The north-east end of the Wairau Fault, Marlborough, New Zealand

Abstract

In terms of movement over the last 10 kyr, the Wairau is one of the main dextral faults in the northern part of the South Island of New Zealand. It is 130 km long and as a clearly deined trace ends near Renwick, about 15 km from the Cook Strait coast at Cloudy Bay. The previously unmapped NE extension of the Wairau Fault in the lower Wairau Valley is described here. It strikes at 065° true through late Holocene beach ridges for about 3 km to within 800 m of the coast. The NW side of the fault is upthrown by about 2 m and there is 10 m dextral displacement of three minor topographic features associated with one of the beach ridges. By interpolation from radiocarbon dated shorelines, the beach ridge is inferred to be about 1.5 kyr old, which gives a dextral rate of faulting of about 6.7 m/kyr. The youngest beach ridges span the last 800 years and are unfaulted. Hence, although the Marlborough Earthquake of 1848 took place in the lower Wairau Valley, it was not caused by movement on the Wairau Fault. The Wairau Fault is the continuation of the Alpine Fault and in terms of the total dextral displacement of 500 km we suggest that up to 1 Myr ago the line of extension was along the North Island Shear Belt. The 065° trend of the newly mapped north east extension of the Wairau Fault implies the existence of a dextral jog fault through the northern part of Cook Strait that is complementary to the sinistral jog at the Glenroy Fault. The 150 km wide block between the two jog faults is assumed to have moved north at some time prior to 1 Myr ago.