The influence of organic-rich shear zones on pelagic sediment deformation and seismogenesis in a subduction zone

We have examined the mineralogy and deformation of black shear zones containing abundant carbonaceous materials (CM) and clay minerals in bedded ribbon cherts in a Jurassic accretionary complex, central Japan. Microtextural observations indicate that pressure solution and cataclastic deformation were the primary deformation mechanisms in the cherts. Whole-rock mineral compositions were quantitatively investigated using X-ray diffraction, Raman spectroscopy, and a CHN elemental analyzer. The results show that the samples contain variable amounts of CM and clay minerals, up to 17 wt% and 60 wt%, respectively. Moreover, the clay and CM contents in the samples, including the host rock cherts, show a positive correlation represented by a single compositional trend, and this may be explained by the progressive concentration of clays and CM due to pressure solution and the removal of soluble quartz or mass transfer processes associated with deformation. Intact cherts dominated by quartz seem to provide plausible source rocks for the nucleation of seismic slip including slow slip events, while the abundant CM and clays as observed in the black shear zones may have effectively weakened and stabilized the sliding behavior. These results are important for understanding deformation processes in the Japan Trench.