SUBDUCTION, PLATFORM SUBSIDENCE, AND FORELAND THRUST LOADING - THE LATE TERTIARY DEVELOPMENT OF TARANAKI BASIN, NEW-ZEALAND

Borehole, seismic, and gravity data are used to investigate deformatio n of continental lithosphere at a Miocene collisional zone. Deformatio n is manifested in the three following principal forms: a long wavelen gth (>500 km) platform subsidence ascribed to mantle convection; flexu ral deformation on a scale of 100-200 km due to crustal thrusting at t he eastern boundary of the Taranaki Basin; and a ductile thickening, e vident on the deep seismic section of Taranaki Basin, that occurs on a scale of similar to 10 km. Evidence for flexural deformation principa lly comes from the deep seismic section that shows a 150-km wavelength bending of the Moho down toward the major zone of thrusting within th e Taranaki Fault Zone. Paleowater depths, however, provide evidence fo r an initial early Miocene regional subsidence that is too long in wav elength 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 th e mantle. By similar to 22-19 Ma, 1-2 km of water existed over most of the area now occupied by South Taranaki Basin. By similar to 19-17 Ma the water depth in the zone east of the Taranaki Basin, the Taranaki Fault Zone, had been replaced by rock due to submarine thrusting and c rustal thickening. This build up of submarine topography in the Tarana ki Fault Zone constitutes part of the load (25 +/- 8 MPa) that created and maintains South Taranaki Basin. Gravity data place further constr aints on loading at the thrust front and point to an additional intrac rustal loading, equivalent to 15 +/- 7 MPa over a 50-km-wide zone. Thi s intracrustal load is explained as being due to thick-skinned thrusti ng bringing denser, lower-crustal rocks nearer to the surface in the t hrust zone. The complete load on the Taranaki foreland is therefore in three parts; the submarine-topographic load, the intracrustal load, a nd the loading of infilling sediments.