Stages of Gas-Hydrate Evolution on the Northern Cascadia Margin

Natural gas hydrate occurs beneath many continental slopes and in arctic permafrost areas. Recent studies have indicated that the largest deposits of gas hydrate might lie in nearly horizontal layers several hundred meters beneath the seafloor of continental slopes, especially in the large, accre-tionary sedimentary prisms of subduction zones. Expedition 311 of the Integrated Ocean Drilling Program (IODP) investigated the formation of gas hydrate in the accretionary prism of the Cascadia subduction zone (Fig. 1). The primary objectives of Expedition 311 were to test and constrain geological models of gas hydrate formation by upward fluid and methane transport in accretionary prisms. We specifically sought to (a) determine the mechanisms that control the nature, magnitude, and distribution of the gas hydrate, (b) find the pathways of the fluid migration required to form large concentrations of gas hydrate, (c) examine the effects of gas hydrate on the physical properties of the host sediment, and (d) investigate the microbiology and geochemistry associated with the occurrence of gas hydrate. Furthermore, we concentrated on the contrast between methane transport by focused flow in fault zones and by dispersed pervasive upward flow at various scales of permeability.