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Triggers, mechanisms and frequency of slope instability processes on mid-Norway’s continental margin

Jing, Song 2022. Triggers, mechanisms and frequency of slope instability processes on mid-Norway’s continental margin. PhD Thesis, Cardiff University.
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This thesis uses high-quality 3D seismic data from the mid-Norwegian margin to investigate the triggers, mechanisms and frequency of slope instability in the poorly investigated northwestern flank of the Storegga Slide, near the Modgunn Arch, a regional anticline formed by Cenozoic tectonic inversion. Chapters 4, 5 and 6 in this thesis focus on specific aspects of slope failure, from tectonic processes such as uplift and faulting to instability features such as slide blocks and sediment creeping zones. The ultimate aim is to improve the present understanding about distinct aspects of slope failure, investigating at the same time the geological conditions that contribute to recurring submarine landslides. The control of extensional faults and older landslides, or mass-transport complexes (MTCs), on long-term slope instability is also discussed in this thesis, together with other potential preconditioning factors. Chapter 4 uses a comprehensive seismic and borehole dataset to investigate how fault movement, growth and local uplift were able to precondition slope instability around the South Modgunn Arch. In addition, features leading to fluid migration and accumulation near the sea floor were identified and related to local tectonism. Seismic and borehole data suggest that hydrothermal vents associated with the intrusion of sills have generated a family of radial faults due to differential compaction above the latter intrusions. In parallel, the interpreted data show that a NE-SW oriented compressional stress field generated a family of dense polygonal faults during a phase of uplift of the South Modgunn Arch. A second family of polygonal faults was formed after tectonic uplift ceased, overlapping and partly linking with the first family. Chapter 5 focuses on characterising the initiation, development and modern slope instability conditions around the South Modgunn Arch. In detail, the recognition of fluid pipes and evidence of fluid migration through faults, together with the sudden deposition of marine deposits on the southern limb of the Modgunn Arch, were considered to be key precondition factors for the long-term slope instability interpreted on seismic data. An interesting aspect verified in this chapter is that density reversal contributed to the evacuation of material on the continental slope, with several evacuation structures being now observed around the Storegga Slide. Furthermore, long-term slope instability was likely preconditioned by multiple weak layers and episodic slope undercutting because seafloor fractures and scarps were found above slide blocks, as well as interactions between two overlapping intervals with slide blocks. The influence of weak layers on the behaviour of slide blocks along gentle continental slopes is addressed in Chapter 6. Detailed seismic interpretation and mapping allowed the collection of sinuosity data for multiple lineations on the sea floor, and to gather geometric information about two units with slide blocks. Both the seafloor lineations and the blocks were interpreted as evidence for successive (recurrent) episodes of slope instability, as these lineations and blocks were deformed after they were first formed. The overlapping of two groups of lineations (with distinct sinuosity values) on the modern sea floor suggests important sediment creeping. A numerical simulation (Computational Fluid Dynamics) approach has highlighted the significance of weak layers, pre-existing fractures and MTCs as features capable of preconditioning regions of the continental slope to long-term instability. Consequently, scenarios were modelled to explain the development of both creeping MTCs and sliding blocks around the South Modgunn Arch. In conclusion, this thesis explores the complex processes leading to long-term slope instability on continental margins. The case study in this thesis, the northwest flank of the Storegga Slide, is a key area to investigate the precondition factors - and triggers – of such long-term instability. This thesis ends by extrapolating its findings - gathered in a thus far poorly studied region of the Storegga Slide - to similar geological environments around the world. Such an extrapolation is important to predict future geological hazards on continental margins.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Date of First Compliant Deposit: 28 April 2022
Last Modified: 28 Apr 2022 10:58

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