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Abstract Using numerical models, we compute the evolution of the mantle flow field and the crystal preferred orientation (CPO) of mineral aggregates in the mantle wedge of generic subduction systems from their nascent to mature stage and investigate shear wave splitting (SWS) through the forearc mantle wedge corner and overriding crust. Upon subduction initiation, the maximum depth of slab‐mantle decoupling (MDD) is relatively shallow (∼20 km depth), resulting in mantle flow and CPO development in the wedge corner. As subduction continues, the MDD deepens, the wedge corner cools and stagnates, and the olivine CPO becomes frozen‐in. In the cool wedge corner, antigorite can form if water is available. In non‐deforming mantle, antigorite CPO develops relative to the host olivine CPO through topotactic growth. We calculate splitting parameters of synthetic local S waves based on the model‐predicted A‐ and B‐type olivine CPOs and topotactically grown antigorite CPO that replaces A‐type olivine CPO in the wedge corner. The fast direction is trench‐normal for A‐type olivine and antigorite CPOs and trench‐parallel for B‐type. When the delay times are long enough (>0.1 s), we find them positively correlated with the thickness of the mantle wedge corner. In NE Japan, where the results of detailed analyses on the spatial variation of the SWS parameters are available, such correlation is not observationally reported. However, the addition of an anisotropic overriding crust provides delay times (∼0.1 s) and trench‐normal fast directions that are consistent with the local SWS observations. 

Abstract. Fluid and melt transport in the solid mantle can be modeled as a two-phase flow in which the liquid flow is resisted by the compaction of the viscously deforming solid mantle. Given the wide impact of liquid transport on the geodynamical and geochemical evolution of the Earth, the so-called “compaction equations” are increasingly being incorporated into geodynamical modeling studies. When implementing these equations, it is common to use a regularization technique to handle the porosity singularity in the dry mantle. Moreover, it is also common to enforce a positive porosity (liquid fraction) to avoid unphysical negative values of porosity. However, the effects of this “capped” porosity on the liquid flow and mass conservation have not been quantitatively evaluated. Here, we investigate these effects using a series of 1- and 2-dimensional numerical models implemented using the commercial finite-element package COMSOL Multiphysics®. The results of benchmarking experiments against a semi-analytical solution for 1- and 2-D solitary waves illustrate the successful implementation of the compaction equations. We show that the solutions are accurate when the element size is smaller than half of the compaction length. Furthermore, in time-evolving experiments where the solid is stationary (immobile), we show that the mass balance errors are similarly low for both the capped and uncapped (i.e., allowing negative porosity) experiments. When Couette flow, convective flow, or subduction corner flow of the solid mantle is assumed, the capped porosity leads to overestimations of the mass of liquid in the model domain and the mass flux of liquid across the model boundaries, resulting in intrinsic errors in mass conservation even if a high mesh resolution is used. Despite the errors in mass balance, however, the distributions of the positive porosity and peaks (largest positive liquid fractions) in both the uncapped and capped experiments are similar. Hence, the capping of porosity in the compaction equations can be reasonably used to assess the main pathways and first-order distribution of fluids and melts in the mantle. 

Abstract The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi. 

This package of codes uses a 3-D velocity flow field to calculate crystal preferred orientation (CPO) using a modified version of D-Rex (Kaminiski et al., 2004), and then calculates local shear wave splitting (SWS) parameters using MSAT (Walker & Wookey, 2012). It includes the codes needed for the plotting D-Rex output (GMT5, Wessel et al., 2013), the scripts and general workflow to process the elastic tensors from D-Rex before using them in the SWS code, and multiple README files containing more details on each code. The mantle wedge flow from a 45 degree obliquity subduction zone is provided as an example. 

Abstract Despite the critical role of subduction in plate tectonics, the dynamics of its initiation remains unclear. High-temperature low-pressure metamorphic soles are vestiges of subduction initiation, providing records of the pressure and temperature conditions along the subducting slab surface during subduction initiation that can possibly differentiate the two end-member subduction initiation modes: spontaneous and induced. Here, using numerical models, we show that the slab surface temperature reaches 800–900 °C at ~1 GPa over a wide range of parameter values for spontaneous subduction initiation whereas for induced subduction initiation, such conditions can be reached only if the age of the overriding plate is <5 Ma. These modeling results indicate that spontaneous subduction initiation would be more favorable for creating high-temperature conditions. However, the synthesis of our modeling results and geological observations indicate that the majority of the metamorphic soles likely formed during induced subduction initiation that involved a young overriding plate. 

Abstract Despite extensive modeling efforts, the dynamics of subduction initiation (SI), including the role of elasticity, are not fully understood. Using two‐dimensional thermomechanical models with visco‐plastic (VP) and visco‐elasto‐plastic (VEP) rheologies, we systematically investigate the role of elasticity in intraoceanic SI using two model setups: spontaneous initiation without imposed convergence and induced initiation with imposed convergence. In spontaneous models, the overriding plate age of <20 Ma and the subducting plate age of >50 Ma generally lead to vertically driven SI with either rheology, but for a given age contrast, SI is easier to occur with the VEP rheology. In induced model with either rheology, when the two plates are young and have a small age contrast, the resulting SI is horizontally driven, and elasticity does not affect SI significantly, regardless of the convergence rate. However, when the thermal age contrast is large and a convergence rate is relatively low, the SI in induced models is vertically driven and similar to that in the spontaneous models, and the VEP rheology leads to faster SI than the VP rheology. This effect of elasticity becomes smaller with increasing initial horizontal compressional stress but does not become fully negated by the initial stress of <∼50 MPa. Therefore, inclusion of elasticity with reasonable shear modulus and initial stress values results in a weaker slab, making it easier for vertically driven SI to occur when the age contrast is relatively large. 

Scientists from different disciplines are working together to identify common challenges in and techniques for modeling fluid migration associated with subduction zone processes.