ZHIBIN LEI, J. Huw Davies 

The evolution of dual inward dipping subduction (DIDS) is crucial to understand multiple slab interaction. Yet, how DIDS influences the thermo-mechanical behaviour of the overriding plate remains unclear, as previous DIDS investigations all applied a compositional or Newtonian rheology that excludes temperature dependency. Here we apply a composite rheology, including temperature dependent creep deformation mechanisms, in 2-D thermo-mechanical numerical modelling to investigate how DIDS modifies the rheological structure of the overriding plate. Three variables on plate sizes are investigated to understand what may control the maximum degree of plate weakening. We find that reducing the initial length or initial thickness of the overriding plate and increasing the initial thickness of the subducting plate can enhance the viscosity reduction within the overriding plate. The progressive weakening can result in a variety of stretching states ranging from 1) little or no lithosphere thinning and extension (<5% accumulation of strain), to 2) limited thermal lithosphere thinning (<30% accumulation of strain), and 3) localised rifting followed by spreading extension. Compared with single sided subduction, DIDS further reduces the magnitude of viscosity in the overriding plate. It does this by creating a dynamic fixed boundary condition for the overriding plate and forming a stronger upwelling mantle flow, both of which promote the progressive weakening in the overriding plate. The result implies that these generic DIDS effects are important aspects to consider to understand extension developed in natural DIDS cases. We also demonstrate that both temperature dependent creep rheologies and yielding deformation mechanism contribute significantly to the continuous viscosity reduction. The finding may also have a broader implication for more general processes that involve plate scale weakening, strain localisation and the formation of new plate boundaries.