Description: The greater area of the Lau Basin consists of a mosaic of microplates with various spreading centers and arc rifts. Repeated crustal recycling and addition of volatiles and metals in this tectonic setting leads to fertilization of the crust. This assimilation of mineral deposits is associated with building blocks of continental crust, and is thus an important setting to study crustal growth on Earth. During research cruise SO267 in 2018/2019, multi method geophysical and geological data were acquired on the newly formed Niuafo’ou microplate in the Lau Basin to study the links of rifting, magmatism and hydrothermal circulation in the early evolution of back arc spreading. The geophysical data encompass refraction- and reflection seismic, seismological, backscatter, gravity, magnetic and magnetotelluric data. Here we present a first electrical conductivity model derived from 3D inversion of marine magnetotelluric (MT) data. Electrical conductivity is a key proxy for imaging hydrothermal circulation and magmatic processes, since both processes effect the bulk electrical conductivity strongly. Our model exhibits three larger scale crustal conductivity anomalies. Based on the electrical conductivity model alone, we cannot discern whether the anomalies are caused by hydrothermal activity through pathways created by tectonic spreading or by melt accumulation caused by magmatic processes. However, a spatial correlation of our major conductivity anomaly with a seismicity cluster and a comparison with seismic velocity and backscatter data allows us to hypothesizeinfer about the geodynamic processes acting in the region. Our study documents the benefit of integrating different geophysical methods to understand rift arc evolution.

Description: Integration of multiple geophysical data is a key practice to reduce model uncertainties and enhance geological interpretations. Electrical resistivity models resulting from inversion of marine magnetotelluric (MT) data, often lack depth resolution of lithological boundaries and distinct information for shallow model parts. This is due to the diffusive nature of electromagnetic fields, enhanced by deficient data sampling and model regularization during inversion. Thus, integrating data or models to constrain layer thicknesses or structural boundaries is an effective approach to derive better constrained and more detailed resistivity models. We investigate the different impacts of three cross-gradient coupled constraints on 3D MT inversion of data from the Namibian passive continental margin. The three constraints are a) coupling with a fixed structural density model; b) coupling with satellite gravity data; c) coupling with a fixed gradient velocity model. Here we show that coupling with a fixed model (a and c) improves the resistivity model the most. Shallow conductors imaging sediment cover are confined to a thinner layer in the resulting resistivity models compared to the MT-only model. Additionally, these constraints help to suppress vertical smearing of a conductive anomaly attributed to a fracture zone, and clearly show that the seismically imaged Moho is not accompanied by a change in electrical resistivity. All of these observations help to derive an Earth model, which will form the basis for future interpretation of the processes that lead to continental break-up during the early Cretaceous.

Description: When interpreting geophysical models, we need to establish a link between the models’ physical parameters and geological units. To define these connections, it is crucial to consider and compare geophysical models with multiple, independent parameters. Particularly in complex geological scenarios, such as the rifted passive margin offshore Namibia, multi-parameter analysis and joint inversion are key techniques for comprehensive geological inferences. The models resulting from joint inversion enable the definition of specific parameter combinations, which can then be ascribed to geological units. Here we perform a user-unbiased clustering analysis of the parameters electrical resistivity and density from two models derived in a joint inversion along the Namibian passive margin. We link the resulting parameter combinations to break-up related lithology, and infer the history of margin formation. This analysis enables us to clearly differentiate two types of sediment cover. Namely, one of near-shore, thick, clastic sediments, and a second one of further offshore located, more biogenic, marine sediments. Furthermore, we clearly identify areas of interlayered massive, and weathered volcanic flows, which are usually only identified in reflection seismic studies as seaward dipping reflectors. Lastly, we find a distinct difference in the signature of the transitional crust south of- and along the supposed hot-spot track Walvis Ridge. We ascribe this contrast to an increase in magmatic activity above the volcanic centre along Walvis Ridge, and potentially a change in melt sources or depth of melting. This characterizes a rift-related southern complex, and a plume-driven Walvis Ridge regime. All of these observations demonstrate the importance of multi-parameter geophysical analysis for large-scale geological interpretations. Furthermore, our results may improve future joint inversions using direct parameter coupling, by providing a guideline for the complex passive margins parameter correlations.

Description: Passive continental margin research amalgamates the investigation of many broad topics, such as the emergence of oceanic crust, lithospheric stress patterns and plume-lithosphere interaction, reservoir potential, methane cycle, and general global geodynamics. Central tasks in this field of research are geophysical investigations of the structure, composition, and dynamic of the passive margin crust and upper mantle. A key practice to improve geophysical models and their interpretation, is the integrated analysis of multiple data, or the integration of complementary models and data. In this thesis, I compare four different inversion results based on data from the Namibian passive continental margin. These are a) a single method MT inversion; b) constrained inversion of MT data, cross-gradient coupled with a fixed structural density model; c) cross-gradient coupled joint inversion of MT and satellite gravity data; d) constrained inversion of MT data, cross-gradient coupled with a fixed gradient velocity model. To bridge the formal analysis of geophysical models with geological interpretations, I define a link between the physical parameter models and geological units. Therefore, the results from the joint MT and gravity inversion (c) are correlated through a user-unbiased clustering analysis. This clustering analysis results in a distinct difference in the signature of the transitional crust south of- and along the supposed hot-spot track Walvis Ridge. I ascribe this contrast to an increase in magmatic activity above the volcanic center along Walvis Ridge. Furthermore, the analysis helps to clearly identify areas of interlayered massive, and weathered volcanic flows, which are usually only identified in reflection seismic studies as seaward dipping reflectors. Lastly, the clustering helps to differentiate two types of sediment cover. Namely, one of near-shore, thick, clastic sediments, and one of further offshore located, more biogenic, marine sediments.

Description: Offshore freshened groundwater (OFG) and submarine groundwater discharge (SGD) are important components of coastal hydrologic systems. A lack of understanding of offshore groundwater systems and their interactions with onshore systems along the majority of global coastlines still exists due to a general paucity of field data. Recently, controlled-source electromagnetic (CSEM) techniques have emerged as a promising noninvasive method for identifying and characterizing OFG and SGD. Unfortunately, only a few systems are available in academic and research institutions worldwide, and applications are limited to specific regions. These systems are often limited by relatively high deployment costs, slow data acquisition rates, logistical complexity, and lack of modification options. A relatively inexpensive and user-friendly CSEM system is needed to overcome these limitations. We present the initial theoretical and practical developments of SWAN — a low-cost, modular, surface-towed hybrid time-frequency domain CSEM system capable of detecting OFG and SGD to water depths of 100 m. A field test of the system was carried out in the central Adriatic Sea at water depths between several tens to approximately 160 m to illustrate its capabilities. Through its ability to facilitate continuous measurements in both the time and frequency domain, the system has demonstrated its effectiveness in acquiring high-quality data while operating at towing speeds ranging from 2.5 to 3 kn. The resulting data coverage enables the system to detect variations in subsurface resistivity to depths of approximately 150–200 m below seafloor. With its modular, user-friendly design, SWAN provides an accessible, cost-efficient means to investigate the hydrogeology of shallow offshore environments.