Subduction erosion versus sediment offscraping at the toe of the Middle America Trench off Guatemala

Abstract

High‐resolution surface ship and deep‐tow data from the Middle America Trench off Guatemala demonstrate that structures at the base of the landward slope are most simply interpreted as resulting from the offscraping and accretion of the uppermost trench sediments. There is a 250 to 300‐m‐wide ridge elevated 40–60 m above the trench floor at the toe of the trench slope. Uplifted trench sediments are resolved on the ridge in one deep‐tow profiler record. Trench strata beneath the ridge are imaged on migrated seismic reflection profiles, which show evidence for folding of the trench strata. Vergence of the structures is consistent with folding above a landward dipping thrust fault. We therefore interpret the ridge at the base of the slope as the surface expression of folded trench sediments that are presently being offscraped and accreted to the toe of the trench slope. Only the upper hundred or so meters of trench strata are offscraped; the remainder of the trench strata and the underlying plate deposits are subducted beneath the toe of the slope. Trench sediment fill buries much of the horst topography, which is then passively subducted without eroding the base of the slope. Our results indicate that (1) non‐steady state accretion has occurred at this margin, even though Deep Sea Drilling Project drilling suggested otherwise, and (2) the lower‐most slope has been affected by compressional deformation rather than extension or slumping.