Transient creep in subduction zones explained by reaction-induced rheological switches

Despite extensive research over the years, the weakening mechanisms that govern strain localization along deep subduction interfaces are still debated. These mechanisms span from the downdip boundary of the seismogenic zone (∼350°C) to the mechanical coupling transition with the upper plate mantle near sub-arc depths (>600°C). Current thermo–mechanical models posit that rock rheology is primarily stress- and rate-temperature-sensitive in the absence of mineral reactions. Strain is accommodated by stable creep, within several km-thick shear zones and at very low strain rates (< 10^-11 s^-1). However, geophysical observations of active subduction zones have outlined, over the last two decades, that deep plate interfaces are likely to be dominated by unstable creep characterized by episodic events of aseismic slips (“slow slip events”) occurring at relatively high strain rates (> 10^-7 s^-1). Meanwhile, geological (i.e. petro-structural) observations of deep subduction interfaces have shown that strain is generally localized within < 10–100’s m-thick shear zones. These shear zones are also known to concentrate metamorphic reactions and episodic fluid flow that have both significant influence on the rock strength. Yet, quantifying the effects of these chemo–mechanical transformations on the transient aseismic slips of deep plate interfaces remains hindered by the complexity of integrating geophysical and geological observations and the general lack of high-pressure deformation experiments.

 

Drawing on novel deformation experiments conducted at 2 GPa (eclogite-facies conditions) using a new generation Griggs-type apparatus, we reveal that unstable creep can be steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC) during mineral reactions. These changes of rheology can cause rock weakening by several orders of magnitude if intergranular fluid transfer is efficient. Such a weakening is a transient process since reaction rates tend to be intermittent / episodic at great depths. Moreover, we show that fluid concentration during viscous strain localization promotes extensive fracturing that may correspond to tremors (i.e., low frequency earthquakes) observed during slow slip events. Indeed, thermodynamic modeling of mafic and sedimentary rocks along pressure/temperature (P/T) gradients of active subduction zones worldwide reveals that slow slip events and tremors preferentially occur in horizons undergoing major dehydration reactions, and thus potential transient changes in rock rheology.