Fluid, methane, and energy flux in an active margin gas hydrate province, offshore Costa Rica

Joint analysis of seafloor temperature data, borehole temperature measurements, and observations that constrain the position of the top and base of the gas hydrate zone (GHZ) at sites on the non-accretionary Costa Rican margin provide quantitative constraints on the flux of energy, fluid, and methane through the sediments on the overriding plate. A comparison of borehole temperature data obtained in the trench, at the toe of the deformed sedimentary wedge, and at a midslope location indicates that the thermal regimes are perturbed by appreciable advective flux at ∼1 km oceanward (19 mm yr−1) and ∼1.4 km landward (5–7 mm yr−1) of the deformation front. This observation is loosely consistent with models that predict the most rapid dewatering and highest rates of fluid expulsion within a few kilometers of the deformation front. Using data that constrain the thickness of the GHZ at three sites on the forearc, we estimate methane and fluid flux rates through application of a one-dimensional analytical model that constrains the nature of the plumbing system for the gas hydrate reservoir. Our results indicate a two-fold decrease in methane flux between a site located ∼400 m from the toe of the wedge and a site 12 km from the toe. An intermediate site located 1.4 km from the toe of the wedge has a thick GHZ (extends to at least 340–365 m below seafloor) that requires a very high methane supply rate (>345 mol m−2 kyr−1 for a mass flux of 5 mm yr−1). We conclude that this site has the most two-dimensional fluid flux regime, with fluids being fed into the zone laterally by conduits and fractures and vertically by expulsion during compaction of underthrusting sediments.