as constituting an arc, but the existence of such an arc by no means invalidates Suess's idea of the north-east trend continuing towards the western edge of the Tonga deep. A glance at the new Geological Survey map gives no indication of the major structural swing postulated by Macpherson: instead, a major anticline striking north-east appears to be abruptly broken by west-north-westerly faults. To quote Ongley and Macpherson again: “The structure of the Tertiary beds in the north-east of the subdivision is dominated” [the writer's italics] “by faults rather than folds”, but the faults actually chop abruptly across the folds and there is no clear evidence of a swing in strike. Furthermore, if we accept a dominant pattern of folds and faults developed in the course of several consecutive fold movements as constituting an arc, the term syntaxis must be equally valid for apparently branching folds formed during several similar movements. Moreover, Fleming (unpublished) has recently cited important evidence, based on submarine contours, for believing that the Taupo graben can be traced as a trench for hundreds of miles towards the Kermadec Islands. Following Healy's discovery of marine Pliocene strata at Matata and Ohiwa, on the Bay of Plenty, Fleming considers that the trench must, at least in part, have been formed very late in Pliocene or even in Pleistocene times.
The Taitai Nappe
Macpherson regarded the Taitai rocks (of Aptian age), apparently overlying younger Cretaceous rocks, as evidence for an eastern over-thrust of some forty miles, also noting that the klippe-like masses appear to be concentrated in or near the Tapuwaeroa syncline. The emplacement of this nappe is regarded by Macpherson, very tentatively, as immediately anterior to the later Senonian, and there is stratigraphic evidence for a regional orogeny at this time, although there is no very pronounced early-formed north-east or north-striking fold and fracture pattern in the Cretaceous rocks such as might be expected with Macpherson's postulated thrust. The school of Grenoble, and particularly Schneegans, have returned to the concept of immense gravity slides of rock-masses as an explanation of some nappe structures, a process which was originally suggested in Schardt's revolutionary papers on the Pre-Alps and which Lugeon and Schneegans describe. If the Taitai rocks really represent klippen, such a process* can readily be imagined as causing their emplacement as a result of an east-south-easterly tilt suggested by the pitch of folds affecting only Cretaceous rocks. The concentration of the klippen in the Tapuwaeroa syncline is also readily explained. This process
[Footnote] * Wellman invokes a Miocene landslide to explain the position of curious outcrops of coal measures at the Fox River head waters. Nelson Province (1946).
seems more in keeping with the regional tectonics than one of tremendous compressive folding in late Cretaceous time.
The interpretation of the Taitai rocks in Waiapu as klippen may, however, be completely denied, for the evidence is doubtful, and if the rocks are truly Aptian a diapir remains a possible explanation.
The example of Waiapu serves as a warning not to be dogmatic regarding constancy of trend throughout the movements of Cretaceous and Tertiary times. It is important, nevertheless, that where the Cretaceous and Tertiary strata show a dominant north-east strike, a comparatively simple pattern results with little or no cross-folding or faulting. On the other hand, where a north-west strike is common in Cretaceous and Tertiary rocks the pattern is always more complex, with some evidence of movement producing two strikes, often at right angles to each other. The Waiapu district also introduces us to Tertiary structural patterns that, although dominated by north-east strike, show minor transverse faults and folds.
The North Auckland Peninsula and the Auckland Region
The North Auckland Peninsula has been covered in some detail by Geological Survey bulletins, but both the structure and stratigraphy remain obscure in many places. The strata include Mesozoic greywackes, Cretaceous argillites, and Tertiary sandstones and mudstones, with also many Tertiary lava flows, and the Tertiary strata show many overlaps and unconformities. The outcrops, moreover, are very deeply weathered.
At the time of mapping, correlation by Tertiary fossils was on uncertain grounds. To add to the difficulties, it is likely that in some places the Tertiary beds were deposited in earlier-formed fault-angle depressions, whereas in other places the faulting followed the deposition of the Tertiary strata; but it is difficult to separate everywhere the structures according to age.
The dominant faults, those bounding the main outcrops of Mesozoic greywacke and adjacent Tertiary beds, strike mostly north-west, but there are numerous important cross fractures, some striking northeast and many roughly east-north-east. Comparatively few strikes of strata are recorded on the map and those few have a bewildering diversity, suggesting folding along varying trends, but some of the strikes and dips recorded in the Tertiary strata may well be primary dips due to sedimentation or compaction. The relations of the lavas to underlying rocks are also obscure and the whole region calls for a synthetic statement by a worker knowing the field intimately; but at the moment it seems clear that the north-west fabric is very considerably complicated by a cross fabric. Ferrar's (p. 33, 1925) description of quartzite “coulisses”, nearly vertical beds in the Triassic-Jurassic rocks, implies that their north-west trend was imparted in an orogeny preceding the deposition of the late Cretaceous strata, presumably the post-Hokonui (early Cretaceous) orogeny.
The vicinity of Auckland city has been described by Professor Bartrum and his students over a period of years (Turner, Searle, Laws, Firth, Lyons, and the authors of several unpublished theses). Firth's description of the Papakura-Hunua region immediately south
of Auckland City exemplifies clearly the Tertiary structures typical of the whole region. He describes two sets of faults, forming on the map a set of rough parallelograms, but he recognises clearly that the movements on faults parallel to one direction are in some places earlier, in others later, than movements on faults belonging to the other set. For such a structural pattern the term “block-faulting” in its strict sense seems very apt, as indeed it does also for the Tertiary structures over most of the North Auckland Peninsula. Bartrum in a synopsis (1949) of the Tertiary history of Auckland City gives evidence for alternating submergence and emergence. The Pareora series is absent and the base of the Waitemata sandstones (of early Miocene age) consists of great boulder beds containing igneous material from the “lost” hinterland which lay west of New Zealand. The Waitemata beds near Auckland show in places conspicuous but irregular folds formed possibly by sub-aqueous slumping, as suggested by Kuenen and Shepard during a visit to the city.
The Hauraki graben is bounded by faults striking approximately north-south, and these are cited by Macpherson as evidence for the general swing in strike which appears clearly along the west coast, and whose existence we have queried on the east coast. East of the Hauraki graben lies the Coromandel Peninsula, largely formed of Tertiary volcanic rocks, and the Geological Survey Bulletins on this region have generally agreed that the volcanism is likely to have commenced along early-formed fractures on the lines of those which now bound the graben. (A convenient and elementary summary by Bartrum, 1930, is listed.)
No attempt is made here to describe the Tertiary and Recent volcanicity of the North Island other than to note that the general concensus of opinion dates the graben of Taupo as formed later than the earliest volcanic outbursts, with fault movement continuing till the present day (Grange, p. 55, 1937). On the other hand it is evident from Healy's description (cyclostyled itinerary for the Pacific Science Congress, 1949) that considerable faulting occurred in the early Pliocene, and that the late Pliocene volcanicity is likely to have originated along such fractures.
The West Coast of the North Island
The West Coast between the Waikato and Mokau rivers is remarkable for the strong meridional strike shown by the Mesozoie greywacke (including fosailiferous Triassic and Jurassic beds). The Tertiary strata, dipping much less steeply, also show the same strike. The latter form several fault-bounded folds clearly marked on Macpherson's map (1946), which distinguishes three principal anticlines, with also one prominent syncline of Tertiary strata as described in the recent Te Kuiti bulletin by Marwick (1946). This field has been covered in detail by several workers, including Ferrar, Grange, Henderson, Marwick, Ongley, Taylor, and Williamson. Their results from adjoining areas tally closely in stressing the great thickness of Mesozoic strata (approximately 28,000 feet), with a lesser thickness of Tertiary beds (approximately 4,000 feet in Te Kuiti region). Cretaceous strata, of considerable thickness on the eastern side of the island, are entirely absent to the west, and the Upper Oligocene (Pareora Series), absent to the east, outcrops on the western side.
It is worthy of note that the cross fractures throughout the southern part of the coastal belt covered by the Te Kuiti bulletin, some 30 miles wide and 30 miles long, are subordinate and of little throw, but increase in number and size north of Kawhia Harbour (i.e. approaching Auckland), where they appear as two sets of faults showing a strike either dominantly east-north-east or north-north-east.
The stretch of comparatively homogeneous fold pattern seen in the Te Kuiti district is the most convincing evidence for the swing of strike required by Macpherson in his postulated arc structure.
Macpherson prolongs the axis of the Tertiary folds southward and east of Mount Egmont and runs them out to sea with a slight curvature.
Wellman, in oral description, has stressed stratigraphical similarities between the Mesozoic rocks of this Waikato-Mokau stretch and the Mesozoic rocks near Nelson, in the South Island, and considers them to have formed part of one geosyncline existing in both Mesozoic and Tertiary times. The rocks of both groups show in general parallel strikes, although the Mesozoic beds are more highly folded than the Tertiary. Wellman's interpretation seems acceptable, and there appears to lie between the Te Kuiti region and Nelson a tract of country where the north-west element in the folds affecting the Mesozoic is entirely suppressed.
Marwick (1946, p. 11) points out that in Te Kuiti district the principal fold movements appear to have occurred in late Miocene post-Tongopurutan times: “The movements presumably represent the Kaikoura Orogeny (Cotton, 1916) and indications are that, here, they were completed relatively early, perhaps before the end of the Miocene and at the latest in the early Pliocene (Waitotaran). After the orogeny, there ensued a fairly long period of stability during which the soft Tertiary rocks were stripped from the high blocks and peneplained… Then came an uplift of several hundred feet… In the Upper Pliocene the vast quantities of ignimbrite that invaded the subdivision from the south-east filled the main valleys and submerged much of the low country…”
South of Mount Egmont, only Pliocene and later beds are visible and they show a general slight south-westerly down-tilt, to be correlated with some differential uprise in the axial range in the vicinity of the Taupo region (M. Te Punga, unpublished thesis). Fleming has redescribed the whole classic Wanganui area of Pliocene sediments in an unpublished bulletin, and has told the writer that although the Pliocene beds must be generally interpreted as smoothly dipping, there are traces of minor folds with axes likely to be coincident with those of folds mapped by Macpherson (1946) to the north. Fleming therefore deduces some very late, but minor, folding.
The Axial Chain in the North Island
The structure of the axial chain is most imperfectly known and the dominant trends on maps are obviously north-east, following enormous faults, many of them formed in very late Tertiary times (post-Castlecliffian, i.e. late Pliocene or even Pleistocene).
A few scattered observations made by the writer suggest that within the least disturbed greywackes and argillites of the axial range there are stretches showing a general north-west strike, which may vary from
west-north-west to north-north-west, although the more sheared grey-wackes on the margins of great late Tertiary faults appear to show a north-easterly strike. This slight evidence is too inconclusive as yet, however, to substantiate a theory of the existence of a general ancient north-west strike within the greywackes forming the axial chain of the North Island.
These greywackes are certainly in part Mesozoic in age, as indicated by the discovery of a single vertebra of an ichthyosaur at Wellington (Benson, 1921), but the stratigraphy is for the most part obscure. Forming a tectonic unit quite distinct from those on the west coast in the Mokau-Waikato area, these rocks may be partly older and perhaps include some Palaeozoic strata.
East Coast of the North Island, from Hawke Bay to Cook Strait
East of the axial chain, the north-east trend already seen in the Tertiary strata north of Poverty Bay continues markedly impressed on all the upper Cretaceous and Tertiary strata from Poverty Bay to Cook Strait. This part of the country is already covered by reconnaissance reports by McKay and more recently, in detail, by Henderson and Ongley in Gisborne and Eketahuna subdivisions, by several other officers of the Geological Survey, and by oil geologists.
In general, from Hawke Bay to Cook Strait, the facies of late Cretaceous and Tertiary sediments appear to be aligned in belts roughly parallel to the north-east strike of the late Tertiary folds, indicating that some folding or faulting along this trend had probably already occurred in early Cretaceous and/or earlier times.
The structure revealed by Cretaceous and Tertiary beds in many places consists essentially of very elongated domes which, although broken by great faults on their eastern limbs, show a truly anticlinal disposition of the erosion surface, which separates the greywacke cores of the folds from the Tertiary cover.
The region constituting the Dannevirke Subdivision, east of the Manawatu Gorge and the Ruahine Range, is of stratigraphic interest in containing localities which show a complete passage from the late Cretaceous through Paleocene to Eocene stages as demonstrated by the detailed examinations of foraminifera carried out by Finlay. The localities of complete passage are concentrated on the margins of a major anticline in the centre of Dannevirke subdivision, while anticlines east and west give good evidence of non-sequences, unconformities and clastic deposition in the late Cretaceous and early Tertiary. It appears likely that during pre-Pliocene time there was progressive overlap of Tertiary sediments towards the axial chain of the Ruahine Range, with the deepest sedimentation near the major central anticline.
The Upper Oligocene (Pareora Series) is completely absent south of Hawke Bay, and its presence further north is doubtful. In parts of Waiapu and Poverty Bay district, Henderson, Macpherson, and Ongley have found prominent conglomerates developed at the base of beds representing the lower Miocene (Southland Series). The Pareora Series is absent in many other parts of New Zealand and obviously about Late Oligocene time some fold movement occurred which, although subordinate to the post-Pliocene movements, must
yet have been considerable. Again the influx of coarse clastics in the late lower Miocene (Upper Southland) makes a second period of widespread precursor movement at that time very likely.
The concentration of the greatest thickness of Pliocene beds west of the major anticline, and in places the eastward overstep and truncation of early Pliocene members by later ones, suggest that the main trough of Tertiary deposition migrated markedly westwards, with movement at the beginning of the Pliocene, through the influence of early development of the major anticline in the centre of the Dannevirke subdivision. Such movement seems to have continued intermittently during the Pliocene. The principal trough of Pliocene deposition seems to have lain between the early-rising central anticline and the Ruahine Range already largely emergent. Here, then, is a tract of country where the concept of the persistent “high” appears to be invalid, although there is good evidence of an anticline persistent throughout late Cretaceous and Eocene times on the present coast east of the major anticline.
This widespread early Pliocene movement inferred by the writer in Dannevirke subdivision can be compared with the movements described by Marwick in Te Kuiti as “before the end of the Miocene and at the latest in the early Pliocene”. They can be accepted as approximately contemporaneous, and both areas flank the axial chain, although the Te Kuiti subdivision is more distant from it. Now, in all the region from Hawke Bay to Cook Strait, the Pliocene beds have been very considerably folded and faulted by very late Pliocene movement and the writer has in this field restricted the term Kaikoura to these late movements. It appears, therefore, that along the axial chain and near it, great post-Pliocene fold movements occurred, but that farther away, on the west coast, these movements had the effect only of imparting a westward down-tilt, with folding very subordinate.
The axial chain shows remnants of an old erosion surface to which Waghorn, Cotton, and Ongley (1935) have called attention, and this is dated by the fact that it is covered by very shallow water Pliocene sediments in a few places. At the Manawatu Gorge these Pliocene beds form an anticline on an axial depression on the range, and Fleming and the writer (1941) have considered this to be a strait in Pliocene time. In another paper (as yet unpublished) the writer calls attention to this transverse depression, which appears to be ancient and yet has continued as a region of maximum axial pitch until recent times. The local strike within the greywacke strata is roughly parallel to the course of the transverse river. The Dannevirke major syncline flanking the Ruahine Range appears to have been already an elongated basin at the end of Pliocene time and the writer interprets the course of the Manawatu in this syncline east of the Ruahine as directly consequent. In its early history it is inferred to have drained the shrinking mud-flats of the infilled Pliocene basin towards and westward through the strait.
Lower Cretaceous (Aptian) Strata and the Post-Hokonui Orogeny
Macpherson's conception of the Taitai overthrust has already been discussed, but the Aptian age ascribed to the rocks of the Taitai series is of more general importance as it may affect the dating of the great
post-Hokonui Orogeny. The genera Maccoyella and Aucellina, considered to be Aptian (Finlay and Marwick, p. 17, in “The Outline of the Geology of New Zealand,” 1948), have not been found at Taitai, but at Koranga, some thirty-five miles west-north-west of Poverty Bay, and Aucellina in rocks of similar lithology in southern Hawke Bay. Large tracts of unfossiliferous greywacke and argillite on the south-east side of the North Island have been provisionally assigned to the Taitai series owing to resemblances in lithology and degree of induration. An abundance of basic igneous pebbles is supposed tentatively to characterise the conglomerates among these strata, but in degree of induration and of deformation, and by general lithology the rocks ascribed to the Taitai series over large tracts are indistinguishable from those which in the axial range are ascribed to the comprehensive Trias-Jura group. If the Aptian age is valid, some of the post-Hokonui movements may well be of age as late as post-Aptian, judging from the deformed state of the Taitai rocks. But it may well be that two orogenies were concentrated into the period between late Jurassic and post-Aptian time.
Outliers of Tertiary near Wellington: Wellington: Cook Strait
Apart from the Pliocene beds at the Manawatu Gorge, two other occurrences of Tertiary beds are known in the axial ranges. One of these, described by Macpherson (1949), is near Paraparaumu on the west coast some 37 miles north-east of Wellington, where beds of Lower Oligocene age occur in an infaulted block bounded by greywacke on all sides. The other beds, at Makara, near Wellington, appear to be of Pliocene age (Gage, 1940) and may be infaulted in the greywacke. These outcrops give evidence of the former extent of Tertiary beds now almost completely removed and of the great Tertiary movements which have dislocated them.
Cotton has described many of the geomorphic features around Wellington. Both the peneplains and the faults there are likely to be largely of post-Pliocene date and give evidence of great differential movement. But little is known of the earlier geological history of the Wellington district, since the approach appears to lie only in a patient mapping of highly crumpled and sheared greywackes or in a geomorphic attack through the correlation of peneplain remnants which lack Tertiary cover.
A zone, or zones, of red rocks, some of which are variolites, appears at several points along the Rimutaka Ranges, but at the moment it is premature to link all the outcrops of red rocks as marking an approximate strike line. H. Fyfe (unpublished) has recently shown that some of these red rocks in the Rimutaka Ranges are pillow lavas, in occurrence similar to those at Wellington (Broadgate, 1916; Benson, 1921, p. 18; Wellman, 1949), but at other localities they are jaspillites and possibly radiolarites.
The recently published geological maps indicate that it is quite feasible to conceive of the axial ranges as continuing across Cook Strait, and the latter may mark either a region of marked axial pitch or some transverse fracture. Certainly, L. C. King's (1939) suggestion of a lateral displacement in the nature of continental drift is unnecessary and is based more on correlation by physiography than on correlation of structural features. Some more detailed pictures of the relations of
structural elements in the North and South Islands to each other is, nevertheless, desirable, and C. A. Fleming has developed views (as yet unpublished) based partly on submarine morphology of Cook Strait which will help to fill this need.
The South Island: Canterbury, Marlborough, and Westland
The whole of the north-east side of the South Island, including Marlborough and most of the Canterbury province, is dominated by north-east-striking folds and faults formed chiefly by the late Tertiary movements and broken by a few cross faults of almost east-west trend. Indeed, it was in this region that McKay first recognised the importance of the late Tertiary orogeny. Reconnaissance reports by Fyfe and Healy give further descriptions of stratigraphy and structure.
Cretaceous and Tertiary stratigraphy has been described in considerable detail, but is outside the scope of this note; and the structural implications of the stratigraphy must be left to some other worker more conversant with the region.
For the region west of the Alpine Fault, the writer is unable to add much to the outline given earlier. The Greymouth coalfield has been surveyed in detail by officers of the Geological Survey and a detailed bulletin by Gage awaits publication. H. W. Wellman's studies on rank in the West Coast coals are also unpublished. The Tertiary strata of the West Coast yield good evidence indicating considerable mid-Tertiary earth movements. Wellman (1945) describes at Ross, south of Hokitika, a section through the Tertiary beds in which Pliocene (Waitotaran) strata rest with marked unconformity on Lower Oligocene, these earlier sediments, according to Wellman's tentative section, being folded into a recumbent syncline formed by movements preceding the Pliocene (Waitotaran). Gage (1945) and Gage and Wellman (1944) discuss further Tertiary sequences in Westland which contain conglomerates indicating earth movement during Tertiary time, as does also Wellman's description of the Fox River headwaters (1946). It seems likely that this area suffered locally movements comparable to those cited for Late Oligocene (Pareora), for Miocene (Upper Southland), and for early Pliocene times in southern Hawke Bay. These writers alone are able to synthesise this great field adequately.
Otago and Southland
Macpherson's views on the Otago and Southland districts have already been briefly outlined, and it is interesting to consider his views on late Cretaceous and Tertiary diastrophism in relation to earlier fold trends in that region.
It has long been known that the north-west trend of the Otago schists and the Hokonui syncline is of ancient origin. Cotton (1917, pp. 429–430) early recognised pre-Cretaceous faulting at the Shag Valley. Benson (1941, p. 216) summarises the position as regards the early history of the Shag Valley fault-zone, which strikes north-west and reaches the east coast some thirty miles north of Dunedin, as follows: “The fact that it has brought into apposition the Palaeozoic (?) Otago or Maniototo schist on the south-west with the Early Mesozoic (?) greywackes, etc., on the north-east, which were reduced
to a common peneplain level by the earlier Cretaceous erosion, indicates movement with a southerly upthrow during the closing Jurassic orogeny. Paterson (1941) pointed out that the semi-talus character of the oldest Upper Cretaceous rocks near Shag Point indicates the existence of a rejuvenated fault-scarp on the north-east side of this fault-zone.” Late Cretaceous and Tertiary marine sediments were later deposited across this ancient fault-zone. “Renewed movement, this time with a northerly upthrow, occurred in mid-Tertiary times,” and after the upthrown greywacke and the marine Tertiary sediments had been reduced to a common peneplain level, a “flow of basalt flooded over this peneplain crossing the (by then) obliterated fault trace …”
This north-west trend can be followed a considerable distance south and west from the Shag Valley Fault into the Otago schists and into the great Hokonui syncline of Triassic and Jurassic rocks. That the trend is in the first place of ancient origin is demonstrable in many places; the lineation of the Otago schists, both throughout Central and Eastern Otago as well as in the Shag Valley, as Turner and Paterson have shown by petrofabric analysis, is between north-west and north-north-west. The main metamorphism of the Otago schists has been reasonably placed by Turner (pp. 75, 189, 190, 1940) as late Palaeozoic or early Triassic, but he does consider it possible that part of the metamorphism may have been produced during the great post-Hokonui Orogeny. There is a distinct difference in the degree of metamorphism shown by the schists and by all rocks of known Triassic age, so that if we were to assume (an assumption not necessarily valid) the periods of deposition of the sediments now appearing as Otago schists to be approximately coeval, it may be suggested as most likely that metamorphism preceded the deposition of the Clinton conglomerate of Permian or even Carboniferous age (Ongley, 1939).
Mackie (1936), by a detailed study of bedding planes, schistosity planes, and joints within the schists and greywackes of Northern Otago, also demonstrates a north-west strike in that region.
Both the Otago schists and the Trias-Jura greywackes showing the north-west strike are in many places unconformably overlain by late Cretaceous beds showing a marked north-east strike. To the north of the Shag Valley the strike of the Cretaceous and Tertiary beds appears to persist as approximately north-west for some considerable distance through North Otago and South Canterbury; but south of Dunedin the strike in Cretaceous and Tertiary beds becomes very markedly north-east to north-north-east and this strike continues along the coastal belt to Kaitangata. Benson (1941) describes this region, summarising the work of others and giving as well much new structural detail. Turner (1940, p. 189) notes the tilting of the schists across a general north-east to north-north-east strike and observes that it has not affected the quartz fabrics of the Otago schists except, possibly, in the immediate vicinity of late Tertiary faults.
Cotton (1917) and Benson (1935) drew attention to widespread remnants of the great Cretaceous peneplain carved on the Otago schists, where is has been buried and exhumed. The plateau so formed is broken in many places so that Cretaceous and/or Tertiary strata are found in fault-angle depressions. In some places as at the Shag Valley already cited, it is evident that much faulting of the peneplain preceded as
well as followed the deposition of these later sediments; but at other places the faulting and folding may be entirely late Tertiary in date. Along the eastern fringe of Otago the peneplain surface carved on the schist shows a general south-easterly dip, in places interrupted by north-east-striking faults, and the schists dip under Cretaceous and Tertiary sediments which also strike north-east. The lowest of these beds, of upper Cretaceous age (at Kaitangata), consist largely of coarse conglomerates with interstratified sands, and thick seams of sub-bituminous coal; they are evidently terrestrial, but are only locally developed and are succeeded in some places (e.g. Kaitangata) conformably, in others unconformably, by a widespread thick series of quartz conglomerates containing rare marine fossils. These strata represent the denudation of an immediately adjacent land mass consisting almost entirely of the Otago schists, which are rich in quartz layers. Ongley's mapping for the Kaitangata bulletin, supported by detailed study of the coalfield (Lillie and Jenkins, unpublished report), shows that a major anticline follows the coast, with a core of schist well exposed in places. The thickest concentration of Cretaceous beds appears to lie between this anticline and the gently-south-east-dipping peneplain carved on the Otago schists to the east of a broad topographic depression which continues from Kaitangata almost to Dunedin. This depression, elongated along a north-east trend, is essentially a faulted syncline, and it formed the chief area of deposition in Cretaceous and Tertiary times. Benson remarks that “the present depression … is parallel to and at most a few miles west of the mid-Cretaceous faultbounded depression. Once again, the tectonic character of eastern Otago seems remarkably persistent”. Throughout his paper he points out evidence of repeated faulting and folding such as that already cited at the Shag Valley.
Benson (1935, 1941) has recognised also a late Tertiary peneplain which he considers, and Cotton admits (1938), may be widespread, though evidence may yet be forthcoming that very considerable areas of the interior plateau of Otago are formed by the exhumed fossil surface (of pre-Miocene, possibly Cretaceous, age) mentioned earlier instead of by the late-Tertiary peneplain (Raeside). In the Dunedin district Benson traces the faulted portions of this late Tertiary peneplain by means of covering lava flows. The surface, also with a gentle south-eastward dip, truncates the Cretaceous fossil peneplain generally at a slight angle, and may have been originally continuous with the peneplain described by Wellman and Willett as extending over the Southern Alps east of the Alpine Fault.
A further feature of Benson's paper (1941), whose main object is a detailed description of the igneous rocks, is the separation of eastern Otago into three tectonic districts according to the amount of deformation suffered. The least deformed district, occupying the eastern part of the field, is almost devoid of late Tertiary igneous rocks. The moderately deformed district which roughly follows the line of the great depression already described, contains many igneous rocks, chiefly of normal basaltic composition, and also many alkaline basic rocks, but the variety is not as great as in the strongly deformed central district around Dunedin. In this last, the alkaline rocks, earlier described by Marshall, occupy an important place and the rock types vary through numerous varieties from very basic olivine basalts to
phonolites, probably including also hybrid rocks. There are also many agglomerates and tuffs, giving evidence that this district constituted the main centre of volcanicity coincident with the maximum tectonic disturbance.
Benson's conception of the regional tectonics is “a series of broad but broken asymmetric folds of late or post-Tertiary age, in which many of the steeper limbs have been more or less replaced by faults,” as was shown by Cotton (1917, b, p. 252), in other parts of Otago. Benson notes that “usually the western limbs of the anticlines have the gentler slopes and provide the stripped and partially dissected back-slopes of broken anticlines or fault-blocks, the eastern and steeper limbs passing into fault scarps.” In the southern portion of the field, at Kaitangata, however, he remarks that the coastal anticline shows a steeper limb dipping to the west. Possibly this feature results from a depression of the basin to the west caused by the infilling load of Cretaceous conglomerates. The Tapanui Range also presents its steep face towards the west (Cotton, 1948, Otago's Physiography, pl. II) at the base of which a meridional strip of Tertiary beds is preserved in a syncline or fault-angle depression of the Otago plateau surface.
One of the most peculiar tectonic accidents in New Zealand has been re-examined by C. O. Hutton (1939). On the shores of Lake Wakatipu, at Bob's Cove, is an overturned sequence of Tertiary strata, 1,450 feet thick, forming, according to Hutton's interpretation, an upfold and a downfold, and flanked by the Otago schists. “Extending from the edge of Lake Wakatipu, for approximately 22 miles in a direction slightly east of north, is a narrow strip of the Tertiary sediments, never more than 150 feet thick, which have been caught in and thrust under the schists, by, it is believed, an easterly directed overthrust” (p. 73). The schist on each side of this Moonlight Thrust fault dips west at approximately 60°. Hutton's view that these sediments are compressed in a great thrust fault seems the only reasonable explanation. If the sequence at Bob's Cove is really inverted, as Hutton shows, then the upfold and downfold, which appear to have strong south-westerly pitch, represent respectively a syncline and an anticline, both upside down. The most logical conclusion to be drawn from Hutton's map is that the Tertiary beds at Bob's Cove represent only the inverted sedimentary cover of the western mass of schist. The Tertiary strata appear to be involved in schist, possibly to a depth of 3,000 feet or more (p. 85), and since the overpush appears to be directed eastwards it may be an important tectonic feature whose full significance in the regional tectonic pattern has been rather neglected.
Further Comments on the Relations of Newer to Older Trends
Macpherson, in postulating a swing in the strike of the late Cretaceous and Tertiary folds to assume a north-west trend throughout Otago and Southland, is obviously reasoning from the regional structural plan and partly from the evidence of tectonics, i.e. folds in growth during these periods. But it remains an open question to what extent the latter evidence supports the north-west swing of strike as being a dominant feature of the late Cretaceous and Tertiary diastrophism. The north-east strike of eastern Otago seems to be relegated to the category of minor cross fracture and folding accom-
panying principal movements along a north-west strike, whose former existence he admits.
Besides those in the eastern Otago region, however, many other infaulted blocks of Tertiary sediments located as basins within the broad mass of the Otago schists are elongated in a direction between north-east and north. The Bob's Cove Tertiary beds fall in this category. The structure at such localities is very often one of fault-folds, as in eastern Otago, rather than simple block-faulting. In Southland, the Waiau Valley follows a prominent syncline whose strike is very roughly north-south. Further east, also in Southland, Macpherson (1937) has described in the Centre Bush district a syncline of Tertiary beds which strikes very slightly west of north, very roughly parallel to the trend of the Waiau syncline. This syncline seems to cut quite athwart the north-west strike shown in the more steeply dipping Mesozoic rocks on its flank.
Macpherson has recognised these complications in writing his synthesis, but has regarded them as details. Nevertheless, in general, the north-west recurving of the are has less to commend it in terms of deformed Tertiary strata than has a general north-east to north trend for late Cretaceous and Tertiary diastrophism. It may be suggested, indeed, as for the Waiapu Subdivision, that when the late Cretaceous and Tertiary movements are grouped together there is no one dominant trend over the whole period and that what we see as a dominant north-west structure is essentially old and only partly resuscitated. Although outstanding, it is not immediately indicative of the magnitude of the fold movements confined within late Cretaceous and Tertiary times.
If a dominant and essentially Tertiary trend is sought in this cross-cross pattern, it seems preferable to imagine that it is east of north, swinging in places to north, continuing the enormous displacement represented by the Alpine Fault probably along the edge of the Waiau Valley for some distance. It is considerably modified by renewed north-west fold movements and the northerly swing may be a resultant between the two trends. The main folding of the basement of Otago schists and Trias-Jura sediments can then be relegated to an early Cretaceous or even older orogeny.
Connecting the Waiau Valley with the Southland Plains to the east, the graben of the Ohai coalfield appears to represent a Tertiary north-west valley as well as a Recent drainage course, and there has been great renewed late Tertiary movement here along an old trend of Cretaceous or earlier origin. This trend cuts across the principal Tertiary trend of the Waiau Valley. Here as in Otago there seems to be close relationship between a geomorphic feature and the growing fold.
The strike pattern in several parts of New Zealand then appears to be extremely complex: in some places a persistent axial strike may exist for a long time with the facies belts aligned roughly parallel to the strike along which movement is continually renewed. In other places, as in Waiapu and between Kaitangata and Dunedin, folding and faulting took place along one trend, followed by some planation, then tilting of the planed surface, so that an axial pitch was imparted to the folds already formed, and this tilting was followed by folding at right angles to the former trend. The time intervals between such oscillation of trends varies considerably from place to place.
The frequency of change of trend from time to time is indicated by the degree of complexity shown by a pattern of cross fractures, etc. The chief trend imparted by the late Pliocene movements is usually very markedly north-east or north-north-east. The swings of the New Zealand recurved are indicate areas where renewed movement along the north-west trends probably occurred fairly late in the Tertiary, but it is not yet definitely established that such movement is synchronous with the late Pliocene movements.
Writers who discuss the lineament pattern of the earth (e.g. Sonder) have sometimes attached especial importance to the directions north-east and north-west, suggesting that they are of planetary significance. The position is summarised by Umbgrove (p. 298): “There are good reasons to believe that the planetary lineaments date from a very early period of the earth's history. If this assumption is correct, it follows that these features, when buried under younger strata or tectonic structures become revealed again in the overlying cover. In this way some of the oldest features of our globe have been repeatedly rejuvenated. And they have to be considered as active elements even in the youngest tectonic zones of our globe.” The remark seems very appropriate to the structure of New Zealand. “It is a rather widespread belief that the origin of faults with a certain well-defined strike dates from a special period, whereas faults with a markedly different strike would date from another well-defined period. In certain areas this conviction is founded on sound arguments …” “If, however, the set of dislocations is not merely of local interest, but belongs to a system of planetary significance, then … we are only justified in saying that at a special moment a certain set of dislocations with a special direction of strike was inherited by the rocks under consideration from some older tectonic structure hidden in the structural units underlying them.”
Although hypotheses of thrusting and over-riding movement during late Cretaceous and Tertiary times are advanced to explain the structures in different parts of New Zealand, well-described examples of great overthrusts are surprisingly lacking. Low angle thrust planes formed in earlier orogenies are well known, particularly in the Mesozoic greywackes and in the Ordovician of Fiordland: most described examples have been inferred to be comparatively small, possibly only because the stratigraphy of the older rocks is too little known to indicate the existence of overthrusts of large dimensions. Overthrusts formed during late Cretaceous and earlier Tertiary movements may have been concealed by later Tertiary sediments, although more evidence of such thrusts might be expected to appear on maps if they existed. But for the latest Tertiary movements—those of late Pliocene age or later—there is little authentic evidence of great thrust planes, although much of the country where dominant faults separate Tertiary and undermass rocks has been mapped in detail. The great faults termed “thrusts” are all reverse faults and the majority of the faults appear at the moment to be very nearly vertical. The apparent exceptions of the Taitai klippes (emplaced during the Cretaceous) and the example quoted by Wellman can be interpreted as gravity-slide structures.
immediately applicable to the New Zealand pattern. Indeed, assuming a wildly speculative attitude, one might suppose that New Zealand lies on the intersection of two of Lake's great circles, those on which he located the centres of his ares of small circles.
Geomorphology in Relation to Structural Studies
The field worker in New Zealand carries a considerable amount of geomorphological ideas in his mental baggage, although his object may be solely structural and stratigraphical description. Many of the faults are first recognised by studying the physiography, and only later are they proved to dislocate strata. The recognition of ancient peneplains and fossil erosion surfaces, both Tertiary and early Cretaceous, is found to be essential in structural analysis in New Zealand, and the dislocations suffered by these surfaces are in many places the only reliable evidence bearing on the late Tertiary movements.
Recent transcurrent movement on faults in New Zeland is best demonstrated by the curious drainage pattern adjacent to the Alpine Fault near Jackson Bay. Wellman and Willett describe the Recent river courses along this fault as showing a remarkable displacement to the north-east immediately west of the Alpine Fault, the displacement for twelve rivers giving an average figure of 0·8 miles. Such transcurrent displacement over a long period could have produced lateral displacement on a great scale (compare Kennedy, 1946). H. Wellman has recently advanced the suggestion that dextral movement of approximately 300 miles along this fault has displaced the rocks west of the fault to the north-east. Thus he would account for the great distance separating areas of similar structure and stratigraphy on each side of the fault. An assessment of Wellman's view must await publication of his detailed evidence. Cotton (1947), in discussing the Alpine Fault, points out that some of the transcurrent movement along this north-east-striking fault could be absorbed by buckling along axes at right angles to the fault, and he cites the gentle south-westerly dip of a freshly cut marine platform, and other geomorphic evidence of local drowning, in support of this theory. It is instructive to compare the fold sequence of Waiapu with Cotton's suggestion for the recent pattern of warping. Such a mechanism can explain much of the New Zealand fold and fault pattern. Cotton (1948) also describes very clear geomorphic evidence at the Hope Fault near Hanmer, which shows transcurrent displacement of at least 8 feet, and for the fault in the axis of the valley of the Silver Stream near Wellington City, which shows a horizontal displacement of 130 feet. Faulting has continued intermittently on this line to the present day, and earthquake traces have been formed in very recent times.
This note has discussed geomorphology only insofar as it bears directly on the larger structures affecting Pliocene and older rocks and where other methods of approach have also been employed. There remains a wide field of geomorphological description covered by a voluminous literature, mostly by Cotton, and all more or less influenced by his teachings. The writer finds it impossible to summarise this aspect of the work. Certain of these papers are fundamental to understanding the large-scale structures, particularly in districts where the other
geological evidence is obscure. The vicinity of Wellington City is a typical example of such a district, and Cotton has devoted a number of papers to describing the downwarped Port Nicholson depression with a great late Tertiary fault bounding its western edge and running from the coast of Wellington along the Hutt Valley. Other important geomorphological papers bearing directly on the geological structure are cited in the list of references given below.
These topics are covered at length in Cotton's books, and a short paper by that author divides New Zealand into Geomorphic Provinces (1945).
In general, most recent papers tend to see a closer connection between tectonics and physiography. Thus, Cotton interprets the Hutt River as a stream initially installed in a fault-angle depression and thinks many major streams may be installed in early formed wrinkles of the earth's crust. From such streams directly reflecting tectonic pattern, he distinguishes others which he would define as structurally controlled, streams which have “worked down to find structural weaknesses” and, therefore, not directly indicative of the first-formed terrestrial pattern (1947). He has recently recognised a number of warpings of recent surfaces, some of them taking the form of gentle anticlines and synclines, but most of these have not yet been described in the literature. Interesting evidence of such warpings is cited by Cotton in the itinerary for a geological excursion in the North Island (Pacific Science Congress, 1949). This warping, or buckling, takes the form of gentle undulations with axes roughly oriented north-west, and is well seen near the Mohaka River (Hawke Bay). These recall the buckling which Cotton describes in discussing the Alpine Fault; but they have not been correlated with active trans-current faulting.
By combining geomorphology with detailed structural studies, there is good reason to hope that some continuity will be established between the palaeogeography of the later Tertiary and the morphological features of Recent times, even in those places where, as in large parts of the New Zealand region, fault and fold movements have intervened.
The structural plan of New Zealand is dominated by great folds and faults formed in late Tertiary times and generally ascribed to the “Kaikoura Orogeny”. Increasing evidence indicates that in some places the most considerable movements were concentrated into late Pliocene, possibly even early Pleistocene time, but that important precursor movements also took place earlier in the Tertiary, In other places the principal folding seems to have ceased in early Pliocene or even Miocene time, with only minor tilt and warping movements in the late Pliocene.
The rocks most deformed by Tertiary movements appear to be those which show evidence of late Pliocene movement, particularly in regions flanking the axial chain where Mesozoic and Palaeozoic strata are exposed.
The dominant strike of the Tertiary folds, particularly where of late Pliocene age, is north-east to north-north-east, but locally the
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|N.Z. Stratigraphic Terms||Continued earth movements along faults|
|local tilts and gentle warpings|
|major orogeny—Kaikoura orogeny||Folds, generally of considerable wave length, and great faults formed.|
|(Wanganui) Pliocene||Castleolifflan Nukumaruan|
|(Wanganui) Pliocene||minor fold movements and tilting||e.g. in Hawkes Bay.|
|(Wanganui) Pliocene||Waitotaran Opoitian|
|minor fold movements and tilting||e.g. in Te Kuiti. Taranaki is locally transgressive over Southland.|
|Miocene||Southland||minor orogeny in Upper Southland widespread||Great influx of coarse conglomerates in Upper Southland which locally (e.g. Waiapu) rests unconformably on Lower Southland.|
|transgression of Lower Southland very extensive|
|Oligocene||Landon||uplift or minor folding widespread||Local absence of Pareora (e.g. Hawkes Bay) indicates emergence at beginning of Pareora or its removal immediately after Pareora time.|
|Eocene Paleocene||Arnold Dannevirke|
|minor local fold movements, ultra-basics in Auckland||Locally (e.g. Eastern Hawkes Bay) Eocene conglomerates rest unconformably on Upper Cretaceous.|
|Cretaceous||Senonlan||Mata||minor orogeny, probably widespread. The north-east elongation of Cretaceous and Tertiary belts facies may be due to this orogeny.||Upper Senonian in some places marked by unconformity with great boulder beds resting on Lower Senonian.|
|Cretaceous||Neocomian||Major orogeny Hokonui or post-Hokonui orogeny, or two orogenies—one late Jurassic and the other post-Aptian not yet separated.||Close isoelinal folding and thrusting N.W. strike in Permian, Trias and Jurassic of Hokonui syneline and Otago schists. This orogeny usually attributed to very late Jurassic or early Cretaceous, but Aptian strata appear to be equally deformed.|
|Jurassic||Hokonui System||Conglomerates with large boulders at base of Carnic (e.g. Te Kuiti).|
|minor orogeny widespread|
|Permian||Maitai System||Carboniferous strata not yet separated. Marked meridional strike in lower Paleozoic, steep folds and great faults. Devonian probably unconformable on Ordovician at certain localities (e.g. Mt. Arthur).|
|Devonian||minor or major movements widespread|
|Silurian*||Inferred from presence of coarse conglomerates in Mid Cambrian.|
The table of orogenies is a very generalised statement and the detailed stratigraphic evidence bearing on the minor “orogenies” is only slightly covered in the text of this paper. It seems likely that, as more precise evidence becomes available, the “major orogenies” will have to be spread out to cover longer periods of time, thus losing much of their present “sharpness,” which is partly subjective. Although many of the “minor orogenies” as a result may fit into a picture of continuous movement leading to a final “major orogeny,” the latter must be retained as representing maxima of fold movement concentrated into comparatively short time.
[Footnote] *Not definitely known to exist in New Zealand.
Tertiary strata strike north-west, and in several places the evidence suggests that such a strike follows that of an older pre-Tertiary fold pattern.
The great faults formed by the late Tertiary movements have throws often exceeding 3,000 feet, and remarkably straight traces, appearing to be mostly vertical or high-angle reverse faults: there is no evidence suggesting great recumbent folding or nappe-formation in the Tertiary orogenic sequence, although minor overthrusts are known.
The Triassic and Jurassic strata are strongly folded and broken by faults formed in pre-Senonian time. The formation of these has been ascribed to the post-Hokonui Orogeny to which is generally ascribed a late Jurassic or early Cretaceous age. Since more recently an Aptian age has been ascribed to beds which are hardly less deformed than the Jurassic strata, it seems possible that this post-Hokonui Orogeny may be in part immediately post-Aptian in age. In many places the strata deformed by these Mesozoic movements show a north-west strike which may be cut across by later Tertiary north-east striking folds and faults. But this north-west strike is not shown everywhere by the Triassic and Jurassic strata. As yet it has been impossible to advance any adequate synthesis of the Mesozoic fold pattern and information on earlier orogenies is even more obscure.
It seems likely that there was a great late Devonian or immediately post-Devonian orogeny imparting a generally meridional strike to the lower Palaeozoic strata, but the latter are so much cut by Tertiary faults that the suggestion can only be very tentatively advanced.
A general tendency for folds of one age to show a common trend is accepted, although there are many local exceptions. Attention is focused on certain regions where the evidence suggests a considerable change in the trend of folds and faults formed even within a comparatively short space of time, because the time factor in tectonics combined with the idea of posthumous folding appears to shed light on these more abrupt swings in trend ascribed to “syntaxes” and to “arcuate structure”. In the north-east of the North Island, folding of thick Cretaceous and early Tertiary beds along an approximately north-west trend was followed by emergence, planation, subsidence, and sedimentation, and then, after an interval of approximately four Tertiary stages, by folding along a north-east trend, but it is likely that during this interval minor folding along both trends occurred locally. Nevertheless, the dominant trend of the late Tertiary folds in this part of the North Island is north-east and is consistent with the old idea of Marshall and Suess that an anticlinal ridge is likely to continue towards the Kermadec and Tonga Islands. There is no clear evidence of an arcuate swing in the north-east part of the country, because the major fold pattern is only broken by great cross-faults, striking west-north-west. The north-east strike formed by late Tertiary folding is dominant throughout the whole length of the eastern side of the North Island, formerly the site of a geosyncline.
An axial chain of Mesozoic greywacke and the great Tertiary and Recent volcanic centre of Rotorua separate the eastern belt from the western Tertiary geosyncline of Macpherson. This axial chain was partly emergent during long periods of Tertiary sedimentation in the western and eastern geosynclines. In the western belt a true arcuate
swing in strike of both the Triassic and Jurassic strata of the under-mass and of the covering Tertiary beds can be observed; to the south the beds strike approximately north-south and, on going northwards, swing more and more towards a north-westerly trend. But the amount of cross-faulting at Auckland and in North Auckland suggests that the strata also adjusted themselves along a roughly north-east line of faulting and minor fold-movements: the result in Auckland is a pattern of parallelograms. The region where the trend of the western belt diverges from that of the eastern must be located very roughly at the north end of the South Island. The term syntaxis can probably be applied to this convergence of trend if one considers only late Miocene to early Pliocene fold movements because a late (post-Pliocene) fold movement affecting the eastern belt profoundly had much less effect in the western belt.
Suess (p. 233) accepted the idea that two independent dissymmetrical mountain chains converged in the southern part of the South Island, forming a syntaxis (schaarung). Morgan, on the other hand, rightly insisted that on the West Coast the north-east striking trends cut across an older north-west trend at the great north-east-striking Alpine Fault, and it is now quite clear that the north-east trend was largely formed in a late Tertiary orogeny, although this trend may also follow an older one. According to Suess (p. 131) the superposition of a newer set of folds over more ancient folds did not constitute a syntaxis (schaarung), so that the idea of syntaxis would be inapplicable to the West Coast, for the time gap in this case is very considerable.
In the southern part of the South Island—which is the part chiefly cited by Suess (p. 233)—the evidence for syntaxis on the map appears to be stronger, but the outstanding folds affecting Tertiary strata trend dominantly north on the west side and north-east on the east side. The northern trend indicates a swing in the strike of Tertiary strata deposited in a western geosyncline and this swing is likely to be dictated by later movement along an old Palaeozoic trend, perhaps Late Devonian, known in the rocks of the Palaeozoic undermass which are exposed to the west. In addition, there have been local movements, in places considerable, along a north-west strike, but as great north-west folding occurred in early Cretaceous time, the movements are essentially posthumous and the amount of Tertiary regional movement along these trends may not have been so very great, although the north-west direction appears conspicuously on a map.
In Southland, a north-west-striking graben cutting across an anticline with a general northern orientation appears to mark a Tertiary and Recent valley as well as probably an old tectonic feature. The Manawatu Gorge also marks a Recent river coincident with an axial depression and a Tertiary strait. Paréjas has indicated that in parts of Europe points of maximum axial pitch or elevation may be aligned transverse to principal folds, and these points may all show some palaeogeographic characteristic in common. These lines he has named transversals. Both the examples cited are likely to represent small transversals. Paréjas's findings may be significant, not only in palaeogeography, but in deciphering the older tectonics of a belt showing one very outstanding strike formed in a later orogeny.
The amount of renewed movement along north-west trends in the southern part of the South Island is so marked that the Tertiary grain can in places be regarded as showing two trends at right angles to each other. To what extent movement occurred along both trends simultaneously is unknown. Conceivably movement along the Alpine Fault and renewed movement along the north-west trend of the Hokonui syncline occurred at the same time, giving a “syntaxis”.
It is possible that a dynamics of folding, planation, tilt along the old fold axes and later folding or faulting at right angles to them may be repeated many times. Cotton's suggestion of warping to absorb trans-current movement along great faults fits into the same picture. Pro-
bably for one particular age of folding one trend is markedly dominant, but the folding over several ages—say within the late Cretaceous and Tertiary—may show many fluctuations from one extreme to the other. It seems likely that the repetition of folding along old trends may, particularly when the trends are so commonly north-east and north-west, be associated with the influence of some deeper texture in the earth.
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