Michael J. Steventon, Christopher Aiden-Lee Jackson , David Hodgson, Howard D. Johnson

Strain style, magnitude, and distribution within mass-transport complexes (MTCs) is important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally-driven passive margins have been shown to approximately balance between updip extensional and downdip compressional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1500 mbsf) and well-imaged MTC, offshore Uruguay using a high-resolution (12.5 m vertical and 15x12.5 m horizontal resolution) 3D seismic-reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic-attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub-orthogonal shear zones, and fold-thrust systems within the basal shear zone beneath rafted-blocks. These features suggest multiple phases of flow and transport directions during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and compressional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra-MTC rafted-blocks due to kinematic interaction between these features and the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is excluded, a strain difference between extension (1.6-1.9 km) and contraction (6.7-7.3 km) is calculated. We attribute this to a combination of distributed, sub-seismic, ‘cryptic’ strain, likely related to de-watering, grain-scale deformation, and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits.