Tertiary tectonic development of Whangaroa district, northeastern Northland, New Zealand

Abstract

In the Kaeo‐Whangaroa district, northeastern Northland, Permian‐Jurassic basement (Waipapa Group) is unconformably overlain by an autochthonous, mid‐late Eocene, transgressive sequence of shelf to bathyal sediments (Te Kuiti Group). At around the Oligocene‐Miocene boundary, allochthonous, deep‐marine rocks of Cretaceous and early Tertiary age (Northland Allochthon) were tectonically emplaced across the Te Kuiti Group. Neoautochthonous early Miocene andesitic breccias, flows, and intrusives (Wairakau Volcanics), late Miocene‐Pliocene volcanics, and Quaternary alluvial, coastal, and terrestrial deposits, intrude or unconformably overlie all of the above.The basement is cut by high‐angle faults upon which displacement probably started before Te Kuiti sedimentation. Changes in stratigraphy and in thicknesses of formations of the Te Kuiti Group across some of the faults indicate that offset of the basement affected, and possibly continued during, Te Kuiti Group deposition; the fine‐grained nature of the sediments and the lack of tectonic disturbance within them implies that they were deposited upon a relatively stable platform.At about the Oligocene‐Miocene boundary, further offset upon these high‐angle faults took place, concurrently with low‐angle emplacement of the nappes of the Northland Allochthon. Crosscutting relationships between the two sets of structures indicate that nappe emplacement was sequential from bottom to top: this is in contrast to the top‐to‐bottom younging of thrust sequences in compressive regimes, and supports the hypothesis that the Northland Allochthon was emplaced by gravity sliding.Emplacement of the allochthon was followed by regional uplift, which was accompanied by continued high‐angle faulting, and by local folding of the basement and its autochthonous and allochthonous cover. The early Miocene Wairakau Volcanics, erupted on to and intruded beneath a subaerial surface, cut (and therefore postdate) all of the low‐angle emplacement structures and most of the high‐angle faults. The late Miocene‐Quaternary volcanics and sediments overstep all structures, confirming that little or no deformation has taken place since the early Miocene.The structural patterns are interpreted as recording the gradual subsidence of a microcontinental platform in the Eocene and Oligocene, followed by the initiation of convergent margin tectonics and the emplacement of the allochthon at bathyal depth at about the Oligocene‐Miocene boundary. Emplacement of the allochthon was followed by uplift and emergence. Subduction failed and tectonism ceased soon after the onset of subaerial arc volcanism in the early Miocene.