Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Voring Plateau, offshore Norway

This paper presents a new data set, including single-channel airgun seismic lines, OKEAN long-range side-scan sonar data and gravity cores, acquired on the edge of the V phi ring Plateau and in the region of the Storrega Slide. The data acquisition was part of the 8th Training Through Research (TTR-8) expedition with R.V. Professor Logachev. The acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom-simulating reflectors (BSRs) at about 350 ms TWT subbottom depth. The lower BSR zone is located on the slope of the V phi ring Plateau in the immediate vicinity of the northern headwall of the Storegga Slide and, in some places, below the slide deposits. This zone runs parallel to the general trend of the continental slope. The spatial distribution of the upper BSR zone, located upslope in an area where diapir-like structures are found, does not demonstrate any topographic control. Interpretation of high-backscatter patches on the OKEAN sonographs associates the observed structures with fluid escape features on the seabed. Most of them are pockmarks, but in a few places, diapirs are cropping out and form dome-like elevations. After analysing the behaviour of the BSR (sometimes crosscutting) and its acoustic characteristics (reversed polarity), and after applying seismic inversion processing to estimate the acoustic velocity change across the BSR, this reflector is interpreted to represent the base of the local gas hydrate stability field (GHSF). This information was used to derive the regional geothermal gradient. Spatial variations of the inferred geothermal field appeared to be negligible. The observation of two separated BSR zones suggests different environmental controls for the growth of the hydrates. Enhanced reflectors observed in the intermediate zone can be explained by the presence of strata-bound free gas accumulations and migration combined with overlying permeability barriers. Therefore, a model for gas hydrate formation in relation to directional fluid migration processes, fluid escape features and the presence of diapiric structures is proposed in this paper. Our model is supported by sedimentological analyses of seafloor samples, which demonstrate the presence of stiff clays with evidence of gas migration, and by paleontological studies of the cores retrieved from the pockmarks. The presence of a BSR in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event. This observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm/year. Finally, the effect of hydrate dissociation on slope instability is considered. Assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re-distributed through the whole sedimentary section above the paleo-BSR (with consuming heat and increasing pore pressure, dissociation tends to shift the PT-conditions back to equilibrium values), it appears that the dissociation of hydrates due to sliding would cause only 0.2% increase of the pore pressure, which would hardly contribute to further slope instability. (C) 2000 Elsevier Science B.V. All rights reserved.