ALBERT

Description: The Lau basin is an active back arc system with several spreading centres and microplates rapidly evolving in time. In the northeast basin, we observe a complex setting of a back arc rifting system and a volcanic arc. The Fonualei Rift and Spreading Center (FRSC) is a part of this complex setting, striking northwest and accommodating eastwest extension between the Niuafo'ou microplate and the Tonga plate. The volcanic arc strikes northeast southwest, in order to this striking direction the Fonualei Rift offset to the volcanic arc decreases to the south. This results in an minimal offset off 10-15km to the active volcanic arc. The interplay of the FRSC and the Tofua volcanic arc are not fully understood. Therefor we want to use the microseismicity as a tool to understand these active tectonics. One goal was to record seismicity data and create an event catalogue. We deployed a network of 16 ocean bottom seismometers (OBS) across the FRSC which recorded approximately 750 local events within a time span of 31 days. The events were located with the NonLinLoc (Non-Linear Location) software package. Further more we determined local and moment magnitudes. The seismicity catalogue shows an average event rate of 24 events per day. The events are focused beneath the network and in a region south of the network. Especially in the southern region we recognised a highly increased event rate depending on time and locality. In a time span of 6 days we observed 600 events resulting in a event rate of up to 100 events per day. This high event rate is an indicator for an earthquake swarm. For the magnitudes frequency distribution we determined a b-value for the entire catalogue of 1.05 with a magnitude of completeness of 1.0. With this microseismicity we were able to create an event catalogue which can be used as a starting point for more investigation within this region. For example we will analyse the focal mechanisms of selected events which will allow insights to the tectonics of the region.

Description: The source of magmatic features along the Namibian continental margin, and therefore the processes which lead to the opening of the South Atlantic ocean, are still debated controversially. One big question is weather hotspot volcanism was fed by a deep reaching plume or by heterogeneities of the middle and upper mantle. In an attempt to gain a better understanding of the involved magmatic processes, a 3D inversion of magnetotelluric data with an integrated seismically constrained density model was conducted. Integration was accomplished by adding a cross-gradient constraint of the density model to the inversion, which enforces model resemblance at structural boundaries. The impact of this cross-gradient constraint with a preexisting density model is limited to this model's resolution, because the cross-gradient only works at structural boundaries within the constraint model. Its benefits include enhancing of resistivity structures in the inversion model. Additionally, the density constraint does not overprint resistivity structures which are not imaged by gravimetric methods (i.e. resistivity variations due to mineral composition). An observed high resistivity anomaly below the continental margin and Walvis Ridge coincides well with seismically observed high velocity underplating. This feature is interpreted to mark magmatic intrusions from a plume source, initiating continental breakup. The eastern termination of the high resistivity structure correlates with the onset of seaward dipping reflectors in seismic data. Therefore, it marks the transition from continental to oceanic regime (continent-ocean boundary). The theory of a plume source of the magmatic features is supported by the local planar extent of roughly estimated 70 000km² and the deep reaching form of the underplating. This form with three arms at a 120° spread is suggestive for the rift arms of a hot spot impingement into the crust. However, since the aforementioned amount of intrusive material is rather small for the impact of a road plume head, the preferred model includes a plume that stopped ascending in the mid-mantel. The underplated magmatic features would then be fed by smaller plumes or hotspot-like dikes rising from this deep mantle plume. A continental breakup solely driven by plate-forces is unlikely for the South Atlantic, as lithospheric thinning and subsequent magmatism would have resulted in a larger volcanic area due to mantle heterogeneities, than the observed local magmatic underplating.

Description: Over 70 % of Earth’s surface is covered by water and inaccessible to standard methods of satellite geodesy. The emerging field of seafloor geodesy aims to provide methods to resolve seafloor deformation with high accuracy. In this thesis data from the first GEOMAR Helmholtz Centre for Ocean Research Kiel acoustic network deployment across the North Anatolian Fault in the Sea of Marmara will be analyzed and discussed. The dextral strike-slip fault system has produced a series of large devastating earthquakes over the last century such as the Izmit (Mw 7.6) earthquake in 1999. However, the Istanbul-Silivri fault segment in the Marmara Sea has not ruptured since 1766 and remains in the interseismic phase. The acoustic seafloor geodetic network is designed to measure strain between transponders on the seafloor. The network consists of six autonomous transponders installed on the seafloor and measures sound velocity, tilt, temperature, pressure and time of flight between the transponders. The sound speed sensors show a long term drift resulting in apparent o sets in baselines. Therefore, an approach is developed which uses constant salinity values for the estimation of sound speed. The resolution of a baseline measurements is defined as the standard deviation over time and increases linearly to ranging distance up to 1 km. Synthetic baselines are estimated in order to compare the di erence of baselines calculated using a water column of constant sound velocity gradient with the measured data including spatial heterogeneity along the ray path. About 65 % of the baseline fluctuations are suggested to originate from spatial heterogeneity along the ray path. Time series of 18 months reveal the absence of deformation estimates beneath the geodetic array within the resolution of 5 mm/a. The slip estimate from far field geodetic land stations as well as the absence of deformation from the acoustic geodetic seafloor data indicate that the North Anatolian Fault is highly locked and accumulating strain. The single baseline located in the nework’s west is showing deformation at a rate of 7 mm/a corresponding to the movement of a potential normal fault imaged by AUV Bathymetry.