Gas Hydrates Along the Peru and Middle America Trench Systems

Abstract

The convergent margins of Peru and Middle America are dominantly shaped by subduction erosion, which causes tectonic subsidence of large areas. Both margins are largely non-accretionary and most of the incoming trench sediment is being subducted. Gas hydrates are ubiquitous across the Peru margin with its high organic carbon content. Strong bottom simulating reflections (BSRs) are present on the lower slope. A regional lack of BSRs further upslope in Lima Basin may be explained by rapid subsidence leading to a downward movement of the base of gas hydrate stability and absorption of free gas into the gas hydrate stability zone. BSRs in small areas in Lima Basin are perhaps supported by high rates of methane supply offsetting the absorption of gas. Off Costa Rica, seamount subduction promotes slope failure. A general lack of BSRs in sediment sections that were destroyed by slumping suggests no or a very slow re-formation of BSRs after slumping. Above a subducted seamount, sediments containing a BSR remain intact despite inferred strong uplift. The associated pressure decrease should lead to dissociation of gas hydrates, generation of gas, and overpressuring. We speculate that the gas may escape through faults above the BSR. Thermo-hydraulic modeling based mainly on BSR heat flow indicates high frictional heating in the Peru subduction zone causing a landward increase of the thermal gradient. Off Costa Rica, a shallowing of BSR depth reflects locally elevated heat flow coinciding with expected expulsion of fluids at the landward boundary of the frontal prism.