Sandbox analogue experiments for subduction of trench-fill sediments beneath accretionary wedge and backstop

We conducted sandbox analogue experiments for subduction of trench-fill sediments beneath accretionary wedge and backstop in order to explain how protoliths of high-pressure/low-temperature (HP-LT) metamorphic rocks are transported to high pressure environment. At accretionary-type subduction zones, it is commonly difficult that coarse-grained sandy trench-fill deposits subduct deeper than high pressure environment (>10 km in depth), because they are accreted at the shallower part of the wedge (<5 km) in association with stepping down of decollement due to progressive dewatering under the accretionary wedge. However, ancient exhumed accretionary complexes sometimes accompany with low-grade accretionary rocks from trench-fill turbidites and HP-LT metamorphic rocks including psammitic and even conglomeratic schists, whose provenance and depositional ages are similar to each other. Therefore, we need a model to explain growth of accretionary wedge and subduction of coarse-grained trench-fill sediments beneath the wedge at the same time. In this study, we attempt to identify an importance of seafloor roughness for transportation of trench-fill sediments to deep during subduction. For this purpose, we performed sandbox analogue experiments by using an unfixed rigid backstop on a subduction channel with the cases of smooth surface (Exp. A) and rough surface representing a seamount or ridge on subducting lower plate (Exp. B). The results of Exp. A showed progressive thickening of the accretionary wedge pushed the backstop down, meaning stepping down of the decollement and narrowing the subduction channel. On the other hand, Exp. B showed a subducting seamount lifted up the backstop, stepped up the decollement, and then widened the subduction channel. Subduction of a rigid material like seamounts is a possible mechanism to open subduction channels for transportation of terrigenous sediments from the trench to high-pressure condition. Significant sediment supply to the trench and rough surface of subducting oceanic plate are required to enable subduction of protolith of HP-LT metamorphic rocks and accretion of trench-fill sediments at the shallow part.