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