Grevemeyer, Ingo
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Grevemeyer, Ingo
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Grevemeyer, Ingo
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Item-typ:Veröffentlichung, Mud volcanic episodicity : subduction zone water budget, long-term monitoring and laboratory case studies(2017-07-29); ; ; Mud volcanism is a widespreaded phenomenon that has been reported both on land and on the seafloor. Despite their worldwide occurrence, more than 60% of mud volcanoes are distributed in proximity of subduction zones. In these tectonically active areas, the presence of overpressured fluids and sediments released through the accretionary prism fuels the formation of such piercement structures. Hence, the mud volcanoes act as windows to depth, making available a wealth of material that otherwise could not be sampled by scientific drilling due to technical limitations. The nature of mud volcano ejecta is various since a lot of different processes interact during the ascent to the seafloor. Nevertheless, studies on mud volcanoic products contributed to elucidate the dynamics of the so-called subduction factory shedding some light on the fluid cycling in subduction zones. At the same time, in order to understand the role of mud volcanoes as fluid expulsion structures, their formation as well as evolution has to be constrained. The main mechanisms, which have been hold responsible for that, are either diapiric ascent of a buoyant body of fluidized mud or hydrofracturing in areas of structural weekness, setting the path to mud intrusions. Once they break trough the seafloor, mud volcanoes are subject to numerous internal and external triggers, which hamper or enhance their activity. Earthquakes, as well as episodic inflation and deflation of an underlying mud reservoir might play a major, concomitant role in the release of accumulated pressure at depth. This role as pressure release valves for deeper sediments makes mud volcanoes complex, dynamic environments governed from a multitude of different processes. The aim of this dissertation is to enhance the understanding of mud volcanism by summarizing the current knowledge on the matter with new insights from case studies, i) explaining the role of mud volcanoes in the Nankai subduction zone offshore Japan, ii) focussing on factors that govern mud volcanic activity at the Athina MV, in the Eastern Mediterranean and finally iii) answering the open questions regarding origin and evolution of these sedimentary structures through laboratory analog experiments. Thirteen mud volcanoes have been reported in the Kumano Basin, landwards of the Nankai Trough. Their role in the water budget of the Nankai subduction zone have been evaluated creating a model of water circulation in the upper and lower plates using constraints from IODP holes of the NanTroSEIZE project, in situ geochemical and heat flow data and water isotopes measurements. Clay minerals have been identified as an important origin of water release in the region whereas mud volcanoes are deemed only minimally responsible for water expulsion, which seems to be diffusively emitted through the whole basin. Admittedly, paroxysmal phases in mud volcanoes activity could not be taken into account in this study. However this aspect of mud volcanism have been tackled with the deployment of a multiparametric observatory on top of the Athina MV, south of Turkey. More than two years of pore pressure, tilt and temperature data revealed an unprecedented connection (in the submarine environment) between variations of those parameters and earthquake occurrence. Such external triggers of mud volcano activity have been linked to magnitude/distance ratio, ground motion and local factors like geology and tectonic structures. Moreover, signals not related to external triggers gave hints on the internal dynamics of the Athina system. Finally, to assess the existing concepts of formation and development of mud volcanoes, a set of analogue experiments on mud breccia samples have been conducted. The unicity of these tests lies in the material used to model hydraulic failure: sediment samples from three different submarine mud volcanoes (Japan, Costa Rica and Mediterranean Sea). The experiments, which showed the characteristics of the plumbing system and the surficial structures forming before and after hydraulic failure of the samples, confirmed hydrofracturing as formation mechanism responsible for mud volcano formation and demonstrated how episodicity in and around mud volcanoes can be the result of inflation and deflation of a mud reservoir at depth.Dissertation442 320 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Magmatic versus amagmatic : a study of local seismicity and lithospheric structure at two contrasting Southwest Indian Ridge segments(2017-11-06); ; ; New sea oor is constantly being created at the global spanning system of mid-ocean ridges (MOR). Over a wide range of spreading rates is the produced oceanic crust of surprisingly uniform thickness and composition. An exception are ridges with spreading rates slower than 15-20 mm yra 1, so-called ultraslow spreading MORs, at which the crustal thickness and composition drastically change. A totally different mode of sea oor spreading occurs, with discontinuous crustal accretion along-axis. Entire sections of the ridge axis lack an igneous crust and the neighbouring volcanoes receive more melt than the regional average. The Gakkel Ridge in the ice-covered Arctic Ocean and the Southwest Indian Ridge (SWIR) in the stormy Southern Ocean are the main representatives of the ultraslow spreading ridges. At present, the processes of lithosphere formation at ultraslow spreading rates are still poorly understood. The investigation of microearthquakes, with networks of ocean bottom seismometers (OBS) or hydrophones, has signi cantly progressed our understanding of the lithosphere accretion processes at faster spreading ridges. Until recently, the rough environmental conditions prevented the long-term deployment of OBSs at the main representatives of ultraslow ridges. This thesis makes use of the rst one-year long records from two OBS networks with comparable extent that had been placed at different sections of the SWIR from 2012 to 2013. The chosen sites are characterized by contrasting crustal thickness, lithology and morphology. The Oblique Supersegment network was deployed near 13Adegree30a E where peridotites are the dominant sea oor lithology and a typical igneous crust is absent. The Segment 8 network was deployed near 65Adegree30a E around the volcanic center of the SWIR Segment 8 where the crust is locally thickened and the sea oor consists exclusively of basalts. I picked and located the microearthquakes for the Oblique Supersegment network and compiled an 11-month long catalogue that contains 2000 microearthquakes. This catalogue was subsequently used to calculate so-called yield-strength envelopes of the lithosphere that provide constraints on the rheological strength and the geothermal gradient below the axial valley. I further used an existing microearthquake catalogue of the Segment 8 network for a local earthquake tomography to image the 3-dimensional structure of seismic velocities below the SWIR Segment 8. The combined seismicity catalogues from both networks showed systematic undulations in the maximum depth of faulting (brittle-ductile transition) along the SWIR axis; with deeper hypocenters below amagmatic ridge segments and shallower hypocenters below magmatic segments. The brittle- ductile transition is mainly temperature related. Thus, its position provides insight into the sub- axial thermal structure. The undulating hypocenter depths imply parallel undulations of the deeper lithosphere-asthenosphere boundary, under which molten material is constantly present. Previous studies postulated such a topography of the lithosphere-asthenosphere boundary that guides the buoyantly owing mantle melts from amagmatic segments towards magmatic centers and thereby maintains the pattern of uneven melt supply along the SWIR. The combined microseismicity results from both OBS networks strongly support this hypothesis and provide the missing geophysical proof of this concept. An extensive aseismic region extending to 15 km depth was found in the upper lithosphere at the Oblique Supersegment, where peridotite is the dominant lithology. The aseismic behaviour is best explained by weakening of the lithosphere by serpentinization, likely focused in aseismic shear zones that constitute the rift valley bounding faults. Geochemical sampling revealed enhanced diffusive uxes near the scarp of a bounding fault and increased methane concentrations in the valley waters that likely stem from abiotic, serpentinization-related processes. The local earthquake tomography of the SWIR Segment 8 network showed a prominent low-velocity anomaly below the segmenta s volcanic center that indicates the presence of partial melts. In addition, preceding teleseismic activity and recorded microearthquake swarms with simultaneous intrusion tremor pointed to an ongoing spreading event. It turned out that this magmatic spreading episode likely lasted already over a decade and thereby vastly exceeds the duration of all previously documented magmatic spreading episodes at active MORs. In summary, this thesis provides for the rst time a detailed insight into the microearthquake activity at two SWIR segments. The analysis and interpretation of the data presented in this thesis signi cantly contribute to a better understanding of the lithospheric structure and the sea oor accretion processes at ultraslow spreading MORs.Dissertation438 185