Stress amplification around weak inclusions can trigger earthquakes in a dry subducting oceanic slab

The origin of intermediate-depth seismicity in subducting oceanic lithosphere is still debated. A key for interpretation is provided by deep-seated pseudotachylytes (quenched frictional melts produced during seismic slip along a fault), exhumed counterparts of the actual deep seismicity, that can record relevant information on the seismic processes at hypocenter depths. Pseudotachylytes crosscutting the dry ophiolitic peridotite/gabbros of Moncuni (Lanzo ultramafic Massif, W. Alps)^[1] have been interpreted to have formed at intermediate-depth (ca. 70 km) conditions under high differential stress and proposed as an analogue for the lower plane of the double seismic layer of subducting plates.

Moved by these observations, we investigated by numerical simulations the potential of a subducting dry slab to achieve the high differential stress required for brittle failure in absence of fluid-mediated embrittlement during the plate bending and unbending. We performed pseudo-2D thermo-mechanical simulations of free subduction of a dry slab considering a visco-elasto-plastic rheology. We tested a homogeneous dry plate and a dry plate with weak circular inclusions representing partially hydrated volumes in the first 40 Km of the slab. The effect of low temperature plasticity (LTP) in olivine was also tested. In the unbending portion of the subducting slab the stress field describes two arcs - the outer one in compression and the inner one in extension - matching the two planes of seismicity. However, the homogeneous slab can only reach a differential stress of around 1 GPa, that is not high enough for triggering earthquakes. The presence of weak inclusions, with degraded elastic properties, but still high viscosity, induces a local amplification of the stress field. Differential stresses in excess of 4 GPa are obtained considering inclusions with a shear modulus decreased by 60-70% relative to the surrounding material but similar viscosity. Increase of the spatial density of inclusions determines a general increase of stress due to local interactions of the stress fields. The LTP of olivine, when considered in the simulations, introduces a stress cut off hampering the differential stress around weak inclusions to rise above 1.5 GPa. However, if the effect of pressure and strain hardening are considered, differential stresses above 3 GPa are achieved, that are high enough for brittle failure at intermediate-depth conditions. The modeled slab with scattered weak inclusions is compatible with a dry and strong peridotitic mantle with partially serpentinized domains, most likely related to faulting during slab bending.

Our results show that brittle failure can occur at intermediate depths during subduction in relatively dry rocks, confirming the hypothesis developed from field interpretations. Further advanced microstructural investigations (e.g. TEM, HR-EBSD) on selected mantle pseudotachylytes, as well as on experimental analogues, can help to better understand the behavior of intermediate-depth earthquakes, hopefully providing new insights into the processes of stress accumulation and release during the seismic cycle.

 

[1]: Pennacchioni et al., 2020, Record of intermediate-depth subduction seismicity in a dry slab from an exhumed ophiolite, Earth Planet. Sc. Lett. 548, 116490