FEEDBACKS BETWEEN TECTONICS AND VOLCANISM ALONG THE VOLCANIC ARC OF NEW ZEALAND
The onshore part of the arc-rift system can be divided in three sectors, based on magma composition, and on the spatial relationships between active volcanism and faulting. In the northern sector, andesitic volcanism occurs at the eastern margin of the rift, and further north transitions into a well-established back-arc basin (the Havre trough). In the central sector, rhyolitic volcanic centres alternate with fault-dominated rift segments, along the axis of the rift. In the southern sector, the andesitic volcanoes are located along the axis of the rift. The northern and central sectors of the rift/arc system are more evolved (~2 Ma old) than the southern sector (~400-300 ka old). In the central sector, rhyolite volcanism now dominates over an earlier andesitic phase, and faulting and volcanism have migrated towards the rift axis.
While the regional location of volcanism and the general eastward migration tendency of volcanism and faulting is controlled by plate tectonics (location of the 100-km depth contour of the subducting slab and slab rollback), it is also likely that crustal thinning associated with extension, as the Havre trough propagated from oceanic into continental crust, dictated the initial location of volcanism and faulting. Also the spatial coincidence of the volcanic arc and rift axis with a major basement structure in the southern, less evolved sector suggests basement structure control on localisation of volcanism at its initial stages.
In the more evolved, rhyolitic sector of the arc, the spatial pattern of volcanic centres and faulting suggests that heat flow and voluminous rhyolitic centres have localised faulting between the volcanic centres. In the southernmost, young, andesitic sector, volcanism is not voluminous, the crust has not been thinned and bedrock controls are important to the continued development of the volcanic arc and the rift.