Aggradation surfaces and implications for displacement rates along the Wairarapa Fault, southern North Island, New Zealand

Abstract

Three Last Glaciation aggradation surfaces, Waiohine (11±1ka), Rata (c30 ka) and Porewa (c60 ka), are displaced by the Wairarapa Fault, Wairarapa Valley, southern part of the North Island of New Zealand. Dextral displacements of125±5m (Waiohine),385±5m (Rata) and c770–800 m (Porewa) indicate an average uniform dextral slip rate along the Wairarapa Fault of 11.5 mm/yr during at least the last 60 ka. This rate accounts for a third of the total oblique convergence rate of 33 mm/yr for the Australian and Pacific Plates along the Hikurangi Trough to the east of the North Island. Vertical displacement rates along the Wairarapa Fault are variable and range from 0.1 to 1.9 mm/yr. Where the Wairarapa Fault cuts aggradation gravels on greywacke basement this variation is controlled by growing bulges developed on the upthrown NW side as a result of left sidesteps of the fault trace. Where the fault cuts gravels overlying Tertiary rocks it is sometimes upthrown to the SE and at one locality the aggradation surfaces indicate the possibility of two reversals of vertical movement within the last 60 ka. Reversals in throw may be related to differential movement on fault clusters at depth that are located near or on growing anticlines in Tertiary strata above upbulged greywacke undermass. The pattern of long term (<100 ka) uplift is a general northeasterly decrease along the Wairarapa Fault. This is consistent with the uplift along the fault during the last earthquake in 1855 and, on a larger scale, the NE divergence of the Wairarapa Fault from the axes of greatest uplift of the Rimutaka and Tararua Ranges to the west.