Silica Kinetics and Subduction Zone Slip Behavior
Abstract
Observations of ancient subduction fault zones are used to constrain a new model for the kinetics of silica redistribution in the seismogenic zone and to characterize the role that geochemical processes play in the mechanical behavior of the subduction interface. The fault zones of this study, from the Kodiak Accretionary Complex and Shimanto belt of Japan, are regionally extensive (100's to 1000's of km along strike) mélange belts, with sandstone blocks in a pervasively sheared mudstone matrix. Typically, the mélange belt is 10's-100's of meters thick and is capped by discrete faults that are characterized by a narrow gouge layer. Simple shear is accommodated in mudstones by development of scaly fabric that shows evidence for pressure solution and concentration of fluid-immobile elements (e.g. HFSE, Ti). Sandstone blocks embedded in the shearing mudstones experience hydrofracturing and mode I failure, with vein textures that record repeated cracking and sealing. We construct a kinetic model to investigate the rate of mass transfer from the mudstone scaly slip surfaces to the sandstone fractures based on dissolution, diffusion, and precipitation of silica in an aqueous medium. Our model is based on the driving force for silica redistribution coming from the difference in strength between mudstone scaly shears and the sandstone blocks, a value that is largely constant and equivalent to 10's of MPa difference in mean stress. Cracks heal at rates relevant to the seismic cycle and with a temperature dependence on healing that should vary with subduction thermal structure from margin to margin. This kinetic model describes one component of the bimodal slip behavior inferred from observations of natural systems that show slow shear across a wide zone and coseismic slip in a narrow gouge zone. Such a behavior is used to inform a 2-D numerical model for the subduction interface with stochastic nucleation, growth, and failure of chemically healed "asperities" at rates determined by Arrhenius-equation silica kinetics, leading to 1) supercycles of buildup and release of elastic strain, 2) a temperature-based up-dip limit to genesis of large earthquakes, and 3) a power law size distribution of earthquakes that varies as a function of temperature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T11C..08F
- Keywords:
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- 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICS