The cenozoic structure of the Murchison & Moutere Basins, South Island, New Zealand

Abstract

Structural mapping in the Murchison Basin demonstrates that faults follow a NNE structural grain, which is inherited from Paleozoic sutures that were zones of weakness prior to the Late Eocene inception of the basin. The Tainui Fault may be the southern extension of the Anatoki Thrust separating Paleozoic sedimentary belts and was reactivated during Late Cenozoic compression. A dextral component of movement on the Tainui Fault has caused transport of the Mount Murchison Block southwestwards along a basement ramp thrust and folded detachment in basal Late Eocene sediments. Similarly, the Owen Fault was also reactivated so that it now juxtaposes Tertiary sequences of the Murchison and Moutere Basins. The Matiri and Maunga Faults are Late Eocene normal faults which have been reactivated and overturned. They mark the western margin to the basin and the junction with a relatively high Karamea Batholith, across which there are distinct thickness and facies changes, as well a marked decrease in percent shortening, calculated from cross-sections to be 5% across the basment high compared with 50% within the Murchison Basin. The eastern boundary is formed by the Tutaki Fault, a southeast-dipping thrust fault which separates the main bodies of the Separation Point Batholith and the Rotoroa Complex. Folds within the basin also follow the NNE trend and are non-concentric, doubly-plunging, steeply-inclined to upright folds. The anticlines have tightly-folded cores, facilitated by fracturing of basement. The Longford Syncline reaches a depth of 8km and shows a change in geometry to a chevron style to the south, due to a small proportion of incompetent material in the Longford Formation. Some folds have formed as drape folds over a warped basement surface and are controlled by a master fault at depth. The dominant control on the development of folds within the Murchison Basin is WNW-oriented compression and basement block-faulting, rather than shearing associated with dextral or sinistral transpression. Reinterpretation of an open-file, reconnaissance seismic reflection survey of the Moutere Depression, integrated with information from two exploration wells, geological maps of the region and a geophysical survey, has revealed three sub-basins beneath post Late Miocene gravels; they are subparallel to Tertiary basins of the West Coast Region and have preserved thicknesses of up to 2500m. One sub-basin is directly related to Late Tertiary outcrop geology, but its extent and geometry was formerly unknown; the other sub-basins are Early Tertiary half-grabens. Their southeastern margins have been overthrust during Mid-Miocene to Pliocene WNW-directed compression. An axial zone of shortening can be traced from the coast to the Kawatiri Fault and represents the northern extension of the Tutaki Fault, separating Rotoroa Complex and Separation Point Batholith basement rocks in the Moutere Depression. Shortening across the Depression was previously more widely distributed, but is now concentrated along the Waimea Fault System, where tectonic loading since the Plio-Pleistocene has caused gravels of this age to dip towards the eastern margin. Comparison of the deformational histories reveals that there was pervasive and more intense deformation in the Murchison Basin during the Late Miocene to Pliocene, whereas deformation in the Moutere Depression was more localised. Deposition in the Moutere Depression was essentially continuous during the Late Miocene and Pleistocene and the area was a major depocentre for material shed off the rising Southern Alps, whereas the Murchison Basin was an area of non-deposition or uplift at this time. While basement block-faulting was an important control on the development of both the Murchison and Moutere Basins, basement sutures beneath the Murchison Basin were weaker in extension and compression than those in the Moutere Depression, resulting in the Murchison Basin being the deepest and most intensely deformed basin of the West Coast Region.