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Correlation between tectonic stress regimes and methane seepage on the western Svalbard margin

Abstract. Methane seepage occurs across the western Svalbard margin at water depths
ranging from < 300 m, landward from the shelf break, to
> 1000 m in regions just a few kilometres from the mid-ocean
ridges in the Fram Strait. The mechanisms controlling seepage remain elusive.
The Vestnesa sedimentary ridge, located on oceanic crust at a depth of
1000–1700 m, hosts a perennial gas hydrate and associated free gas system.
The restriction of the occurrence of acoustic flares to the eastern segment
of the sedimentary ridge, despite the presence of pockmarks along the entire
ridge, indicates a spatial variation in seepage activity. This variation
coincides with a change in the faulting pattern as well as in the
characteristics of the fluid flow features. Due to the position of the
Vestnesa Ridge with respect to the Molloy and Knipovich mid-ocean ridges, it
has been suggested that seepage along the ridge has a tectonic control. We
modelled the tectonic stress regime due to oblique spreading along the Molloy
and Knipovich ridges to investigate whether spatial variations in the
tectonic regime along the Vestnesa Ridge are plausible. The model predicts a
zone of tensile stress that extends northward from the Knipovich Ridge and
encompasses the zone of acoustic flares on the eastern Vestnesa Ridge. In
this zone the orientation of the maximum principal stress is parallel to
pre-existing faults. The model predicts a strike-slip stress regime in
regions with pockmarks where acoustic flares have not been documented. If a
certain degree of coupling is assumed between deep crustal and near-surface
deformation, it is possible that ridge-push forces have influenced seepage
activity in the region by interacting with the pore-pressure regime at the
base of the gas hydrate stability zone. More abundant seepage on the eastern
Vestnesa Ridge at present may be facilitated by the dilation of faults and
fractures favourably oriented with respect to the stress field. A modified
state of stress in the past, due to more significant glacial stress for
instance, may explain vigorous seepage activity along the entire Vestnesa
Ridge. The contribution of other mechanisms to the state of stress (i.e.
sedimentary loading and lithospheric flexure) remain to be investigated. Our
study provides a first-order assessment of how tectonic stresses may be
influencing the kinematics of near-surface faults and associated seepage
activity offshore of the western Svalbard margin.
