Post-Eocene Development of the Taranaki Basin, New Zealand: Convergent Overprint of a Passive Margin: Chapter 7: Southwest Pacific and Eastern Indian Ocean Margins

The Taranaki Basin has undergone a complex sedimentary and tectonic history since the late Oligocene, reflecting the impingement of deformation associated with the evolving Pacific and Australian convergent plate boundary. At various times through the Neogene, elements of a passive continental margin, retro-arc foreland basin, foreland fold and thrust belt, inverted subbasin, back-arc rift and back-arc contractional basin have been variously displayed within the Taranaki Basin. Following its initial Cretaceous development within a synrift setting, the Taranaki Basin evolved as a passive margin. Postrift cooling and foundering of the New Zealand continental plateau effected a widespread transgression, and by early to mid-Oligocene times, the Taranaki Basin lay almost submerged. A dramatic increase in subsidence in the late Oligocene is interpreted as foreland basin-type deepening. A basement block bounded by the Taranaki fault began to progressively overthrust westward to ultimately form the present-day eastern margin of the Taranaki Basin. This initially transpressive tectonic regime was a manifestation of a protoplate boundary through western New Zealand. Sedimentary response to this change in tectonic style was at first muted. Carbonate deposition was enhanced as a result of the increased intrabasinal submergence. Thereafter, the supply of terrigenous sediment into the basin increased in the early Miocene coincident with main overthrusting of the Taranaki fault. In the south and southeast parts of the basin, a steady northwestward progradation of the shelf edge began in mid-Miocene times as sediment supply exceeded subsidence. Submarine fans were deposited on the lower slope of the migrating shelf. End_Page 93------------------------- On the basin's eastern flank, compression along the outer edge of the foreland thrust belt continued until around 10 Ma. This was expressed as ongoing growth of thin-skinned overthrusts within the Tarata thrust zone. Elsewhere in the east, foreland subsidence and bathyal water depths prevailed; whereas in the north, late Miocene volcanism occurred. These high-potash andesites are presumed to have deep subduction origins. Around 10 Ma, the compressive focus in the Taranaki Basin shifted from its eastern to its south and southeastern margins where former rift half-grabens were inverted in the latest Miocene. This is related to a shift in the direction of convergence across and alignment of the plate boundary. At the start of the Pliocene, accelerated progradation of the shelf began in the Taranaki Basin, and giant clinoform-bounded sediment wedges were built out to the north and west. The progradation is attributed to an excess of sediment supply over accommodation, caused by a marked increase in hinterland uplift and erosion. Relative sea-level lowstands were a less important cause of sedimentary imbalance and shelf outbuilding. Pliocene rifting of the North Taranaki Graben and downwarping of the South Taranaki Graben are back-arc effects of Pacific plate subduction.