Abstract:
This study investigates the nature, origin, and distribution of Cretaceous to Recent
sediment fill in the offshore Taranaki Basin, western New Zealand. Seismic attributes
and horizon interpretations on 30,000 km of 2D seismic reflection profiles and three 3D
seismic surveys (3,000 km²) are used to image depositional systems and reconstruct
paleogeography in detail and regionally, across a total area of ~100,000 km² from the
basin's present-day inner shelf to deep water. These data are used to infer the influence
of crustal tectonics and mantle dynamics on the development of depocentres and
depositional pathways.
During the Cretaceous to Eocene period the basin evolved from two separate rifts into a
single broad passive margin. Extensional faulting ceased before 85 Ma in the present-day
deep-water area of the southern New Caledonia Trough, but stretching of the lithosphere
was higher (β=1.5-2) than in the proximal basin (β<1.5), where faulting continued into
the Paleocene (~60 Ma). The resulting differential thermal subsidence caused northward
tilting of the basin and influenced the distribution of sedimentary facies in the proximal
basin. Attribute maps delineate the distribution of the basin's main petroleum source and
reservoir facies, from a ~20,000 km²-wide, Late Cretaceous coastal plain across the
present-day deep-water area, to transgressive shoreline belts and coastal plains in the
proximal basin.
Rapid subsidence began in the Oligocene and the development of a foredeep wedge
through flexural loading of the eastern boundary of Taranaki Basin is tracked through the
Middle Miocene. Total shortening within the basin was minor (5-8%) and slip was
mostly accommodated on the basin-bounding Taranaki Fault Zone, which detached the
basin from much greater Miocene plate boundary deformation further east. The imaging
of turbidite facies and channels associated with the rapidly outbuilding shelf margin
wedge illustrates the development of large axial drainage systems that transported
sediment over hundreds of kilometres from the shelf to the deep-water basin since the
Middle Miocene.
Since the latest Miocene, south-eastern Taranaki Basin evolved from a compressional
foreland to an extensional (proto-back-arc) basin. This structural evolution is
characterised by: 1) cessation of intra-basinal thrusting by 7-5 Ma, 2) up to 700 m of
rapid (>1000 m/my) tectonic subsidence in 100-200 km-wide, sub-circular depocentres
between 6-4 Ma (without significant upper-crustal faulting), and 3) extensional faulting
since 3.5-3 Ma. The rapid subsidence in the east caused the drastic modification of shelf
margin geometry and sediment dispersal directions. Time and space scales of this
subsidence point to lithospheric or asthenospheric mantle modification, which may be a
characteristic process during back-arc basin development.
Unusual downward vertical crustal movements of >1 km, as inferred from seismic facies,
paleobathymetry and tectonic subsidence analysis, have created the present-day
Deepwater Taranaki Basin physiography, but are not adequately explained by simple rift
models. It is proposed that the distal basin, and perhaps even the more proximal Taranaki
Paleogene passive margin, were substantially modified by mantle processes related to the
initiation of subduction on the fledgling Australia-Pacific plate boundary north of New
Zealand in the Eocene.
