Landward-dipping reflectors in accretionary wedges: Active dewatering conduits?

Abstract

Landward-dipping seismic reflectors (LDRs) within the accretionary wedge at convergent plate margins have generally been interpreted as tilted-bedding or thrust-fault surfaces (e.g., Mexico, northern Japan). This interpretation accords with models in which trench-axis sediments are scraped off the descending plate to form a series of imbricate thrust sheets. However, intensely disturbed melanges with a regionally extensive landward-dipping foliation are found beneath slope sediments in many ancient and active subduction complexes. This observation, combined with the fact that large volumes of fluid are released during dewatering of subducting and previously accreted sediments, leads to the proposal that some of the seismic reflectors are porous, laterally extensive fractureways filled with dewatering fluids derived from deeper underthrust water-rich sediments. These fractureways trend subparallel to the regional foliation of the accreted melange wedge. The contrast in acoustic impedance between the high-porosity fluid-filled fractureways and the melange wall rocks is enhanced whenever methane and/or carbon dioxide exsolve from rising waters. Ancient LDRs may be recognized where mineralized veins trend subparallel to the regional foliation of melange belts generated during subduction accretion. 42 references, 2 figures.