RECURRENT VOICES: Offshore Fault-Zone Mapping Provides Insights and Highlights New Mysteries – PART 4 of Evolving perspectives on the Pacific-North American plate boundary near Haida Gwaii, BC
July 26, 2016
***See Parts 2 & 3 for figures 2 – 4***
Offshore Fault-Zone Mapping Provides Insights and Highlights New Mysteries
July 26, 2016
by Sean Mullan
Multibeam sonar hydrographic surveys conducted with the CCGS Vector off the west coast of Haida Gwaii have revealed, in high-resolution, the surface expression of the Queen Charlotte Fault and associated landforms (Barrie et al., 2013). Multibeam bathymetry obtained off the west coast of central Haida Gwaii (figures 5 and 6) allows the visualization of a major valley attributable to the Queen Charlotte Fault. The lack of sediment deformation along this seafloor valley is indicative of pure strike-slip motion. Small depressions on the upper slope may be the result of strike-slip right-step offsets formed by realignment during oblique convergence (Barrie et al., 2013).
Given the compelling evidence for an underthrusting Pacific Plate described earlier, some may find it unusual that the linear strike-slip trace of the Queen Charlotte Fault is so apparent in this area. Hyndman (2015) suggests that this is due to the partitioning of relative plate motion by two major structural features: 1) the steeply-dipping strike-slip Queen Charlotte Fault, with its surface expression as the seafloor valley described by Barrie et al. (2013); and 2) the shallowly dipping “Haida Gwaii” thrust fault, located at the base of the accretionary prism (Queen Charlotte Terrace).
Additional mysteries are bubbling towards the surface. During a 2011 NRCan expedition on CCGS Tully, a large conical seabed feature was observed off the southwest tip of Haida Gwaii, near the seabed expression of the Queen Charlotte Fault (position indicated by “V1” in figure 1). Sonar profiles over this feature revealed the emission of a gas plume from its summit at about 775 m depth. A similar summit-venting hill, at about 1000 m depth, was just discovered during a 2015 CCGS Tully transit across the seaward side of Dixon Entrance near the BC-Alaska border (position indicated by “V2” on figure 1). A screenshot of the acoustically detected plume in Dixon Entrance is shown in figure 7.
These two venting landforms morphologically resemble volcanoes. However, they are probably the result of non-volcanic processes due to: 1) their local tectonic positions along the continental slope and the Queen Charlotte-Fairweather fault system; and 2) their general presence in a non-volcanic arc forming tectonic convergence zone. Although the Kodiak-Bowie and Tuzo Wilson seamount chains are located offshore of this region, the limited heat-flow data available near the south coast of Haida Gwaii (Hyndman et al., 1982) is consistent with an underthrusting slab of oceanic crust (Hyndman, 2015). Drs. Gary Greene (Moss Landing Marine Laboratories, Santa Cruz, California) and Vaughn Barrie (NRCan) saw no evidence of volcanic materials when examining seabed digital photographs of the Dixon Entrance site. They hypothesize that the feature and other hills around it may constitute a mud volcano field.
A well-known origin of gas seeps along active margins is the compaction and dewatering of accreted sediments containing methane-rich pore fluids (Johnson et al., 2003). These methane rich fluids are formed in buried sediments by the biological decay or thermogenic degradation of organic matter (e.g. Coleman and Ballard, 2001). Fluid circulation through substrata can supply seafloor vents with dissolved and free gas. Seeps such as these have been extensively studied in the accreted sediments along the Cascadia Margin west of Vancouver Island (e.g. Riedel et al., 2006; and ongoing monitoring by Oceans Network Canada at Clayoquot Slope). The possibility of seabed organic hydrocarbon seeps along the margin of Haida Gwaii should be further explored. Heat-flow in the Queen Charlotte Fault region is known to be high offshore and decrease in a landward direction (Hyndman et al., 1982). It is quite possible that the circulation of hydrothermal fluids has an influence on venting.
A notable location with some morphological similarities to the Haida Gwaii vents is Hydrate Ridge, off the west coast of Oregon. There, a portion of accretionary prism has been heavily deformed by the compression of the Juan De Fuca Plate’s oblique subduction beneath North America (Johnson et al., 2003). Methane hydrates, the site’s namesake, are methane molecules trapped in an ice-like cage of water molecules. They develop under low-temperature and high-pressure situations and their dissolution can result from a change in these temperature-pressure conditions. At Hydrate Ridge, the spatial distribution of seafloor pockmarks indicates that the gas hydrate stability limit is found at 550-750 m depth (Johnson et al., 2003). The presence of gas plumes emanating from hilltops along Hydrate Ridge is explained by the focusing of subsurface fluid flow towards the crests of anticlinal structures (Johnson et al., 2003).
This structural fluid-focusing process, and the previously mentioned mud volcano hypothesis, should both be investigated as possible mechanisms for the development of volcano-like morphologies at the large vents near Haida Gwaii. The general investigation of sub-seafloor fluids along the Queen Charlotte-Fairweather fault system will aid geohazard assessments related to both fault-lubrication and submarine slope stability.
Barrie, J. V., Conway, K. W., and Harris, P. T. (2013). The Queen Charlotte Fault, British Columbia: Seafloor anatomy of a transform fault and its influence on sediment processes. Geo-Marine Letters. 33. pp. 311 – 318.
Coleman, D.F. and Ballard, R.D. (2001). A highly concentrated region of cold hydrocarbon seeps in the southeastern Mediterranean Sea. Geo-Marine Letters. 21. pp. 162 – 167.
Hyndman, R. D. (2015). Tectonics and structure of the Queen Charlotte Fault Zone, Haida Gwaii, and large thrust earthquakes. Bulletin of the Seismological Society of America. 105-2B. pp. 1058-1075.
Hyndman, R. D., Lewis, T. J., Wright, J. A., Burgess, M., Chapman, D. S., and Yamano, M. (1982). Queen Charlotte fault zone: Heat flow measurements, Canadian Journal of Earth Sciences. 19. pp. 1657 – 1669.
Johnson, J.E., Goldfinger, C. and Suess, E. (2003). Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin. Marine Geology. 202. pp. 79-120.
Riedel, M., Novosel, I., Spence, G.D., Hyndman, R.D., Chapman, R.N., Solem, R.C., and Lewis, T. (2006). Geophysical and geochemical signatures associated with gas hydraterelated venting in the northern Cascadia margin.
Geological Society of America Bulletin. 118. 1-2. pp. 23-38.
*Part 4 of 4
**article originally released in the Geological Association of Canada (GAC) Marine Geoscience newsletter