Effect of Buoyant Sediment Overlying Subducting Plates on Trench Geometry: 3D Viscoelastic Free Subduction Modeling

초록

Buoyant trench sediment generates an isostatic restoring force that opposes the slab pull of subducting plates; however, the influence of this force on subduction dynamics is poorly understood. Here, we performed three-dimensional free subduction simulations adopting a variety of sediment distributions along a trench to investigate the correlation between trench motions and heterogeneous buoyant forces. Two endmembers, sediment-rich and sediment-starved centers, induced convex and concave trenches, respectively. The trench curvatures obtained from the natural subduction zones are well constrained by our models over wide magnitudes (1-9 km) and wavelengths (100-400 km) of deficient and excess sediments. Conversely, a uniform sediment distribution leads to an extremely narrow range of trench curvatures. These results imply that trench sediment contributes significantly to deviated curvatures along trench strikes. Finally, we observed stress localization at shallow slab hinges that lie beneath abundant sediments, clarifying the relationship between sediment thickness and subduction earthquakes. Plain Language Summary The role of trench sediment in subduction dynamics as a lubricant or restoring force against denser oceanic crust remains controversial. Buoyant sediment induces a restoring force and variations in sediment thickness may lead to laterally complicated force balances. However, the effect of variations in sediment distribution along strikes on subduction dynamics is underexplored. We conducted three-dimensional free subduction modeling to analyze the factors that affect trench morphology and stress states within the subducting slab. Our results show that trench motion is strongly affected by differences in local isostatic restoring forces associated with sediment thickness variations. Locally thin trench sediment leads to a more rapid trench retreat and vice versa. The resultant differential trench motion along strike yields complicated trench shapes which exhibit a wide range of curvatures. By comparing the trench curvatures derived from our models to those obtained from natural subduction zones, we confirmed that the models with differential sediment distribution explained the patterns observed in major subduction zones. In contrast, models with uniform sediment thickness did not reflect natural trench curvature. Our study provides a fresh perspective on the trench shape of subduction zones (e.g., the Tonga-Kermadec) with highly variable along-strike sediment thickness. Key Points We performed three-dimensional free subduction simulations of trenches with variable along-strike sediment distributions Locally excessive and deficient trench sediments induce convex and concave trenches, respectively The models with differential sediment distributions explain a wide range of trench curvatures of modern subduction zones

키워드