5:15 PM - 6:45 PM
[SCG40-P31] Deformation mechanisms and rheology of chlorite-actinolite schist: Implications for slow slip near the mantle-wedge corner
Keywords:actinolite, slow slip events, dissolution-precipitation creep, rheology
In warm subduction zones such as the Nankai subduction zone, the slab-mantle interface near the mantle wedge corner is thought to be the source region of slow slip events (SSEs). Chlorite-actinolite schist (CAS) has been proposed as one of the rocks responsible for SSEs. However, it remains uncertain how the deformation mechanisms and rheology of CAS affect SSEs. We examine the subduction mélange in the Nishisonogi metamorphic rock, Japan, exhumed from the source depth of SSEs, comparable to the Nankai subduction zone beneath Shikoku. Field observations indicate that viscous shear in the ~90 m thick subduction mélange is concentrated along the anastomosed CAS intercalated in metapelite, metasandstone, and metabasite. Microstructural observations indicate that dissolution-precipitation creep of fine-grained actinolite in the CAS matrix and dislocation creep of quartz lenses were dominant deformation mechanisms. Rheological analysis, considering the coexistence of actinolite dissolution-precipitation creep and quartz dislocation creep, indicates that viscous shear in CAS occurred at stress ~40 MPa and strain rate of ~10-10 s-1. If plate boundary slip is accommodated by anastomosing shear of CAS in the ~90 m thick subduction mélange, the slip rate is ~10-8 m/s, comparable to slow slip rates observed in modern subduction zones. Alternatively, the contrast in deformation mechanisms between the CAS matrix and quartz lenses may be due to stress amplification in the quartz lenses imposed by faster strain rates and lower shear stress in the surrounding weaker actinolite, which could also be caused by SSEs. We suggest that slow slip near the mantle wedge corner is accommodated by anastomosing viscous shear along the chlorite-actinolite schist.