Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

Integrated Ocean Drilling Program Expedition 311 is based on ex- tensive site survey data and historic research at the northern Cas- cadia margin since 1985. This research includes various regional geophysical surveys using a broad spectrum of seismic tech- niques, coring and logging by the Ocean Drilling Program Leg 146, heat flow measurements, shallow piston coring, and bottom video observations across a cold-vent field, as well as novel con- trolled-source electromagnetic and seafloor compliance surveying techniques. The wealth of data available allowed construction of structural cross-sections of the margin, development of models for the formation of gas hydrate in an accretionary prism, and es- timation of gas hydrate and free gas concentrations. Expedition 311 established for the first time a transect of drill sites across the northern Cascadia margin to study the evolution of gas hydrate formation over the entire gas hydrate stability field of the accre- tionary complex. This paper reviews the tectonic framework at the northern Cascadia margin and summarizes the scientific stud- ies that led to the drilling objectives of Expedition 311 Cascadia gas hydrate.