Geology of the Tinui district

Abstract

The Tinui District is assumed to be typical of the more deformed part of the New Zealand Mobile Belt. It contains an unusually complete stratigraphic record, rocks representing most stages from Upper Jurassic to Recent being present. Although the rocks are strongly deformed, the complex diapiric structures that occur in the northeast of the mobile belt are absent. The stratigraphy is described in terms of formations which are then used to infer the paleogeography for eight periods of time. An attempt is made to treat the structure according to its development with time. The main conclusion is that there was a change in the strike of the fold axes and in the sense of movement of the faults. Strong folds, striking approximately northeast, are Paleocene in age and weak folds, striking approximately north, are post-Miocene. There are two fault trends, one NNE and the other ENE. The ENE striking faults were dominant in the Early Cenozoic and the NNE striking faults were dominant in the Late Cenozoic. The sense of movement on the NNE faults changed from sinistral to dextral. The change in the direction of the axes and in the sense of movement on the faults can be expressed as a change in the direction of maximum horizontal shortening, which is inferred to have changed with time. It is also found that the rates of tilting, and probably faulting, have not been constant with time, but occurred as bursts (disturbances) in the Paleocene, Early Miocene Late Pliocene, and Late Quaternary. The Mesozoic part of the geological history of the Tinui District is scrappy and far less complete than the Cenozoic part. In order to place the Tinui District in a broader setting, the central part of the New Zealand landmass in the Cenozoic, called the New Zealand Mobile Belt, is discussed in some detail. The mobile belt consists of fault blocks which form a geanticline along the New Zealand landmass and a geosynclinal trough between the east coast and the Hikurangi Trench. It is shown that a clear distinction has to be made between tilting and uplift. A main feature of the New Zealand Mobile Belt is the dextral faulting, on major NNE striking faults, in the Late Cenozoic. A major reversal in the direction of maximum horizontal shortening was found in the Tinui District to have taken place at the beginning of the Miocene or in the Oligocene. The reversal indicates that the dextral faulting of the New Zealand Mobile Belt may have started at that time, and that earlier strike-slip movement had been sinistral. This conclusion contradicts existing reconstructions of the New Zealand landmass with time, and a more complex reconstruction is required to satisfy the tectonics of the Tinui District.