Preferential Acceleration of Pressure Solution Creep of Shear Zones in Shale of Subduction Zone

We have estimated activation energy of natural pressure solution deformation in shales of the Shimanto accretionary complex in southwestern Shikoku Island, Japan. Pressure solution deformation affected two types of shales: shear-dominated (type S) and compaction-dominated (type C) shales. Additionally, within each type of shale, pressure solution seams (PSS) density, indicative of the intensity of the deformation, is highly variable in relation with a relatively wide range of paleotemperature [Kawabata et al., 2007]. Correlating the PSS density and the paleotemperature with an Arrhenius expression, we extracted low activation energies of 18 kJ/mol for type S and 45 kJ/mol for type C. The good agreement between these values and the activation energy of diffusion controlled pressure solution creep (approximately 35kJ/mol), which is sum of activation energy of diffusion coefficient through grain boundary fluid (15kJ/mol) [Nakashima, 1995] and equilibrium constant of silica (20kJ/mol) [Rimstidt and Barnes, 1980], as well as our microstructural observations, show that diffusion is indeed the controlling process of pressure solution deformation. Difference in activation energy between Type S and Type C is probably caused by grain boundary structure, which can be either wetted (connected) or non-wetted (non-connected) [Nakashima, 1995]. Shear deformation, active in type S shales, creates more wetted (connected) grain boundary than compaction in type C shales, resulting in smaller activation energy. Such interplay between shear deformation and the fluid phase structure promotes the localization of deformation within shear zones deforming by pressure solution.