Thrasher, G.P.; Viskovic, G.P.D.; Sagar, M.W.; Seebeck, H. 2024 Subsurface igneous rocks of the Taranaki peninsula. GNS Science report 2023/47. 83 p.; doi: 10.21420/8BEH-6P24
Abstract
The igneous centres within and adjacent to the Taranaki Peninsula represent the southwestern-most extent of arc volcanism associated with the Tonga-Kermadec-Hikurangi subduction system. The Taranaki Peninsula, on the west coast of New Zealand’s North Island, is dominated by the conical form of Taranaki Maunga (mountain). This subduction-related volcano is the largest and youngest of three volcanos on the Peninsula, the others being the eroded remains of Kaitake and Pouakai. Also exposed on the peninsula’s northern coast near Port Taranaki are a series of locally concentrated plutonic bodies composing the Sugar Loaf Andesites. There are also several subsurface igneous features associated with the peninsula. Combined, these igneous bodies represent the last c. 3 Myr of volcanic activity in the region. Beneath the peninsula, the ring plain created by Taranaki Maunga and its predecessors lies on a west-dipping angular unconformity cut into Pliocene marine strata. Many petroleum exploration wells have penetrated the surficial volcanic and pyroclastic rocks, which rarely extend more than a few hundred metres below sea level. Here, we combine existing well data and the dating of subsurface samples with seismic reflection data to better constrain the age of the angular unconformity and its relationship to igneous activity. Hornblende from 14 igneous samples from onshore petroleum exploration wells have yielded 40 Ar/39 Ar plateau ages ranging from c. 2.9 Ma to 0.15 Ma. Seven 40 Ar/39 Ar samples from near the base of the Taranaki Maunga ring plain (previously dated at <0.2 Ma) ranged from 0.515 Ma in the Kapuni-15 well on the south side of Taranaki to 0.39 Ma in Rahotu-1 on the western side. These data suggest that surficial volcanism, including Taranaki Maunga, initiated between 0.5 to 0.4 Ma. Samples from the western tip of the peninsula returned ages of 2.9 and 2.3 Ma, significantly older than the exposed Sugar Loaf Andesites (1.8 Ma). Three subsurface samples from below the outcrop area of the Maitahi Formation, which surrounds Pouakai and Kaitake (previously dated between 0.6 to 0.2 Ma), returned ages of 1.1, 0.8 and 0.6 Ma, indicating initiation ages in excess of 1 Ma for Kaitake and up to 0.9 Ma for Pouakai. These ages are in broad agreement with an age estimate based on volcanic production rates. The present study estimates a total volume of the Egmont Volcanic Centre to be c. 780 km3 , the majority of which are the products of Kaitake and Taranaki. Using previously published volcanic production rates of 530 km3 /Myr, up to 1.5 Myr is required to accumulate the present mountain. Seismic-reflection mapping also reveals several subsurface igneous features in close association to the Taranaki Peninsula. Offshore, immediately west of the peninsula, there are three small buried volcanic cones. Each of these features appears to have been active for a few hundred thousand years, with overall activity ranging from c. 2.5 Ma to c. 0.5 Ma. Onshore, at least two small cones underly the ring plain, resting on the unconformity that Kaitake, Pouakai and Taranaki are founded upon. In summary, this study indicates igneous activity in the vicinity of the Taranaki Peninsula has been continuous over the last 3 Myr.
