付加体の構造侵食による前弧の構造発達

Abstract

This paper proposes a new model for the evolution of geologic structures in forearcs concerning subduction erosion after tectonic accretion. The model was generated from a high-pressure (HP) metamorphic unit, in which the early history of frontal offscraping accretion is recorded, in the Kamuikotan Zone of central Hokkaido. It was transported from the trench to the interior of the subduction zone during a non-accretionary stage, and the process is regarded to be subduction erosion and subsequent underplating re-accretion at greater depths. This mode of material transport persuasively explains the common and abundant occurrences of terrigenous meta-clastic rocks in HP subduction complexes, in spite of common evidence that sediments had been scraped off at much shallower depths as seen in coeval non-metamorphic accretionary units. It is thus suggested that significant amounts of HP metasediments originated from tectonically eroded materials of the frontal accretionary complex, instead of rocks directly underplated from the subducted slab. Subduction erosion in Hokkaido occurred contemporaneously with the exhumation of a higher-grade HP unit with similar rates of vertical movement of opposite senses. It suggests that exhumation was a counter flow of subduction erosion: the higher-grade rocks were lifted up by the insertion and accumulation of tectonically eroded rocks at depths, and were unroofed by lateral extension of the oversteepened non-accretionary wedge. This model can also explain common flat-lying structures of accretionary and subduction complexes exposed on land. They originally had rather high-angle structures formed by lateral shortening during early accretionary stages, and then were tilted trench-ward by the removal of materials near the trench by subduction erosion and their underplating at rear-side portions during the subsequent non-accretionary stages.