Shallow structure and active tectonism of an offshore continental back-arc spreading system: the Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ), the predominant active volcano-tectonic structure within the New Zealand sub-continent, marks the zone of Quaternary back-arc spreading and calc-alkaline volcanism associated with the convergence of the Pacific and Australian plates at the Hikurangi plate-boundary margin, North Island, New Zealand. New bathymetric, shallow seismic reflection, magnetic anomaly, and GLORIA data presented here accurately define for the first time the extent, structure and tectonism of the poorly known offshore portion of the TVZ. These data show that nearly half of the entire ca. 40 km wide TVZ lies offshore, extending northeast for some 150 km seaward to 36°45′S. The southeastern and northwestern boundaries of the offshore TVZ are significant tectonic and bathymetric features, which comprise inward facing normal fault zone (the White Island and Tauranga Fault Zones, respectively). Three, essentially parallel, tectono-morphological units are identified along the length of the offshore TVZ. They comprise (from southeast to northwest) a frontal non-volcanic graben, a volcanic ridge, and a volcanic “back-arc” graben. The spatial position of these units with respect to the Hikurangi subduction margin is not a simple “arc” and “back-arc” relationship. Tectonism within the offshore TVZ, although pervasive, increases from northwest to southeast, so that the area of highest extension and subsidence of the non-volcanic graben occurs in front of the volcanic ridge. Geophysical data also suggest the possible tectonic ablation of blocks of the forearc hinterland into the offshore TVZ along its southeastern margin. The southern Havre Trough and northern TVZ are offset sinistrally by 45–50 km. They overlap by 40 km, and differ in orientation by ca. 7°. Dextral displacement between these two extensional regimes is accomodated by a series of dextrally oblique and en-échelon “bookshelf” faults. The orientation of extensional structures within the offshore TVZ is also dextrally oblique, by 10°–15° to the regional 030° trend of the bounding faults, and agree with axes of maximum extension determined elsewhere from geodetic and seismological data.