Structural Profile and Development of the Accretionary Complex in the Nankai Trough, Southwest Japan: Results of Submersible Studies - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 189

5.4 Roles of Carbonate Cementation in Concentration

of Fluid and Strain

Carbonate cementation is distributed abruptly along the faults. Observations from
the 5th ridge clarified that the distribution of fluid flow is concentrated around the
cemented zone. Our observations suggest that cementation and hardening of ridge
front or deformation front may play an important role in the concentration of strain,
localization of fluid flow and mineral precipitation, thickening of accretionary
complex, and development of trench-slope-break. Our scenario is summarized in
schematic sketches in Fig. 11 .
Initial channels of fluid flow must be formed due to brittle faulting (Fig. 11a ).
Nakanishi et al. ( 2008 ) suggested the presence of a thin low-velocity layer above
the megasplay fault with velocities 0.5-1.5 km/s slower than the surrounding rock,
implying elevated fluid pressure in the fault zone that resulted in mega-thrust
earthquakes earlier. Fault rupturing and subsequent inter-seismic deformation of
accretionary complex including very low frequency earthquakes (e.g. Kodaira et al.
2004 ; Obara and Ito 2005 ; Ito and Obara 2006 ) allow CaCO 3 -saturated fluid to
migrate along the fault planes (e.g. Davis et al. 2006 ). Sandstones with high pore
connectivity and permeability nearby the faults were then hardened by the carbon-
ate cementation (Fig. 11b ). The cementation decreases the porosity and pore-space
connectivity in sandstones, prevents diffusive fluid flow, guides and concentrates
flow around the cemented zone (Fig. 11b-d ). The distribution of such hardened
zones and increased pore pressures around them may also control the position and
patterns of fault propagation. A hardened ridge front will form by repeated ruptur-
ing, fluid flow and cementation, which in turn act as an indenter for the inner
wedge. It is evidenced by the presence of folds with wavelengths of ~200 m devel-
oped behind the hardened zone (Fig. 7a ), implying that strain was accumulated as
the cemented zone was transported landward with the subducting Philippine Sea
plate. A new bifurcation fault may be formed behind the hardened zone, whereas
slips on the megasplay fault likely took place repeatedly as fluids migrate prefera-
bly upward and cement upper part of the fault (Henry et al. 2002 ; Fig. 11d ).
The hardened ridge front may also act as a backstop for the outer wedge and
frontal imbrications may propagate forward in the outer wedge. Cemented zones
are also present in the 1st ridge, although deformation and strain concentration
behind or in front of it was not obvious. This is perhaps due to immaturity of the
tectonic deformation in the frontal thrust zone.
Recently, Park et al. ( 2010 ) reported detailed seismic velocity structures in the
accretionary complex and the presence of a low-velocity zone of maximum of ~2
km thick (Fig. 11e ). They attributed the low velocity zone to an underplated, fluid-
rich underthrust package, which may supply a significant amount of fluid to the
megasplay fault zone, elevate fluid pressure, and eventually foster tsunami genera-
tion. It is noteworthy that their velocity structure also indicates the presence of a
seaward-dipping high-velocity zone just above the megasplay fault zone (Fig. 11e ).
Our observations further imply that the presence of such a high-velocity zone may