Roles of Hydrous Lithospheric Mantle in Deep Water Transportation and Subduction Dynamics. Issue 11 (2nd June 2018)

• Record Type: Journal Article
• Authors: Nakao, Atsushi; Iwamori, Hikaru; Nakakuki, Tomoeki; Suzuki, Yujiro J.; Nakamura, Hitomi
• Is Part Of: Geophysical research letters. Volume 45:Issue 11(2018)
• Page Start: 5336
• Page End: 5343
• Publication Date: 2018-06-02
• Subjects: mantle convection -- subduction dynamics -- water transportation -- plate velocity -- stagnant slab -- serpentinite; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2017GL076953
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: We used two‐dimensional simulations to investigate the effects of a hydrous lithospheric mantle (HLM) on subduction dynamics. It was demonstrated that the thickness of a subducting HLM strongly controls the motion and deformation of converging plates. When the HLM thickness is lower than a critical value, for example, ∼5 km for a 100‐Ma slab, all serpentine in the HLM decomposes, and the liberated fluid enters the mantle wedge. The fluid weakens the overlying plate, causing trench retreat, rapid plate convergence, and slab stagnation. In contrast, when the HLM thickness exceeds the critical value, dense hydrous magnesium silicates appear in cold parts of the HLM at high pressures. The buoyant dense hydrous magnesium silicates reduce the slab pull force, weakening trench retreat, rapid convergence, and slab stagnation. A low HLM thickness may account for tectonics in some actual subduction zones characterized by trench retreat, rapid convergence, and stagnant slabs, for example, in Northeast Japan. Plain Language Summary: Rocks on the Earth's surface are cooled, hardened, eventually forming rigid plates that move around relative to one another. When two plates converge, one plate overrides the other, which sinks into the Earth's deep mantle. The sinking plate carries water, which softens rocks and also affects the behavior of the sinking/overriding plates and surrounding mantle flows ("subduction dynamics"). To investigate the role of water in subduction dynamics, 2‐D fluid dynamical simulations were performed. The simulations suggest that subduction dynamics change significantly with the level of hydration of the sinking plate, which is represented by the thickness of a hydrous layer. When the hydrous layer is thin, the plate sinks rapidly with a shifting boundary and stagnates above the lower mantle. In contrast, when the hydrous layer is thick, plate convergence is sluggish, the plate boundary remains stationary, and the sinking plate penetrates into the lower mantle. These results indicate that a small amount of water is expected for the northwest part of the Pacific Plate, characterized by the rapid convergence, plate boundary shifting, and stagnation of the sinking plate. Key Points: Numerical simulation shows how a subducting hydrous lithospheric mantle affects plate velocity and slab geometry A thin (thick) hydrous layer within the lithospheric mantle enhances (weakens) subduction, trench retreat, and slab stagnation A thin hydrous layer within the lithospheric mantle may explain the behavior of converging plates in Northeast Japan