Natural and experimental faulting of rocks, Nankai Accretionary Prism. Follow-up work to lODP Expeditions 315 and 316.
Zusammenfassung der Projektergebnisse
Presently the Nankai Accretionary Prism offshore Japan is the stage of a most ambitious research venture, the NanTroSEIZE Project, geared at understanding fundamental processes of faulting and earthquake generation in a transect of shallow non-riser and deep riser drillholes. IODP Expeditions 315 and 316 investigated the shallow frontal thrusts, and the hangingwall to a major active splay fault, to document near-surface active deformation in front of, and above the seismogenic zone. IODP Expedition 333 concentrated on drillholes to reveal and document the nature of the so-called subduction input, i.e. the sediments that arrive on the incoming plate and travel to depth in the course of subduction. The geomechanical behaviour, composition, and fabrics of both rock associations play a key role in defining the way deformation is achieved and localized in the plate boundary. Our work was guided by the hypothesis that the observed partitioning of deformation in the Nankai accretionary wedge reflects the capability of the rocks involved to generate seismic faulting and related tsunamis. Is brittle faulting in the accretionary wedge in response to a large seismic event capable of producing dramatic surface breaks, or are the rocks more amenable to slow, stable slip, strain weakening, and distribution of deformation within large volumes? We investigated this by conducting a series of deformation experiments that provided data on strength, stress paths, and pore pressure evolution in muds and mudstones from the drill cores. We found that two groups of sediments exist. One shows significant strain weakening, the other one displays strain hardening. It ist he weakening group of rocks that is associated with the development of discrete thrust zones and very localized deformation, like expressed in the megasplay fault that occurs updip from the Nankai Trench. Strain weakening causes mechanical runaway, and these faults are amenable for catastrophic shallow co-seismic slip, and earthquake faults breaking out at the seabed. On the other hand the sediments located at the deformation front at the trech itself show strain hardening; an observation that explains why deformation there is more distributed to produce folds and fault-fold packets. Principal cause for this may be overconsolidation, produced by excess loading during the process of frontal accretion. We also show that the slope, trench and incoming plate sediments have specific acoustic signatures, both, regarding P- and S-wave velocities, and petrophysical parameters like Poisson Ratios. Furthermore, we document that deformation of the rocks, be it by tectonic shearing or natural compaction, introduces shape and crystallographic preferred orientations that, in turn, greatly affect the mechanical behavior. To make predictions about those parts that were, and possibly will not be reached by NanTroSEIZE drilling, we have supplemented the Nankai data base by studying samples from two important analogue cases on land: the Shimanto Belt and the Bozo accretionary wedge. Comparison of natural and experimental deformation structures and fabrics guided extrapolation of the laboratory data to the natural case.
Projektbezogene Publikationen (Auswahl)
- (2013) Strength, textures, microfabrics and acoustic properties of active plate margin sediments on- and offshore SW Japan. – Doctoral Thesis, Christian-Albrechts- Universität Kiel, 341 pp.
Schumann, K.
- (2013) Strong sediments at the deformation front, and weak sediments at the rear of the Nankai accretionary prism, revealed by triaxial deformation experiments. - Geochemistry Geophysics Geosystems, 14
Stipp, M., Rolfs, M., Kitamura, Y., Behrmann, J.H., Schumann, K., Schulte-Kortnack, D. & Feeser, V.
(Siehe online unter https://doi.org/10.1002/ggge.20290) - (2014) P and S wave velocity measurements of water-rich sediments from the Nankai Trough, Japan. – J. Geophys. Research
Schumann, K., Stipp, M., Behrmann, J.H., Klaeschen, D. & Schulte-Kortnack, D.
(Siehe online unter https://doi.org/10.1002/2013JB010290) - Geotechnical behavior of mudstones from the Shimanto and Boso accretionary complexes, and implications for the recently active Nankai Accretionary Prism. Earth, Planets and Space, Vol. 66. 2014: 129.
Schumann, K., Behrmann, J.H., Stipp, M., Yamamoto, Y., Kitamura, Y. & Lempp, Ch.
(Siehe online unter https://doi.org/10.1186/1880-5981-66-129) - Texture development in naturally compacted and experimentally deformaed silty clay sediments from the Nankai Trench, Japan. Tectonophysics, Vol. 636. 2014, pp. 125–142.
Schumann, K., Stipp, M., Leiss, B. & Behrmann, J.H.
(Siehe online unter https://doi.org/10.1016/j.tecto.2014.08.005)
