Abstract
Identifying and examining geological processes that have occurred in sediment‐starved trenches of ancient non‐accretionary subduction zones exposed on land are still challenging, because such style of subduction is believed to leave scarce rock records. Our new geological mapping, petrography of coarse clastic rocks, radiolarian and zircon U–Pb dating of the Shiriya accretionary complex in Northeast Japan suggest that the younger parts (Sr1 unit) of the complex formed in a sediment‐starved trench. The key observation is that debris derived from the inner trench slope directly overlies pelagic chert in many sections where trench turbidites are lacking. A significant mass of turbidites occurring in other sections are also considered to represent distal facies of the debrites from the adjacent inner trench slope, rather than normal trench‐fill turbidites directly supplied from the continental landmass. These recycled materials from the inner trench slope comprised an imbricate frontal accretionary wedge, together with pelagic siliceous deposits of ocean floor origin. This accretionary wedge of debrite origin suggests that mass wasting on inner slopes of sediment‐starved trenches does not always result in tectonic erosion (removal of materials from the upper plate), but results in material recycling to reconstruct a new frontal accretionary wedge. The spatial dimensions of the recycling cell may be one of the critical differences between accretionary and non‐accretionary margins. Recycled continental materials transported by turbidites from remote landmasses construct frontal wedges in accretionary margins, whereas mass wasting on adjacent slopes and re‐accretion at the trench recycle continental materials in non‐accretionary margins. The transition from an accretionary to non‐accretionary (i.e. sediment‐starved) trench detected in the Shiriya Complex potentially records tectonic response of the inner trench slope to increased friction along the subduction interface.