ANATOMY OF A SUBDUCTION PLATE INTERFACE DOWNDIP OF THE SEISMOGENIC ZONE: THE LEECH RIVER SHEAR ZONE, VANCOUVER ISLAND

The downdip limit to the seismogenic zone of subduction megathrusts is defined as the region where stick-slip behavior transitions to steady creep. This behavior is consistent with the transition from frictional sliding to deformation accommodated predominantly by crystal plastic deformation mechanisms. However, due to the relative scarcity of deeply exhumed examples, the distribution of strain across the plate interface, the lithologies and minerals that accommodate the deformation, and the fluid-rock interactions in the deformation zone are not well understood. Here, we document the Leech River Shear Zone on Vancouver Island, British Columbia, a 600-m wide mylonitic shear zone that represents a paleo-plate boundary from the Cascadia subduction zone. Deformation at lower amphibolite facies conditions was distributed across the upper plate Leech River metapelitic accretionary complex and the Metchosin metabasalt of the downgoing plate. Interconnected, anastomosing layers of biotite, muscovite, and chlorite in the accretionary prism rocks deformed through kinking and dislocation glide. In the metabasalt, amphibole and chlorite formed an SC fabric in which asymmetric amphibole porphyroclasts deformed by pressure solution creep. Available flow laws indicate the presence of phyllosilicates resulted in a weak bulk rheology compared to quartz-dominated rocks. As deformation was distributed across both lithologies, we suggest hydration, alteration, and fabric development in the basaltic rocks resulted in a similar weakness to the phyllosilicate-rich rocks. The strength of the plate interface was therefore controlled predominantly by the presence of phyllosilicates and amphibole. These hydrous minerals resulted in low viscosities that permitted aseismic creep at low stress along the plate interface.