Mantle constraints on the plate tectonic evolution of the Tonga-Kermadec-Hikurangi subduction zone and the South Fiji Basin region

Abstract

The Tonga–Kermadec–Hikurangi subduction zone is a major plate boundary in the Southwest Pacific region, where the Pacific plate subducts westward underneath the Australian plate. Considerable controversy exists regarding the Cenozoic evolution of this subduction zone, its connection with the Vitiaz–Solomon trench, the opening of adjacent backarc basins (South Fiji Basin, Norfolk Basin), and the obduction of ophiolites in New Caledonia and New Zealand. We analyse three tectonic reconstructions from 45 Ma to the present that represent three end-member tectonic scenarios. The first model involves an approximately west-dipping Tonga–Kermadec–Hikurangi subduction zone that rolls back clockwise, and an approximately east-dipping New Caledonia–Northland subduction zone that rolls back anticlockwise until ca 21 Ma. The other models involve only the Tonga–Kermadec–Hikurangi subduction zone that rolls back clockwise in one, and anticlockwise in the other model. We tied the three reconstructions to an Indo-Atlantic moving hotspot reference frame, indicating how the subduction zones migrated with respect to the lower mantle at 45–0 Ma, thereby predicting the location of slab material in the mantle. The predicted slab locations have been compared with a global P-wave tomography model (UU-P07) that gives insight into mantle structure and fossil slab locations. Minor agreement is achieved for the anticlockwise model, which fails to predict most of the lower mantle Tonga–Kermadec–Hikurangi slab anomaly below the South Fiji Basin and North Fiji Basin. Moderate agreement is achieved for the clockwise model, which predicts most of the lower mantle Tonga–Kermadec–Hikurangi slab anomaly below the South Fiji Basin, but not its northernmost extent below the North Fiji Basin. The model involving two oppositely dipping subduction zones shows good agreement, predicting slab anomalies at the right depth and geographical location for the Tonga– Kermadec–Hikurangi and Solomon–Vitiaz subduction segments, as well as for the New Caledonia– Northland subduction zone. Connection between the Tonga and Vitiaz subduction segments until ca 20–15 Ma is predicted and agrees with tomography. Predicted average upper mantle and lower mantle sinking velocities are 3.3–7.0 cm/yr and 0.8–1.7 cm/yr, respectively, with variation occurring at different segments along the subduction zone owing to variation in subduction velocity and subduction partitioning. Our analysis couples plate tectonics to mantle evolution and demonstrates that reconstructions tied to an Indo-Atlantic moving hotspot reference frame better agree with mantle structure than those tied to a Pacific fixed hotspot reference frame