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

Abstract

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.