Fluid flow at the toe of convergent margins: interpretation of sharp pore-water geochemical gradients

Fluid expulsion greatly impacts chemical and mass budgets at subduction zones, particularly if it is focused along faults and stratigraphic conduits. Geochemical anomalies centered at décollement zones, such as pore-water freshening and the presence of thermogenic hydrocarbons, indicate long-distance, focused flow of deeply sourced fluids. The sharp gradient of these anomalies below the décollement zone has been interpreted as evidence for recent pulses of fluid flow, initiating a few to tens of ka. However, this interpretation does not consider that underthrust sediments are moving arcward beneath the décollement zone. In addition, upward flow from the compacting underthrust sediments can modify chemical profiles. Here, we use a simple model that couples fluid flow and solute transport to evaluate these sharp chemical gradients. We find that observed geochemical anomalies at the Northern Barbados and Costa Rican subduction zones can be explained either by recent pulses of flow, or by sustained flow along the décollement zone coupled with modest vertical fluid expulsion from consolidating underthrust sediments. The latter interpretation is consistent with estimates of upward flow rate at Costa Rica based on estimated pore pressure gradients and measured permeabilities within the underthrust sediments. One important implication is that recent pulses of flow along fault conduits may not be required to explain the geochemical anomalies. Furthermore, mixing of locally derived fluids flowing upward from the underthrust sediments and deeply sourced fluids flowing along the décollement zone provides an explanation for the observed changes in pore-water freshening along the décollement at Costa Rica.