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
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|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.