The raw 2D multichannel seismic reflection data presented in this report were acquired during expedition SO301 north of the Rodrigues Triple Junction. Data collection targeted the Central Indian Ridge, including the 25°S Oceanic Core Complex (OCC) and the Kairei hydrothermal field, to investigate crustal structure and faulting processes at a slow spreading ridge. A total of 29 seismic lines were recorded using an 8 km, 756-channel streamer and an air gun source array. The dataset covers approximately 1871 line kilometers and includes profiles across ridge segments, the OCC, and conjugate flanks. The data are provided in raw SEG-D format with associated navigation and standardized metadata. Detailed information on the acquisition can be found in the SO301 cruise report (https://doi.org/10.48433/cr_so301). This technical report provides further information on the acquisition of the 2D multichannel seismic reflection data set.

Three tectonic plates meet at the Rodriguez Triple Junction in the Central Indian Ocean. The plates are separated by the Central Indian Ridge (CIR), the South-East Indian Ridge (SEIR) and the South-West Indian ridge (SWIR), which all show highly different spreading behaviours. While the northernmost segment of the SEIR is magmatically robust, the eastern tip of the SWIR is highly amagmatic. The CIR appears to oscillate between opening mechanisms, associated either with magmatic or magma-starved spreading processes, which can be observed over a very confined stretch of crust. Even though the area has been studied thoroughly, using a variation of geophysical and geological methods in the past decades, seismic images of the region were missing. From November 2023 to January 2024, RV Sonne (SO301 - SCIROCCO) set out for a seismic reflection and refraction survey to fill this gap and to provide a database for a better understanding of the tectonic setting and evolution of the area. A special focus was put on studying the structure and extent of the Oceanic Core Complex (OCC) at 25 °S.Here we present preliminary results of an east-west trending 150 km long profile crossing the OCC and the CIR. Along the profile, 33 ocean bottom seismometers were deployed with a spacing of 4-5 km that grew denser over the OCC. The shot spacing was between 50-110 m. Clear crustal refracted P- and S-phases were observed to offsets of up to 40 km in the shot sections and mantle reflections, as well as Pn-phases could be identified sporadically. First results of travel time tomographies, which were executed separately for P- and S-waves, and used for the calculation of a Vp/Vs-ratio section indicate a strongly variable crustal construction. Highly fractured areas seem to interchange with highly hydrated areas within short distances. Correlations of the new bathymetric data to the seismic images and the integration of the new gravimetric and magnetic data will sharpen the geophysical image and its tectonic interpretation along the profile.

The Java-Lesser Sunda margin exhibits different topography of subducting oceanic basement relief and diverse upper plate tectonic processes, ranging from neutral characteristics offshore Lombok and Sumbawa to erosional features offshore Central Java to Bali, distinct from its accretionary counterpart off Sumatra. Despite this classification, a comprehensive understanding of how the subduction of oceanic basement relief influences the plate boundary and upper plate structure across the transition from neutral to erosional remains elusive. In our investigation, we illuminate the tectonic parameters governing the margin's classification by integrating multi-channel reflection seismic images obtained through a grid-based P-wave velocity inversion and high-resolution multibeam bathymetric maps. Our dataset reveals the nuanced modifications to seafloor morphology, upper plate structure, and décollement position brought about by various scales of subducting topography. Large-scale subducting features prompt a landward shift of the deformation front, leading to a shortened accretionary wedge and heightened seafloor incline at the relief's trailing edge. Conversely, small-scale subducting ridges predominantly impact the frontal prism, causing over-steepening at the trench and localized slope failures. Deformation of the accretionary wedge ahead of subducting relief is characterized by intensified compression and reduced seafloor slope, seemingly independent of the relief's size. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P-wave velocity models reveal a notably lower rigidity of the upper plate's base along the eastern Sunda margin compared to the global trend. This lower rigidity is a crucial factor favoring the occurrence of tsunami earthquakes on the Java margin. In conclusion, our study provides a comprehensive analysis of the complex interplay between subducting oceanic relief and tectonic processes, shedding light on the factors that dictate the margin's transition from neutral to erosional characteristics and the associated seismic implications.

We analyzed refraction and reflection seismic data covering the Central Lau Spreading Center (CLSC) and the Tonga volcanic arc at 18 degrees S in the Pacific Ocean to investigate tectonic inactivity in actively extending back-arc systems. Our P-wave travel time tomography shows a characteristic 6-8 km thick back-arc crust beneath and around the CLSC and a similar to 13 km thick arc-crust at the eastern end of our profile, which corresponds to the Tonga volcanic arc. Lower crustal velocities increase to 7.5 km/s toward the volcanic arc, marking the transition from back-arc to arc crust. These high P-wave velocities can be explained by a high pyroxene content in the lower crust originating from depleted melts. Our seismic reflection data show steep normal faults below the CLSC and volcanic structures closer to the volcanic arc, without a tectonically inactive zone (also knows as the "diffuse plate boundary") in between. Based on our results, we suggest that the Niuafo'ou and Tonga microplates should be treated as one tectonic plate with local zones of intra-plate deformation that are separated by zones that are tectonically quiet. In a subduction zone one tectonic plate sinks below another. A retreating downgoing plate causes tension in the overriding plate, which leads to thinning behind the volcanic arc: a back-arc basin is born. We investigate the crustal structure of a seismically inactive region within a back-arc basin in overall extension. Our aim is to understand why there is no seismic activity in this specific area. The Lau Basin is our case study, because it is a type-example for back-arc basin tectonics. We have acquired and interpreted seismic data to analyse the structure of the crust along a profile in the Lau Basin. We observe crustal thickness changes from typical back-arc crust (6 and 7 km) to arc-crust (13 km), and volcanic remnants in the crust with higher crustal velocities. Current tectonic activity only occurs at the Central Lau Spreading Center (CLSC), not in between the CLSC and the volcanic arc. We therefore reject the idea of a diffuse plate boundary, and propose that the Niuafo'ou and Tonga microplates should be treated as one tectonic plate that consists of localized zones that deform. In between those actively extending zones, there are areas that do not deform and that are tectonically quiet. The crustal structure of the central Lau Basin at 18 degrees S complies with typical back-arc/arc crustal velocities and thickness South of the FRSC no plate boundary is observed, suggesting one Niuafo'ou-Tonga plate with zones of intraplate deformation High crustal P-wave velocities result from the crystalisation of hydrous melts (upper crust) and pyroxene-rich material (lower crust)

Three tectonic plates meet at the Rodriguez Triple Junction in the Central Indian Ocean. The plates are separated by the Central Indian Ridge (CIR), the South-East Indian Ridge (SEIR) and the South-West Indian ridge (SWIR), which all show highly different spreading behaviours. While the northernmost segment of the SEIR is magmatically robust, the eastern tip of the SWIR is highly amagmatic. The CIR appears to oscillate between opening mechanisms, associated either with magmatic or magma-starved spreading processes, which can be observed over a very confined stretch of crust. Even though the area has been studied thoroughly, using a variation of geophysical and geological methods in the past decades, seismic images of the region were missing. From November 2023 to January 2024, RV Sonne (SO301 - SCIROCCO) set out for a seismic reflection and refraction survey to fill this gap and to provide a database for a better understanding of the tectonic setting and evolution of the area. A special focus was put on studying the structure and extent of the Oceanic Core Complex (OCC) at 25 °S.Here we present preliminary results of an east-west trending 150 km long profile crossing the OCC and the CIR. Along the profile, 33 ocean bottom seismometers were deployed with a spacing of 4-5 km that grew denser over the OCC. The shot spacing was between 50-110 m. Clear crustal refracted P- and S-phases were observed to offsets of up to 40 km in the shot sections and mantle reflections, as well as Pn-phases could be identified sporadically. First results of travel time tomographies, which were executed separately for P- and S-waves, and used for the calculation of a Vp/Vs-ratio section indicate a strongly variable crustal construction. Highly fractured areas seem to interchange with highly hydrated areas within short distances. Correlations of the new bathymetric data to the seismic images and the integration of the new gravimetric and magnetic data will sharpen the geophysical image and its tectonic interpretation along the profile.

ABSTRACT Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak (‘Entenschnabel’) reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source–receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo‐landscape reconstruction and identification of shallow gas for hazard assessments, for example.

The Java ‐ Lesser Sunda margin, which features multi‐scale subducting oceanic basement relief, is classified as neutral (Lombok and Sumbawa) to erosional (Central Java to Bali) in comparison to its accretionary counterpart offshore Sumatra. However, a comprehensive analysis of how plate boundary and upper plate structure across the neutral to erosional transition are modulated by the subduction of oceanic basement relief is lacking to date. To shed light on the tectonic parameters that push the margin into the neutral or erosional domain, we combine multi‐channel reflection seismic images derived through a grid‐based P‐wave velocity inversion, and multibeam bathymetric maps. The data document how different scales of subducting topography modify seafloor morphology, upper plate structure, and décollement position. Large‐scale subducting features cause a landward shift of the deformation front, shortening of the accretionary wedge, and seafloor steepening at the relief's trailing edge. Small‐scale subducting ridges primarily impact the frontal prism resulting in over‐steepening at the trench and localized slope failure. Ahead of subducting relief, deformation of the accretionary wedge encompasses enhanced compression and a reduction in seafloor slope but appears independent of the size of the relief. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P‐wave velocity models indicate that the rigidity of the upper plate's base along the eastern Sunda margin is significantly lower than the worldwide trend. We conclude that this favors the genesis of tsunami earthquakes that have occurred on the Java margin.

Due to the complexity of 2D magnetic anomaly maps north of 18 degrees S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi-method approach to reconstruct the formation of the basin at similar to 16 degrees S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km-long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response. Plain Language Summary The opening of back-arc basins is often described as analogy to mid-ocean ridge spreading, where the only difference is the force driving the extension. However, the northern Lau Basin is a prime example for the shortcomings of this analogy since its crust preserves an image of its complex tectonic history. The complexity results from the short-lived nature of zones of active rifting and spreading in the northern Lau Basin, which is very different from the temporally and spatially steady nature of spreading centers at mid-ocean ridges. The analysis of different methods (wide angle seismic data using ocean bottom seismometers, multi-channel seismic, magnetic, gravity, and geochemical data) has led us to conclude that the Lau Basin's crust at 15 degrees 30-17 degrees 20'S was formed by a dynamically changing, both in regard of magma composition and position, extensional system that consists of the Fonualei Rift and Spreading Center and the Mangatolu Triple Junction. Nevertheless, the crustal zonation, formed by the varying subduction influence during its formation, is still preserved and affects the stress response of the crust and thus the present-day tectonic behavior.

The Mangatolu Triple Junction (MTJ) and the Fonualei Rift and Spreading Center (FRSC) are two prominent bathymetric features in the northern Lau Basin in the southwest Pacific Ocean. We present the results of six W-E running Multi-Channel Seismic (MCS), magnetic and sediment echo sounding profiles acquired during the ARCHIMEDES-I expedition. These profiles cover the MTJ, the FRSC and the region just south of the FRSC to investigate the tectonic history and current tectonic activity of the Lau Basin. On all MCS profiles, we observe a heavily faulted basement on both sides of the MTJ and FRSC with faults that are covered with sediments, confirmed by the sediment echo sounding data. We consider these buried faults inactive today. We also observe faults that reach the seafloor. These faults are generally located closer to the MTJ and the FRSC and they correlate well with seismic activity recorded in the region. We thus consider these faults currently active. Seismically transparent bodies are observed on most profiles as well. We have interpreted those as volcanic intrusions, i.e. sills, or as volcanoes that pierce through the stratigraphy, especially closer to the volcanic arc. The two sets of faults, the notion that extension rates are higher at the MTJ (32 mm/yr) than at the southern tip of the FRSC (8 mm/yr) and the results from our newly acquired and interpreted magnetic data, have led to the interpretation that an earlier rift phase accommodated extension in a wide rift tectonic setting between 2.15 Ma and 0.85 Ma at the MTJ and 2.15 and 1.61 Ma at the FRSC. Today, the extension is accommodated in a narrow rift tectonic setting close to the MTJ and FRSC with a higher extension rate at the MTJ than at the southern tip of the FRSC. These findings suggest that the MTJ and FRSC are one, single intra-plate extension zone that is in the process of breaking apart the overriding Niuafo’ou-Tonga microplate along the MTJ and FRSC.

The Cretaceous and Cenozoic evolution of the North German Basin is shaped by complex processes involving basin inversion, uplift and erosion, extension and several periods of Quaternary glaciations. Based on a densely spaced long-offset 2D seismic profile network covering the Bays of Kiel and Mecklenburg, we employ a Machine Learning algorithm to pick refracted first-arrival travel-times. These travel-times are used in a travel-time tomography to derive velocity models for the approximately upper 800 m depth of the subsurface. Investigating velocity-depth relations within the Upper Cretaceous strata and analyzing lateral velocity anomalies within shallow depths provide new insights into the magnitude of the Cenozoic basin exhumation and the locations of glacial tunnel valleys. Our findings suggest that previously observed bent-up structures in seismic images are caused by heterogeneous velocities in the overburden and do not represent actual reflectors. We provide strong indications that these misinterpretations of imaging artifacts are related to tunnel valleys even though these valleys might not always be resolvable in seismic reflection or sediment sub-bottom images. Comparing Upper Cretaceous velocity-depth trends to reference trends reveals significantly higher velocities in our study area. We interpret these differences as overcompaction and estimate the apparent Cenozoic exhumation in the Bay of Mecklenburg to be about 475 m. Within the Bay of Kiel, we observe an increase of the apparent exhumation from about 385 m (south) to about 480 m (north). Our study demonstrates the importance of near surface velocity analysis for the investigation of geological processes in shallow marine settings.

The dataset contains the multi-beam bathymetry map, merged with GEBCO global grid, offshore the eastern Java margin, Sunda arc. The multi-beam data was collected by GEOMAR and BGR in 2006 during Sonne cruise SO190. The merging of the grid file is done by using QGIS 3.16 and saved in NetCDF format.

The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. As a result of the asymmetric roll back of the Pacific plate, the Lau basin’s divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. One of these centres of extensional motion, and the subject of this study, are two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC).In 2018, the research vessel Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basinat ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.The results of our Monte-Carlo P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradientin the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and thereforeincreasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.

The northern Lau Basin in the southwest Pacific Ocean is one of the fastest opening back-arc basins on Earth, resulting in a mosaic of microplates, including the Niuafo’ou and Tongan microplates. The Fonualei Rift and Spreading Center (FRSC) is the eastern plate boundary that separates the Niuafo’ou from the Tongan microplate. The northern part of the FRSC is actively spreading, whereas the southern part is rifting. What is unclear, however, is how extension of the Lau Basin is accommodated north and south of the FRSC. We present the results of six Multi-Channel Seismic profiles acquired during the ARCHIMEDES-I expedition and show an analogue lithosphere-scale model example of our proposed tectonic evolution. Profiles P1 (oriented NW-SE) and P2 (oriented W-E) cover the Mangatolu Triple Junction (MTJ) and the northern part of the FRSC. P3 and P4 (both oriented W-E) cover the southern Niuafo’ou microplate. P5 and P6 (both oriented W-E) cover the area south of the FRSC. The northern profiles (P1 and P2) reveal a thick package of sediment towards the east, covering a heavily faulted basement over a wide area. Some indication for intrusive material is observed closer to the volcanic arc, but also further towards the western end of P2. Faults cross-cutting the basement but that do not reach the surface are considered inactive today. Faults reach the surface close to the MTJ and the northern tip of the FRSC and are considered active today. This leads to the interpretation that an earlier rift phase accommodated extension in a wide rift tectonic setting, whereas today, the extension is accommodated in a narrow rift or spreading tectonic setting. We will show an analogue model example that demonstrates this wide-to-narrow extensional tectonic evolution. The profiles that cover the southern extent of the FRSC (P3, P4, P5 and P6), show that active faulting occurs towards the west, close to the Central Lau Spreading Center. Hidden faults that have deformed the basement, but do not affect the surface today anymore are observed in the abyssal parts of P3, P4, P5 and P6. Active faults that reach the surface are also observed towards the east. Recent volcanism is observed, both in the form of intrusive bodies, i.e. sills, as well as volcanoes that pierce through the stratigraphy. The observations lead to the conclusion that south of the FRSC an earlier (wide) rift system affected a larger area in the current abyssal parts of the profiles, whereas extension is currently accommodated through spreading in the CLSC, west of the southern tip of the FRSC.

The geometry of sedimentary basins is normally described by the interpretation of seismic reflectors. In addition to that, rock properties of the sedimentary successions between these reflectors give further insight into the subsurface geology. Here, we present a model for the Bay of Mecklenburg, situated at the northeastern margin of the North German Basin. The model consists of eight layers; it covers seismic velocities of sediments from the Neogene down to the base of the Permian Zechstein. We use eight seismic profiles for model building and apply seismic migration velocity analysis in combination with pre-stack depth migration. The results are interval velocities down to a depth of 5000 m. A further aim of the study is to investigate the sensitivity of these indirectly deduced velocities in comparison to direct measurements within drill holes. The velocities from this study are in good agreement with earlier results from vertical seismic profiling at a nearby well. Cenozoic and Mesozoic strata within the Bay of Mecklenburg show clear depth-dependent velocity trends. A comparison of these trends with predicted compaction trends shows that burial anomalies within Lower Triassic units are significantly higher than in Upper Cretaceous units. This finding could be explained by a greater amount of erosion during Upper Jurassic/Lower Cretaceous times than during Cenozoic times. The Zechstein layer shows a decreasing interval velocity with increasing thickness. Our study demonstrates that seismic velocities deduced from surface-based measurements are of high value in areas with sparse drilling coverage.

The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin. As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge. Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired. The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget. The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.

The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin. As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge. Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired. The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget. The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.

We resolve a previously unrecognized shallow subducting seamount from a re-processed multichannel seismic profile crossing the 1994 M-w 7.8 Java tsunami earthquake rupture area. Seamount subduction occurs where the overriding plate experiences uplift by lateral shortening and vertical thickening. Pronounced back-thrusting at the landward slope of the forearc high and the formation of splay faults branching off the landward flank of the subducting seamount are observed. The location of the seamount in relation to the 1994 earthquake hypocentre and its co-seismic slip model suggests that the seamount acted as a seismic barrier to the up-dip co-seismic rupture propagation of this moderate-size earthquake.

Accurate subsurface velocity models are crucial for geological interpretations based on seismic depth images. Seismic reflection tomography is an effective iterative method to update and refine a preliminary velocity model for depth imaging. Based on residual move-out analysis of reflectors in common image point gathers, an update of the velocity is estimated by a ray-based tomography. To stabilize the tomography, several preconditioning strategies exist. Most critical is the estimation of the depth error to account for the residual move-out of the reflector in the common image point gathers. Because the depth errors for many closely spaced image gathers must be picked, manual picking is extremely time-consuming, human biased, and not reproducible. Data-driven picking algorithms based on coherence or semblance analysis are widely used for hyperbolic or linear events. However, for complex-shaped depth events, purely data-driven picking is difficult. To overcome this, the warping method named non-rigid matching is used to estimate a depth error displacement field. Warping is used, for example, to merge photographic images or to match two seismic images from time-lapse data. By matching a common image point gather against its duplicate that has been shifted by one offset position, a locally smooth-shaped displacement field is calculated for each data sample by gather matching. Depending on the complexity of the subsurface, sample tracking through the displacement field along predefined horizons or on a simple regular grid yields discrete depth error values for the tomography. The application to a multi-channel seismic line across the Sunda subduction zone offshore Lombok island, Indonesia, illustrates the approach and documents the advantages of the method to estimate a detailed velocity structure in a complex tectonic regime. By incorporating the warping scheme into the reflection tomography, we demonstrate an increase in the velocity resolution and precision by improving the data-driven accuracy of depth error picks with arbitrary shapes. This approach offers the possibility to use the full capacities of tomography and further leads to more accurate interpretations of complex geological structures.

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is fairly well understood, the structure and evolution of the back-arc crust are not. We present refraction seismic, multichannel seismic and gravity data from a 300 km long east-west oriented transect crossing the Niuafo’ou Microplate (back-arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17°20’S. Our P wave tomography model shows strong lateral variations in the thickness and velocity-depth distribution of the crust. The thinnest crust is present in the Fonualei Rift and Spreading Center, suggesting active seafloor spreading there. In the much thicker crust of the volcanic arc we identify a region of anomalously low velocities, indicative of partial melts. Surprisingly, the melt reservoir is located at ~17 km distance to the volcanic front, supporting the hypothesis that melts are deviated from the volcanic arc towards the FRSC in sub-crustal domains. We identify two distinct regions in the back-arc crust, representing different opening phases of the northeastern Lau Basin. During initial extension, likely dominated by rifting, crust of generally lower upper-crustal velocities formed. During an advanced opening phase, likely dominated by seafloor spreading, crust of higher upper-crustal velocities formed and is now up to 11 km thick. This thickening is the result of magmatic underplating, which is supported by elevated upper mantle temperatures in this region.