High resolution bathymetric survey on the NW slope of Walvis Ridge, offshore Namibia

Expedition 17/1 of the German research vessel R/V MARIA S. MERIAN, carried out geophysical surveys and experiments between November and December 2010 in the area around Walvis Ridge, Southeast Atlantic Ocean. Among the data collected, a high-resolution bathymetric dataset aquired on the northwestern slope of the ridge offers some important preliminary insights into the tectonic evolution of the ridge and the adjoining lower continental slopes and ocean basin. The NE-SW trending Walvis Ridge has a trapezoid shape and is likely built up by thick sequences of plateau basalts, with top of basement rocks inclined to the south. Sediments are almost absent on the NW side of the ridge, preserving a fascinating mountainscape formed early in the tectonic history, most probably on-land. This interpretation is supported by clear denudational features, like steep cliffs up to 150 m high, and deeply incised valleys, defining paleo-drainages. Isolated, flat-topped guyots seaward of the ocean-continent boundary attest to a later history of wave abrasion and progressive subsidence of Walvis Ridge

Seismic explosion sources on an ice cap – technical considerations

Controlled source seismic investigation of crustal structure below ice covers is an emerging technique. We have recently conducted an explosive refraction/wide-angle reflection seismic experiment on the ice cap in east-central Greenland. The data-quality is high for all shot points and a full crustal model can be modelled. A crucial challenge for applying the technique is to control the sources. Here, we present data that describe the efficiency of explosive sources in the ice cover. Analysis of the data shows, that the ice cap traps a significant amount of energy, which is observed as a strong ice wave. The ice cap leads to low transmission of energy into the crust such that charges need be larger than in conventional onshore experiments to obtain reliable seismic signals. The strong reflection coefficient at the base of the ice generates strong multiples which may mask for secondary phases. This effect may be crucial for acquisition of reflection seismic profiles on ice caps. Our experience shows that it is essential to use optimum depth for the charges and to seal the boreholes carefully

Subduktionszone Segmentierung entlang des Sunda-Margin, Indonesien

The Sunda margin marks the eastern termination of the Indian Ocean along the Indonesian archipelago, where the subduction of the Indo-Australian plate below Eurasia is taking place. The convergence direction changes from orthogonal in the eastern part of the margin to a more oblique convergence towards the north-west offshore Sumatra. Along the margin, a wide range of subduction scenarios is observed, which makes this margin a perfect natural laboratory to study the role of various physical and geological parameters on subduction dynamics. This work builds on geophysical and tectonic interpretations of data acquired along eight seismic/gravity and several MCS marine profiles, distributed along the Sunda margin. The data include wide-angle seismic records, multi-channel seismic data, and gravity data. Data interpretation and modeling include a joint reflection/refraction seismic tomography used to constrain geophysical models across the subduction complex at several locations, further constrained by geological information from the multichannel seismic data. Additionally, other geophysical information including gravity modeling and bathymetry data is employed to refine geodynamic models based on seismic interpretations. Excellent data quality allows detailed structural models of the subduction complex to be constrained in several locations along the Sunda margin. The geophysical models obtained in this study show significant variations of the crustal and upper mantle structure of the subduction complex along-strike and across-strike of the margin. The study revealed an increased thickness of the crystalline crust in the Savu Sea, attributed to the approach of the Australian shelf to the trench. Offshore Lombok Island, the oceanic crust thickness is found to be 7 km, and heavily fractured by normal faults. The crustal structure of the Roo Rise oceanic plateau was modeled for the first time, revealing the crustal thickness of 15 km. Its subduction is causing inhomogeneous deformation of the forearc and complex evolution of the entire subduction processes. In addition, large variability in the amount of sediment on the incoming plate is found in adjacent sectors of the margin, ranging from ~3 km in the Savu Sea to almost absence in the Lombok and East Java areas. Offshore north Sumatra, comparison of the crustal-scale profiles located on different sides of the segment boundary between 2004 and 2005 earthquakes rupture areas, showed variability of the structure of the oceanic plate and the amount of sediment present at the trench, resulting in the principally different sizes of the accretionary prisms and the width of the seismogenic zone. A comparison of the subduction complex along the adjacent profiles shows that changes in the structure of the incoming oceanic plate result in drastic changes in the structure of the subduction complex and its evolution. New crustal-scale geophysical models enable analysis of the geodynamic effects caused by the transition from oceanic subduction to continental shelf collision, the presence of anomalous relief on the oceanic plate, variations in the morphology of the incoming oceanic plate, and the presence of segment boundaries, and thus provide a solid basis for geodynamic interpretations of subduction processes and the associated geohazards.Der Sunda Kontinentalrand bildet die östliche Begrenzung des Indischen Ozeans entlang des indonesischen Archipels, wo die Subduktion der Indo-Australischen Platte unter Eurasien statt findet. Die Konvergenzrichtung variiert von orthogonal im östlichen Bereich des Kontinentalrandes zu einer schrägen Konvergenzzone im Bereich vor Sumatra. Die Variation von Subduktionsszenarien entlang dieses Kontinentalrandes ermöglicht das Studieren von unterschiedlichen Subduktions-Parametern und deren Auswirkung auf die Dynamik der Subduktionszone. Diese Dissertation integriert geophysikalische und tektonische Interpretationen von acht Seismik/Gravimetrie- und etlichen marinen Mehrkanalseismik - Profilen, die den Sunda Kontinentalrand erfassen. Die Daten umfassen Weitwinkelseismik, Mehrkanalseismik, und Gravimetriedaten. Die Modellierung einer refraktionsseismischen Tomographie inkludiert geologische Informationen aus Mehrkanalseismiken und ermöglicht die Erstellung von geophysikalischen Modellen entlang mehreren Lokalitäten des Subduktionskomplexes. Zusätzliche geophysikalische Daten aus Gravimetriemodellen und bathymetrische Datensätze erlauben eine Verbesserung des ursprünglichen geodynamischen Modells. Durch die exzellente Datenqualität konnten hochauflösende strukturelle Modelle des Subduktionskomplexes im Streichen und quer zum Sunda Kontinentalrand erstellt werden. Diese Modelle zeigen signifikante Unterschiede im strukturellen Aufbau der Kruste und des oberen Erdmantels innerhalb des Subduktionskomplexes. Die Studie zeigt eine Zunahme der kristallinen Krustenmächtigkeit in der Savu See, aufgrund der Annäherung des Australischen Schelfs in Richtung Tiefseerinne. Die ozeanische Kruste vor der Insel Lombok zeigt eine Mächtigkeit von 7 km, die durch starke tektonische Abschiebungsbrüche charakterisiert ist. Die Krustenstruktur des ozeanischen Roo Rise Plateaus wurde in dieser Arbeit erstmalig modelliert und weist eine Mächtigkeit von 15 km auf. Die Subduktion des Plateaus führt zur inhomogenen Deformation des Forearcs sowie zu einer komplexen Entwicklung des gesamten Subduktionsprozesses. Weiterhin wurden große Unterschiede in der Sedimentmächtigkeit auf der einfahrenden Platte beobachtet. Diese reichen von ~3 km in der Savu See bis nur geringfügiger Sedimentbedeckung in den Bereichen um Lombok und Ost-Java. Ein Vergleich verschiedener Krustenprofile vor Nord-Sumatra, entlang der Segmentgrenze der Erdbeben von 2004 und 2005, zeigt Unterschiede in der Struktur der ozeanischen Platte sowie Unterschiede in der Sedimentmächtigkeit in der Tiefseerinne. Dies resultiert in den unterschiedlich großen Akkretionskeilen und der Breite der seismogenen Zone. Ein Vergleich der unterschiedlichen Profile des Subduktionskomplexes zeigt, dass strukturelle Variationen der subduzierten ozeanischen Platte dramatische Veränderungen in der Struktur und Entwicklung des Subduktionskomplexes hervorbringen. Unsere krustenmaßstäblichen geophysikalischen Modelle zeigen die geodynamischen Effekte, die durch den Übergang von ozeanischer Subduktion zu kontinentaler Schelfkollision, das anomale Relief der ozeanischen Platte, Veränderungen in der Morphologie der subduzierten ozeanischen Kruste, sowie die Präsenz von Segmentgrenzen hervorgerufen werden. Diese Modelle bilden eine solide Basis für geodynamische Interpretationen von Subduktionsprozessen und den damit verbundenen Naturgefahren und Georisiken

Crustal domains in the Western Barents Sea

The crustal architecture of the Barents Sea is still enigmatic due to complex evolution during the Timanian and Caledonian orogeny events, further complicated by several rifting episodes. In this study we present the new results on the crustal structure of the Caledonian–Timanian transition zone in the western Barents. We extend the work of Aarseth et al. (2017), by utilizing the seismic tomography approach to model Vp, Vs and Vp/Vs ratio, combined with the reprocessed seismic reflection line, and further complemented with gravity modelling. Based on our models we document in 3-D the position of the Caledonian nappes in the western Barents Sea. We find that the Caledonian domain is characterized by high crustal reflectivity, caused by strong deformation and/or emplacement of mafic intrusions within the crystalline crust. The Timanian domain shows semi-transparent crust with little internal reflectivity, suggesting less deformation. We find, that the eastern branch of the earlier proposed Caledonian suture, cannot be associated with the Caledonian event, but can rather be a relict from the Timanian terrane assemblance, marking one of the crustal microblocks. This crustal block may have an E–W striking southern boundary, along which the Caledonian nappes were offset. A high-velocity/density crustal body, adjacent to the Caledonian–Timanian contact zone, is interpreted as a zone of metamorphosed rocks based on the comparison with global compilations. The orientation of this body correlates with regional gravity maxima zone. Two scenarios for the origin of the body are proposed: mafic emplacement during the Timanian assembly, or massive mafic intrusions associated with the Devonian extension.publishedVersio

Subduction zone segmentation along the Sunda Margin, Indonesia

The Sunda margin marks the eastern termination of the Indian Ocean along the Indonesian archipelago, where the subduction of the Indo-Australian plate below Eurasia is taking place. The convergence direction changes from orthogonal in the eastern part of the margin to a more oblique convergence towards the north-west offshore Sumatra. Along the margin, a wide range of subduction scenarios is observed, which makes this margin a perfect natural laboratory to study the role of various physical and geological parameters on subduction dynamics. This work builds on geophysical and tectonic interpretations of data acquired along eight seismic/gravity and several MCS marine profiles, distributed along the Sunda margin. The data include wide-angle seismic records, multi-channel seismic data, and gravity data. Data interpretation and modeling include a joint reflection/refraction seismic tomography used to constrain geophysical models across the subduction complex at several locations, further constrained by geological information from the multichannel seismic data. Additionally, other geophysical information including gravity modeling and bathymetry data is employed to refine geodynamic models based on seismic interpretations. Excellent data quality allows detailed structural models of the subduction complex to be constrained in several locations along the Sunda margin. The geophysical models obtained in this study show significant variations of the crustal and upper mantle structure of the subduction complex along-strike and across-strike of the margin. The study revealed an increased thickness of the crystalline crust in the Savu Sea, attributed to the approach of the Australian shelf to the trench. Offshore Lombok Island, the oceanic crust thickness is found to be 7 km, and heavily fractured by normal faults. The crustal structure of the Roo Rise oceanic plateau was modeled for the first time, revealing the crustal thickness of 15 km. Its subduction is causing inhomogeneous deformation of the forearc and complex evolution of the entire subduction processes. In addition, large variability in the amount of sediment on the incoming plate is found in adjacent sectors of the margin, ranging from ~3 km in the Savu Sea to almost absence in the Lombok and East Java areas. Offshore north Sumatra, comparison of the crustal-scale profiles located on different sides of the segment boundary between 2004 and 2005 earthquakes rupture areas, showed variability of the structure of the oceanic plate and the amount of sediment present at the trench, resulting in the principally different sizes of the accretionary prisms and the width of the seismogenic zone. A comparison of the subduction complex along the adjacent profiles shows that changes in the structure of the incoming oceanic plate result in drastic changes in the structure of the subduction complex and its evolution. New crustal-scale geophysical models enable analysis of the geodynamic effects caused by the transition from oceanic subduction to continental shelf collision, the presence of anomalous relief on the oceanic plate, variations in the morphology of the incoming oceanic plate, and the presence of segment boundaries, and thus provide a solid basis for geodynamic interpretations of subduction processes and the associated geohazards