Fracture and Weakening of Jammed Subduction Shear Zones, Leading to the Generation of Slow Slip Events

Geodetic data have revealed that parts of subduction interfaces creep steadily or transiently. Transient slow slip events (SSEs) are typically interpreted as aseismic frictional sliding. However, SSEs may also occur via mixed viscobrittle deformation, as observed in shear zones containing mixtures (melange) of strong fractured clasts embedded in a weak viscobrittle matrix. We test the hypothesis that creep in a subduction melange occurs through distributed matrix deformation, where flow is intermittently impeded by load-bearing clast networks (jamming). Our numerical models demonstrate that bulk melange rheology can be dominated by the strong clasts in the absence of fracturing, while at high driving stresses or low frictional strength, clast fracturing redistributes deformation into the matrix, leading to high bulk strain rates. Because melange stress is heterogeneous, fracturing of clasts occurs throughout jammed melange when the driving stresses are only similar to 20% of the clast yield strength. The effective rheology of jammed melange can be characterized by a logarithmic dependence of stress on strain rate, which is used to explore strain rate transients caused by temporal variation in clast friction. Clasts must weaken significantly (similar to 75%) in order to increase strain rate by 8 times. Spring-block slider models with a more idealized viscobrittle rheology also demonstrate that strain rate transients can be generated when rate-and-state friction is incorporated. We outline a model where high bulk strain rates are generated when pervasive fracturing occurs, but further slip is limited by viscous processes. Incorporating such viscous damping into models may widen the conditions under which SSEs can occur while preventing development of seismic slip.