The South Westland Basin: seismic stratigraphy, basin geometry and evolution of a foreland basin within the Southern Alps collision zone, New Zealand

This paper develops further the case for a foreland basin origin of South W estland Basin, located adjacent to the Southern Alps mountain belt. Geohist ory analyses show Middle Miocene initiation of subsidence in the basin, wit h marked increases at 5-6 Ma. Five seismic reflection horizons, including b asement,Middle Miocene (top Awarua Limestone), top Miocene, mid-Pliocene (P PB) and mid-Pleistocene (PPA) have been mapped through the grid of seismic data. A series of five back-stripped structure contour maps taken together with five isopach maps show that prior to the Middle Miocene, subsidence an d sedimentation occurred mainly along the rifted continental margin of the Challenger Plateau facing the Tasman Sea; subsequently it shifted to a fore deep trending parallel to the Southern Alps and located northwest of them. Through the Late Miocene-Recent this depocentre has progressively widened, and the loci of thickest sediment accumulation have moved northwestward, mo st prominently during the Late Pliocene and Pleistocene with the progradati on of a shelf-slope complex. At the northern end of the basin the shelf-slo pe break is currently located over the forebulge, which appears not to have migrated significantly, probably because the mountain belt is not advancin g significantly northwestwards. Modelling of the lithospheric flexure of th e basement surface normal to the trend of the basin establishes values of 3 .1 to 9.8 x 10(20) Nm for the flexural rigidity of the Australia Plate. Thi s is at the very low end of rigidities for plates, and 1-2 orders of magnit ude less than for the Australia Plate beneath the Taranaki Basin. Maps of t ectonic subsidence where the influence of sediment loading is removed also clearly identify the source of the loading as lying within or beneath the m ountain belt. The basin fill shows a stratigraphic architecture typical of underfilled ancient peripheral foreland basins. This comprises transgressiv e (basal unconformity, thin limestone, slope-depth mudstone, flysch sequenc e) and regressive (prograding shelf-slope complex followed by molasse depos its) components. In addition the inner margin of the basin has been inverte d as a result of becoming involved in the mountain building, as revealed ea rlier by fission track thermochronological data. The timing and degree of i nversion fits well with the geometrical and stratigraphic development of th e basin. That the inversion zone and the coastal plain underlain by molasse deposits are narrow and most of the basin is beneath the sea, highlights t his as an underfilled active foreland basin. The basin is geodynamically pa rt of the Southern Alps collision zone. (C) 1998 Elsevier Science B.V. All rights reserved.