Subduction, platform subsidence, and foreland thrust loading: The late Tertiary development of Taranaki Basin, New Zealand

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Abstract
Borehole, seismic, and gravity data are used to investigate deformation of continental lithosphere at a Miocene collisional zone. Deformation is manifested in the three following principal forms: a long wavelength (>500 km) platform subsidence ascribed to mantle convection; flexural deformation on a scale of 100–200 km due to crustal thrusting at the eastern boundary of the Taranaki Basin; and a ductile thickening, evident on the deep seismic section of Taranaki Basin, that occurs on a scale of ∼10 km. Evidence for flexural deformation principally comes from the deep seismic section that shows a 150‐km wavelength bending of the Moho down toward the major zone of thrusting within the Taranaki Fault Zone. Paleowater depths, however, provide evidence for an initial early Miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading. Instead, we propose that this broad "platform subsidence" was driven by loading from a deep source, probably subduction‐induced flow in the mantle. By ∼22–19 Ma, 1–2 km of water existed over most of the area now occupied by South Taranaki Basin. By ∼19–17 Ma the water depth in the zone east of the Taranaki Basin, the Taranaki Fault Zone, had been replaced by rock due to submarine thrusting and crustal thickening. This build up of submarine topography in the Taranaki Fault Zone constitutes part of the load (25±8 MPa) that created and maintains South Taranaki Basin. Gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading, equivalent to 15±7 MPa over a 50‐km‐wide zone. This intracrustal load is explained as being due to thick‐skinned thrusting bringing denser, lower‐crustal rocks nearer to the surface in the thrust zone. The complete load on the Taranaki foreland is therefore in three parts; the submarine‐topographic load, the intracrustal load, and the loading of infilling sediments.