@phdthesis{Bredow2017,
  author    = {Bredow, Eva},
  title     = {Geodynamic models of plume-ridge interaction},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-411732},
  school      = {Universit{\"a}t Potsdam},
  pages     = {104},
  year      = {2017},
  abstract  = {According to the classical plume hypothesis, mantle plumes are localized upwellings of hot, buoyant material in the Earth's mantle. They have a typical mushroom shape, consisting of a large plume head, which is associated with the formation of voluminous flood basalts (a Large Igneous Province) and a narrow plume tail, which generates a linear, age-progressive chain of volcanic edifices (a hotspot track) as the tectonic plate migrates over the relatively stationary plume. Both plume heads and tails reshape large areas of the Earth's surface over many tens of millions of years. However, not every plume has left an exemplary record that supports the classical hypothesis. The main objective of this thesis is therefore to study how specific hotspots have created the crustal thickness pattern attributed to their volcanic activities. Using regional geodynamic models, the main chapters of this thesis address the challenge of deciphering the three individual (and increasingly complex) Réunion, Iceland, and Kerguelen hotspot histories, especially focussing on the interactions between the respective plume and nearby spreading ridges. For this purpose, the mantle convection code ASPECT is used to set up three-dimensional numerical models, which consider the specific local surroundings of each plume by prescribing time-dependent boundary conditions for temperature and mantle flow. Combining reconstructed plate boundaries and plate motions, large-scale global flow velocities and an inhomogeneous lithosphere thickness distribution together with a dehydration rheology represents a novel setup for regional convection models. The model results show the crustal thickness pattern produced by the plume, which is compared to present-day topographic structures, crustal thickness estimates and age determinations of volcanic provinces associated with hotspot activity. Altogether, the model results agree well with surface observations. Moreover, the dynamic development of the plumes in the models provide explanations for the generation of smaller, yet characteristic volcanic features that were previously unexplained. Considering the present-day state of a model as a prediction for the current temperature distribution in the mantle, it cannot only be compared to observations on the surface, but also to structures in the Earth's interior as imaged by seismic tomography. More precisely, in the case of the Réunion hotspot, the model demonstrates how the distinctive gap between the Maldives and Chagos is generated due to the combination of the ridge geometry and plume-ridge interaction. Further, the Rodrigues Ridge is formed as the surface expression of a long-distance sublithospheric flow channel between the upwelling plume and the closest ridge segment, confirming the long-standing hypothesis of Morgan (1978) for the first time in a dynamic context. The Réunion plume has been studied in connection with the seismological RHUM-RUM project, which has recently provided new seismic tomography images that yield an excellent match with the geodynamic model. Regarding the Iceland plume, the numerical model shows how plume material may have accumulated in an east-west trending corridor of thin lithosphere across Greenland and resulted in simultaneous melt generation west and east of Greenland. This provides an explanation for the extremely widespread volcanic material attributed to magma production of the Iceland hotspot and demonstrates that the model setup is also able to explain more complicated hotspot histories. The Iceland model results also agree well with newly derived seismic tomographic images. The Kerguelen hotspot has an extremely complex history and previous studies concluded that the plume might be dismembered or influenced by solitary waves in its conduit to produce the reconstructed variable melt production rate. The geodynamic model, however, shows that a constant plume influx can result in a variable magma production rate if the plume interacts with nearby mid-ocean ridges. Moreover, the Ninetyeast Ridge in the model is created by on-ridge activities, while the Kerguelen plume was located beneath the Australian plate. This is also a contrast to earlier studies, which described the Ninetyeast Ridge as the result of the Indian plate passing over the plume. Furthermore, the Amsterdam-Saint Paul Plateau in the model is the result of plume material flowing from the upwelling toward the Southeast Indian Ridge, whereas previous geochemical studies attributed that volcanic province to a separate deep plume. In summary, the three case studies presented in this thesis consistently highlight the importance of plume-ridge interaction in order to reconstruct the overall volcanic hotspot record as well as specific smaller features attributed to a certain hotspot. They also demonstrate that it is not necessary to attribute highly complicated properties to a specific plume in order to account for complex observations. Thus, this thesis contributes to the general understanding of plume dynamics and extends the very specific knowledge about the Réunion, Iceland, and Kerguelen mantle plumes.},
  language  = {en}
}
@phdthesis{Dannberg2016,
  author    = {Dannberg, Juliane},
  title     = {Dynamics of mantle plumes},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-91024},
  school      = {Universit{\"a}t Potsdam},
  pages     = {162},
  year      = {2016},
  abstract  = {Mantle plumes are a link between different scales in the Earth's mantle: They are an important part of large-scale mantle convection, transporting material and heat from the core-mantle boundary to the surface, but also affect processes on a smaller scale, such as melt generation and transport and surface magmatism. When they reach the base of the lithosphere, they cause massive magmatism associated with the generation of large igneous provinces, and they can be related to mass extinction events (Wignall, 2001) and continental breakup (White and McKenzie, 1989). Thus, mantle plumes have been the subject of many previous numerical modelling studies (e.g. Farnetani and Richards, 1995; d'Acremont et al., 2003; Lin and van Keken, 2005; Sobolev et al., 2011; Ballmer et al., 2013). However, complex mechanisms, such as the development and implications of chemical heterogeneities in plumes, their interaction with mid-ocean ridges and global mantle flow, and melt ascent from the source region to the surface are still not very well understood; and disagreements between observations and the predictions of classical plume models have led to a challenge of the plume concept in general (Czamanske et al., 1998; Anderson, 2000; Foulger, 2011). Hence, there is a need for more sophisticated models that can explain the underlying physics, assess which properties and processes are important, explain how they cause the observations visible at the Earth's surface and provide a link between the different scales. In this work, integrated plume models are developed that investigate the effect of dense recycled oceanic crust on the development of mantle plumes, plume-ridge interaction under the influence of global mantle flow and melting and melt migration in form of two-phase flow. The presented analysis of these models leads to a new, updated picture of mantle plumes: Models considering a realistic depth-dependent density of recycled oceanic crust and peridotitic mantle material show that plumes with excess temperatures of up to 300 K can transport up to 15\% of recycled oceanic crust through the whole mantle. However, due to the high density of recycled crust, plumes can only advance to the base of the lithosphere directly if they have high excess temperatures, high plume volumes and the lowermost mantle is subadiabatic, or plumes rise from the top or edges of thermo-chemical piles. They might only cause minor surface uplift, and instead of the classical head-tail structure, these low-buoyancy plumes are predicted to be broad features in the lower mantle with much less pronounced plume heads. They can form a variety of shapes and regimes, including primary plumes directly advancing to the base of the lithosphere, stagnating plumes, secondary plumes rising from the core-mantle boundary or a pool of eclogitic material in the upper mantle and failing plumes. In the upper mantle, plumes are tilted and deflected by global mantle flow, and the shape, size and stability of the melting region is influenced by the distance from nearby plate boundaries, the speed of the overlying plate and the movement of the plume tail arriving from the lower mantle. Furthermore, the structure of the lithosphere controls where hot material is accumulated and melt is generated. In addition to melting in the plume tail at the plume arrival position, hot plume material flows upwards towards opening rifts, towards mid-ocean ridges and towards other regions of thinner lithosphere, where it produces additional melt due to decompression. This leads to the generation of either broad ridges of thickened magmatic crust or the separation into multiple thinner lines of sea mount chains at the surface. Once melt is generated within the plume, it influences its dynamics, lowering the viscosity and density, and while it rises the melt volume is increased up to 20\% due to decompression. Melt has the tendency to accumulate at the top of the plume head, forming diapirs and initiating small-scale convection when the plume reaches the base of the lithosphere. Together with the introduced unstable, high-density material produced by freezing of melt, this provides an efficient mechanism to thin the lithosphere above plume heads. In summary, this thesis shows that mantle plumes are more complex than previously considered, and linking the scales and coupling the physics of different processes occurring in mantle plumes can provide insights into how mantle plumes are influenced by chemical heterogeneities, interact with the lithosphere and global mantle flow, and are affected by melting and melt migration. Including these complexities in geodynamic models shows that plumes can also have broad plume tails, might produce only negligible surface uplift, can generate one or several volcanic island chains in interaction with a mid-ocean ridge, and can magmatically thin the lithosphere.},
  language  = {en}
}