Abstract:

The Tonga Subduction Zone, located in the southwestern Pacific, is characterized by the global most typical trench-arc-basin system. It not only has the fastest plate convergence rate globally but also features the fastest back-arc basin, the Lau Basin. The Samoan mantle plume, situated on the northeastern side of the Tonga Subduction Zone, interacts strongly with the subducting system, making the mantle dynamics in this region even more complex. Many previous geophysical investigations of the region have been conducted, but the fine three-dimensional structure of the upper mantle, especially its anisotropic structure, is still not very clear. Here we apply azimuthal anisotropy tomography using amplitude and phase data of teleseismic fundamental-mode Rayleigh waves at periods of 20-150 s to further constrain the upper mantle structure beneath Tonga. Our results show the following features: (1) The southward inflow of the Samoan mantle plume material into the Lau Basin mainly occurs at depths less than 50 km. (2) There is a west-east mantle flow beneath the northern part of the Lau Basin, while a south-north mantle flow exists beneath the southern part, possibly resulting from passive mantle flow to accommodate asymmetric rollback of the subducting slab. (3) Within the subducting slab, the fast-velocity direction is nearly north-south, possibly caused by the presence of subduction-related normal faults. (4) In the asthenosphere beneath the outer-rise region, there is a lateral mantle flow parallel to the trench, probably formed because the asthenospheric material being extruded by the rollback of the subducting slab.

Key words: anisotropic tomography, mantle convection, subducting slab, Tonga subduction zone, Lau basin