Constraints on Element Mobility During Deformation Within the Seismogenic Zone, Shimanto Belt, Japan

Abstract

Subduction interfaces exhibit a variety of slip behaviors, including megathrust and slow earthquakes. Field observations are consistent with crack‐seal deformation, in which tensile cracks are sealed by fluid‐transported solute. However, there are few constraints on the mass fluxes and length and time scales of such deformation and attendant increases in cohesion within the seismogenic zone. Here, we present a systematic geochemical investigation of mass transport associated with development of crack‐seal veins in the Shimanto Belt—an accretionary complex that preserves a record of plate boundary slip behavior at temperatures relevant to the seismogenic zone: 150–350°C. These mélanges show evidence for shear across decameter‐scale zones of deformation dominated by anastomosing scaly fabrics and pervasive veins. We use meso‐ and microstructural observations with geochemical observations of scaly fabrics and crack‐seal veins to evaluate the role of silica redistribution in healing fracture porosity along the plate interface and modulating slip behavior. Crack‐seal veins contain primarily quartz with albite and calcite, and vein textures provide evidence for partial sealing. Bulk rock analyses determined that the amount of phyllosilicates, specifically illite and chlorite, increases with temperature. A simple mass‐balance model based on the immobile chemical component TiO2 shows a systematic trend in mobility for all three mélanges and increased element mobility as a function of temperature. Scaly fabrics and veins show compositional evidence for locally sourced mineral redistribution. This study supports a model where development of scaly slip surfaces and fracture healing through temperature‐dependent mineral redistribution can impact slip behavior and fluid flow along the subduction plate interface.

Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

“…6B), which is consistent with slip dissolution, compactive strain, and volume loss in mudstones (Ramirez et al, 2021). Increasing temperature within the underthrusting sediment pile, from ~150-300 °C, is depicted in X-ray diffraction patterns of mudstones by a prograde decrease in the proportion of smectite, changes in the proportion of illite polytypes, and the increasing abundance of chlorite (Ramirez et al, 2021). These observations show that noncoaxial strain in subduction mélange is accomplished through a combination of quartz and feldspar dissolution and metamorphic equilibration of phyllosilicates.…”

“…earth elements (REEs), high field strength elements (HFSE), and Ti (Fig. 6B), which is consistent with slip dissolution, compactive strain, and volume loss in mudstones (Ramirez et al, 2021). Increasing temperature within the underthrusting sediment pile, from ~150-300 °C, is depicted in X-ray diffraction patterns of mudstones by a prograde decrease in the proportion of smectite, changes in the proportion of illite polytypes, and the increasing abundance of chlorite (Ramirez et al, 2021).…”

“…In the subduction mélanges studied that were underthrust to temperatures characteristic of the seismogenic zone, the scaly fabric is characterized by narrow bands of silica depletion relative to wall-rock lithologies (Kawabata et al, 2007;Fisher et al, 2019a;Ramirez et al, 2021) (Fig. 6A).…”

“…6A). Detrital feldspars throughout the matrix underwent dissolution within scaly folia, and there is evidence for the growth of illite, and in some cases, additional Fe-Ti phases (Ramirez et al, 2021). Shear failure of scaly folia at temperatures typical of the seismogenic zone is followed by low-grade metamorphic reactions, dissolution along scaly microfaults, and depletion of fluid-mobile chemical components.…”

Metamorphism and Deformation on Subduction Interfaces: 2. Petrological and Tectonic Implications

“…Microstructural observations from HPLT rocks imply that dissolution‐precipitation creep (DPC) also plays an important role in deformation of quartz‐rich rocks of the subduction interface (e.g., Behr & Bürgmann, 2021; Condit et al., 2022; Wassmann & Stöckhert, 2013b). Unequivocal evidence for DPC includes truncated intracrystalline chemical zonation (e.g., Wassmann & Stöckhert, 2013a), dissolution seams that are enriched in insoluble components (e.g., Ramirez et al., 2021) and the development of textural fabrics that indicate mineral growth into an open fluid‐filled cavity (e.g., Fletcher, 1977). We investigate DPC using the thin‐film model (Rutter, 1976; Weyl, 1959) which has the form $$$\dot{\varepsilon }=\tau \left(\frac{F{V}_{s}{\rho }_{f}S}{R{\Theta}{\rho }_{s}}\right){D}_{0}\,\mathrm{exp}\left(\frac{-{E}_{d}}{R{\Theta}}\right)\left(\frac{w}{{d}^{3}}\right),$$$ with the variables as defined in Table 1.…”