Mid-Mantle Interaction Between the Big, Active Samoan Plume and the Tonga-Kermadec Slabs

Abstract:

Mantle plumes play an efficient role in transferring heat from the core-mantle boundary to the surface, where they significantly influence plate tectonics. It is well known that, upon impinging on the lithosphere at spreading ridges or intra-oceanic settings, mantle plumes generate hotspots, Large Igneous Provinces and considerable dynamic topography. However, it is still poorly understood which is the active role of mantle plumes on subducting slabs. Here we show that the stagnancy and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the big Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3-D anisotropic tomography images and 3-D petrological-thermo-mechanical models, which show complex mantle flow around the slab and intense deformation and anisotropy in the transition zone, explaining several unique features in the Fiji-Tonga area self-consistently. We also found that horizontally polarized shear waves (SH) are faster than vertically polarized shear waves (SV) in the mid mantle beneath the Tonga slab, which may indicate a dominant dislocation creep mechanism during the slab-plume interaction. We propose possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy with thermo-mechanical calculations.