New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 908 bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back-arc domain, the active deformation is characterized by propagation of the back-arc spreading ridge into the Hunter volcanic arc. The N-S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike-slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction-related magmas has been produced including adakites, island arc tholeiites, back-arc basin basalts, and medium-K subduction-related lavas.

Seismic investigation in marine gas-bearing sediments fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra-high resolution seismic imaging, marine electrical resistivity tomography (MERT) and core sampling to study the physical properties of gas-bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P-) wave velocitvalues where free gas was identified in seismic data. We tested a joint processing workflow combining the 1D inversion of the MERT data with the 2D P-wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with. The resulting models showed the continuity of the geological units below the acoustic gas fronts which is associated with paleo-valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale then compared the results to data from core sampliand porosity). We inferred the porosity distribution from the MERT data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas-bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60-70%) down to about 10-20 m in depth in contrast with the surrounding medium with porosity less than 55%. These results support the hypothesis that failures inside the paleo-valley sediment could control the gas migration

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<p>Understanding and quantifying the migration of free-gas in hydrate-bearing sediments through time is particularly compulsive along continental margins, where gas hydrate dissociation could have triggered some of the largest submarine landslides observed on Earth. Offshore Romania, high-resolution seismic profiles reveal low reflective or low-velocity zones, which are indicative of free gas, beneath vertical stacked Bottom Simulating Reflectors (BSRs). To further understand the occurrence of double BSRs in the area and the possible effect of gas hydrate dynamics on slope instability and free gas releases, we performed a numerical 2D transient modelling of the evolution of the thermodynamic stability of gas hydrates, integrating in-situ measured physical data and indirect assessments of sea-bottom temperature, thermal conductivity, salinity and sea-level variations. We found that the shallowest BSR matches well with the current Base of the Gas Hydrate Stability Zone (BGHSZ) and the deeper one with the Last Glacial Maximum (LGM) base of GHSZ. The reduction of the GHSZ extension subsequently led to widespread gas hydrate dissociation associated with warming conditions and an increase in Black Sea salinity. However, this dissociation is only responsible of some very superficial submarine landslides (< 30 mbsf and 3 m thick in average) that occurred during this same period. These new constraints improve our understanding of the sliding mechanisms on the Romanian slope that have been ongoing since the LGM and support less catastrophic scenarios than those suggested previously in the case of active gas hydrate dissociation. These results also allow solving the mystery of the double BSR, which here corresponds to a relic of the LGM BGHSZ.</p>

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