Seafloor geology and petrology in the oceanic to continental transition zone of the Kermadec‐Havre‐Taupo Volcanic Zone arc system, New Zealand

Abstract

To the north of New Zealand, the oceanic Tonga‐Kermadec island arc system strikes roughly NNE‐SSW and intersects the continental shelf of New Zealand in the vicinity of the Whakatane arc volcano, a submarine edifice which rises around 1000 m from the surrounding seafloor. A major bathymetric slope‐break here coincides with the proposed oceanic‐continental transition zone. Northwest of the Whakatane volcano, dredging on Colville Knolls recovered samples of deformed Mesozoic(?) metasedimentary rocks, whose petrography and geochemistry resemble Waipapa Terrane rocks of onshore New Zealand, thereby confirming the extent and “continental” status of the offshore region. The transition zone from continental crust to oceanic crust is delineated by a marked slope‐break at the 2000 m isobath, oriented roughly parallel to the Bay of Plenty coastline. The linearity of this boundary is broken by a well‐defined reentrant structure, the Ngatoro Basin, which here is recognised as a southern extension of the oceanic back‐arc basin, the Havre Trough.

Volcanic rocks dredged from a number of stations parallel to and normal to the arc front, in both oceanic and continental settings, show petrography and chemistry in keeping with their location. In the oceanic area to the north, basalts, basaltic andesites, and andesites predominate, whereas in the continental area (south of the Whakatane volcano and Colville Knolls), compositions range from basalt to rhyolite, similar to the onshore Taupo Volcanic Zone. We interpret this relative distribution of essentially coeval volcanic rocks to result from the migration of a volcanic arc system from an oceanic to a continental setting. The observed higher proportion of felsic rocks in the continental setting reflects the complex interplay between ascending melts, thicker continental lithosphere, and convecting asthenospheric mantle in the wedge between subducting slab and sub‐arc lithosphere. In addition, our work highlights significant chemical and petrographic variations over relatively short length scales (c. 10 km) where volcanic rocks of arc affinity give way to MORB‐like rocks of back‐arc basin affinity. Such variations are significant as they point to distinctive reservoirs in the mantle wedge and, possibly, to vertical zonation of the melting column.

Key words:

• offshore Taupo Volcanic Zone
• southern Kermadec Arc‐Havre Trough
• tectonics
• back‐arc rifting
• igneous petrology