Tectonic melange as fault rock of subduction plate boundary

An assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone. The research object is the latest Cretaceous Mugi melange in the Shimanto Belt, southwest Japan. Systematic age younging from pelagic to terrigenous through hemipelagic sediments is well-documented even though original stratigraphy is disrupted. Systematic shear fabric consistent with ancient plate convergence is reconstructed. The melange was formed at temperatures of similar to 130-200 degrees C by cataclastic comminution of sandstone layers accompanied by tensile cracking, and plastic deformation and the dehydration of clayey shale matrix, with subsequent peeling off and underplating of the oceanic basement. The temperature setting for the Mugi melange indicates around the up-dip limit of the seismogenic zone, therefore includes various fault rocks suggestive of earthquake fault; pseudotachylyte, fluidized ultracataclasite with heating evidence, amorphous silica and so on. These suggest that fluid induced lubrication was dominated. Localized cataclastic shear, which is a candidate of small earthquake or very low frequency earthquake, is also recognized especially in sandstone blocks dominated portion in melange. These observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone. The quantitative examination of the Mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt. (c) 2011 Elsevier B.V. All rights reserved.