TEMPORAL VARIATIONS IN LATEST QUATERNARY SLIP ACROSS THE AUSTRALIAN-PACIFIC PLATE BOUNDARY, NORTHEASTERN SOUTH ISLAND, NEW-ZEALAND

Rates of latest Quaternary slip from stream terraces and moraines displaced by faults of the Alpine shear system in NE South Island, New Zealand, vary in space and time. Detailed histories of fault slip are obtained from combining displacement data with estimates of the age of the surface from weathering characteristics. Precision in surface ages is 5-20% using rock weathering finds and 15-50% using soil properties. The oldest surfaces examined are 15-20 ka and have right-lateral fault offsets up to 400-600 m. The main faults of the in the NE South Island (the Wairau, Awatere, Clarence, Hope, Kekerengu, and Porters Pass faults) have average latest Quaternary right-lateral slip rates of 3-10, 5-10, 7-10, 25-40, 5-7, and 3-4 mm/yr respectively. Every fault has undergone a substantial decrease in lateral slip rate in the last 3-5 kyr. Summed across the plate boundary, the average latest Quaternary slip rates are comparable to long-term rates of horizontal slip across the Australian-Pacific plate boundary (around 40 mm/yr parallel to the boundary) and rates of geodetic strain and seismic moment release over the last 50-100 years (approximately the same). However, sums of lateral fault-slip rates over the interval from 15 to 5 ka exceed the plate motions, whereas late Holocene lateral fault-slip rates are less than half the long-term average. The best explanation of these variations is slip across the plate. boundary is episodic, varying over perhaps 5-kyr intervals. This implies that 15-20 kyr is the time interval necessary to average out shorter, 5-kyr episodic variations in plate boundary motions.