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: The northern Lau Basin, at the Australian-Pacific plate boundary, is one of the fastest opening back-arc basins on earth. An amalgamation of active rifts and spreading centers accommodates the extension. The current configuration of micro-plates, motions and plate-boundaries within the northern Lau Basin has been studied, but remains complex. Especially in the southern part of the Lau Basin questions remain about the crustal structure. In this area, 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 seismic refraction and reflection data of a 200-km long transect in the transition zone from the eastern side of the CLSC to the southern tip of the FRSC. The seismic data recording was accompanied by parametric sediment echosounder data, gravimetric and magnetic measurements and dredged samples of the seafloor in the vicinity of the profile. A 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 that vary between 300m to 1000m depth and are located on both the thinner back-arc crust and thicker arc-crust. Rock sampling along the transect retrieved predominantly massive aphyric basalts from the back-arc-crust in the west. Ol-Px-Pl-phyric basalts, andesites, and a broad spectrum of volcaniclastic rocks are the most common rock-type collected from the arc-crust in the east. These rocks are currently analyzed to determine the age and geochemical (major, trace element and Sr-Nd-Pb-Hf isotopic) composition of the sampled structures. The lack of a thinner crust near the southern tip of the FRSC is located and a wide distribution of normal faults in the sedimentary basins that reach the surface suggest that the southern plate boundary of the Niuafo’ou microplate actively accommodates extension in a wide rift setting.

Description: The 2 June 1994 Java (Indonesia) tsunami earthquake ruptured in a seismically quiet subduction zone and generated a larger-than-expected tsunami. Since the peak of the co-seismic slip occurred underneath a local bathymetric high, the 1994 event was previously interpreted as being caused by a subducting seamount. Combining a re-processed seismic reflection line across the rupture area with a refraction tomography P-wave velocity model, multibeam bathymetry, and gravity data suggests that rupture over a subducted seamount is unlikely to explain the seismo-tectonic genesis of the event. The forearc high is rather related to the enhanced back-thrusting activity and an island arc crust backstop in the upper plate. We newly resolve a shallow subducting seamount seaward of the forearc high and up-dip of the rupture area. We propose that this seamount acted as a seismic barrier and may have diverted the co-seismic rupture into the overlying splay faults, which may have contributed to the larger-than-expected tsunami

Description: Since the last glaciation the Canadian Arctic Beaufort Shelf is subjected to marine transgression. From subaerial mean annual temperatures during terrestrial exposure of ≤ -20°C, thermal conditions changed up to present submarine bottom water temperatures near -1°C. While conditions during the Pliocene favoured extensive formation of permafrost and gas hydrates, present occurrences are exposed to degradation due to the warmer climate. Today, submerged offshore permafrost is still responding to this thermal change. Ongoing degradation creates the potential of methane release of previously trapped biogenic gas within the relic permafrost and from gas hydrate dissociation. The mobilisation of methane and its possible release to the atmosphere plays a significant role in climate change. Yet, both the extent of permafrost and underlying gas hydrates is still poorly known. Here, we present seismic indicators for offshore permafrost and gas hydrates in 2D multichannel reflection seismic data acquired in the Canadian Beaufort Sea. Seismic lines that run from the shallow shelf towards deeper water show layer-crossing reflections that become gradually shallower towards the north-west into deeper water. These reflections show an amplitude-varying characteristic and are phase-reversed. We first use shot gathers from a synthetic model based on the field seismic acquisition characteristics and borehole geophysical data to verify our general ability to detect permafrost-and gas hydrate-related reflections. The synthetic data were processed using the same data processing applied to the field data and reveal clear top and base of permafrost and gas hydrate reflections. With this encouraging result, we can exclude any potentially misleading processing artefacts in the field seismic data. We interpret the amplitude-varying, phase-reversed and layer-crossing reflections seen in the field data as seismic indicators for the base of permafrost and base of gas hydrates. In contrast to the synthetic data, top of permafrost and top of gas hydrates are not clearly identified in the field data. However, additional seismic indicators support the interpretation of the presence of permafrost including attenuation of acoustic penetration and velocity pull-up effects at presumably horizontal strata. Furthermore, strong amplitude variations beneath the current base of gas hydrates and bright spots indicate trapped free gas accumulations from possible previously dissociated gas hydrates.

Description: Over the last two decades sea bed drilling technology has proven to provide a valuable complement to the services of classical drill ships. Especially for shallow drillings up to 200 mbsf and when working in remote areas difficult to access, sea bed drill rigs are a cost-effective alternative. Recent developments especially concerning borehole logging techniques add to the capabilities of sea bed drilling technology. The MARUM-MeBo is a robotic drilling system that is developed since 2004 at the MARUM Center for Marine Environmental Sciences at the University of Bremen (Freudenthal and Wefer, 2013). The drill rig is deployed on the sea bed and remotely controlled from the vessel. It is used for core drilling in soft sediments as well as hard rocks in the deep sea. Especially since an upgrade in 2007/2008 for the use of wireline drilling technique, the first-generation drill rig MARUM-MeBo70 with a drilling capacity of about 70 m was successfully deployed on more than 15 research expeditions. Since 2014 the second-generation drill rig MARUM-MeBo200 with an increased drilling capacity of up to 200 m below sea floor is successfully in operation. In this presentation we focus on results of three recent drilling campaigns, exemplifying the exploitation of the potential of the sea bed drilling technology: In early 2017 the MeBo70 was deployed from the ice breaking vessel RV POLARSTERN on the West Antarctic shelf (Gohl et al., 2017), an area difficult to access by a drill ship. We were able to recover a sedimentary sequence of the upper Cretaceous time period as one of the very few terrigenous records from this time in Antarctica. This sequence indicates that about 92 to 83 Mio years ago at a paleolatitude of about 82°S this area was covered by a temperate coastal rain forest, making any Antarctic ice sheet formation at this time period unlikely (Klages et al., in press). Also, in 2017 the MeBo70 was deployed in the Arctic off Svalbard. Next to coring a temperature probe was used to assess in situ temperatures and local geothermal gradients (Riedel et al. 2018). Combining these temperature data with the porewater geochemistry of the drilled cores Wallmann et al (2018) were able to prove the effect of isostatic rebound after deglaciation on gas hydrate dissociation. In late 2017 the MeBo200 was deployed in the Black Sea. Geophysical borehole log data of P-wave velocity, electrical resistivity, and spectral gamma ray were combined with core-derived physical properties of porosity, magnetic susceptibility, and bulk density and compared with seismic data of the region (Riedel et al., in press). This study shows the potential of core-log seismic integration for shallow drilling campaigns conducted with a sea bed drill rig. References: Freudenthal, T and Wefer, G (2013) Geoscientific Instrumentation, Methods and Data Systems, 2(2). 329-337. doi:10.5194/gi-2-329-2013 Gohl, K, et al. (2017) Geochemistry, Geophysics, Geosystems, 18, 4235–4250. https://doi.org/10.1002/2017GC007081 Klages, JP et al. (in press) Nature, 2019-10-14805B Riedel, M et al. (2018) Geochemistry, Geophysics, Geosystems, 19, 1165–1177. doi:10.1002/2017GC007288 Riedel, M et al. (in press) Marine and Petroleum Geology, doi.org/10.1016/j.marpetgeo.2019.104192 Wallmann, K et al. (2018) Nature Communications, 9:83, DOI: 10.1038/s41467-017-02550-9

Description: Mega‐scale glacial lineations formed by the raking of ice shelves across the seafloor have been reported from multiple polar regions. Here, we present the first evidence of continental slope situated buried lineations in the southern Canadian Beaufort Sea in present‐day water depths of 220 to 800 m. Three separate surfaces with lineations are defined at sub‐seafloor depths of 40 m to 390 m. All lineations are mostly parallel to the general trend of slope contours. The uppermost surface is recognized over a distance of 56 km. In water depths 〉 500 m the lineations are parallel to each other at a consistent direction (43°–44°). The second lineated surface is a regionally occurring erosional unconformity. This event has two sub‐sets of lineations: mid‐slope situated lineations oriented at 42°–48°, and lineations closer to the continental shelf break at 55°–59°. The third lineated surface is an unconformable horizon buried up to 390 m below seafloor with lineaments oriented between 30° and 55°. All three sets of lineations are interpreted to have been produced by ice‐ploughing on the paleo‐seafloor through the grounding of an ice shelf. Our observations are similar to those documented along the slope off northern Alaska, Chukchi Rise, and Lomonosov Ridge. Collectively, these observations support the concept of an extensive ice shelf across the Arctic Ocean that grounded locally along its margins during multiple glaciations, including during the penultimate (or an earlier) glaciation. The youngest set of lineations indicates ice movement to the southwest with a suggested source in Amundsen Gulf and/or M'Clure Strait. Tentative age considerations for these youngest lineations indicate the first evidence for an analogous extensive ice shelf configuration for the Last Glacial Maximum.

Description: We present the first evidence of continental slope situated buried lineations in the southern Canadian Beaufort Sea in present‐day water depths of 220 to 800 m. Three stratigraphically separate surfaces with lineations have been defined from three‐dimensional seismic data. All three sets of lineations are interpreted to have originated from interactions of an ice‐shelf that locally grounded with soft sediments at the paleo‐seafloor.

Description: Ministry of Oceans and Fisheries, Korea http://dx.doi.org/10.13039/501100003566

Description: Geological Survey of Canada‐Pacific

Description: British Petroleum Canada

Description: Imperial Oil Resources Canada Ltd