Strength of Strained Two-Phase Mixtures: Application to Rapid Creep and Stress Amplification in Subduction Zone Melange

Aseismic creep may occur by distributed deformation in melange shear zones comprising weak matrix and stronger clast materials. Slow slip events or steady tectonic displacement can be distributed over <100-m thick shear zones if weak matrix controls bulk shear zone deformation. We use 2-D numerical models to quantify the rheology of moderately strained (shear strain <1.75) melange for various volumetric proportions of competent clasts. Melange deformation with <50% clasts is matrix dominated and can accommodate steady creep. At higher clast proportions melange viscosity increases more than tenfold after small strains, because strong clasts interact and form force chains. Clast shear stress is amplified above the imposed shear stress, by a factor of <14 where force chains develop. Slow slip events may occur due to a temporary absence of force chains, while localized regions of high shear stress generate coincident fracturing and potentially tremor events. Plain Language Summary Some subduction shear zones creep without generating large earthquakes. It is unclear which deformation process at depth allows this creep to occur. We compute numerical models to explore whether creep is promoted by having a particular proportion of weak materials within subduction fault zones, which typically consist of mixtures (melanges) of weak and strong components. We demonstrate that in a melange that consists of <50% weak minerals, forces are concentrated into force chains of strong materials when the melange is slightly deformed, resulting in an overall strength which is much greater than previously predicted. Subduction zone creep events therefore occur in melanges either with a high proportion of weak minerals, or in the temporary absence of such force concentrations. Swarms of very small earthquakes are often associated with creep events, which likely require failure of the stronger melange materials. We show that large forces are generated in the strong materials, plausibly leading to this failure, even when the overall melange is very weak.