Abstract

Muddy chaotic deposits, now melanges, are common throughout the Shimanto Belt, southwest Japan, a Cretaceous-Neogene forearc extending for more than 1800 km along strike, and also in the older, more northerly, arc-related belts. Many zones within these melanges show features that are consistent with an early, localized genesis as mud injections, seepages, or mud diapirs synchronous with early slope and accretion-related deformation. These have subsequently undergone considerable postlithification deformation. Melanges containing basalt, limestone, and chert associations may have resulted partly from the explosive fragmentation and fluidization of parts of accreted ocean seamounts or ridges as pore fluids escaping from overpressured shales moved along faults and fractures within the basalt-limestone-chert units to form large mud injections. Using field examples from the Shimanto and related belts, we review criteria for recognition of a variety of mud injections in the light of previous work. These criteria include the nature of contacts, the geometry and internal deformation of clasts or blocks, the matrix fabric and orientation of clasts or blocks in the matrix, and anomalous age relations. The incorporation of basalt-limestone-chert associations in some mud injections in southwest Japan is thought to result from the penetration of overpressured, fluidized muds through accreted seamount or ridge lithologies. This generates a variety of textural relations which may be further complicated by subsequent tectonic shearing.

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