The interaction of fluid processes and progressive deformation during shallow level accretion: Examples From the Shimanto Belt of SW Japan

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Abstract
The interaction of deformation and fluid flow is recorded in the mesostmcture and microstructure of the Shimanto Belt in the eastern Hata Peninsula of SW Japan. Large areas of disrupted strata, including the Okitsu mélange have been generated during the progressive deformation associated with shallow level accretion. The resulting assemblage of structures and their crosscutting relations reflect the complex interplay between pore fluid pressures, lithification states, and strain rates. Evidence for the relative timing of structures, diagenesis, and modes of fluid migration has been used to constrain a generalized model for the nature of fluid outflow during the accretion and burial of trenchslope lithologies. Early outflow was diffuse, but the flow direction was controlled mainly by a regionally pervasive structural anisotropy and the predominance of independent paniculate flow. As the sediments became more lithified, fluid flow was focused along faults in which transient microfractures controlled the permeability. Fractures released pore fluid pressure promoting further deformation by diffusive mass transfer. This in turn resulted in the buildup of pore fluid pressures and tectonic stresses and a recurrence of fracturing. In some fault zones and in the Okitsu mélange several such deformation mechanism cycles can be identified. The development of a complex array of faults during the late stages of accretionary complex evolution resulted in fluid flow paths becoming multidirectional.