TECTONICS, VOL. 13, NO. 5, PAGES 1068-1092, OCTOBER 1994 Subduction,platform subsidence, and foreland thrust loading: The late Tertiary developmentof Taranaki Basin, New Zealand w. E. Holt DepaRment of Earth andSpace Sciences, State University of New York at Stony Brook T. A. Stem Research School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand 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 principalforms: a long wavelength (>500 km) platform subsidence ascribed to mantle convection; flexural deformation on a scale of 100-200km due to crustal thrusting at the easternboundary of the Taranaki Basin; and a ductile thickening, evident on the deep seismicsectionof Taranaki Basin, that occurs on a scale of -10 km. Evidence for flexural deformation principallycomesfrom the deep seismic section that shows a 150-km wavelength bendingof 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 sheetloading. Instead,we propose that this broad "plafforln 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 SouthTaranakiBasin. By -19-17 Ma the water depth in the zone east of the Taranaki Basin, the Taranaki Fault Zone, had been replaced by rock due to submarine thrusting and crustal thickening.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 sublnarine topography in the Taranaki Fault Zone constitutes part of the load (25 _+ 8 MPa) that created and maintains SouthTaranaki Basin. Gravity dataplacefurtherconstraints 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 intracrustalload is explained as being due to thick-skinned thrusting bringing denser, lower-crustal rocks nearer to the surface in the thrustzone. The complete load on the Taranaki foreland is therefore in threeparts; the submarine-topographic load, the intracrustal load, and the loading of infilling sediments. Introduction Foreland basins are an isostatic response to e•nplaced thrust loads [Price, 1973] and are colnmonly found adjacent to fold and thrust belts [Coney, 1973; Dickinson, 1974]. Well- known examples of coupled foreland basin-activefold and thrust belt pairs are the Alberta Basin-Rocky Mountains [Beaumont, 1981], Molasse Basin-European Alps [Karnerand Copyright 1994by theAmerican Geophysical Union. Paper number 94TC00454. 0278-7407/94/94TC-00454 $10.00 Watts, 1983], and the basins of the Brazilian shield-Bolivian Andes [Lyon-Caen et al., 1985]. There has also been a recognition that forelandbasinsdo not necessarily have to be coupled to a growing, subaerial orogen, as thrusting and loading can occur below sealevel on a passive margin andstill inflict the same order of deformation associated with supracrustal thrust sheets [Quinlan and Beaumont, 1984; Stockreal et al., 1986]. This study focuses on such a case, where much of the loading was brought about by submarinethrusting onto a subsided continental platform. Sediment isopachs, paleobathymetric, and gravity data are the principaldata used to analyze the loading. We show that when the complete sediment load and initial loading conditions are accounted for, the estimated thrust load is consistent with the shapeof the Oligocene-Miocene isopach and the observed gravity anomaly field. Two possibly general findings for foreland basinsevolve from our study. First, because the basin is overfilled with sediments,there is poor control on rigidity. This finding confirms what Flemings and Jordan [1989] predict on theoretical grounds. Second,at least three modesof crustal deformationare apparent at and adjacent to the thrustfront. Theseincludethe following: (1) an initial regional platform subsidence on a wavelength >200 km, (2) a flexural response of wavelength -150-200 km, and a forin of ductilethickening in the lower crust on a scale of-10 km. Structure of Taranaki Basin In this studywe describe and interpretthe crustalstructure and geohistory of a submarine, continental basin; South Taranaki Basin, New Zealand(Figuresla and lb). The early history of the South Taranaki Basin was dominated by Cretaceous rifting, associated with the opening of the Tasman Sea [Laird, 1980]. Most of the sedi•nentary sequence in South Taranaki Basin was, however, deposited during a compressional phase from the Oligocene to late Miocene. For example,Pilaar and Wakefield[1978] mapped a 4- to 5- km thick, wedge-shaped package of mainly Miocene sediments that are bounded by steep thrustfaults at the eastern edge of the basin. A recentdeep seismic reflectionsurvey by Stern and Davey [1990] revealed at least 10 km of crustal thickening beneath the eastern margin of South Taranaki Basin (Figure 2), and therefore they propose that during the Miocene, SouthTaranakiBasindeveloped as a classic retroarc (in the sense definedby [Beaumont [1981]) foreland basin. 1068