Description: Upper‐plate normal faults are a widespread structural element in erosive plate margins. Increasing coverage of marine geophysical data has proven that similar features also exist in accretionary margins where horizontal compression usually results in folding and thrust‐faulting. There is a general lack of understanding of the role and importance of normal faulting for the structural and tectonic evolution of accretionary margins. Here, we use high‐resolution 2D and 3D seismic reflection data and derived seismic attributes to map and analyze upper‐plate normal faulting in the marine forearc of the accretionary Hikurangi margin, New Zealand. We document extension of the marine forearc over a wide area along the upper continental slope. The seismically imaged normal faults show low vertical displacements, high dip angles, a preference for landward dip and often en echelon patterns. We evaluate different processes, which may cause the observed extension, including (1) stress change during the earthquake cycle, (2) regional or local uplift and decoupling of shallow strata from compression at depth, as well as (3) rotation of crustal blocks and resulting differential stresses at the block boundaries. The results suggest that normal faults play an important role in the structural and tectonic evolution of accretionary margins, including the northern Hikurangi forearc.

Description: ALKOR cruise AL600 served as a marine geophysical field course for ‘Physics of the Earth System’ bachelor students at Kiel University. Beside taking an active role in planning and realization of the individual geophysical measurements, the students also performed some first processing and interpretation of the obtained data. This work had to be documented in form of a scientific presentation as well as writing of the respective chapter in this cruise report. For the following chapters, we (Sebastian Krastel, Jens Schneider von Deimling) decided to only slightly modify the text and figures provided by the students. This should emphasize the student’s achievements, and underline the overarching aim of the cruise to train the students in acquisition, processing, and documentation of marine geophysical data.

Description: The South Chilean marine fore arc (35°S–40°S) is separated into four tectonic segments, Concepción North, Concepción South, Nahuelbuta, and Tolten (from north to south). These are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent Nazca and South American plates. Splay faults that cut through continental framework rock are seismically imaged in both Concepción segments and the Tolten Segment. Additionally, the Concepción South Segment exhibits prominent upper plate normal faults. Normal faults apparently relate to uplift caused by sediment underthrusting at depth. This has led to oversteepening and gravitational collapse of the marine fore arc. There is also evidence for sediment underthrusting and basal accretion to the overriding plate in the Tolten Segment. There, uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km. In the Nahuelbuta Segment transpressive upper plate faults, aligned oblique to the direction of plate motion, control the seafloor morphology. Based on a unique acoustic data set including 〉90% of bathymetric coverage of the continental slope we are able to reveal an along‐strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin.

Description: A site at the gas hydrate stability limit was investigated offshore northwestern Svalbard to study methane transport in sediment. The site was characterized by chemosynthetic communities (sulfur bacteria mats, tubeworms) and gas venting. Sediments were sampled with in‐situ porewater collectors and by gravity coring followed by analyses of porewater constituents, sediment and carbonate geochemistry, and microbial activity, taxonomy, and lipid biomarkers. Sulfide and alkalinity concentrations showed concentration maxima in near‐surface sediments at the bacterial mat and deeper maxima at the gas vent site. Sediments at the periphery of the chemosynthetic field were characterized by two sulfate‐methane transition zones (SMTZ) at ~204 and 45 cm depth, where activity maxima of microbial anaerobic oxidation of methane (AOM) with sulfate were found. Amplicon sequencing and lipid biomarker indicate that AOM at the SMTZs was mediated by ANME‐1 archaea. A 1D numerical transport reaction model suggests that the deeper SMTZ‐1 formed on centennial scale by vertical advection of methane, while the shallower SMTZ‐2 could only be reproduced by non‐vertical methane injections starting on decadal scale. Model results were supported by age distribution of authigenic carbonates, showing youngest carbonates within SMTZ‐2. We propose that non‐vertical methane injection was induced by increasing blockage of vertical transport or formation of sediment fractures. Our study further suggests that the methanotrophic response to the non‐vertical methane injection was commensurate with new methane supply. This finding provides new information about for the response time and efficiency of the benthic methane filter in environments with fluctuating methane transport.

Description: Submarine landslides can be several orders of magnitude larger than their terrestrial counterparts and can pose significant hazards across entire ocean basins. The landslide failure mechanism strongly controls the associated tsunami hazard. The Tampen Slide offshore Norway is one of the largest landslides on Earth but remains poorly understood due to its subsequent burial beneath up to 450 m of sediments. Here, we use laterally extensive (16,000 km2), high‐resolution processed 3D seismic reflection data to characterize the upper Tampen Slide. We identify longitudinal (downslope, movement‐parallel) chutes and ridges that are up‐to‐40 m high, as well as extensional and compressional (cross‐slope) ridges. This is the first time that longitudinal ridges of such size have been imaged in a deep marine setting. The first phase of the Tampen Slide involved the simultaneous translation of over 720 km3 of sediments along a single failure plane. This was followed by spreading along the head‐ and sidewall, and the formation of a retrogressive debris flow and slump, the volumes of which are insignificant compared to the first failure. The process responsible for movement of such a large sediment volume along a single glide plane differs significantly from that of other passive margin megaslides, which typically comprise numerous smaller landslides that fail retrogressively along multiple glide planes. The trigger mechanism (e.g. an earthquake), the presence of mechanically strong obstructions (e.g. igneous topographical high), and the number and location of weak layers may be key factors that determine whether megaslides develop along a single plane or retrogressively.

Description: The Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross-cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models. Key Points Seismic reflection data reveal occurrences and extent of submarine permafrost and associated gas hydrates at the Canadian Beaufort Shelf Synthetic modeling of permafrost and gas hydrate is required to assess seismic processing minimizing the potential for misinterpretation Indicators of top and base of permafrost and the base of gas hydrate stability support previously published numerical models

Description: Research cruise AL579 is part of the bachelor course "Physics of the Earth System - Geophysics, Meteorology and Oceanography" at the University of Kiel. It is the field exercise for marine geophysics and hydroacoustics. The aim of the annually recurring cruise is to give students a practical insight into the acquisition, processing, documentation, and interpretation of marine geophysical data. AL579 took place from August 20th -28th 2022 with the main study areas in Eckernförde Bay and the Bay of Mecklenburg. Parts of the scientific crew changed during a stopover in Kiel on Wednesday, 24.8.2022. In Eckernförde Bay we mainly collected Multibeam Echosounder (MBES) and INNOMAR Subbottom Echosounder (SES) data calibrated by CTD measurements close to the pockmark field off Mittelgrund. On Wednesday, 24.8.2022 we tested a new Ocean Bottom Seismometer (OBS) prototype. In the Bay of Mecklenburg, the focus was on Blinkerhügel and the seafloor structures further west where an enigmatic stone structure was discovered in 2021. This area was surveyed with Sidescan Sonar, MBES, SES, and CTD measurements and several video transects with an underwater drone. We also collected two sets of multi-channel seismic data to investigate the deeper structures of the Western Baltic Sea and the Bay of Mecklenburg.