Subduction of trench-fill sediments beneath an accretionary wedge:
insights from sandbox analogue experiments
Abstract
Ancient exhumed accretionary complexes are sometimes associated with
high-pressure–low-temperature (HP–LT) metamorphic rocks, such as
psammitic schists, which are derived from sandy trench-fill sediment. At
accretionary margins, sandy trench-fill sediments are rarely subducted
to the depth of HP metamorphism because they are commonly scraped off at
the frontal wedge. This study uses sandbox analogue experiments to
investigate the role of seafloor topography in the transport of
trench-fill sediment to depth during subduction. The experiments were
conducted with a detached, rigid backstop to allow a topographic high
(representing a seamount) to be subducted through a subduction channel.
In experiments without topographic relief, progressive thickening of the
accretionary wedge pushed the backstop down, leading to a stepping down
of the décollement, narrowing the subduction channel, and underplating
the wedge with subducting sediment. In contrast, in experiments with a
topographic high, the subduction of the topographic high raised the
backstop, leading to a stepping up of the décollement and widening of
the subduction channel. These results suggest that the subduction of
topographic relief is a possible mechanism for the transport of
trench-fill sediment from the trench to HP environments through a
subduction channel. A sufficient supply of sediment to the trench and
topographic relief on the subducting oceanic plate might enable
trench-fill sediment to be accreted at various depths and deeply
subducted to become the protoliths of HP–LT metamorphic rocks.