Description: Highlights • Low upper mantle seismic velocity indicates mantle hydration in the Porcupine Basin. • Crustal stretching factors suggest crustal break up in the Porcupine Basin. • Fault-controlled mantle hydration explains across-axis mantle velocity variations. • Along-axis variations in mantle hydration control the development of low-angle faults. Abstract Mantle hydration (serpentinisation) at magma-poor rifted margins is thought to play a key role in controlling the kinematics of low-angle faults and thus, hyperextension and crustal breakup. However, because geophysical data principally provide observations of the final structure of a margin, little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension. Here we present new observational evidence on how crustal strain-dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the Porcupine Basin, offshore Ireland. We present three new P-wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the Moho across and along the basin axis. We use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching ( ) of ∼2.5 in the north at 52.5° N and 〉10 in the south at 51.7° N. These values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting may have progressed to crustal breakup in the southern part of the study area. We observed a decrease in mantle velocities across the basin axis from east to west. These variations occur in a region where is within the range at which crustal embrittlement and serpentinisation are possible ( 3–4). Across the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that the degree of serpentinisation, together with the amount of crustal faulting, increases southwards along the basin axis. Seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 〉15%. This observation is consistent with laboratory measurements that show that at this degree of serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. Based on these results, we propose two alternative formation models for the Porcupine Basin. The first involves a northward propagation of the hyperextension processes, while the second model suggests higher extension rates in the centre of the basin than in the north. Both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation, which eventually controls the development of detachments faults with advanced stretching.

Description: Highlights • Elongated fault structures are conduits for focused fluid flow. • Gas migration occurs only along a sub-set of faults across Opouawe bank. • Stress state deduced from 3D fault structures appears partially stratigraphically controlled. Abstract High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridge-perpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal- and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it ‘self-generates’ its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.

Description: In many places along the central and southern Chilean active continental margin sedimentary successions covering the forearc contain methane hydrate, resulting from a mixture of biogenic and thermogenic processes. Here, we report the spatial distribution of gas hydrate in the accretionary prism and forearc sediments offshore western Patagonia (50°S and 57°S), landward of the Antarctica-South America plate boundary. Knowledge of the forearc structure here is limited, owing to the small number of reflection seismic profiles available, lack of high-resolution bathymetry data and the absence of scientific drillholes. However bottom-simulating reflectors (BSR) indicative of gas hydrate occur regionally extensive below about one third of the forearc slope, between about 280 and 630 m below sea floor. BSR-derived heat flow was calculated at about 30 and 70 mWm−2. These are typical values above subduction zones of oceanic crust older than 10 Ma, where vigorous fluid flow above young and hot subducting oceanic crust has leveled off. To move towards an estimate of gas hydrate present in the sediments, the velocity model was converted into a gas-phase concentration model using data from one of the seismic sections. Average thickness of gas hydrate is about 290 m, and average concentrations estimated are in a range of 3.4%–10%. If we use the minimum value of 3.4%, the amount of methane present in the region is about 3.0 × 1013 m3 at standard pressure-temperature conditions. We conclude that the Pacific forearc of Patagonia area is an important reservoir of methane hydrates and we propose this area be considered as a potential methane hydrate concentrated zone and a key area to be investigated in the future.

Description: Highlights • Plate boundary re-organization in the central Mediterranean Sea • Segmentation of the subduction complex along lithospheric transverse faults • STEP faults in the Ionian Sea • Pleistocene active faulting and Mt. Etna formation Abstract The Calabrian Arc is a narrow subduction-rollback system resulting from Africa/Eurasia plate convergence. While crustal shortening is taken up in the accretionary wedge, transtensive deformation accounts for margin segmentation along transverse lithospheric faults. One of these structures is the NNW-SSE transtensive fault system connecting the Alfeo seamount and the Etna volcano (Alfeo-Etna Fault, AEF). A second, NW-SE crustal discontinuity, the Ionian Fault (IF), separates two lobes of the CA subduction complex (Western and Eastern Lobes) and impinges on the Sicilian coasts south of the Messina Straits. Analysis of multichannel seismic reflection profiles shows that: 1) the IF and the AEF are transfer crustal tectonic features bounding a complex deformation zone, which produces the downthrown of the Western lobe along a set of transtensive fault strands; 2) during Pleistocene times, transtensive faulting reactivated structural boundaries inherited from the Mesozoic Tethyan domain which acted as thrust faults during the Messinian and Pliocene; 3) the IF and the AEF, and locally the Malta escarpment, accommodate a recent tectonic event coeval and possibly linked to the Mt. Etna formation. Regional geodynamic models show that, whereas AEF and IF are neighboring fault systems, their individual roles are different. Faulting primarily resulting from the ESE retreat of the Ionian slab is expressed in the northwestern part of the IF. The AEF, on the other hand, is part of the overall dextral shear deformation, resulting from differences in Africa-Eurasia motion between the western and eastern sectors of the Tyrrhenian margin of northern Sicily, and accommodating diverging motions in the adjacent compartments, which results in rifting processes within the Western Lobe of the Calabrian Arc accretionary wedge. As such, it is primarily associated with Africa-Eurasia relative motion.

Description: Highlights • A stack of four BSRs were identified in levee deposits of the Danube deep-sea fan. • The multiple BSRs are not caused by overpressure compartments. • The multiple BSRs reflect stages of stable sealevel lowstands during glacial times. • Gas underneath the previous GHSZ does not start to migrate for thousands of years. Abstract High-resolution 2D seismic data reveal the character and distribution of up to four stacked bottom simulating reflectors (BSR) within the channel-levee systems of the Danube deep-sea fan. The theoretical base of the gas hydrate stability zone (GHSZ) calculated from regional geothermal gradients and salinity data is in agreement with the shallowest BSR. For the deeper BSRs, BSR formation due to overpressure compartments can be excluded because the necessary gas column would exceed the vertical distance between two overlying BSRs. We show instead that the deeper BSRs are likely paleo BSRs caused by a change in pressure and temperature conditions during different limnic phases of the Black Sea. This is supported by the observation that the BSRs correspond to paleo seafloor horizons located in a layer between a buried channel-levee system and the levee deposits of the Danube channel. The good match of the observed BSRs and the BSRs predicted from deposition of these sediment layers indicates that the multiple BSRs reflect stages of stable sealevel lowstands possibly during glacial times. The observation of sharp BSRs several 10,000 of years but possibly up to 300,000 yr after they have left the GHSZ demonstrates that either hydrate dissociation does not take place within this time frame or that only small amounts of gas are released that can be transported by diffusion. The gas underneath the previous GHSZ does not start to migrate for several thousands of years.

Description: Highlights • Dual-vergence structure is observed for the first time on the northern Cascadia margin. • Around central Vancouver Island, vergence switches from seaward in the south to landward in the north. • First OBS migration study imaging the top of the igneous oceanic crust using only a small airgun source (120 in.3). • OBS migration indicates that an OBS, in water depths up to 2.5 km, can image up to 5 km on either side of its seafloor position. Abstract The detailed structure of the northern Cascadia basin and frontal ridge region was obtained using data from several widely spaced ocean bottom seismometers (OBSs). Mirror imaging was used in which the downgoing multiples (mirror signal) are migrated as they provide information about a much larger area than imaging with primary signal alone. Specifically, Kirchhoff time migration was applied to hydrophone and vertical geophone data. Our results indicate remarkable structures that were not observed on the northern Cascadia margin in previous single-channel or multi-channel seismic (MCS) data. Results show that, in these water depths (2.0–2.5 km), an OBS can image up to 5 km on either side of its position on the seafloor and hence an OBS spacing of 5 km is sufficient to provide a two-fold migration stack. Results also show the top of the igneous oceanic crust at 5–6 km beneath the seafloor using only a small airgun source (120 in.3). Specifically, OBS migration results clearly show the continuity of reflectors which enabled the identification of frontal thrusts and a main thrust fault. These faults indicate, for the first time on this margin, the presence of a dual-vergence structure. These kinds of structures have so far been observed in 〈 0.5% of modern convergent margins and could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Reanalysis of previous MCS data from this region augmented the OBS migration results and further suggests that the vergence switches from seaward to landward around central Vancouver Island. Furthermore, fault geometry analyses indicate that the total amount of shortening accommodated due to faulting and folding is about 3 km, which suggest that thrusting would have started at least ∼ 65 ky ago.

Description: Volcanic island flank collapses have the potential to trigger devastating tsunamis threatening coastal communities and infrastructure. The 1888 sector collapse of Ritter Island, Papua New Guinea (in the following called Ritter) is the most voluminous volcanic island flank collapse in historic times. The associated tsunami had run-up heights of more than 20 m on the neighboring islands and reached settlements 600 km away from its source. This event provides an opportunity to advance our understanding of volcanic landslide-tsunami hazards. Here, we present a detailed reconstruction of the 1888 Ritter sector collapse based on high-resolution 2D and 3D seismic and bathymetric data covering the failed volcanic edifice and the associated mass-movement deposits. The 3D seismic data reveal that the catastrophic collapse of Ritter occurred in two phases: (1) Ritter was first affected by deep-seated, gradual spreading over a long time period, which is manifest in pronounced compressional deformation within the volcanic edifice and the adjacent seafloor sediments. A scoria cone at the foot of Ritter acted as a buttress, influencing the displacement and deformation of the western flank of the volcano and causing shearing within the volcanic edifice. (2) During the final, catastrophic phase of the collapse, about 2.4 km³ of Ritter disintegrated almost entirely and travelled as a highly energetic mass flow, which incised the underlying sediment. The irregular topography west of Ritter is a product of both compressional deformation and erosion. A crater-like depression underlying the recent volcanic cone and eyewitness accounts suggest that an explosion may have accompanied the catastrophic collapse. Our findings demonstrate that volcanic sector collapses may transform from slow gravitational deformation to catastrophic collapse. Understanding the processes involved in such a transformation is crucial for assessing the hazard potential of other volcanoes with slowly deforming flanks such as Mt. Etna or Kilauea.