Timing of Cretaceous extension and Miocene compression in northeast south island, New Zealand: Constraints from Rb-Sr and fission-track dating of an igneous pluton

The Tapuaenuku Plutonic Complex is a large alkaline intrusion that was emplaced into Upper Jurassic-Lower Cretaceous Torlesse greywacke/argillite near the axis of the Inland Kaikoura Range (northern South Island, New Zealand). Concordant Rb-Sr, K-Ar, and titanite/zircon fission-track cooling ages of circa 105 to 93 Ma record a phase of igneous activity that was related to a period of mid-Cretaceous extension in South Island, New Zealand. The updoming and uplift which accompanied the extension and igneous activity are also reflected by the uplift of the marine Split Rock Formation above sea level where it was eroded. On this was deposited a terrestrial sequence followed by shallow marine deposits which may represent the onset of a long period of transgressive marine sedimentation, the result of slow thermal subsidence after magmatism and rifting, from Late Cretaceous to Oligocene. This burial of the pluton is identified not only from the sedimentary relationships of the local stratigraphy but also by the apatite fission-track ages which record later exhumation. During this time, New Zealand remained passively part of the Pacific Plate. In the early Miocene at circa 22 Ma the intrusion cooled through the partial annealing zone of apatite (approximately 120 degrees-60 degrees C). Exhumation was the result of early Miocene thrusting and shortening in Marlborough and inception of the present day transpressive plate boundary between the Indo-Australian and Pacific Plates through New Zealand. Further evidence for this period of uplift and accompanying denudation is seen through the large quantities of coarse-grained elastic sediments which were deposited in Marlborough at this time. The uplift rate required to raise the top of pluton to its present altitude (2885 m), based on a constant rate from 22 Ma until the present day, is approximately 0.2 mm yr(-1). However, estimates of present-day uplift rates of 10 mm yr(-1) from summit heights and raised marine benches suggest that two periods of late Cenozoic uplift must have occurred with a long period of quiescence between, a conclusion supported by a major mid-Miocene unconformity below Quaternary sediments in Marlborough.