Seismic anisotropy and microseismicity: from crustal formation to subduction termination

• Main Author: Bacon, Conor
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Cambridge 2022
• Subjects: seismic anisotropy; seismology; tectonics; volcano seismology; microseismicity; Iceland; Southeast Asia; Mid-Atlantic Ridge
• Online Access: https://www.repository.cam.ac.uk/handle/1810/334765; https://doi.org/10.17863/CAM.82196

The plate tectonic cycle is fundamental to our dynamic Earth, encompassing the formation and evolution of new lithosphere at divergent, mid-ocean ridges, all the way to its eventual return to, and re-equilibration with, the mantle in subduction zones. I investigate the structure of the crust and upper mantle of the Earth in two regions that represent different endmembers in this cycle through the analysis of microseismicity and seismic anisotropy, seeking to learn more about the stresses in these environments and how they are manifest in the structure of the subsurface. In Iceland, new oceanic crust is accreted episodically within the volcanic rift zones that delineate the subaerial portion of the Mid-Atlantic Ridge, the divergent margin between the North American and the Eurasian plates. Northern Borneo, conversely, exhibits the tectonic signatures of not one but two terminated subduction zones, where oceanic lithosphere was once being actively recycled into the Earth’s mantle. As part of this work, I have helped to deploy and service two passive seismic experiments—the Cambridge Volcano Seismology network in Iceland and the northern Borneo Orogeny Seismic Survey (nBOSS) network in Sabah—from which I have derived my results. Seismic anisotropy is manifest on a vast range of scales, from swathes of the crust and mantle, all the way down to the scale of single mineral crystals. It has the potential to inform on the dynamic state of the mantle, the structural fabric of fault zones, layering in sedimentary basins, and the distribution of partial melt in the subsurface, to name a few applications. Here, I seek new insights into the stress state and structure of nascent oceanic crust as it is accreted at a mid-ocean ridge, and the volcanic systems found therein, in Iceland. I also look to piece together the interplay between past tectonic events, subduction termination, and the present-day state of the mantle in northern Borneo. In the first part of my dissertation, I apply shear-wave splitting analysis to a ...