PALEOSEISMOLOGY OF AN ACTIVE REVERSE-FAULT IN A FORE-ARC SETTING - THE POUKAWA FAULT ZONE, HIKURANGI FORE-ARC, NEW-ZEALAND

The Poukawa fault zone, on the North Island of New Zealand within the forearc of the Hikurangi subduction zone, consists of a series of en e chelon reverse faults and companion hanging-wall anticlines. The geomo rphically expressed length of the fault zone is 34 km, However, on the basis of coseismic deformation associated with an M-s 7.8 earthquake in 1931 and the presence of blind faults north of the geomorphically e xpressed fault zone, it appears that the seismogenic length of the fau lt zone may be as much as 130 lan. On the basis of chronostratigraphic horizons identified in each of three trenches evenly distributed alon g the exposed fault zone, from which a paleoseismological record for t he past similar to 25 k.y. can be determined, there is not a character istic rupture length for earthquakes, Some slip events are confined to the similar to 10-20-km-long southern part of the fault zone,,whereas other slip events may have ruptured the entire 34 km length of the ge omorphically expressed fault zone. At least two slip events that occur red in the northern part of the fault zone did not occur in the southe rn part of the zone. The largest earthquake recorded in the trenches h ad a maximum reverse slip in excess of 10 m, We infer that this prehis toric earthquake, similar to the 1931 earthquake, entailed slip on fau lts along the geomorphically expressed fault zone and on blind faults to the north. This prehistoric earthquake may have had a rupture lengt h (surface plus subsurface) in excess of 100 km. Average earthquake re peat times on the fault zone range from 3-7.5 k.y. for the southern an d middle part of the zone to 7-12 k.y. for the northern part of the fa ult zone. Average single-event slip ranges from 3 m to as much as 6 m, Slip was initially accommodated at the surface primarily by folding. With successive slip events, however, coseismic displacements propagat ed to the surface and surface deformation became increasingly dominate d by reverse slip on fault planes. The Poukawa fault zone is part of a foreland-propagating fold and thrust belt in the forearc of the Hikur angi subduction zone. Older, actively eroding hanging-wall anticlines are present to the west of the fault zone toward the volcanic are, whe reas younger folds are developing above blind reverse faults east of t he main fault trace. In addition to propagating to the east, the fault zone is propagating northward beneath the Heretaunga Plains. This act ive propagation testifies to ongoing and evolving contractional forear c deformation in response to oblique plate convergence.