Gravity expression of the Patea-Tongaporutu High eastern Taranaki, North Island, New Zealand

Abstract

The Patea-Tongaporutu High, western North Island of New Zealand, is delineated and the associated uplift and erosion of the Taranaki and Wanganui Basins are estimated on the basis of gravity, borehole and geological data. The regional gravity component used was derived from the gravitational effects of a large scale 3-dimensional lithospheric model proposed by Stern. After residual field modelling, selection of this regional component enables the entire Bouguer Anomaly to be explained in terms of geologic structure. Two-dimensional modelling of the residual anomaly defined the structure of the Patea-Tongaporutu High. The High shallows and undergoes a eastward, step-like, migration relative to 174 30' to the north. Both the eastern (Strathmore Fault) and western (Taranaki Fault Zone) margins of the High are modelled as medium angle (45) reverse faults. Basement for the Patea-Tongaporutu High and Wanganui Basin was modelled at the standard density of 2.67 Mg/m for Mesozoic greywacke. Beneath Taranaki Basin it was necessary to model denser (2.72 to 2.75 Mg/m) basement; this has been interpreted as corresponding to a Northwest Nelson type plutonic and metamorphic basement. Within the study area evolution of the Patea-Tongaporutu High started with Cretaceous to late Oligocene extension. Following this, until the late Miocene, the area underwent a compressive phase that increased in magnitude to the north. This event caused the Taranaki Fault Zone to rotate to its present reverse style, the Strathmore Fault to develop as a back thrust feature, and the Patea-Tongaporutu High to thrust westward over the suture between Northwest Nelson and Torlesse type basement lithologies.