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.