Differential uplift of middle and late quaternary shorelines, northwest South Island, New Zealand

Abstract

Discontinuous Pleistocene cliff-backed shorelines up to 220 m in altitude, and Holocene shorelines up to 12 m, border much of 125 km of coast from Westport to Hokitika in northwest South Island, New Zealand. The coastal terraces are up to 10 km wide where cut on soft Tertiary sediments but are narrow or absent on gneiss. By analogy with post-glacial shoreline development, Pleistocene shorelines are accepted as having been formed at the times of attainment of high interglacial sea levels. Past intraregional correlations assumed minimal differential uplift, but, as with glacial outwash surfaces inland, the raised shorelines are deformed by folding. Correlations are helped by the relations of shoreline deposits to glacial outwash gravels in the south, and by a few radiometric, mainly radiocarbon, dates. Correlations with high sea levels of deep-sea Oxygen Isotope Stages are made using the best fits of the altitudes of local sequences of shorelines to the altitudes expected assuming constant rates of uplift for each sequence. Uplift rates are between 0.5 and 0.2 m/kyr, and the uplift pattern substantially matches that of uplift of Miocene to lower Quaternary sediments. The shorelines correspond to high sea levels within Oxygen Isotope Stages 15, 13, 11, 9 (two), 7 (two), 5 (two) and 1.