Lithification processes in subduction zones and their influence on the structural style of accretionary prisms
Final Report Abstract
The largest earthquakes in the world occur along subduction zones where an oceanic plate is thrust beneath an adjacent continental plate. The up-dip limit of catastrophic slip along the plate interface is located in ~5-10 km depth and associated with temperatures of 100-150°C as shown by integration of thermal models and focal depth data. The cause for the onset of unstable slip on the subduction thrust under these conditions is still subject to debate, because the seismogenic zone of an active megathrust zone has not been sampled so far. The overarching goal of the funded study was to test the hypothesis that sediment lithification caused by late stage diagenesis drives the transition from stable to unstable, seismogenic slip and affects the structural style of subduction zones. The research focused on the Nankai Trough subduction zone (SW Japan) and was divided in three topics: 1) Identification and spatiotemporal characterization of important lithification processes in the accretionary prism, 2) Evolution of physical properties (porosity, strength and permeability) as a function of lithification progress in subduction zone sediments and 3) Correlation of lithification processes and their progress with important features such as taper angle of the accretionary prism and depth of seismogenesis. In the Nankai Trough subduction zone the smectite-to-illite diagenesis is a major diagenetic process. A multi-methodological approach was used to characterize the illitization progress in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. For this study cutting and core samples from IODP Site C0002 were employed, where the inner accretionary prism of the Nankai Trough subduction zone was sampled down to 3058.5 m below seafloor (mbsf). The mineralogical and isotopic/geochemical composition of sediment revealed an advanced degree of illization compared to the illitization state as predicted by kinetic reaction modeling. The difference in the degree of illitization can be related to the uplift of inner prism material along the megsplay fault zone – a steeply dipping outof-sequence thrust in the Nankai accretionary prism not considered in the model. The difference between modeled and measured illitization state suggests a vertical throw of 1-3 km along the megasplay fault, which indicates a major discontinuity in the degree of compaction and diagenesis across this fault. The advanced illitization state probably also affected the pore space decrease of finegrained silty claystone samples from 34% porosity at 1.2 km below seafloor to 13% porosity at 3 km below seafloor. The pore space reduction is associated with a decrease in permeability on the order of 1 magnitude. Coarser grained silt- and sandstone samples revealed a dual porosity character of which only pores associated with pore throats >0.2 µm contribute to compactional porosity loss. Below 2 km below seafloor2000 mbsf inter-particle cement observed in µCT images also contribute to porosity loss. The higher permeability of the coarse-grained material is related to large well-connected inter-granular pores. In combination with borehole logging data a high-resolution in situ permeability profile of Site C0002 was created that revealed two low permeable horizons. These two horizons provide barriers to drainage of the inner accretionary prism and compartmentalize elevated fluid-pressure in the accretionary prism. Both, characterization of smectite-to-illite diagenesis and physical properties provide evidence for a well lithified sediment at depth landward of the megasplay fault suggesting an abrupt change in compaction and diagenesis across the megasplay. This implies a major change in friction behavior as shown by laboratory friction experiments on subduction zone sediments exhumed in the hinterland of the Nankai accretionary prism. Whereas powdered rock samples as an analog of unlithified material usually exhibits stable sliding behavior, friction experiments on intact rocks provide laboratory evidence for unstable slip behavior in subduction zone sediments that have preserved diagenetic and low-T metamorphic effects. These abrupt changes in the mechanical properties across the megasplay fault zone coincide with a change in the prism geometry and seaward extent of seismogenesis in the Nankai accretionary prism. The results may have implications for other subduction zone forearcs where megasplay faults have been observed, such as in Cascadia or south-central Chile.
Publications
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2015, Friction properties of rocks sampled from the Shimanto Belt (Japan) - a fossil accretionary prism, 20th International Conference on Deformation Mechanisms, Rheology and Tectonics, RWTH Aachen University, GER, 7-11 September
Hüpers, A., Trütner, S., Ikari, M.J., and Kopf, A.
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2015. Lithification facilitates frictional instability in argillaceous subduction zone sediments. Tectonophysics 665, 177-185
Trütner, S., Hüpers, A., Ikari, M.J., and A.J. Kopf
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2016, Friction properties of rocks in forearcs of accretionary convergent margins. IODP-ICDP Colloquium, Heidelberg, GER., 14-16 March
Hüpers, A., Trütner, S., Ikari, M.J., Kopf, A.
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2017, Clay mineralogy and composition of sediments sampled at Integrated Ocean Drilling Program Site C0002: Implications for the lithification of prism sediments in the Nankai Trough subduction zone. IODP-ICDP Colloquium, Braunschweig, GER., 14-16 March
Hüpers, A., Warr, L., Grathoff, G., Wemmer, K. and Kopf, A.
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2017. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra. Science, 356 (6340). 841-844
Hüpers, A., Torres, M.E., Owari, S., McNeill, L.C., Dugan, B., Henstock, T.J., Milliken, K.L., Petronotis, K.E., Backman, J., Bourlange, S., Chemale, F., Chen, W., Colson, T.A., Frederik, M.C.G., Guèrin, G., Hamahashi, M., House, B.M., Jeppson, T.N., Kachovich, S., Kenigsberg, Abby R., Kuranaga, M., Kutterolf, S., Mitchison, F.L., Mukoyoshi, H., Nair, N., Pickering, K.T., Pouderoux, H.F.A., Shan, Y., Song, I., Vannucchi, P., Vrolijk, P.J., Yang, T. and Zhao, X.
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GSA books. Lithostratigraphic controls on dewatering and fluid pressure in the western Nankai subduction zone: Implications for drainage behavior and consolidation state of the underthrust sequence
Hüpers, A., Saffer, D.M., and Kopf, A.J.
