Abstract
The Tonga-Kermadec subduction zone exhibits the fastest observed trench
retreat (up to 16 cm/yr) and convergence rate (up to 23 cm/yr) near its
northern end. However, it exhibits a paradox: despite this rapid trench
retreat, the Tonga slab maintains a relatively steep dip angle (53°)
above 400 km. The slab turns flat around 400 km, then steepening again
until encountering a stagnant segment near the 670 km discontinuity.
Despite its significance for understanding slab dynamics, no existing
numerical model has successfully demonstrated how such a distinct slab
morphology can be generated under the fast convergence. Our mantle
convection models successfully reproduced the observed slab geometries
while incorporating the observed subduction rate. A key element of
achieving a qualitative match lies in the implementation of a hybrid
velocity boundary condition, which proves crucial for handling the fast
trench retreat. Our investigation explains how the detailed slab
structure is highly sensitive to physical parameters including the
seafloor age and the mantle viscosity. Notably, a nonlinear rheology,
where dislocation creep reduces upper mantle viscosity under strong
mantle flow, is essential. The weakened upper mantle allows for a faster
slab sinking rate, which explains the large dip angle. Our findings
highlight the utilizing rheological parameters that lead to extreme
viscosity variations within numerical models to achieve an accurate
representation of complex subduction systems like the Tonga-Kermadec
zone. Our study opens new avenues for further study of ocean-ocean
subduction systems, advancing our understanding of their role in shaping
regional and global tectonics.