Slab dynamics in the transition zone

Seismic images of horizontal slab segments in, across or below the transition zone invoke scenarios in which slabs are laid down in the mantle through progressive trench retreat. However, observations of subduction characteristics do not exhibit a clear correlation between trench retreat and slab dip in the transition zone. Instead analysis of a range of subduction characteristics demonstrates that while transition-zone slabs include slabs of many ages and subduction velocities, with both retreating and advancing trench motion, they also have several enigmatic characteristics such as faster sinking rates and a larger range in slab dip than slabs that extend to either shallower or deeper depths. Comparison of subduction characteristics with several analytical and numerical models of subduction dynamics suggests that many of the possible mechanisms for trapping slabs in the transition zone (e.g., positive buoyancy sources, viscous resistance, slab weakening) are only viable if the slab is already shallow-dipping. Two scenarios for the formation of stagnant slabs are proposed: (1) trench retreat prior to slabs entering the transition zone or caused by the negative buoyancy forces associated with the wadsleyite and ringwoodite phase transitions and (2) slow, lateral migration of slabs in stable subduction zones.