Alternating convergent and non-convergent tectonics, 100 million years to present, Puketoro area, northeastern New Zealand

Abstract

The area mapped comprises 180 km2 of rolling hill country between Waitahaia and Ihungia, 70 km north of Gisborne, in the extreme northeast of the North Island of New Zealand. The region is within Cretaceous and Tertiary rocks of the northern part of the northeast-trending East Coast Deformed Belt. Six formations and one group (containing seven lithofacies) have been recognised. Mokoiwi Formation, Taitai Sandstone Member, Karekare, Whangai and Ihungia Formations have been retained. Waitahaia, Owhena and Tikihore Formations, and Mangatu siltstone, Wheturau siltstone and Te Waka greensand lithofacies are names introduced from the upper Waipaoa, Mangatu and Waitahaia Catchments. Smectite mudstone ("bentonite") is given lithofacies status here. The term Mangatu Group has been reinstated (Kenny 1984a) and the following lithofacies have been erected - Whakoau limestone, Opossum Creek sandstone and Hauturu greensand. New Rakauroa and Upper Calcareous Members and two other lithofacies are described for the Whangai Formation. Two conglomerate lithofacies, a breccia and two limestone lithofacies (Kouetumarae and Bexhaven) are recognised in the Ihungia Formation. Two autochthonous domains and three stacked allochthonous sheets are established. The autochthonous "basement" domain consists of highly deformed, sandstone-dominated submarine fan sediments of Ngaterian (Late Albian to Early Cenomanian) age (Waitahaia Formation), accreted to an eastward-younging series of subduction-related imbricated packets. The relatively undeformed mudstone-dominated, fining upwards "cover" domain has accumulated in slope basins upon the accretionary prism (older Karekare Formation - Arowhanan and Mangaotanean; Late Cenomanian to Turonian) eventually burying the "basement" inter-basin structural highs (younger Karekare Formation, Owhena and Whangai Formations - Teratan to Teurian; Senonian to Paleocene). An allochthonous domain was emplaced possibly from the northwest at approximately 25 Ma (Oligocene-Miocene boundary), scraping off uppermost autochthonous sediments in its path, and incorporating some as lubricants. The domain contains Mokoiwi and Waitahaia Formations, both of which are comparable to the autochthonous "basement"; but in addition they have been rotated during emplacement. Many smaller sheets of sandstone, siltstone, mudstone, greensand and limestone are included in the overlying Mangatu Group domain, also emplaced from the northwest, probably in Tongaporutuan (latest Miocene) times. In this domain mixed lithologies within mélange and "bentonite" represent a fining upwards, passive margin sequence, poor in terrigenous detritus, of Late Cretaceous to Oligocene age. The uppermost domain is thought to have been emplaced "piggy-back" upon the Mangatu Group domain. In the study area it is composed entirely of gently folded, mudstone-dominated Miocene Ihungia Formation strata. It is proposed that the formation was deposited in basins formed in accreted Karekare and Whangai Formations during early development of the Miocene to Recent Hikurangi subduction system. Post-emplacement megascopic faulting and folding affect the region.