Abstract - Temporal changes in the geometry of the subducted plate along the Hikurangi margin, New Zealand, are estimated using a combination of historical intra-slab earthquakes (to constrain current arc-slab spatial relations) and the south-eastward migration of the ancient volcanic arcs since 20 Ma. The slab hinge retreated south-eastwards towards the trench at rates that varied in space and time. Between 20-18 Ma the change in trend of the volcanic arc indicates a twisting, clockwise rotation of the subducting plate associated with greater slab steepening and rollback in the north than the south. After 16 Ma changes in slab geometry were mainly achieved by southeast migration of the subduction hinge. Slab-hinge migration rates were approximately uniform along the volcanic portion of the margin and varied from 3-8 mm/yr (~16-6 Ma) to 15-20 mm/yr (~6-0 Ma). The acceleration of slab-hinge migration rates were accompanied by the introduction of a hot low viscosity mantle wedge resulting in uplift of the central North Island. Changes in the magnitude of upper-plate extension along the margin and associated vertical-axis rotation (greater than or equal to 350/20 Myr) contrast with the uniform rates of slab hinge migration and the absence of slab rotation about vertical axes since ~16 Ma. The plates were decoupled beneath the North Island on million-year timescales, permitting the over-riding plate to rotate independent of the subducting plate. The rates and amount of upper plate extension do not provide a proxy for slab rollback because upper plate deformation is also controlled by continental collision at the southern termination of subduction.
Journal of the Geological Society, Longon, 170, 951-962, 2013.