ASYMMETRIC RIFTING IN A CONTINENTAL BACK-ARC ENVIRONMENT, NORTH-ISLAND, NEW-ZEALAND

Marine seismic reflection data have been used to investigate the structure and evolution of the continental back-arc basin forming the volcanic region of central North Island, New Zealand. The seismic reflection data defines two parts to the volcanic region, in the west an older region showing narrow back tilted blocks, and a younger rift basin, delineated by the White Island and Tauranga Fault Zones, to the east. The younger rift coincides with the offshore trend of the active Taupo Volcanic Zone (TVZ). Listric faulting of basement in the eastern part gives rise to about 80% extension for the basin which is consistent with the thinning of the crust from 25 km along the margins of the basin to 15 km under the basin. Basement in the region is interpreted to be highly extended greywacke. The nature of the faulting and the seismic stratigraphy demonstrates that the extension has been asymmetric in nature, being progressively younger towards the plate boundary in the east. Active faulting offsets the seafloor in the younger, eastern part of the basin. Eight major sedimentary sequences, delineated by unconformities, are recognised in the basin. These sequences, based on correlation with major onshore rhyolitic eruptions, are considered to be up to about 1.6 Ma old. A major unconformity within the sequences may coincide with a hiatus in volcanic activity from 0.7-0.4 Ma ago. A caldera structure is tentatively identified under the western part of the younger rift basin (offshore TVZ) from seismic and other data. Its age is not well constrained but an age corresponding to the initiation of the younger extension episode (1.6 Ma ago) is preferred. Older volcanic sequences underlie the oldest TVZ volcanic units to the west of the younger basin and probably correspond to the Pliocene-Miocene volcanics onshore in the Coromandel Peninsula.