Discovery of 3 km long seafloor fracture system in the Central North Sea

We have discovered a 3 km long seafloor fracture system in the Central North Sea (block 16/4). The discovery was made using a synthetic aperture sonar (HISAS) mounted on a Kongsberg Hugin AUV. The surface expression of the structure - named the Hugin Fracture - changes along the strike and it is characterized by: 1) linear; 2) en echelon; and 3) branching segments. Ring-structures, that typically are 5-10 meters across, are common along the feature. Micro-bathymetry acquired using the HISAS system demonstrates that sub-meter scale elevation changes occur across the fracture. Microbial mats occur along different parts of the structure showing that active seepage is taking place. AUV based photo-imaging of parts of the structure shows that the microbial mats predominantly are associated with ring structures and some of the linear fracture segments. Sediment pore waters extracted from push cores show Na, Cl and Mg contents that are 10-15% lower compared to background pore fluid concentrations. This points towards a fresh water input. The fracture pore fluid compositions are also characterized by elevated methane, ammonium and hydrogen sulphide contents compared to the background seawater. The presence of these volatiles is likely caused by subsurface microbial activity, and carbon isotope analyses confirm a biological source of the detected methane. However, the presence of ethane (CH4/C2H6 of 126) indicates a small input of a thermogenic carbon to these fluids. Subsurface imaging using a hull mounted parametric sub bottom profiler reveal Holocene stratified sediments overlying quaternary moraine in the area. The sub bottom profiler data show sub-meter scale vertical movements along the fracture. Associated with the structures are small bright spots that may reflect gas accumulations. No deep-seated fault system is apparent below the fracture in 3D seismic data from the area. However, the 3D seismic data show that the structure is located above the boundary of a channel-like feature present around 150m subseafloor. This subsurface feature probably represents a shallow fluvial channel that formed during glaciation/deglaciation. A structural connection between the seafloor seepage structure and a fluvial channel is supported by the pore water chemistry at the investigated seepage sites. High undrained shear strengths (up to 180 kN/m2) have been measured in the shallow clay-rich sediments that are present in this area (Sejrup et al. 1987). We therefore propose that the Hugin Fracture formed by brittle failure of such stiff sediments as a result of differential compaction located to the margin of the shallow fluvial channel. The discovery of the Hugin Fracture demonstrates that the clay-rich and impermeable sediments that represent the uppermost seal of the Utsira Formation may be broken as a result of brittle failure, and that km-scale fracture systems may form as a result. The fracturing of this impermeable top layer may connect permeable fluvial channels to the seafloor. The presence in the fluids of hydrocarbons (ethane) that typically form by deep thermogenic processes suggests that this shallow fluid flow system is connected, or has been connected, to sedimentary sequences that are present below the Utsira Formation.