Late Cenozoic vertical crustal movements in the southern North Island, New Zealand

Abstract

Four benches of the Last Interglacial, and the Present Interglacial bench are identified on most parts of the south-east coast of the North Island. Inferred ages for the back of the benches are 125 000, 100 000, 84 000, 80 000, and 6000 years. The heights of the benches define a pattern of folds and faults which can be traced inland by contouring the summit heights. In the coastal belt the summit height surface is similar in pattern, but twice as high as the 100000-year surface. It is found that the best overal1 fit with the 100 000-year surface is by assuming an age of 200 000 years for the summit height surface. It is thought that uplift was smal1 until 200 000 years ago and then increased to the average rate for the last 100 000 years. Rates are compared in a combined isobase map, and the uplift rates and flank tilt rates for the main anticlines and synclines set out in a table for the last 6000 years, 100 000 years, and summit-height surface. On the marine benches uplift rates range from 0 · 75 m to 4 · 0 m per thousand years for the growing anticlines, and from 0 · 5 m to 2 · 2 m per thousand years for the growing synclines. For the summit height surface, uplift rates on the anticlines range from 3 · 0 m to 4 · 5 m per thousand years. In terms of classical description, there are three phases in the uplift history of the district: an epeirogenic phase (very slow subsidence without much folding) from 14 mil1ion years (m.y.) to 1 · 3 m.y.; a phase of tectogenesis (folding with low rate of uplift) from 1 · 3 m.y. to 0 · 2 m.y.; and an orogenic phase (folding and intense uplift) from 0 · m.y. to the present day.