LATE QUATERNARY FAULTING OF THE OFFSHORE WHAKATANE GRABEN, TAUPO VOLCANIC ZONE, NEW-ZEALAND

The 15-20 km wide zone of active late Quaternary extensional faulting (the Taupo Fault Belt-Whakatane Graben), within the Taupo Volcanic Zone, extends offshore at least 43 km towards White Island, Bay of Plenty. Within the offshore Whakatane Graben, as defined by the White Island Fault and newly named Rurima Fault, some 50 normal faults displacing post-18 ka transgressive sediments are newly mapped from high-resolution 3.5 kHz seismic profiles. The faults strike northeast, appear laterally discontinuous over distances of > 10 km, generally dip to the northwest, and bound fault blocks tilted to the southeast. Of these, four newly named faults-the Ohiwa, Nukuhou, Pukehoko, and Rangitaiki-show large and repeated movements within the last 18 ka. Post-18 ka seismic stratigraphy, tephrostratigraphy, and C-14 dating indicate relative vertical fault displacement rates vary from 0.2 to 2.8 mm/yr. An extensional block model relating fault geometry, vertical displacements, and extension indicates that absolute subsidence varies within the graben. Subsidence is estimated to average 2-2.5 mm/yr and range locally to a maximum of 3-3.5 mm/yr. Assuming the faults dip at 45 +/- 10-degrees, extension is estimated to be at least 3.5 +/- 1.7 mm/yr, implying that a significant amount of the 7 mm/yr extension of the Taupo Volcanic Zone, near the Bay of Plenty coast, occurs within the 15-20 km wide Whakatane Graben.