Description: The northeastern Lau Basin is one of the fastest opening and magmatically most active back‐arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is reasonably understood, the internal structure and evolution of the back‐arc crust are not. We present new geophysical data from a 290 km long east‐west oriented transect crossing the Niuafo'ou Microplate (back‐arc), the Fonualei Rift and Spreading Center (FRSC) and the Tofua Volcanic Arc at 17°20′S. Our P wave tomography model and density modeling suggest that past crustal accretion inside the southern FRSC was accommodated by a combination of arc crustal extension and magmatic activity. The absence of magnetic reversals inside the FRSC supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion. The back‐arc crust constituting the southern Niuafo'ou Microplate reveals a heterogeneous structure comprising several crustal blocks. Some regions of the back‐arc show a crustal structure similar to typical oceanic crust, suggesting they originate from seafloor spreading. Other crustal blocks resemble a structure that is similar to volcanic arc crust or a “hydrous” type of oceanic crust that has been created at a spreading center influenced by slab‐derived water at distances 〈50 km to the arc. Throughout the back‐arc region, we observe a high‐velocity (Vp 7.2–7.5 km s−1) lower crust, which is an indication for magmatic underplating, which is likely sustained by elevated upper mantle temperatures in this region.

Description: Key Points: First insights into the crustal structure of the northeastern Lau Basin, along a 290 km transect at 17°20′S. Crust in southern Fonualei Rift and Spreading Center was created by extension of arc crust and variable amount of magmatism. Magmatic underplating is present in some parts of the southern Niuafo'ou Microplate.

Description: Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347

Description: New marine geophysical data acquired across the partly ice-covered northern East Greenland continental margin highlight a complex interaction between tectonic and magmatic events. Breakup-related lava flows are imaged in reflection seismic data as seaward dipping reflectors, which are found to decrease in size both northward and southward from a central point at 75°N. We provide evidence that the magnetic anomaly pattern in the shelf area is related to volcanic phases and not to the presence of oceanic crust. The remnant magnetization of the individual lava flows is used to deduce a relative timing of the emplacement of the volcanic wedges. We find that the seaward dipping reflectors have been emplaced over a period of 2–4 Ma progressively from north to south and from landward to seaward. The new data indicate a major post-middle Eocene magmatic phase around the landward termination of the West Jan Mayen Fracture Zone. This post-40-Ma volcanism likely was associated with the progressive separation of the Jan Mayen microcontinent from East Greenland. The breakup of the Greenland Sea started at several isolated seafloor spreading cells whose location was controlled by rift structures and led to the present-day segmentation of the margin. The original rift basins were subsequently connected by steady-state seafloor spreading that propagated southward, from the Greenland Fracture Zone to the Jan Mayen Fracture Zone.

Description: Gas hydrates are naturally-occurring solid compounds of gas and water within almost all sediment-rich continental margins. Due to the large amounts of methane stored in submarine gas hydrates, they might serve as future reservoirs for offshore marine gas production. Assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration, which can be best addressed using geophysical and geological investigations. Here, we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments. Regional 2D multichannel seismic data provide the broad overview of a hydrate-bearing area. High-resolution 2D and 3D seismic reflection data provide detailed images of two working areas, the buried S1 channel-levee system at 1500 m water depth (well within the gas hydrate stability zone) and a slope failure location, located at 665 m water depth (top limit of the hydrate formation) next to the S2 channel. Detailed compressional and shear wave (Vs) velocity-depth models were derived from four component ocean-bottom seismic data, the latter from P- to S-conversion upon reflection. Due to their steep reflection angles, shear wave events result in less resolved Vs models. Nevertheless, in case of a change in elasticity of the sediment matrix due to gas hydrate cementation, shear wave events can be used as an indicator. As such, Vs can give insight into the nature of hydrate formation throughout the GHSZ. We present new developments in the application of common reflection surface, normal-incidence-point tomography and full waveform inversion techniques to enhance model resolution for the seismic data sets. 2D and 3D controlled-source electromagnetic measurements provide volume information of the resistivity-depth distribution models. Electrical resistivity of the sediment formation depends on its porosity and the resistivity of the pore fluid. Gas hydrate and free gas generally have much higher electrical resistivities than saline pore fluid, and can be assessed using empirical relationships if the porosity and pore fluid salinity are known. Calibration with logging data, laboratory experiments on hydrate- or ice-bearing sediments, and resulting velocity and resistivity values, guide the joint interpretation into more accurate saturation estimations. Beyond that, a joint inversion framework supporting forward calculation of specialized geophysical methods at distributed locations is under development. In this paper, we summarize these individual components of a multi-parameter study, and their joint application to investigate gas hydrate systems, their equilibrium conditions and preservation of bottom-simulating-reflectors. We analyze data from two working areas at different locations and depth levels along the slope of the Danube Fan, which are both characterized by multiple bottom simulating reflectors indicating the presence of gas hydrate. In the first working area we located two depth windows with indications for moderate 16%–24% gas hydrate formation, but no vertical gas migration. In the second working area we observed fluid migration pathways and active gas seepage, limiting gas hydrate formation to less than 10% at the BSR. Some discrepancies remain between seismic-based and electromagnetic-based models of gas and gas hydrate distribution and saturation estimates, indicating that further in-situ investigations are likely required to better understand the gas hydrate systems at our study areas and to calibrate the inversion processes, which will be required for a joint inversion framework as well.