[S-MP27] Deformed rocks, Metamorphic rocks and Tectonics

[SMP27-04] A simple 2D visco-elasto-plastic model of subduction channel growth at oceanic-continental convergent boundary

*Kohei Shiseki¹, Tatsuki Tsujimori¹ (1.Tohoku University)

Keywords:subduction channel, high-pressure mélange, subduction angle, subduction velocity, 2D-geodynamic modelling

The exhumation of eclogite or other dense high-pressure metamorphic rocks hardly relies solely on their own buoyancy; this process requires less-buoyant rocks hosting the denser rocks, or alternatively, there can be missing parameters other than buoyancy that can affect the exhumation of eclogite (e.g., Morita, Tsujimori et al. 2022 J. Petrol.). In this study, we carried out a two-dimensional numerical simulation based on the I2VIS code (Gerya and Yuen 2003 EPSL). In order to understand the geodynamics of oceanic plate subduction, we performed numerical simulation to test different input parameters including the plate convergence rate, the subduction angle, the age of subducted slab, and the presence/absence of cessation in convergence. We also evaluated the P–T paths at the top of downgoing slab, and confirmed that the paths in our different models fell within the range of the global subduction zone thermal structure and P–T condition of exhumed rocks (e.g., van Keken et al. 2018 G-Cubed). Our numerical results indicate that there are no straightforward models to explain how the high-pressure and/or ultrahigh-pressure metamorphic rocks can exhume along a subduction channel or trigger subvertical uplifting within the mantle wedge. Nevertheless, our modeling could confirm the following key factors that might affect the exhumation process of high-dense rocks in an oceanic subduction setting: (1) if the subduction angle is large, more sedimental material can be dragged into the subduction channel, which reduces the density of the mélange. In addition, the Poiseuille flow tends to be dominant. This can promote the ascent of the high-pressure metamorphic complexes toward shallower depth; (2) if the convergence rate is large, more sedimentary material can be dragged into the subduction channel, which reduces the density of the mélange. In this case, the hangingwall continental crust deforms to form a fold belt and the width of the subduction channel increases; (3) if plate convergence stops, slab rollback occurs, which can consequently promote widening of the subduction channel and/or exhumation of high-pressure metamorphic complexes. Furthermore, our model demonstrate that subduction of older and colder slab can facilitate the occurrence of slab rollback.