Greater depth samples show the same sediment strength variations across the Nankai forearc offshore as originally found at shallow depth.

Stipp, Michael, Leiss, B. and Behrmann, Jan H. (2016) Greater depth samples show the same sediment strength variations across the Nankai forearc offshore as originally found at shallow depth. [Poster] In: IODP/ICDP Kolloquium 2016. , 14.-16.03 2016, Heidelberg, Germany .

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Abstract

For assessing the tsunamigenic potential, the mechanical strength, composition and fabric of forearc slope and accreted sediments from the Nankai Trough offshore SW-Japan have been investigated (NanTroSEIZE; IODP expeditions 315, 316, 333). Triaxial testing of shallow whole round samples (maximum depth of 130 mbsf) at confining pressures of 0.4-1.0 MPa, room temperature, strain rates of approx. 10-3 to 10-6s-1, and up to 64% axial strain (Stipp et al., 2013) revealed mechanically and structurally weak samples from the upper and middle forearc slope of the accretionary prism and strong samples from the accretionary prism toe (Stipp et al., 2013). In order to constrain these results, three additional experiments on samples from greater depth (211-221 mbsf) at confining pressures of 3.0-8.0 MPa, room temperature, strain rates of approx. 10-5s-1, and up to 54% axial strain were carried out. For these tests a digitally controlled servo-hydraulic triaxial appartus with a maximum load of 100 kN was used. Correspondingly to the shallow samples, these experiments show a deviatoric peak stress after only a few percent strain (<10%) and a continuous stress decrease after this maximum combined with a continuous increase in pore pressure indicative of structurally weak behavior for the two forearc slope samples. The sample from the prism toe area, however, displays a continuous stress increase together with a decrease in pore pressure towards high strain indicative of structurally strong behavior. Synchrotron texture and composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinement program MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth down to 523 m below sea floor corresponding to a shape preferred mineral alignment due to compaction (Schumann et al., 2014). Experimentally deformed samples have generally stronger textures than related undeformed core samples, and they show increasing strength of the illite and kaolinite textures with increasing axial strain. Mechanically weak samples have a bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite (001)-pole figures have maxima >1.5 mrd (multiples of a random distribution). Mechanically strong samples, which were deformed to approximately the same amount of strain (up to 40%) have no calcite and a bulk clay content of 24-36 vol.-%. Illite, kaolinite and smectite (001)-pole figure maxima are predominantly <1.5 mrd. The synchrotron textures indicate that the mineral fabric as a whole (clay and also calcite grains) becomes preferentially oriented in the mechanically weak samples. Reorientation of the mineral grains is an important cause of strain weakening and contraction, persisting to high compressive strains. In contrast, the strong samples from the accretionary prism toe keep their microfabric up to fairly high compressive strain, allowing for strain hardening and dilation. This soft sediment hardening tends to involve increasingly large volumes of sediment into the imposed deformation, permitting strain dissipation as long as the sediments are homogeneous. Deformation will tend to localize into structurally weak sediments if they occur within the lithological sequence. Such weak sediments, which soften further with increasing strain, predominate in the cover units of the forearc slope and around the existing megasplay fault. They may either provoke mechanical runaway situations allowing for earthquake rupture, surface breakage and tsunami generation, or slope destabilization and resulting submarine mass wasting.

Document Type: Conference or Workshop Item (Poster)
Additional Information: Abstract volume pp. 108-109
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Projects: IODP
Expeditions/Models/Experiments:
Date Deposited: 19 Dec 2016 11:16
Last Modified: 19 Dec 2016 11:16
URI: http://oceanrep.geomar.de/id/eprint/35249

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