Unique deformation processes involving the recrystallization of chrysotile within serpentinite: implications for aseismic slip events within subduction zones

Serpentinite bodies within the Franciscan Complex, a Mesozoic accretionary prism located in California, USA, display a unique form of deformation that involves the recrystallization of chrysotile and the formation of a block-in-matrix structure. The phacoidal-shaped blocks have a preferred orientation, and result from the local replacement of serpentine minerals by chrysotile grains that are aligned parallel to ductile shear planes such as S-C foliation; ultimately, some of the rocks evolved into chrysotile schist. The relic blocks are also fragmented into multiple parts, with the spaces between fragments being infilled by recrystallized chrysotile. The low coefficient of friction of chrysotile means that this deformation process acts to suppress the frictional properties of the entire serpentinite body within the forearc mantle. This phenomenon can be attributed to the slip style that occurs in aseismic regions of subduction zones in areas shallower than the stability field of antigorite.