Description: At the Australian-Pacific plate boundary, the northern Lau Basin is one of the fastest opening back-arc basins on earth. The current configuration of micro-plates, plate boundaries and motions within the northern Lau Basin is quite well understood, but in the southern part of the Lau Basin questions remain about the crustal structure. Here, the Central Lau Spreading Center (CLSC) and the southern tip of the Fonualei Rift and Spreading Center (FRSC) define the diffuse southern boundary of the Niuafo’ou microplate. It remains unclear where the southern plate boundary is located and what kind of boundary it is.We present 1) seismic refraction data of a 200-km long, E-W transect acquired in the transition zone from the eastern side of the CLSC to the southern tip of the FRSC and 2) seismic reflection data of four E-W profiles of varying length, acquired in both the southern part of the Niuafo’ou microplate and the transition in between the CLSC and the FRSC. The seismic data acquisition was accompanied by parametric sediment echosounder, gravimetric and magnetic measurements and was complemented by heat flow probes and dredged samples of the seafloor in the vicinity of the profile.Our travel time tomography reveals a pronounced lateral variation in seismic P-wave velocities from west to east, within the 7-8 km thick back-arc crust. Towards the east, the crust gradually thickens to 13 km of arc crust. The reflection seismic data reveals sediment pockets, varying between 300m to 1000m depth, located on both the thinner back-arc crust and thicker arc crust. In the abyssal regions, faults that cross-cut the basement, but do not reach the surface, are observed on all reflection seismic profiles and are considered inactive today. Towards the west of the profiles, faults reach the surface and are considered active. Rock sampling from this area retrieved predominantly massive aphyric basalts from the back-arc crust in the west. Olivine-rich basalts, andesites, and a broad spectrum of volcaniclastic rocks are the most common rock-type collected from the arc crust in the east.The lack of a thinner crust near the southern tip of the FRSC, the presence of inactive faults that cross-cut the basement, and the presence of active faults in the CLSC suggest that the southern plate boundary of the Niuafo’ou microplate accommodated extension in a wide-rift tectonic setting in the past. Today, this extension is accommodated in the CLSC in a narrow extensional tectonic setting.

Description: Hydrocarbon gases are actively seeping from pockmarks in the eastern part of the Vestnesa Ridge, western-Svalbard Margin. One of these is Lunde pockmark which is characterized by a seismic chimney below. Such seismic anomalies are widely believed to represent fluid migration pathways. However, their detailed structure and the physical properties of such structures is poorly understood and might be highly variable. Tomographic seismic velocity analysis can resolve the inner structure of the chimney beneath the Lunde pockmark. The aim is to understand the distribution of gas hydrate, free gas and carbonates within the gas chimney. Here, we present first results of our detailed 3D seismic travel time tomography using newly acquired high-resolution ocean bottom seismometer data guided by high-resolution 3D multi-channel seismic data. These models were generated with the Jive3D software. Our initial results show the variability of the seismic velocity structure beneath the Lunde pockmark. Our analysis, combined with earlier datasets and results shows that fluid pathways through the gas hydrate stability zone are anything but simple and highlights the importance of understanding the evolution of methane seepage pathways through time.

Description: Focused fluid flow shapes the evolution of marine sedimentary basins by transferring fluids and pressure across geological formations. Vertical fluid conduits may form where localised overpressure breaches a cap rock (permeability barrier) and thereby transports overpressured fluids towards shallower reservoirs or the surface. Here, we study field outcrops of an Eocene fluid flow system at Pobiti Kamani and Beloslav Quarry (~15 km West of Varna, Bulgaria), where large carbonate-cemented conduits formed in highly permeable, unconsolidated, marine sands of the northern Tethys Margin. Using an uncrewed aerial vehicle with an RGB sensor camera we produced ortho-rectified image mosaics, digital elevation models, and point clouds of the two km-scale outcrop areas. Based on these data, geological field observations, and petrological analysis of rock/core samples, we mapped and analysed fractures and vertical fluid conduits with centimetre accuracy. Our results show that both outcrops comprise several hundred carbonate-cemented fluid conduits (pipes), oriented perpendicular to bedding, and at least seven bedding-parallel carbonate interbeds which differ from the hosting sand formation only by their increased amount of cementation. From these observations, we conclude that carbonate precipitation likely initiated around areas of focused fluid flow, where methane entered the formation from the underlying fractured subsurface. These first carbonates formed the outer walls of the pipes and continued to grow inward leading to self-sustaining and self-reinforcing focused fluid flow. Our results, supported by literature-based carbon and oxygen isotope analyses of the carbonates, indicate that ambient seawater and advected fresh/brackish water were involved in the carbonate precipitation by microbial methane oxidation. We propose that similar structures may also form in modern settings where focused fluid flow advects fluids into overlying sand-dominated formations, which has wide implications for our understanding of how focusing of fluids works in sedimentary basins with broad consequences for the migration of water, oil, and gas

Description: SO277 OMAX served two scientific projects. The objectives of the first project, SMART, were to develop multi-disciplinary methodologies to detect, quantify, and model offshore groundwater reservoirs in regions dominated by carbonate geology such as the Mediterranean Sea. To this end we acquired controlled-source electromagnetic, seismic, hydroacoustic, geochemical, seafloor imagery data off Malta. Preliminary evaluation of the geophysical data show that there are resisitivity anomalies that may represent offshore freshwater aquifers. The absence of evidence for offshore springs means that these aquifers would be confined and that it will be difficult to use them in a sustainable manner. The objective of the second project, MAPACT-ETNA, is to monitor the flank of Etna volcano on Sicily which is slowly deforming seaward. Here, we deployed six seafloor geodesy stations and six ocean bottom seismometers for long-term observation (1-3 years). In addition, we mapped the seafloor off Mt. Etna and off the island of Stromboli to constrain the geological processes that control volcanic flank stability.

Description: The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning and un-roofing of mantle material little is known about the style of back-arc rifting. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. The majority of the refraction seismic data show mantle phases at offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The mantle shows rather high velocities 〉7.8 km/s. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly does not initiated oceanic spreading before the opening of the Ligurian Basin stopped.