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.