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[1] In order better to understand the physical mechanisms underlying free subduction, we perform three-dimensional boundary-element numerical simulations of a dense fluid sheet with thickness h and viscosity h 2 sinking in an 'ambient mantle' with viscosity h 1 . The mantle layer is bounded above by a traction-free surface, and is either (1) infinitely deep or (2) underlain by a rigid boundary at a finite depth H + d, similar to the typical geometry used in laboratory experiments. Instantaneous solutions in configuration (1) show that the sheet's dimensionless 'stiffness' S determines whether the slab's sinking speed is controlled by the viscosity of the ambient mantle (S < 1) or the viscosity of the sheet itself (S > 10). Time-dependent solutions with tracers in configuration (2) demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides. The extra 'edge drag' exerted by the flow around the sides causes transverse deformation of the slab, and makes the sinking speed of a 3-D slab up to 40% less than that of a 2-D slab. A systematic investigation of the slab's interaction with the bottom boundary as a function of h 2 /h 1 and H/h delineates a rich regime diagram of different subduction modes (trench retreating, slab folding, trench advancing) and reveals a new 'advancing-folding' mode in which slab folding is preceded by advancing trench motion. The solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary.

[1] High-to ultrahigh-pressure (HP-UHP) metamorphic rocks commonly form and exhume during the early continental collision, and many questions related to their origin still remain unresolved. We focus our study on explaining the poly metamorphic origins of many HP-UHP terranes composed of tectonic units having strongly variable ages, peak metamorphic conditions, and P-T paths. These features are especially well characterized for the Sulu UHP terrane in eastern China which we have chosen therefore as the reference case. We conducted 2-D thermomechanical numerical modeling of continental subduction associated with formation and exhumation of HP-UHP rocks. Our experiments suggest existence of several consequent episodes of (U)HP rocks exhumation related to the inherently cyclic origin of continental crust subduction-detachment-exhumation processes. Three major phases of these processes are identified in our reference model for the Sulu UHP terrane: (1) first and (2) second exhumation episodes of HP rocks originated in the subduction channel at lithospheric depths and (3) exhumation of UHP rocks originated at asthenospheric depths. Numerical models also suggest that subducted UHP rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere. This sublithospheric plume may exist for several million years being heated to 800-900°C by the surrounding hot mantle. At the later stage, upward extrusion of hot partially molten rocks from the plume may exhume high-temperature (HT) UHP complexes toward the surface.Citation: Li, Z., and T. V. Gerya (2009), Polyphase formation and exhumation of high-to ultrahigh-pressure rocks in continental subduction zone: Numerical modeling and application to the Sulu ultrahigh-pressure terrane in eastern China,