Tripathi, A. & Kamp, P. J. J. (2008). Timing of initiation of reverse displacement on the Taranaki Fault, northern Taranaki Basin: Constraints from the on land record (Oligocene Te Kuiti Group). In Proceedings of 2008 New Zealand Petroleum Conference, Auckland, New Zealand, 9-12 March 2008.
Permanent Research Commons link: http://hdl.handle.net/10289/3758
Structures associated with the wedge of basement overthrust into Taranaki Basin along the Taranaki Fault, are regarded as hydrocarbon plays and have been tested by drilling through the tip of the overthrust. The timing of initiation of reverse displace ment on Taranaki Fault is difficult to interpret from available seismic reflection data across it because the evidence has been masked by later movements. The record from the basin, as summarised in King & Thrasher (1996), suggests that the fault evolved from normal to reverse character during the mid-Oligocene. This was inferred from formation of a foredeep parallel to, and west of, Taranaki Fault and a marked increase in its paleo-water depth, as indicated by foraminiferal assemblages of Late Oligocene age. A comprehensive re-assessment of the lithostratigraphy and sequence stratigraphy of the Late Eocene-Oligocene Te Kuiti Group exposed on land east of Taranaki Fault in central-western North Island, between Port Waikato and Awakino, provides new constraints on the early history of Taranaki Fault displacement. New age control has been achieved by a review of existing foraminiferal biostratigraphy combined with determination of Sr isotope ages from macrofossil samples. Six unconformity-bound sequences have been identified and mapped within the Te Kuiti Group. A major subaerial unconformity between sequences TK3 and TK4 combined with a basinward shift in the position of onlap for sequence TK4 indicate a dramatic change in stratigraphic development and basin dynamics during the mid-upper Whaingaroan at c. 29 Ma, corresponding to the change from mild extension (sag basin) to shortening across the Taranaki Fault Zone. We consider sequences TK4 – TK6 to each represent tectonic cycles of subsidence and basin inversion and we attribute the origin of these cycles to periodic locking of the Taranaki Fault décollement in underlying Murihiku basement, the accumulating strain causing uplift in the basin east of the fault zone, followed by free displacement, relaxation in the upper crust and subsidence. A 1st order model is presented of the Late Oligocene to earliest Miocene vertical and horizontal displacement of basement on the Taranaki Fault Zone for a west –east transect through Awakino. It implies that the mid- to Late Oligocene displace¬ment on the fault zone in the vicinity of Awakino was episodic, and that the thrust belt was narrow (c. 15 km). North of Kawhia Harbour there will have been a different displacement history with most of the total displacement occurring during the devel opment of the c. 29 Ma unconformity at the base of Sequence TK4, whereas to the south between Awakino and Kawhia Harbour the majority of the total displacement occurred during the Otaian and at the end of it. The model also shows that the start of reverse/thrust displacement on Taranaki Fault must have involved the development of a completely new fault trace(s), rather than involving a change of sense of movement on the pre-existing normal fault. The Manganui Fault is part of the Taranaki Fault Zone and probably became active at c. 27 Ma during development of the unconformity between sequences TK4 & TK5. The model presented here has been validated against the subsurface Oligocene stratigraphy in Taranaki Basin.
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This article has been published in Proceedings of New Zealand Petroleum Conference 2008. Copyright 2008 The Authors.