Seamount Chain – Subduction Zone Interactions: influence of subducting seamount complexes on accretionary and erosive subduction zone behaviour

Abstract

The tectonic processes of subduction erosion — where upper plate material is entrained within the subduction zone fault as it migrates upwards — and accretion — where oceanic material is offscraped or underplated to the upper plate — are traditionally considered to operate over laterally extensive portions of convergent margins, with sediment volume at the trench exerting a strong control on this behaviour (Clift & Vannucchi, 2004). However, detailed examination of forearcs in areas of subducting high relief seafloor show localised enhanced subduction erosion in the wake of subducting seamounts leading to the formation of re-entrant depressions (Ranero and von Huene, 2000). Seismic and on-land observations have inferred the partial or complete off-scraping of seamounts within the subduction channel (Cloos & Shreve, 1996).

Seamount complexes are surrounded by flexural moat basins, formed by the flexural bending of the oceanic plate in response to the mass of the growing seamount (Vening Meinesz, 1941; Gunn, 1943). These are hundred kilometre wide bathymetric depressions that may accommodate the deposition of up to 3 kilometres of seamount-derived sediment and mass wasted igneous blocks (ten Brink and Brocher, 1987). Collapse of the unstable seamount flanks may transport 100 metre – kilometre scale blocks to the moat (Moore et al., 1994).

Our work on the Osa Melange in southern Costa Rica suggests that this unit represents an accreted moat basin. This melange consists of basalt and carbonate blocks in a pelitic matrix comprised of feldspar and pyroxene grains with rare (0 – 5±2%) quartz grains. This is consistent with sediment recovered from the distal portion of the Hawaiian moat by ODP leg 136 (Tribble et al., 1993).

We propose that subduction of flexural moats provides sufficient volumes of sediment to the trench to promote the formation of localised accretionary prisms, even in margins that are otherwise non-accretionary. This occurs immediately adjacent to the subduction erosion caused by the subduction of the seamount. The composition of the subduction channel at depth is determined by the erosive or accretionary behaviour of the subduction zone: where the subduction zone is erosive, its subduction channel contains entrained blocks of upper plate material, whereas where the subduction zone is accretionary its subduction channel is composed of subducted sediment from the incoming plate.