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Small-scale heterogeneities within the seafloor such as glacial boulders, concretions, or unexploded ordnance are of major scientific and economic interest. Because of their small size, such objects are hardly imageable by conventional seismic methods since they produce only faint diffractions. Despite the growing interest, reliable and efficient object detection methods are still not established. So, a marine acquisition system to image objects in the size range 0.3–4.1 m in the near-surface has been designed and workflows have been developed. Source signals with a central frequency of ∼1,000 Hz are necessary to generate diffractions for the object size range. Performing beam pattern analyses, a rigid tow frame with the dimensions 8 × 3 m and attached hydrophones was designed. A synthetic aperture approach is realized to improve the resolution. Reflection events are suppressed in Normal-Move-Out corrected shot gathers by the muting of high singular values to enhance the diffractions. The diffractions are imaged with a beamforming algorithm with a high efficiency, as a total swath angle of about 80° in across-track direction is covered. The processing of synthetic data sets allows an optimization and validation of the workflow and sensitivity analyses. Ground truthing is achieved with 1–2 m large boulders on the seafloor in 22 m depth, which were identified in a Multi-Beam Echo Sounder data set. Although diffractions are only weak events in seismic data sets, 3D object detection with seismic beamforming has been found to be an efficient and reliable method to perform derisking for offshore infrastructure installations, drillings, or geologic interpretation.

Paleo-shorelines on continental shelves give insights into the complex development of coastlines during sealevel cycles. This study investigates the geologic development of the Limpopo Shelf during the last sealevel cycle using multichannel seismic and acoustic datasets acquired on the shelf in front of the Limpopo River mouth. A detailed investigation of seismic facies, shelf bathymetry, and a correlation to sea level revealed the presence of numerous submerged shorelines on the shelf. These shorelines are characterized by distinct topographic ridges and are interpreted as coastal dune ridges that formed in periods of intermittent sealevel still-/slowstand during transgression. The shorelines are preserved due to periods of rapid sealevel rise (melt water pulses) that led to the overstepping of the dune ridges as well as due to early cementation of accumulated sediments that increased the erosive resistance of the ridges. The high along-shelf variability of the submerged dune ridges is interpreted as a result of pre-existing topography affecting shoreline positions during transgression. The pre-existing topography is controlled by the underlying sedimentary deposits that are linked to varying fluvial sediment input at different points on the shelf. The numerous prominent submerged dune ridges form barriers for the modern fluvial sediment from the Limpopo River and dam sediment on the inner shelf. They may also facilitate along-shelf current-induced sediment transport.

The shallow Nice submarine slope is notorious for the 1979 tsunamigenic landslide that caused eight casualties and severe infrastructural damage. Many previous studies have tackled the question whether earthquake shaking would lead to slope failure and a repetition of the deadly scenario in the region. The answers are controversial. In this study, we assess for the first time the factor of safety using peak ground accelerations (PGAs) from synthetic accelerograms from a simulated offshore Mw 6.3 earthquake at a distance of 25 km from the slope. Based on cone penetration tests (CPTu) and multichannel seismic reflection data, a coarser grained sediment layer was identified. In an innovative geotechnical approach based on uniform cyclic and arbitrary triaxial loading tests, we show that the sandy silt on the Nice submarine slope will fail under certain ground motion conditions. The uniform cyclic triaxial tests indicate that liquefaction failure is likely to occur in Nice slope sediments in the case of a Mw 6.3 earthquake 25 km away. A potential future submarine landslide could have a slide volume (7.7 × 106 m3) similar to the 1979 event. Arbitrary loading tests reveal post-loading pore water pressure rise, which might explain post-earthquake slope failures observed in the field. This study shows that some of the earlier studies offshore Nice may have overestimated the slope stability because they underestimated potential PGAs on the shallow marine slope deposits.

International audience; IODP Expedition 354 in the Bengal fan [1] generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and the monsoon. The Bengal fan is predominantly composed of detrital sediments originating from Himalayan rivers, and transported to the N. Bengal shelf. They supply turbidity currents able to spread sediments overs 3000 km. Turbiditic record at a given site being discontinuous, Expedition 354 drilled seven sites along a 320 km E-W transect at 8°N allowing the restitution of an almost complete record of Himalayan erosion at the scale of the Neogene. In spite of the transect extension, a long absence of deposition is observed between 0.6 to 1.2 Ma indicating that turbiditic depocenter was derived more to the West for ca. 600 kyr. Bengal fan sediments have close mineralogical, geochemical and isotopic analogy with sediments of the modern Ganga-Brahmaputra rivers. Altogether they reveal mineralogical sorting associated to turbiditic transports that compares well with that of rivers in the floodplain and delta. Major and trace element geochemistry and clay compositions reveal relatively even and weak conditions of weathering throughout the 20-Ma record. Most variations are controlled by the origin of eroded sources ie. the proportions of sediments derived from Himalaya and Transhimalaya. Source tracers such as Sr-Nd isotopic compositions, and detrital carbonate compositions show organised variations with time. They also imply for some periods compositions with very high proportion of sediment derived from the Transhimalaya suggesting that the Ganga and Brahmaputra sediment delivery to the fan can be decoupled at some periods. Long term source variations imply that exposure to erosion of the different Himalayan formations has evolved as a result of the evolution of the thrusting structures. Data suggest that (1) a component derived from Transhimalayan formation was present even during Lower Miocene, (2) the Tethys Himalaya exposure to erosion was higher during Miocene than during Pliocene and Pleistocene, and (3) that the exhumation of the Lesser Himalaya was initiated around 8 Ma.

International audience; Recent IODP Expedition 354 in the Bengal fan [1] generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and the monsoon. The Bengal fan is predominantly composed of detrital turbiditic sediments originating from Himalayan rivers, and transported through the delta and shelf canyon, supplying turbidity currents loaded with a wide spectrum of grain sizes. Turbiditic deposition makes that record at a given site is discontinuous which was the reason for an E-W transect approach. Expedition 354 drilled seven sites along a 320 km E-W transect at 8°N allowing the restitution of an almost complete record of Himalayan erosion at the scale of the Neogene. In spite of the transect extension, a long absence of deposition is observed between 0.6 to 1.2 Ma indicating that turbiditic depocenter was derived more to the West for ca. 600 kyr. Turbiditic sediments have close mineralogical and isotopic analogy with sediments of the modern Ganga-Brahmaputra rivers. Major and trace element geochemistry show stable compositions throughout the Neogene and Quaternary with most variations controlled by the origin of eroded sources ie. the proportions of sediments derived from Himalaya and Transhimalaya. Although relatively stable, source tracers such as Sr-Nd isotopic compositions, and detrital carbonate compositions show organised variations with time. They imply that exposure to erosion of the different Himalayan formations has evolved as a result of the evolution of the thrusting structures. Data suggest that (1) a component derived from Tranhimalayan formation was present even during Lower Miocene, (2) the Tethys Himalaya exposure to erosion was higher during Miocene than during Pliocene and Pleistocene, and (3) that the exhumation of the Lesser Himalaya was initiated around 8 Ma. Coring of large sand samples also allowed to initiate cosmogenic 10 Be tracing of erosion rates at the scale of the Ganga-Brahmaputra basin. Relatively stable paleo-concentrations reveal relatively steady erosion rates in spite of the Pleistocene climatic changes. Clays and major and trace element geochemistry reveal a very weak regime of chemical weathering with no significant variation through time. Concentrations in mobile elements such as Na and K relative to Al are significantly higher than in modern sediments suggesting that weathering or soil erosion is amplified in the modern time. Compositions are indeed controlled by source and weathering processes, but mineral sorting during turbiditic transport may also alter the geochemical signature of the sediments. Close attention has been given to evaluate the possible loss of clay size particles by large scale dispersion compared to silt and sand. It seems, however, that such processes cannot account for the differences observed with modern rive sediments. Low weathering of the sediments at 8°N indicates that erosion was dominated by physical processes and that transport is rapid enough to prevent evolution of particles in the floodplain. In the modern Himalaya, low weathering is achieved primarily by landslides and rapid transfer through the floodplain, i.e. limited recycling of sediment deposited in the floodplain. Both processes are favoured by the seasonality and the intensity of the monsoon. Overall, the low weathering intensity suggests that comparable erosion regime prevailed since at least the Early Miocene.

In this study we present the first 3D high-resolution multichannel seismic dataset from a (partly) submerged caldera setting, the Campi Flegrei caldera (CFc). Our work aims at examining the spatial and temporal evolution of the CFc since the last caldera-forming event, the Neapolitan Yellow Tuff (NYT, 15 ka) eruption. The main objectives are to investigate the caldera's shallow (< 200 m) subsurface structure and post-NYT-collapse (< 15 ka) deformational processes, the manifestation of magmatic and hydrothermal processes in the subsurface, as well as the volume, dispersal and explosivity of coastal post-collapse eruptions, thereby significantly advancing our current knowledge of the CFc. Our findings confirm the existence of a nested-caldera system comprising two caldera depressions bordered by an inner and a deeper (> 200 m) outer caldera ring-fault zone. The seismic data revealed that the NYT collapse occurred exclusively along the inner caldera ring-fault and that the related NYT caldera depression is filled with on average ~ 61 m of sediment deposited between 15 and 8.6 ka. The geometry of the inner ring-fault, consisting of four fault segments, seems to be strongly influenced by regional NW-SE and NE SW-trending faults. Furthermore, we found that the ring-faults have acted as pathway for the recent (< 3.7 ka) ascent of fluids (gases and liquids) and the emplacement of intrusions. We propose that the outer ring-fault zone, which likely formed in the course of the Campanian Ignimbrite (CI, 39 ka) eruption, has had the main control on the release and ascent of fluids. Overall, the caldera ring-faults represent key locations for the interconnection between the magmatic-hydrothermal systems and the surface and, thus, potentially represent future eruption sites as well as important fluid pathways during the recent unrest episodes. Furthermore, we reassessed the volume, dispersal, and explosivity of the post-collapse Nisida Bank (10.3-9.5 ka), Nisida Island (~ 3.98 ka), and Capo Miseno (3.7 ka) eruptions, yielding DRE values of 0.15 km3, 0.1 km3, and 0.08 km3, respectively, and an explosive magnitude of at least moderate-large scale (VEI 3). Our findings highlight that eruption volumes may be underestimated by 3 to 4 times if the submerged portion of a (partly) submerged caldera is not considered, implying severe consequences for the hazard and risk evaluation. The spatial response of the post-collapse (< 15 ka) depositional environment to volcanic activity, deformational processes and sea-level variations is presented in a comprehensive 3D evolutionary model.

Relative sea level history, which is the result of the combined effects of land subsidence, sediment supply and absolute sea level history may be reconstructed from preserved sediment thicknesses. However, variations in the preserved sediment thicknesses between different sedimentary environments strongly limit the accuracy of this type of geological approach, particularly in fluvial channelized systems, such as delta plains. To address this, we apply three different and independent stratigraphic approaches to the case of the Ganges-Brahmaputra-Meghna Delta (GBMD). Our approach has been made possible by a broad dataset of geological archives we have collected, which includes more than 400 hand-drilled stratigraphic wells, 198 radiocarbon ages, and river seismic reflection data (255 km of high-resolution multichannel seismic images). The seaward gradient of accommodation and the limit of the subsiding delta plain are estimated, assuming that the delta is near or at the base-level, which is considered to be the relative sea-level. First, a statistical analysis of the variability of preserved sediment thicknesses is used to derive the average pattern of accommodation from the Holocene isopach. Secondly, the preserved sediment thicknesses are analyzed by geomorphotectonic domains to estimate an average pattern of accommodation. Thirdly, the burial history of the seismically imaged last glacial incision of the Brahmaputra River is reconstructed. Results suggest that the variability of preserved sediment thicknesses can be up to 35% in a delta plain between river channel and flood plain deposits for the same relative sea-level history. Taking this variability into consideration, the Holocene relative sea-level history of the GBMD and the most likely pattern of subsidence are determined. Results provide evidence of moderate Holocene subsidence over the delta, gently increasing seaward from

Resurgent calderas are among the largest and most dynamic volcanic structures on earth. They are typically associated with major eruptions with considerable volumes of pyroclastic deposits accompanied by large collapse structures and late stage deformation and uplift of the intra-caldera floor region. The Campi Flegrei is a vast volcanic area located on the coastal zone of the Campania region of SW Italy, a large part of which develops off the Naples (Pozzuoli) Bay. The area has been active since at least ~80 ka BP and is structurally dominated by a caldera collapse, ca. 8 km in diameter, associated with the eruption of the Neapolitan Yellow Tuff (NYT), a 30–50 km3 Dense Rock Equivalent ignimbrite dated at ca. 15 ka BP. In the past decades the shallow crustal structure of the NYT caldera has been mostly reconstructed using gravimetric and magnetic data, seismic tomography images and modelling (analogue) experiments, whereas, the structural elements of the caldera collapse in the Pozzuoli Bay have been largely inferred, based on seafloor morphology and associated deposits. Despite the conspicuous research so far conducted off the Campi Flegrei, the stratigraphic architecture of the NYT caldera structure and inner caldera deposits is still poorly understood. This is mostly because of the intrinsic limitations due to the insufficient resolution of previous seismic datasets, as well as to the lack of reliable geologic calibration of the offshore geophysical data. In this study we present a detailed structural and stratigraphic reconstruction of the submerged part of the NYT caldera obtained by full integration of swath bathymetry, high-resolution multichannel and single channel reflection seismics, gravity core, geochemical analysis of marine fluid vents and seafloor temperature profiling, recently acquired from the Pozzuoli Bay, along with existing geological, geophysical and geodetic datasets on land. Particularly, the high-resolution reflection seismic data offer unprecedented detailed insights into the stratigraphy and shallow structure of the NYT caldera collapse–ring fault zone–inner resurgence system. The results of this research provide a contribution to the understanding of structural style and timing of deformation of restless, resurgent calderas that develop along active continental margins.