Abstract
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.
Highlights
Hundreds of gigatonnes of carbon are stored as gas hydrates and shallow gas reservoirs in continental margins (e.g. Hunter et al, 2013)
We focus exclusively on the potential contribution of oblique spreading at the Molloy and the Knipovich ridges to the total state of stress along the Vestnesa Ridge and undertake a qualitative analysis of how stress generated by mid-ocean ridge spreading may influence near-surface faulting and associated seepage activity
The interactions between tectonic stress regimes and pore-fluid pressure we propose for explaining seepage evolution along the Vestnesa Ridge may be applicable to seepage systems along other passive margins, in particular along Atlantic passive margins where leakage from hydrocarbon reservoirs is prominent (e.g. Andreassen et al, 2017; Bünz et al, 2003; Hovland and Sommerville, 1985; Riboulot et al, 2014; Somoza et al, 2014; Vis, 2017)
Summary
Hundreds of gigatonnes of carbon are stored as gas hydrates and shallow gas reservoirs in continental margins (e.g. Hunter et al, 2013). Hundreds of gigatonnes of carbon are stored as gas hydrates and shallow gas reservoirs in continental margins Hunter et al, 2013) The release of these carbons over geological time, a phenomenon known as methane seepage, is an important contribution to the global carbon cycle. Understanding and quantifying seepage has important implications for ocean acidification, deep-sea ecology, and global climate. Periods of massive methane release from gas hydrate systems Methane seepage and near-seafloor gas migration have implications for geohazards, as pore-fluid pressure destabilization is one factor associated with the triggering of submarine landslides It is well known that seepage at continental margins has been oc-
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