Spiegel, Cornelia
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Spiegel, Cornelia
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Spiegel, Cornelia
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Item type:Publication, Palaeoenvironment of the Weddell Sea and Amundsen Sea - eastern Ross Sea basins, Antarctica: Insights from comprehensive seismostratigraphic analysis(2014-05-19); ; ; In combining existing multichannel seismic reflection seismic data in the Weddell Sea deep-sea basin, with existing and newly acquired data in the Amundsen Sea and Ross Sea basins, previously unknown sequences representing the pre-glacial to glacial palaeoenvironmental development of the West Antarctic Margin were identified. Pre-glacial sediment deposition centres seemed to have changed near or after the Eocene/Oligocene boundary (~34 Ma) when the first major ice sheets advanced to and across the shelf. The middle Miocene (~16 Ma) full glacial sequences indicate a new depocentre formed North of the Amundsen Sea Embayment. Smaller depocentres in the Bellingshausen Sea and Antarctic Peninsula basins, shifted eastward. Calculations indicate ~4.6 km (~10.2 million km3) of West Antarctica's landmass were eroded since the Late Cretaceous and deposited in the Southern Pacific. This has implications for the palaeotopographic and palaeobathymetric reconstructions, and ice sheet climate models.doctoral thesis401 135 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Early rifting of Gondwana : Conclusions from the crustal structure of the Falkland Plateau(2018-06-08); ; ; During the Jurassic, the Falkland Plateau was part of Gondwana and occupied a position between the African, South American and Antarctic plates. Several contrasting kinematic models exist for the breakup of Gondwana because of the currently unknown crustal structure of the plateau. In this thesis, I present the results of recently acquired wide-angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions. The new data show that the Falkland Plateau Basin consists of up to 20 km thick oceanic crust. Maurice Ewing Bank is an up to 29 km thick continental fragment, which was directly attached to the Falkland Islands block during the Jurassic. Rifting between the Falkland Islands and the Maurice Ewing Bank started at 178 Ma and ceased at around 154 Ma.doctoral thesis332 160 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Diversity of the crustal structure in the northern Baffin Bay and the southern Nares Strait(2015-11-09); ; ; The crustal structure of the Baffin Bay and southern Nares Strait is studied based on three refraction seismic profiles and corresponding reflection- and gravity datasets. P wave velocity, density and geological models derived from raytracing, modeling and interpretation of these datasets are presented. My findings confirm that a 3.5-7 km thick oceanic crust is present in central northern Baffin Bay, which is covered by up to 6.5 km thick sediments. The partly small thickness of the crust and the underlying serpentinized upper mantle are indications for slow to ultraslow spreading rates during the formation of the oceanic crust. An up to 80 km wide continent-ocean transition zone separates the oceanic crust from stretched and rifted continental crust. The continent-ocean transition in the Melville Bay area has been affected by intrusive and extrusive magmatism, while the transition between continental and oceanic crust in the southern Nares Strait does not show any signs of magmatic activity. Therefore, the margin in the southern Nares Strait can be classified as non-volcanic rifted margin. The margin west of the Melville Bay area can be described as rifted margins with decreasing influence of magmatism towards the north, since clear indications for a classification as non-volcanic or volcanic margins are missing. The up to three-layered crystalline continental crust has a maximum thickness of 30 km and is partly covered by sediments. Steep faults and deep basins characterize the Melville Bay area. Two parallel extending profiles in northern and southern Melville Bay revealed changes in the crustal geometry as well as in basin infill and therefore indicate differences in the genesis of both regions. A comparison of my results on the extent of crustal types in the research area with previous studies shows that the extent of the continent-ocean transition and partly also the eastern extent of oceanic crust were underestimated in the northeastern Baffin Bay.doctoral thesis325 149 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Deformation Microstructures : Antarctic Ice from EPICA Dronning Maud Land and Artificial Creep TestIce(2007-12-20); ; ; The primary objective of this thesis is the investigation of microstructures obtained from samples from the EPICA Dronning Maud Land ice core from Antarctica. The goal is to gain understanding of deformation processes an deformation-related recrystallization mechanisms using these structures. The structures are visualized with the new microstructure mapping method using the preferred sublimation along defect regions in the crystal. This method enables observation in high resolution as well as overview over a significant sample volume. In order to provide unambiguous proof of their deformational origin and to offer interpretation and characterization, experimental reproduction of the microstructural features are performed using creep tests.Subgrain boundaries and grain-boundary morphology are identified as the most direct effects of deformation and recrystallization processes, which are still easily observable. They can be used additionally to the conventional parameters (grain size, crystal-orientation distribution) to determine these mechanisms. Different sbugrain-boundary types observed in experimentally deformed samples as well as in natural ice indicate several formation processes.Results obtained from this new and novel data suggest a profound reconsideration of the classical tripartition of recrystallization regimes described in the literature in ice sheets. Instead, dynamic recrystallization in two of its forms (rotation recrystallization and strain-induced migration recrystallization) dominates the microstructure evolution in all depth regions of the EDML ice core.Results of systematic microstructure analysis of creep-test samples demonstrate the correlation of grain-substructure evolution and strain hardening during primary creep. Subgrain boundaries and dislocation walls acting as obstacles for dislocation motion was identified thereby as a main process of strain hardening in ice.The comparison of results from experimentally and naturally deformed ice enables a specification of the well-known difference in flow behaviour for low- and high-stress regimes: in low-stress regimes dislocation-density decreasing processes (recovery and dynamic recrystallization), which restore the properties of the material, play a significantly more important role than in high-stress regimes.doctoral thesis581 131 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, The mechanics of an accretionary wedge affected by a layer with varying rheology, viscosity, and thickness(2014-02-13); ; ; An accretionary wedge evolves wide extended, flat and with a characteristic thrust system, if the wedge décollement is located in a viscous evaporite. The deformation processes of such wedges are complex and until now not fully understood. Therefore, in this present work 2D numerical sandbox experiments are developed in which an incoming sediment sequence is accreted against a fixed vertical wall due to a moving box bottom. The experiments contain a brittle incoming sequence in which either a brittle Mohr Coulomb layer or a viscoelastic-plastic layer (Burger s rheology) is embedded. The first study focuses on the differences which occur in the mechanics of two wedge experiments if either a weak viscoelastic-plastic or a low-friction brittle layer is embedded between two high friction brittle layers. To find out the viscosity under which the wedge deforms most likely as it would grow on top of an evaporite, the second study analyzes the effects of different viscous materials on wedge mechanics. Finally, the calibrated model is used to study the effects of different incoming sediment thicknesses on the accretion process.doctoral thesis265 209 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, The Central Mozambique continental margin : Its tectonic evolution as the centrepiece of the initial Gondwana break-up(2017-06-09); ; ; A consolidated knowledge of the formation and dispersal of the former supercontinents reveals important evidence for the eartha s climate and biosphere in the past and contribute to the prediction of their future evolution. Nowadays, a main objective is the investigation of the initial break-up of the continental assembly of Gondwana that serves as a constraint for its subsequent dispersal and the evolution of all oceans and seas in the southern hemisphere. Evidence of the early rifting stages are expected at the margins of Southeast Africa and East Antarctica, whereas the latter one is difficult to access, due to its remote position and ice coverage. To understand the driving forces and the chronology of the break-up and of the massive volcanism additional detailed knowledge of the crustal setting along the margins of Southeast Africa is required. Therefor, a new geophysical dataset was acquired with the RV Sonne in the northern Mozambique Basin at the beginning of year 2014. This comprises a deep seismic sounding profile across a so-far unknown structural high, the Beira High. Additional gravity and magnetic data were systematically recorded across the entire northern Mozambique Basin. Based on velocity, amplitude, density and magnetic modelling, a geological model of the continental margin of Central Mozambique was prepared. A new compilation of all available magnetic data in the Mozambique Basin reveals information about the age of the sea floor, which serves as constraint for the reconstruction of the initial Gondwana break-up. The study depicts a continental origin of up to 23 km thick and partly highly intruded crust at Beira High. In the adjacent coastal areas of the south-western part of Central Mozambique, 7 km thin crust is observed, which is covered by more than 11 km thick sediments and implies the continuation of the continent-ocean transition towards onshore Mozambique. This is in clear contrast to the narrow transition observed in the north-eastern part of the margin and reveals a clear asymmetric crustal setting as supposed for the conjugate margin in the Riiser-Larsen Sea in Antarctica and consequently suggests a complex break-up scenario. The presence of a pronounced high-velocity lower crustal body is interpreted as magmatic material, which underplates the crust and extends about 200 km from the Central Mozambican margin towards the Mozambique Basin and testifies for the massive volcanism during the break-up. The distribution of further volcanics along the entire margin clearly depicts the continuation of the north-eastern branch of the Karoo large igneous province and are mainly emplaced between 177-157 Ma. The magmatism in Southeast Africa seemed to be continuous throughout the initial break-up, which points to the presence of either a mantle plume or a thermal anomaly as source of the giant magmatism. An additional late stage of rift-volcanism mainly affected the margin of Dronning Maud Land and causes the difference in the magnetic signature of the conjugate margins. The tracing of continuous fractures throughout the Africa-Antarctica Corridor leads to the reconstruction of a tight Gondwana fit prior rifting, which reveals several geological links between the plates. A main structure of the East African-Antarctic Orogen extends from the Namama Shear Zone in Central Mozambique across the Orvin Shear Zone towards the Forster magnetic anomaly in Dronning Maud Land. During the initial Gondwana break-up at 182 Ma, Beira High started to separate from West Gondwana along this suture until it demerged as well from East Gondwana by a rift jump. The investigation of further partly unknown tectonic structures along the western and southern coast of Mozambique revealed a possible oceanic origin of the southern part of the Mozambique Coastal Plains, due to similarities of the magnetic signature to the oceanic crust south of Beira High as well as the tentative identification of magnetic spreading anomalies. The subsequent emplaced Mozambique Ridge moved southwards as part of a micro plate during an additional active spreading centre in the Northern Natal Valley. The resulting reconstruction of the initial Gondwana break-up in the Africa-Antarctica Corridor accounts for all present-day available geological, geophysical and geodynamic constraints and might serve as a basis for the investigation of the subsequent dispersal of Gondwana.doctoral thesis486 196