The Potential and Limitations of 2D Seismic Experiments for 3D Tomography

The Liguro-Provençal Basin is located in a complex tectonic area, at the junction of the Western Alps and Northern Apennines. Despite its central location within Europe, much about the basin, including the character of the crust, and the continuation of the Alpine orogen offshore, remain ambiguous. The basin began opening in the late Oligocene as a back-arc basin related to the retreat of the Apennine subduction zone. Opening continued into the early Miocene with the counter-clockwise rotation of the Corsica-Sardinia block to its current position. In the southern part of the basin where this rotation opened the widest, seismic tomography has shown evidence of oceanic crust, however, the extent of this spreading zone northward into the Ligurian Sea is poorly mapped. The nature of the crust in the basin, whether atypical oceanic crust or partially serpentinized mantle overlain by sediments or highly thinned continental crust is still a matter of debate. At a larger scale there are still open questions as to the continuation of the alpine orogen offshore, and the change in polarity between the Alp and Apennine subduction zones. As well, present day seismicity with thrust-faulting focal mechanisms have been observed in the basin, indicating that the stress field is now compressive. This could potentially reactivate rift-structures in the basin, which are difficult to map due to thick sediment cover including a layer of Messinian salt with variable thickness. These open questions, and the accessibility of the basin in the heart of Europe, have led to the collection of at least 18 active seismic profiles, and even more multi-channel seismic lines. Each of these studies have contributed to understanding the tectonics of the area through 2D tomography along the profile, but these are small snapshots of a complex setting. The amount of data that has been collected provides a unique opportunity to combine data sets and examine the possibility of gaining new information in the form of 3D tomography from existing 2D data sets. In this project we use active seismic data from the LOBSTER-AlpArray Experiment, the GROSMarin Experiment, and the SARDINIA Experiment, as well as passive seismic data from the AlpArray Experiment and the ISC Bulletin. We explore the potential and limitations of these data sets for use in 3D tomography using two new methods. We first use off-profile stations along a 2D seismic line combined with passive seismicity to provide back-shots for the stations, then in the Gulf of Lion we use two parallel seismic profiles where stations recorded shots from both profiles. This project is part of the DFG Priority Program “Mountain Building Processes in Four Dimensions (4DMB)”

Does gravity modelling justify a rifted "Ligurian Basin"?

The geo-historical development of the Ligurian Basin and the structure of the crust and upper mantle in this area are still being discussed. Yet it remains unclear if rifting caused continental break-up and seafloor spreading and one of the key questions is whether rifting can be identified in geophysical measurements. For our investigations we had the following updated data sets at our disposal: the new gravity maps of the AlpArray Gravity Working Group (complete Bouguer - CBA, Free air, and isostatic anomalies) the seismic results of the Lobster campaigns of our GEOMAR partners in the SPP MB4D as well as the dynamic modelling results from our own subproject. The constraining data are supplemented with seismic profile data from French and Italian offshore campaigns, as far as they are usable in publications for us. The GFZ modelling software IGMAS+ was used for an interactive 3D modelling. The resulting model contains density inhomogeneities in the crust as well as in the upper mantle down to a depth of 300 km following the results of dynamic models of our own subproject. Due to the special hybrid modelling of the crust (by polygonal structures) and the upper mantle (by voxels of recent velocity models), the individual contributions to the gravity field are clearly separable. As a further special feature, we point out that the density model used is based on the gravity modelling from the first phase of the SPP MB4D (our former subproject INTEGRATE). Thus, a largely consistent 3D density model for both the Alps and the Ligurian Sea is available for interpretation. The constrained 3D modelling of the gravity field, as well as numerical analyses of the fields (terracing, clustering, filtering, curvature), calculations of the vertical stress and Gravity Potential Energy (GPE) suggest that a rift structure in the area of the Ligurian Sea can be identified and mapped. The interactive modelling is supported by the use of geological maps in the Ligurian Sea area. By overlaying the model gravity maps and the geological maps, the good agreement becomes visible – refer to the attached figure

Investigations of the Oligocene-Miocene opening of the Ligurian Basin using amphibious refraction seismic data

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian-Maghrebides subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning and un-roofing of mantle material little is known about the style of back-arc rifting. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. Two refraction seismic profiles were shot across and along the centre of the Ligurian Basin. P01 was shot in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. P02 is a transect along the basin in NE-SW direction extending a previous shot seismic profile reaching to the Italian cost near Genua. The majority of the ocean bottom seismometer data show sedimentary and crustal phases of good quality and weaker in amplitude mantle phases to offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results for p01 show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long-lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. This is in good agreement with recent kinematic modelling performed in the second phase of the SPP2017 4D-MB. The modelling results of p01 indicate that continental crust can be stretched over several million years when the opening rate is low, i.e. <2 mm/year, and syn-rift sedimentation rate is high. Subduction initiation could occur in ultra-thinned continental crust as basin inversion has been observed at the northern Ligurian margin as a result of the African-European convergence. Additionally, the observations from the Ligurian Basin might be transferred to the evolution of the Piemont-Liguro Ocean. So far oceanic crust was assumed as initial conditions for the subduction of the Piemont-Liguro Ocean. An ultra-thin continental crust as initial condition would explain the observed thin subducted Piemont-Liguro plate which seemed to be thinner than 6-7 km oceanic crust. Further, a dry continental crust could explain why no back-arc volcanism was observed. The along-basin profile p02 shows a deepening crust-mantle boundary from 11 to 13 km. Based on the retrieved velocity model, gravity modelling and further results from surrounding studies we conclude that the continental crust is thinning from the northeast to the southwest which is related to the increase of extension away from the rotation pole of the anticlockwise rotation of the Corsica-Sardinia block. It remains unclear if at the southern end of the profile the mantle is overlain directly by sediments or by extremely thinned continental crust of up to 2.5 km thickness. The results however document, that seafloor spreading and the formation of mantle-derived oceanic crust was not initiated during the extension of the Ligurian Basin

Agricultural land use changes – a scenario-based sustainability impact assessment for Brandenburg, Germany

AbstractDecisions for agricultural management are taken at farm scale. However, such decisions may well impact upon regional sustainability. Two of the likely agricultural management responses to future challenges are extended use of irrigation and increased production of energy crops. The drivers for these are high commodity prices and subsidy policies for renewable energy. However, the impacts of these responses upon regional sustainability are unknown. Thus, we conducted integrated impact assessments for agricultural intensification scenarios in the federal state of Brandenburg, Germany, for 2025. One Irrigation scenario and one Energy scenario were contrasted with the Business As Usual (BAU) scenario. We applied nine indicators to analyze the economic, social and environmental effects at the regional, in this case district scale, which is the smallest administrative unit in Brandenburg. Assessment results were discussed in a stakeholder workshop involving 16 experts from the state government.The simulated area shares of silage maize for fodder and energy were 29%, 37% and 49% for the BAU, Irrigation, and Energy scenarios, respectively. The Energy scenario increased bio-electricity production to 41% of the demand of Brandenburg, and it resulted in CO2 savings of up to 3.5milliontons. However, it resulted in loss of biodiversity, loss of landscape scenery, increased soil erosion risk, and increased area demand for water protection requirements. The Irrigation scenario led to yield increases of 7% (rapeseed), 18% (wheat, sugar beet), and 40% (maize) compared to the BAU scenario. It also reduced the year-to-year yield variability. Water demand for irrigation was found to be in conflict with other water uses for two of the 14 districts. Spatial differentiation of scenario impacts showed that districts with medium to low yield potentials were more affected by negative impacts than districts with high yield potentials.In this first comprehensive sustainability impact assessment of agricultural intensification scenarios at regional level, we showed that a considerable potential for agricultural intensification exists. The intensification is accompanied by adverse environmental and socio-economic impacts. The novelty lies in the multiscale integration of comprehensive, agricultural management simulations with regional level impact assessment, which was achieved with the adequate use of indicators. It provided relevant evidence for policy decision making. Stakeholders appreciated the integrative approach of the assessment, which substantiated ongoing discussions among the government bodies. The assessment approach and the Brandenburg case study may stay exemplary for other regions in the world where similar economic and policy driving forces are likely to lead to agricultural intensification

Crustal structure of the Niuafo’ou Microplate and Fonualei Rift and Spreading Center in the northeastern Lau Basin, Southwestern Pacific

Key points: First insights into the crustal structure of the northeastern Lau Basin, along a 290 km transect at 17°20’S. Crust in southern Fonualei Rift and Spreading Center was created by extension of arc crust and variable amount of magmatism. Magmatic underplating is present in some parts of the southern Niuafo’ou Microplate 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 reasonably understood, the internal structure and evolution of the back‐arc crust are not. We present new geophysical data from a 290 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 and density modelling suggests that past crustal accretion inside the southern FRSC was accommodated by a combination of arc crustal extension and magmatic activity. The absence of magnetic reversals inside the FRSC supports this and suggests that focused seafloor spreading has until now not contributed to crustal accretion. The back‐arc crust constituting the southern Niuafo’ou Microplate reveals a heterogeneous structure comprising several crustal blocks. Some regions of the back‐arc show a crustal structure similar to typical oceanic crust, suggesting they originate from seafloor spreading. Other crustal blocks resemble a structure that is similar to volcanic arc crust or a ‘hydrous’ type of oceanic crust that has been created at a spreading center influenced by slab‐derived water at distances < 50 km to the arc. Throughout the back‐arc region we observe a high‐velocity (Vp 7.2‐7.5 km s‐1) lower crust, which is an indication for magmatic underplating, which is likely sustained by elevated upper mantle temperatures in this region

Magmatic accretion versus serpentinized mantle exhumation at ultraslow spreading rates: constraints from seismic data and Vp/Vs ratios, Mid-Cayman Spreading Centre, Caribbean Sea

About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the ~60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 1.9. Here, we report results from a seismic refraction survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The ocean-bottom-seismometer and hydrophones provide both P-wave and S-wave refracted arrivals. Travel time data were inverted using seismic tomography. Resulting Vp/Vs ratios suggest that up to 25% of the lithosphere have high ratios of >1.9, supporting serpentinization and exposure of hydrated mantle at the seafloor. Further, the mode of accretion has changed over time, supporting both areas of mantle exposure and magmatic crust. Magmatic crust has a typical layer 2 and layer 3 velocity structure and a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, crustal rocks are characterized by typical crustal-velocities (7.6 km/s. Domains of un-roofed mantle have high Vp/Vs ratios and velocities gradually increasing to 7.4-7.6 km/s. In addition, we will use our results to re-assess the depth distribution of local earthquakes at ultra-slow spreading ridges, including the Cayman Trough and the Southwest Indian Ridge. Most importantly, the high Vp/Vs ratio of >1.9 characterizing serpentinized mantle causes earthquakes to focus at much shallower depth when compared to location procedures using a global average for Vp/Vs of 1.73; the bias in depth might be in the order of 10 km

Ionian Abyssal Plain: a window into the Tethys oceanic lithosphere

The nature of the Ionian Sea crust has been the subject of scientific debate for more than 30 years, mainly because seismic imaging of the deep crust and upper mantle of the Ionian Abyssal Plain (IAP) has not been conclusive to date. The IAP is sandwiched between the Calabrian and Hellenic subduction zones in the central Mediterranean. A NNE–SSW-oriented 131&thinsp;km long seismic refraction and wide-angle reflection profile, consisting of eight ocean bottom seismometers and hydrophones, was acquired in 2014. The profile was designed to univocally confirm the proposed oceanic nature of the IAP crust as a remnant of the Tethys and to confute its interpretation as a strongly thinned part of the African continental crust. A P-wave velocity model developed from travel-time forward modelling is refined by gravimetric data and synthetic modelling of the seismic data. A roughly 6–7&thinsp;km thick crust with velocities ranging from 5.1 to 7.2&thinsp;km&thinsp;s−1, top to bottom, can be traced throughout the IAP. In the vicinity of the Medina seamounts at the southern IAP boundary, the crust thickens to about 9&thinsp;km and seismic velocities decrease to 6.8&thinsp;km&thinsp;s−1 at the crust–mantle boundary. The seismic velocity distribution and depth of the crust–mantle boundary in the IAP document its oceanic nature and support the interpretation of the IAP as a remnant of the Tethys lithosphere with the Malta Escarpment as a transform margin and a Tethys opening in the NNW–SSE direction.</p

Crustal structure of the propagating TAMMAR ridge segment on the Mid-Atlantic Ridge, 21.5°N

Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length