Development of high fluid pressure at the base of the shallow mantle wedge due to presence of an antigorite shear zone

Deep slow earthquakes in subduction zones are concentrated at the base of the wedge mantle, but the reasons for this relationship are not well understood. High fluid pressures are thought to be an essential component of slow earthquakes, enabling slip at low shear stresses characteristic of slow earthquakes and compatible with recorded high vp/vs ratios. One of the prerequisites for high fluid pressure is the presence of a cap rock that can trap fluid in a limited region. Quartz precipitated from slab-derived fluids has been proposed as important component to forming cap rocks at the base of the overriding crust. However, quartz is not stable in the mantle wedge and in this region a different mechanism is required. In the cool mantle wedge, the addition of SiO2 through the influx of aqueous fluids will cause the development of serpentine and the presence of this serpentine may also affect the permeability. Experimental evidence shows foliated antigorite serpentinite has a strong permeability anisotropy with very slow flow perpendicular to the foliation. A laterally continuous layer of foliated antigorite serpentinite could act as a cap rock but its presence is not widely recognized. The subduction-type Sanbagawa metamorphic belt preserves numerous blocks of former mantle wedge and the boundary with the surrounding slab-derived schists represents the paleo subduction boundary. Units that formed at depths of ~35-45 km similar to that shown by deep slow earthquakes show a 100 m-wide strongly-foliated zone of antigorite schist followed by narrow zone of actinolite and talc-bearing lithologies along the boundary with slab-derived metapelite. A estimate of the amount of fluid flow can be derived by using the amount of added silica needed to make the serpentinite compared to the original mantle lithology and the solubility of SiO2 at the recorded metamorphic temperatures. These calculations show good agreement with estimates based on modelling of slab dehydration reactions. A comparison with experimentally determined fluid permeability suggests that the presence of an antigorite shear zone will cause high fluid pressures to develop at around 35-45 km depth. The presence of an antigorite shear zone along the base of the mantle wedge may control the depth of deep slow earthquakes.