Variability of subducting slab morphologies in the mantle transition zone: Insight from petrological-thermomechanical modeling

Abstract

Variable morphologies of subducting slabs are observed in tomographic images of the mantle transition zone (MTZ), where slabs appear to stagnate in the MTZ or enter the lower mantle. These contrasting morphologies of subducting slabs are dependent on the joint effects of various dynamic, kinematic and geometric factors. Force balance analysis indicates that the favorable conditions of slab stagnation in the MTZ include old/cold slab subducting into the mantle with large Clapeyron slopes at 410 km and 660 km discontinuities, as well as the significant trench retreat and shallow dip angle. However, these conditions are often not achieved together for specific subduction zones on the Earth, which thus require systematic studies to distinguish their relative effects. Here, we analyze the slab mode selection in the MTZ based on coupled petrological-thermomechanical numerical model. The model results indicate that (1) water activity and partial melting weaken the subduction channel and form a hot and weak mantle wedge beneath the island arc that affects slab dynamics. (2) The Clapeyron slope of phase transition at 660 km can significantly contribute to the slab stagnation in the MTZ, whereas the Clapeyron slope at 410 km does not change the general mode selection, but does affect the trench motion and further the length of flattened slab. (3) A sharp viscosity jump between the lower and upper mantles can promote slab stagnation in the MTZ, which has a similar effect with a strong viscosity-depth gradient. (4) Fast trench retreat is the most critical factor controlling slab stagnation, especially the long slab flattening in the MTZ. (5) The age/thickness of converging plates can also influence the slab/MTZ interaction by modifying the slab dip angle and trench motion. (6) A thin, weak layer at the bottom of MTZ does not play significant roles in the slab mode selection. The combined force balance analysis and numerical studies are compared with the comprehensive observations of natural subduction zones, which improve understanding of the dynamics of slab/MTZ interaction and the resulting variability of subducting slab morphologies.