Abstract

Based on a three-dimensional high-resolution seismic survey off Vancouver Island, Canada, we show that natural hydraulic fracturing is an efficient process to create permeable pathways for focused fluid upflow at submarine venting sites. The pockmark structure examined in this study is located on top of an accreted ridge, where the low-permeability base of the gas hydrate stability field is also elevated, and fluid overpressure will be induced tectonically. Elongated hydraulic fractures reveal decreased reflection amplitudes and are oriented along the maximum principal stress direction, perpendicular to nearby fault planes. A surface expression is created by fluid and material expulsion, forming an irregular, noncircular rim along the axis of the upflow zone. Our findings confirm a relationship between lateral variations in seismic reflectivity and sediment distribution, regional tectonics, episodic fluid flow, and the formation and dissociation of gas hydrate. They explain the presence of gas hydrate at the top of the upflow zone, which requires transport of large volumes of gas through the gas hydrate stability field. Since hydraulic fracturing is not restricted to compressional regimes, it is a likely explanation for the nature, shape, and orientation of upflow zones as well as for seismic blanking widely observed at both active and passive continental margins.

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