Deformation styles in accretionary wedges at 3D seismic to microstructural scales

Azevedo, Marco ORCID: https://orcid.org/0000-0003-3896-2013 2025. Deformation styles in accretionary wedges at 3D seismic to microstructural scales. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis uses high quality 3D post-stack migrated seismic data from the NanTroSEIZE transect, which images the outer wedge of the NAP (from the MSFZ to the deformation front) in SE Japan. It investigates the strain distribution and deformation styles experienced in the NAP at different scales of observation in order to bring new insights on how deformation within accretionary prisms is influenced by the roughness of their respective incoming subducting plate. In parallel, it investigates how deformation of accretionary prisms adapts and changes to allow the inclusion of asperities within the subduction channel. The first part of the thesis focuses on the holistic mapping and characterisation of the structural framework of the NAP, gathered from a detailed interpretation of the seismic data. The interpreted seismic data show a much higher structural complexity than previously described by other authors - a wide range of new and previously found structures evolve together in the wider context of the accretionary prism but also changed their nature throughout the different evolutionary stages of the NAP, on a smaller scale. Structures identified in the outer wedge of the NAP and their relationships point towards an alternation between deformation styles during distinct seismic and aseismic periods. The overall structural framework provides evidence for a complex transpressional deformation regime in the NAP. Following up on the first part, this thesis focuses on studying the roughness of the subducting Philippines Plate and its influence in the overlying NAP. This section provides a new detailed 3D mapping of the subducting PSP and estimates horizontal shortening across the outer wedge of the NAP within the NanTroSEIZE transect, an approach never attempted for the study area. The subducting oceanic plate and overlying décollement are significantly rougher than previously described and includes important features such as a 1.5 km high seamount and elongated major ridges with heights of up to 950 m. These deep structures influence the erosive – accretional behaviour of the NAP with important outcomes in the shortening and deformation experienced by the overlying accreted sediment. The last part of this thesis explains the results of two sandbox analogue experiments, completed to better understand the interference of individual and combined asperities in the overall strain distribution and deformation within accretionary prisms. The sandbox experiments simulated a simplified, but still similar in the larger scale, asperity/roughness context of the study area, providing further evidence to understand the behaviour and evolutionary stages of the low-velocity unit within the NAP and its influence in the strain distribution along this latter. The new data provided new insights on how accretionary prisms adapt and accommodate deformation in their interior when including asperities, such as seamounts, into the subduction zone to allow the subsequent, and continuous, growth of the accretionary prism’s front. Main results of this thesis are as follows: 1) accretionary prisms document complex structural frameworks where deformation styles may change at different scales as a response to factors such as roughness of the subducting oceanic plate, characteristics of the asperities being subducted, and kinematic relationships between pre-existing and newly-formed structures within the accreted sediments; 2) the roughness of the subducting oceanic plate can be one of the most significant factors in the deformation and structural framework of accretionary prisms, having strong implications on their shortening, fluid migration, pore–fluid pressure distribution, and stress distribution; 3) seismic attributes are a key tool to help imaging structural features and the roughness of accretionary prisms and subducting oceanic plates, all this in order to assist the development of tectonic models in evolving subduction zones; 4) sandbox analogue experiments are key tools for both a quantitative and qualitative analysis of active deformation in accretionary prisms. This allows for a more complete interpretation of processes that lead to modern and future seismic events. i

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Date of First Compliant Deposit:	10 February 2026
Last Modified:	10 Feb 2026 16:37
URI:	https://orca.cardiff.ac.uk/id/eprint/184486

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