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Analysis of the South Westland Basin, New Zealand

Running parallel to the thrust front of the Southern Alps, an elongate, predominantly offshore and relatively deep basin, considered to have originated as a foreland basin, has previously been identified in South Westland, South Island, New Zealand. An integrative approach to basin analysis has been undertaken in this thesis to test the hypothesis that this basin has been formed by the loading of the Pacific Plate onto the Australia Plate during the Late Cenozoic. The two principal approaches to this problem undertaken in this thesis are seismic mapping of industry acquired seismic reflection profiles available for the basin and geodynamical analysis of the basin geometry and its stratigraphy. Seismic mapping reveals a thick sedimentary section that can be divided broadly into five sequences. The thickness distribution of these units have been digitized into a computer database for subsequent modelling and mapping. The five sequences are titled Sequence E through to A from the basement to the surface. The lowermost sequence, Sequence E, is a thin sheet of marginal marine sediments thickening toward the west. It is believed to have formed in a rift margin setting during the Late Cretaceous-Eocene, overlain by a regionally extensive limestone formed during a marine transgression in the Oligocene. Sequence D has a sheet-like form comprising turbiditic systems deposited in bathyal depths between around 30 Ma and 15 Ma. This is coincident with increasing convergence across the Alpine Fault and initial loading of the Australia Plate, and Sequence D may represent the early underfill stage of foreland basin development. Sequence C is wedge-like in form, consisting of deep water turbiditic systems rapidly deposited between 5 Ma and 3.1 Ma. This corresponds with the uplift of the inner margin of the basin along the South Westland Fault Zone. Sequence B is a prominent lensoidal form prograding facies consisting of mudstone with frequent coarser beds deposited between 3.1 Ma and 1.2 Ma. Sequence A tops the section with a possible eustatically controlled and predominantly slumped deposit of mudstone with thick units of bioclasts near the present surface. Geodynamic analysis reveals an early stage of thermal subsidence due to late Cretaceous rifting that preceded Tasman Sea spreading. It is compatible with a lithospheric stretching factor of 1.25. Increasingly rapid subsidence since 15-10 Ma is interpreted as possible fault-controlled subsidence due to strike-slip deformation combined with loading resulting from increased convergence across the Alpine Fault. This is followed by the uplift of the inner margin of the basin around 5 Ma to form the presently exposed coastal strip. This event probably also contributed to the loading of the Australia Plate. Flexural analysis of the basement structure of the Australia Plate reveals an unusually weak lithosphere with a rigidity around 9.79 x 10²⁰ Nm. Several alternative interpretations are presented to explain this unusually weak lithosphere, such as high heat flows, lithospheric decoupling and inherited structures. The complex evolution of the basin from rift margin, through strike-slip to foreland basin is believed to have left behind structures that may constrain and/or enhance the flexural response of the Australia Plate.
Type of thesis
The University of Waikato
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