The tectonic history of New Zealand

Summary The tectonic history of Kew Zealand falls into two periods of major activity, a late Mesozoic‐early Tertiary phase, and a late Tertiary‐Quaternary phase. The first phase is here termed the Rangitata Orogeny and the last phase the Kllikouni Diastrophism. The Rangitata Orogeny started in the southern part of the South Island probably early in the Jurassic and progressed northward bringing more and more of the New Zealand Geanticlinc above sea‐level. The nose of the geanticline reached the North Island area late in the Cretaceous and the geanticline was above sea‐level for the whole of the New Zealand area early in the Tertiary. There is evidence that the New Zealand Geanticline rose above sea‐level earlier in the northern part of the North Island of New Zealand, but this is at present interpreted as a local undulation in the geanticlinal body. On the eastern and western sides of the geanticlinc, secondary, sedimentary basins‐the Eastern Basin and the Western Bacsin‐developed, in which most. of the Cretaceo‐Tertiary sedimentation in New Zealand took place. While the geanticline was being brought above sea‐level from the New Zealand Geosyncline, a force acting from north‐north‐east to south‐south‐west deformed the geanticline and pressed it westward into an· arc‐the Alpine Schist Arc. This internal deformation caused the deve.lopment of the schistose rocks in this arc by stretching and slip in the Southern Alps part of the are, and by crumpling and a minor amount of stretching in the Otago part of the arc. The stretching in the Southern Alps region probably reached its maximum in the late Cretaceous, and thereafter no schistose rocks were formed. During the westward movement of the Alpine Schist Are, the Western Basin was overwhelmed in the area between the arc and the FiordlandStewart Island Complex. The hypothesis that the Alpine Schist Arc is a large nappe is herewith proposed. Ultrabasic rocks are in such a position that they could very well be situated on the thrust plane of the nappe. Gravity anomalies are most likely to have been caused by the overfold. The Alpine Schist Arc was broken up after the Eocene, because, at that time, the deforming movements were active in a more east‐west direction. Immediately prior to this external deformation, the Alpine Arc had been wrenched off the northern part of New Zealand Geanticline in the Marlborough Sounds area, thus causing the different attitudes in the schistosities north and south of the Wairau Fault. The breaking‐up continued until the present, but the speed of deformation increased markedly during the Kaikoura Diastrophism. During the latest stages of the internal deformation and the earliest stages of the external deformation, faults showing dextral transcurrent movement developed parallel to the geantic1inal ridge. These faults swung westward during the Wairau wrench‐off and continued across the geantic1inal body, cutting it into compartments during the external deformation. These faults have cut the Marlborough‐East Coast area into a large number of parallel segments. As the faults are all dextral transcurrent, all the segments must move in a southerly directiolli. One of these segments, the Ruahine—Rimutaka Horst, was uplifted during the development of the WanganuiPetane Trough; its continuation in the Marlborough area is the Spenser Mountain segment between the Wairau and Awatere faults.