Fluid Migration in a Subducting Viscoelastic Slab

Abstract

Metamorphic dehydration reactions in a subducting slab release fluids that trigger arc volcanism and are thought to be responsible for intermediate‐depth seismicity. The fluid flow from the source is controlled by buoyancy and compaction pressure which is modified by viscous and elastic effects. In this paper, we investigate how fluid migrates in viscoelastic slab by using 2‐D and 3‐D numerical models based on a theory of two‐phase flow. When bulk viscosity is sufficiently low, viscosity plays a dominant role and fluid goes up almost vertically soon after its release producing porosity waves. When a higher bulk viscosity is assumed, a large amount of fluid is trapped in a high porosity region produced by the fluid source and migrates along the source except for a case where the ratio of permeability (K) to fluid viscosity (μ) is relatively low. We also find that porosity increases in the deeper part of the fluid source in cases with intermediate and low values of K/μ. In 3‐D, fluid focusing occurs where the slab bends away from the trench causing a local increase in porosity and compaction pressure. These findings may help us explain several types of observations in subduction zones including slow earthquakes at the plate interface, low seismic wave velocities in the oceanic crust, double seismic zones in the slab, and shallow subduction angle at the bend of the slab.

Seismic Evidence for Water Transportation in the Forearc off Northern Japan

“…This interpretation is consistent with our result that large megathrust earthquakes ( M w ≥ 6.0) are mainly located near the low‐V anomalies at the slab interface (Figures e, , S7i, and S7l), which may reflect fluids released from the slab dehydration (Liu et al, ). These vents or fractures at the slab interface may be caused by at least two factors (Halpaap et al, ): the uneven permeability in the crust and at the interface (Faccenda et al, ) and along‐strike 3‐D undulation of the slab interface (Morishige & van Keken, ; Zhao et al, ).…”

Devolatilization of Subducting Slabs, Part II: Volatile Fluxes and Storage

“…We therefore treat the slab as a rigid plate in our model. Slab deformation has also been modeled using viscoelastic (Morishige and van Keken, ) and visco‐elasto‐plastic rheologies (Gerya, ), yielding diverse fluid flow patterns within slab. By treating the slab as a rigid plate, the flow direction can be considered as a free parameter that is to be explored (see below), and we can thus circumvent the uncertainties of slab deformation and coupled fluid flow that are difficult to constrain.…”

“…As briefly discussed in section , the upside is that the large uncertainties associated with the diversity of possible flow patterns yielded by various dynamic considerations is avoided. For example, two‐phase dynamic model using viscous rheology suggests a high‐permeability flow channel at the interface between slab and mantle wedge (Wilson et al, ), whereas those using viscoelastic rheology show the development of porosity waves within the slab (Morishige and van Keken, ). On smaller spatial scales (e.g., centimeter to meter), Plümper et al () and Malvoisin et al () show that flow channels and porosity waves can develop as well.…”

“…In this paper, we provide a two‐dimensional model of reactive fluid flow in subducting slabs. Since the dynamics of within‐slab flow remains highly uncertain (Faccenda et al, ; Morishige and van Keken, ; Plümper et al, ; Wilson et al, ), we prescribe the flow direction in our model and investigate model behavior as a function of this parameter, rather than solving equations for momentum conservation. The model incorporates open‐system equilibrium thermodynamics and enables us to assess the factors controlling slab dehydration and decarbonation.…”

Devolatilization of Subducting Slabs, Part I: Thermodynamic Parameterization and Open System Effects

“…On the dynamic side, the style and pathways of volatile migration is poorly constrained. Field evidence shows that fluid flow proceeds through both porous and fractured rocks (Ague, 2014), and numerical models suggest that fluid migration can take place via porosity waves (Morishige and van Keken, 2018), fractures (Plümper et al, 2017), and large-scale faults (Faccenda et al, 2009). Recent geochemical studies indicate that Geochemistry, Geophysics, Geosystems 10.1029/2019GC008488 fluid explusion during subduction might be episodic rather than steady (John et al, 2012).…”