TECTONIC COMPRESSION AS A SOURCE OF METHANE EMISSIONS ON CASCADIA MARGIN

Recent geophysical surveys have imaged bubble streams within the water column using acoustic methods to show that methane emissions are densely-spaced along most of the Cascadia margin. Survey compilations along the 250 km long Washington segment indicate a total of 492 bubble plume emission sites. Methane emission sites along the accretionary margin sediment wedge are non-uniformly distributed, being generally gathered into clusters located at different water depths. Examination of the depth-distribution of methane emission site density indicates that the vast majority of sites (360 out of 492) are clustered the western edge of the continental shelf within a narrow depth range from 150 to 250 meters. In addition to this primary depth-distribution peak at 175 m depth, there is a minor distribution peak near 500 meters (27 sites), which is the depth of the upper hydrate methane stability zone.

The enormous scale of these methane emissions impacts the continental margin biological and chemical environment, and it is important to determine the hydrocarbon sources and geological pathways. The outer continental shelf-edge is a prominent east-west feature of the Cascadia accretionary wedge. A recent analysis demonstrated that the sediments beneath the entire Washington continental shelf are much more compact than those beneath the upper margin slopes that are located just to the west of the shelf edge. Seismic velocity profiles show that the boundary between sediments with low compaction on the upper margin and highly-compacted sediments beneath the shelf takes place directly at that western shelf edge, occurring within a narrow 10 km horizontal distance. Because the compaction and porosity reduction occurs at the shelf break where methane emission sites are abundant, it is likely that the fluid and gas emissions observed result from this near 30% reduction of sediment pore volume. The transformation of margin slope sediments into the continental shelf sediments on an accretionary wedge is an evolutionary process, where maximum horizontal compressive stress on Cascadia alternates between E-W and N-S, depending on the subduction zone’s timing within the current earthquake cycle. The resulting tectonic compression that occurs at the shelf edge is likely the major source of methane emissions on Cascadia margin.