Double seismic zones (DSZs), parallel planes of intermediate-depth earthquakes inside oceanic slabs, have been observed in a number of subduction zones and may be a ubiquitous feature of downgoing oceanic plates. Focal mechanism observations from DSZ earthquakes sample the intraslab stress field at two distinct depth levels within the downgoing lithosphere. A pattern of downdip compressive over downdip extensive events was early on interpreted to indicate an unbending-dominated intraslab stress field. In the present study, we show that the intraslab stress field in the depth range of DSZs is much more variable than previously thought. Compiling DSZ locations and mechanisms from literature, we observe that the “classical’ pattern of compressive over extensive events is only observed at about half of the DSZ locations around the globe. The occurrence of extensional mechanisms across both planes accounts for most other regions. To obtain an independent estimate of the bending state of slabs at intermediate depths, we compute (un)bending estimates from slab geometries taken from the slab2 compilation of slab surface depths. We find no clear global prevalence of slab unbending at intermediate depths, and the occurrence of DSZ seismicity does not appear to be limited to regions of slab unbending. Focal mechanism observations are frequently inconsistent with (un)bending estimates from slab geometries, which may imply that bending stresses are not always prevalent, and that other stress types such as in-plane tension due to slab pull or shallow compression due to friction along the plate interface may also play an important role.

Many exposed high-pressure meta-serpentinites comprise a channelized network of olivine-rich veins which formed during dehydration at depth and served as pathway for fluid escape. Previous studies showed that the formation of an olivine enriched vein-like interconnected porosity network on the µm-scale is controlled by chemical heterogeneities in the rock. However, the evolution towards larger scale and nearly pure olivine veins is not yet well understood. Here we study the effects of reactive fluid flow on a developing vein system during dehydration. We use thermodynamic equilibrium calculations to investigate the effects of bulk silica content variations in serpentinites on the dehydration reaction of antigorite + brucite = olivine + free fluid and silica content of this fluid phase. We develop a numerical model combining the effects of intrinsic chemical heterogeneities with reactive silica transport. Increasing temperatures lead to local fluid overpressure and the liberation of a silica-poor fluid in a subdomain with initially increased bulk iron and decreased silica content. The fluid overpressure drives fluid flow into other subdomains where the fluid enhances dehydration and leads to olivine enrichment in an iron-enriched vein. Our model shows how reactive silica transport can lead to vein widening and olivine enrichment within the veins as observed in the Erro Tobbio meta-serpentinites. Thus, reactive fluid flow is a critical step in the evolution towards a larger scale vein system and a dynamic porosity evolution by accounting for a chemical feedback between the dehydrating rock and the liberated fluid.