Underplated units in an accretionary complex: Melange of the Shimanto Belt of eastern Shikoku, southwest Japan

摘要

Imbricated thrust stacks composed of melange and oceanic slabs occur in the Shimanto Belt, eastern Shikoku, SW Japan. The melanges are divided into two types: Melange I consists of lenticular sandstone blocks surrounded by scaly shale matrix, and Melange II is composed of blocks of oceanic material, such as basalts, cherts and red shale, in a scaly matrix of shale. Melange II is observed only near the boundaries between Melange I and oceanic slabs. The fabric of Melange I indicates progressive deformation during underthrusting of originally coherent trench‐fill turbidites. Layer‐parallel extension characterizes early deformation and is divided into two stages: an early stage formed during normal faulting, and a subseguent stage formed perpendicular to the early extension. The early extension is interpreted in terms of Riedel shear associated with layer‐parallel shearing and tectonically induced loading normal to the layering. The second extension is due to sticking‐thrust movement. Late deformation of Melange I is represented by folding in association with layer‐parallel shearing. This appears to be related to compression just before or during underplating of Melange I beneath the accretionary prism. Layer‐parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments. Subsequently, the shear zone penetrates pelagic sediments and, finally, oceanic basement. At this time, Melange II is formed by the shearing. An imbricated stack of Melanges I, II and oceanic slabs forms duplexes during the underplating process. The weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench f