From the Forearc Basin to the Accretionary Prism: Fluid Migration in the Nankai Subduction Complex

Figure 1: 3D seismic volume of the IODP Kumano transect, SE offshore Japan (from Bangs et al. 2009). Black lines indicate the Megasplay


Figure 2: Schematic of drilling mud gas monitoring onboard D/V Chikyu (© JAMSTEC).Fluids are of great importance for various geological, geochemical and geophysical processes. Generation and migration of fluids influences heat and solute transport in the earth’s crust. It further plays an important role in the formation and storage of natural resources, and the generation of natural hazards. At the Nankai Trough subduction zone, SE offshore the Japanese Kii Peninsula, seismogenic processes and the spatial and temporal variability of the seismogenic zone were repeatedly discussed in the light of pore fluid pressure and fluid migration (Fig. 1). Of particular interest is a major splay fault, termed “Megasplay”, that is supposed to be hydraulically connected to the seismogenic zone of the thrust fault boundary. If the Megasplay fault can serve as fluid conduit, it might significantly influence the stick-slip behaviour at the plate boundary and thus, would control the updip limit of the seismogenic zone. Further landward, in the Kumano forearc basin, several mud volcanoes indicate past and recent fluid and mud expulsion. However, it is unclear form what depths these fluids are derived, and what mechanisms trigger mud volcanism.
In order to advance our understanding of fluid generation and migration at the Nankai Trough subduction complex, this PhD thesis uses different borehole monitoring techniques. In the course of the IODP (Integrated Ocean Drilling Program) NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment), the Megasplay fault was drilled and instrument with a long-term borehole monitoring device called SmartPlug. The SmartPlug measured fluid pressure (as proxy for strain) and fluid temperature for 15 months. In the Kumano Basin, information about in situ composition of fluids was obtained with drilling mud gas monitoring and sampling during riser drilling with drilling vessel (D/V) Chikyu to ca. 3000 meter below seafloor (Fig. 2).
Evaluation of the long-term pressure data revealed that the fluid pressure data is affected by a wide range of natural signals, including seismic waves radiated by non-volcanic tremor and earthquakes. Hydrogeological formation properties that are based on pressure data and laboratory permeability tests imply the possibility of fluid migration. However, no fluid flow was observed during the monitoring period, nor was any pressure signal detected that would point to co-seismic large-scale deformation in the accretionary prism or the splay fault zone.
By contrast, fluid migration in the accretionary prism is supported by drilling mud gas data which indicated thermogenic and bacterial hydrocarbon gas at depths shallower than 2000 mbsf. At these depths, time-temperature modelling implied no in situ generation of thermogenic gas, thus active migration of thermogenic gas and mixing with bacterial gas must take place. Noble gas isotopes suggest primordial fluids to be present as well, which, following the results of a one-dimensional fluid flow model, are related to episodic, co-seismic fluid migration.
Potential fluid pathways in the accretionary prism and the Kumano Basin, the co-seismicity of fluid migration, and the kind of migration (flow vs. diffusion) remain unclear at this point. In the future, ejecta of the mud volcanoes in the Kumano Basin will be subject to further investigation, and might hopefully help to elucidate source depth and triggering mechanisms. In addition to that, recently developed borehole monitoring systems, namely MeBoCORKs, MeBoPLUGS and MeBoPUPPIs, will be used for long-term observation of fluid expulsion. Monitoring of in situ fluid pressure, temperature and geochemical parameters will improve our understanding of fluid migration, its triggering mechanisms, and might help to understand the seismogenic cycle at the Nankai Trough subduction complex.

References
Bangs NLB, Moore GF, Gulick SPS, et al. (2009) Broad, weak regions of the Nankai Megathrust and implications for shallow coseismic slip. Earth Planet Sci Lett 284:44–49.