Offscraping accretion of Jurassic chert-clastic complexes in the Mino-Tamba belt, central Japan

Abstract

Detailed structural and biostratigraphical analysis of the Jurassic Inuyama Sequence, a coherent chert-clastic complex in the Mino-Tamba Belt, central Japan, clarifies the evolution of accretionary processes at shallow structural levels. The Inuyama Sequence is characterized by a series of stacked thrust sheets. Each sheet consists of an Early Triassic to Middle Jurassic oceanic plate stratigraphy composed of four lithologic units which are, in ascending order: siliceous claystone; ribbon chert; siliceous mudstone; and clastic rocks. The structural features of the Inuyama Sequence demonstrate a four-stage progressive deformation. (1) A décollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front. Clastic dikes and sills formed at the horizon just above the décollement at this time. (2) The stratigraphic section above the décollement was imbricated by in-sequence thrusting in the frontal part of the wedge. This initial stage of thrusting and imbrication was followed by (3) the formation of duplex structures with fault-related folds within the lower stratigraphic section as the décollement stepped down-section to the lowest siliceous claystone interval. Finally, (4) these thrust packages were overprinted by secondary prism thickening in the form of out-of-sequence thrust faulting.