Seamount subduction to the Nankai accretionary wedge and its impact on methane hydrate accumulation: insights from analogue and numerical models

Seamount sudbuction is a common feature at convergent plate margins and several examples can also be seen at the Nankai wedge, but its impact on methane hydrate accumulation has not fully described. In order to understand the accumulation mechanism of methane hydrate, the key issue would be the fluid flow within the sediments. The fluid flow can be classified into two types; the diffusive flow by intergranular porosity and the focused flow along faults (Baba and Yamada, 2004). The diffusive flow can be modeled by conventional reservoir simulator type of approaches, but the focused flow along faults may be difficult. One possible scenario was suggested by Sibson (1995) that fluid may migrate along a fault surface when it slipped (breakage of seal). Following the idea, the focused fluid flow can be evaluated by fault activity that can be modeled and examined by analogue experiments and numerical simulations (Yamada et al., submitted). This research employed analogue experiments that used granular materials (dry sand and glass beads) and numerical simulations that approximate the geologic body as particles (distinct element method). The analogue model results are further analyzed by optical image correlation technique, PIV, to extract faulting events in detail. By using the same tectonic model of sea mount subduction to an accretionary prism, we examined the deformation process particularly the faulting by these two techniques. The results of the analogue experiments and numerical simulations are basically the same, apart from the reproducibility of small structures influenced by the particle size. The models suggest that a sea mount subduction causes segmentation of wedge formation. The segmentation of wedge also produces two types of fault systems; one formed before sea mount subduction and the other afterward. The geometry of these faults illustrates that the fluid from the deeper segment of the sedimentary pile may focus on the faults that formed after sudbuction. These models can provide some ideas to interpret seismic profiles at subduction margins to evaluate the focused fluid flow along the faults at the Nankai accretionary wedge. By combining geochemical and well-log data from the Nankai area, we are gong to construct the next stage model of methane migration at the Nankai wedge.