|The tectonic evolution of the Marlborough region has been studied by application of fission track thermochronology and finite element (FE) methods. The region lying within the Australian-Pacific plate boundary zone is considered to have a transcurrent fault system, known as the Marlborough Faults System (MFS). The MFS is viewed as comprising secondary transforms connecting the Hikurangi subduction margin with the main Alpine Fault oblique-slip boundary. This fault system appears to have developed sequentially towards the southeast during the past 5 million years. Consequently, the subducted Pacific plate has extended beneath the region.
The FE modelling results reveal that the accommodation percentage of total displacements in Marlborough is about 85% of the total plate motion. The instantaneous displacements of the Marlborough faults show successively increase to the southeast away from the Wairau Fault. The contour of instantaneous displacements estimated by the FE modelling can be compared to the topography of Marlborough. According to the results of the FE modelling cases, the main conclusions drawn are: (1) A curved fault (the Alpine Fault) resulted from a change in the plate motion vector and is a manifestation of dextral tectonic rotation. (2) The development of the MFS reflects the continuation of tectonic rotations. (3) Three secondary faults (Awatere, Clarence, and Hope) may have developed within a short period of one another. This suggests that the change in plate motion has impacted in Marlborough. (4) Uplift movements of the Spenser Mountains and Kaikoura Ranges still continue. (5) Suggate’s model (1979) of a pre-existing fault offset of the Alpine Fault is not a unique result in the FE modelling.
The extremely young fission track ages (<10 Ma) in the vicinity of the Alpine Fault bend and Seaward Kaikoura Range coincide with the recent rapid uplift/erosion in these areas. All the apatite ages are less than the corresponding depositional ages of the samples, which indicates that the host rocks in Marlborough have experienced exposure to elevated temperatures in the zone of partial annealing for apatite, some of them having been reset. Except for the samples in the Marlborough Sounds region, zircon fission track ages are older than 119 Ma, reflecting that the host rocks of the samples have not experienced temperatures in the zircon partial annealing zone since the mid Cretaceous. Apatite fission track ages and mean lengths indicate that there are two major cooling events: one occurring from the early Miocene (~20 Ma) and the other in the mid Cretaceous (~100 Ma). Modelled thermal histories indicate that in the Wairau block the timing of the main Neogene uplift/erosion event is earlier (mid to late Miocene) than to the southeast in the Seaward Kaikoura Range (late Pliocene-Pleistocene). The greatest amount of cooling in Marlborough occurs along the Alpine Fault in the vicinity of the big bend, where rocks have cooled from temperatures above 240 ± 25°C (temperature at which fission tracks are reset in zircon). These samples derive from the Alpine Schist belt. Over wide parts of Marlborough the rocks now at the surface have cooled from lower levels of the apatite partial annealing zone (<110°C). Generally, the rocks have cooled from at least 50°C to surface temperatures (10-15°C).
The largest amount of rock uplift (~11.5 km) occurs in the area of the Alpine Fault bend. The amounts of rock uplift and denudation derived from the fission track parameters are in the range 0.7-11.5 km and 0.6-11.0 km, respectively. The amounts of rock uplift in the Inland and Seaward Kaikoura Ranges are about 2.4 and 4.8 km, respectively. The amount of denudation across Marlborough is generally higher in the Wairau block than elsewhere. In the Seaward Kaikoura Range, high elevation coincides with large amounts of denudation. Compared with the region of continent-continent convergence to the south in Canterbury, the amounts of rock uplift and denudation in Marlborough are relatively small, revealing the differences between a fully developed continent-continent collision zone and the continental transform setting in Marlborough.
The horizontal movements determined by the FE modelling can be converted into vertical movements. Both the FE modelling and fission track results show that the pattern of vertical deformation is consistent with the topography in Marlborough: the younger the fission track age, the more uplift/deformation there has been. The FE modelling and fission track results reveal the character of the tectonic evolution of Marlborough and are a step towards its quantification.