Northland Allochthon

The name Northland Allochthon is proposed for the thick and extensive assemblage of Upper Cretaceous to Upper Oligocene sediments, plus Tangihua volcanic masses, occurring on the western side of Northland, shown by drillholes at Ngawha, Waimamaku and Batley to be allochthonous and to contain masses of more‐or‐less coherent sedimentary rock possibly up to 2.5 km thick. The Allochthon is unconformably overlain by the Waitemata and correlative Groups of Waitakian to Southland age (uppermost Oligocene to mid‐Miocene) and was itself emplaced by gravity sliding during the Waitakian. It is thus quite distinct from the post‐or‐syn‐Waitemata Onerahi Chaos‐breccia of Kear and Waterhouse. The Allochthon now extends from the Waikato Fault northwards probably to North Cape, a distance of more than 350 km, and westwards beneath the present continental shelf, a width approaching 100 km. It may originally have been much wider. It contains a stratigraphic sequence with an original thickness of several kilometres, now present as olistoliths of sedimentary and volcanic rocks of all sizes up to hundreds of cubic kilometres, irregularly juxtaposed in terms of stratigraphic age, and separated by zones ot strongly transposed clay‐rich sediment. It is inferred to have been emplaced from the present north, from an everted marine basin with oceanic volcanic basement, which lay to the north of the present Raukumara Peninsula on a pre‐Miocene reconstruction of the North Island. Parts of the Allochthon were transported at least 350 km; the maximum estimate of speed is 350 mm per year, following uplift of the depositional basin of perhaps as much as 14 km. Uplift and perhaps compression of the depositional basin, and downsinking of the receiving basin by 3 km or more, were two of the events marking the first appearance through the North Island of the India/Pacific subducting plate boundary. A probable extension of the Allochthon is present at East Cape (Matakaoa Volcanics), and it can possibly be traced along the east coast of the North Island and into the Kaikoura area of the South Island.