Ryukyu Subduction Zone: 3D Geodynamic Simulations of the Effects of Slab Shape and Depth on Lattice-Preferred Orientation (LPO) and Seismic Anisotropy

Abstract:

At the Ryukyu subduction zone, seismic anisotropy observations suggest that there may

be strong trench-parallel flow within the mantle wedge driven by complex 3D slab

geometry. However, previous simulations have either failed to account for 3D flow or

used the infinite strain axis (ISA) approximation for LPO, which is known to be

inaccurate in complex flow fields. Additionally, both the slab depth and shape of the

Ryukyu slab are contentious. Development of strong trench-parallel flow requires low

viscosity to decouple the mantle wedge from entrainment by the sinking slab. Therefore,

understanding the relationship between seismic anisotropy and the accompanying flow

field will better constrain the material and dynamic properties of the mantle near

subduction zones. In this study, we integrate a kinematic model for calculation of LPO

(D-Rex) into a buoyancy-driven, instantaneous 3D flow simulation (ASPECT), using

composite non-Newtonian rheology to investigate the dependence of LPO on slab

geometry and depth at the Ryukyu Trench. To incorporate the 3D flow effects, the trench

and slab extends from the southern tip of Japan to the western edge of Taiwan and the

model region is approximately 1/4 of a spherical shell extending from the surface to the

core-mantle boundary. In the southern-most region we vary the slab depth and shape to

test for the effects of the uncertainties in the observations. We also investigate the effect

of adding locally hydrated regions above the slab that affect both the mantle rheology and

development of LPO through the consequent changes in mantle flow and dominate

(weakest) slip system. We characterize how changes in the simulation conditions affect

the LPO within the mantle wedge, subducting slab and sub-slab mantle and relate these to

surface observations of seismic anisotropy.