Jamming and mixed brittle-viscous deformation of subduction zone mélange
Abstract
Aseismic creep has been proposed to occur by distributed deformation in mélange shear zones comprising weak matrix and stronger clast materials. A hypothesis is that slow slip events (SSEs) and steady tectonic displacement can be distributed over 100 m thick shear zones if weak matrix minerals, deforming viscously at low stress, control mélange deformation. By contrast, load-bearing by clasts can be expected to promote localised, brittle deformation. Here, numerical models are used to quantify whether the rheology of moderately strained (shear-strain < 1.75) mélange is matrix or clast dominated, for various volumetric proportions of competent clasts. Mélange deformation with < 50% clasts is matrix dominated and can accommodate steady creep. At higher clast proportions, networks of clasts form load-bearing force chains, leading to jamming, with a significant increase of the bulk mélange viscosity (> 10x) in the absence of clast deformation. Shear stress in force chains is typically amplified to 10x the bulk stress state. This stress amplification causes shear and/or tensile failure of clasts with friction coefficients of 0.5, even when bulk stresses are only 10 MPa and pore pressures are moderate.
Numerical models approximating frictional failure demonstrate that fracturing of clasts within force chains leads to unjamming, facilitating distributed brittle-viscous deformation of the surrounding low stress mélange. The formation and destruction of force chains is therefore a viable mechanism for episodic transient creep, perhaps accommodating SSEs. A further effect of unjamming is that clast fracturing and stress amplification at clast edges promote localisation of matrix deformation into connected shear pathways. These pathways likely correspond to slip surfaces, with a length controlled by the fault-zone thickness and force chain spacing. Slip along these connected surfaces could correspond to tremor events associated with SSEs. The maximum slip surface length is estimated for various force chain configurations and compared to area-limited scaling models for low-frequency earthquakes. Localised fracturing of jammed mélange components may therefore trigger distributed viscous and brittle-viscous mélange deformation, generating SSEs and tremor respectively within a low stress environment.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T21E0250B
- Keywords:
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- 3902 Creep and deformation;
- MINERAL PHYSICSDE: 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGYDE: 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICSDE: 8434 Magma migration and fragmentation;
- VOLCANOLOGY