On Venus, the coexistence of large volcanic highlands interpreted as the surface expression of long-lived mantle plumes, alongside coronae, smaller features thought to be caused by transient thermal diapirs, remains enigmatic. Using numerical models of mantle convection with sharp and broad mineral phase transitions, we show that both scales of upwellings are generated in a stagnant lid planet with an interior temperature 400 K warmer than Earth's. The smaller plumes originate from a ~600 km deep internal layer that exists as a consequence of the different sequence of mineral phase transitions that occurs in warmer mantles. These models further produce essential features of Venus' geodynamics, including apparent large scale downwellings, variable heat flow and plains dynamic topography.

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.

We currently have a limited understanding of the tectonic framework that governs Venus. Schubert and Sandwell (1995) identified over 10,000 km of possible subduction sites at both coronae and chasmata rift zones. Previous numerical and experimental studies have shown the viability of regional-scale lithospheric recycling via plume-lithosphere interactions at coronae, yet little work has been done to study the possibility of resurfacing initiated at Venusian rift zones. We created 2D numerical models to test if and how regional-scale resurfacing could be initiated at a lateral lithospheric discontinuity. We observed several instances of peel-back delamination - a form of lithospheric recycling in which the dense lithospheric mantle decouples and peels away from the weak, initially 30 km-thick crust, leaving behind a hot, thinned layer of crust at the surface. Delamination initiation is driven by the negative buoyancy of the lithospheric mantle and is resisted by the coupling of the plate across the Moho, the significant positive buoyancy of the crust arising from a range of crustal densities, and the viscous strength of the plate. Initial plate bending promotes yielding and weakening in the crust, which is crucial to allow decoupling of the crust and lithospheric mantle. When there is sufficient excess negative buoyancy in the lithospheric mantle, both positively and negatively buoyant plates may undergo delamination. Following a delamination event, the emplacement of hot, buoyant asthenosphere beneath the crust may have consequences for regional-scale volcanism and local tectonic deformation on Venus within the context of the regional equilibrium resurfacing hypothesis.