Journal of Geography 115 3 367 382 2006 Current Research Status and Meaning of Fluid Pressure Monitoring at the Nankai Trough Hitoshi MIKADA, Masanori IENAGA, Tada-nori GOTO and Takafumi KASAYA Abstract Experimental monitoring of fluid pressures was initiated in June 2001 at 2 underwater holes drilled during the Ocean Drilling Program ODP Leg 196 to investigate the relationship between deformation and fluid flow processes in the Nankai accretionary prism. ODP Leg 196 visited Sites 1173 drilled on Leg 190 and 808 drilled through the frontal thrust on Leg 131, installing 2 Advanced Circulation Obviation Retrofit Kits ACORKs to monitor fluid pressures along the walls of the drilled holes. Site 808 penetrates the frontal thrust fault and the décollement, while Site 1173 is located about 11 km seaward from the Nankai Trough deformation front. Formation water freshening around the décollement was first observed on ODP Leg 131, and geochemists have been investigating whether or not the freshening was due to the production of deep-sourced dehydration processes. We now know that the role of smectite dehydration and dehydrant quartz-cristobalite phase transition should be estimated quantitatively. One of the important findings of fluid pressure monitoring is that the formation fluid pressures seem to reflect the change in the stress state in and around the accretionary prism. We believe that such fluid pressure monitoring in the accretionary prism and in the sediments on top of the subducting oceanic plate in terms of stress field and dehydrant process of minerals is a key to deepening our knowledge in future investigations of seismogenic processes. Key words ODP, Nankai Trough, ACORK, stress state, fluid circulation, fluid pressure ODP ACORK Depertment of Civil and Earth Resources Engineering, Kyoto University Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology Present Address: SciMarkJ Inc.
I Ando, 1975 Moore et al., 2001 Hyndman et al., 1995; Saffer and Bekins, 1998; Moore and Silver, 2002; Bourlange et al., 2003; Henry et al., 2003; Saffer, 2003; Morgan and Ask, 2004; Ge and Screaton, 2005 Moore et al., 2005 Mikada et al., 2002a Bangs et al., 2004; Moore et al., 2005; Ienaga et al., 2006 Kastner et al., 1993; Brown et al., 2001; Bangs et al., 2004; Moore et al., 2005 2001 Advanced CORK; CORK Circulation Obviation Retrofit Kit ACORK 13 km Mikada et al., 2002b Hucks et al., 2005 Permeability, Zhang et al., 2000; Bernabe et al., 2003 ACORK K. Becker Davis et al., 2006 30 50 http://www.jishin.go.jp/ Savage, 1995; Ozawa et al., 1999
1 Ocean Drilling Program Moore et al., 2005 ODP Embayment 808 1173 I-B Trench, I-B Arc, KPR, KSC Fig. 1 Locations of sites drilled during the Ocean Drilling Program Moore et al., 2005. Numbers are ODP site numbers. Embayment of the accretionary prism due to the subduction of a seamount is clearly visible in the figure. Two formation pressure-monitoring observatories were installed one at Site 808, at the toe of the accretionary prism, and one at Site 1173, at a sea-ward site of the Nankai Trough. I-B, KPR, and KSC depict Izu-Bonin, Kyushu-Palau Ridge, and Kinan Seamount Chain, respectively. ACORK ACORK II 190 196 1946 Ando, 1975 4 cm 310 N Ando, 1975;, 1992; Sagiya and Thatcher, 1999 Mazzotti et al., 2000 1,500 Okino et al., 1994
2 1 Muroto Transect Moore et al. 2005 PTZ BSR Proto-Thrust Zone Bottom Simulating Reflector Fig. 2 Schematic cross-section along the Moroto transect shown in Fig. 1 Modified from Moore et al., 2005. PTZ is the Proto-Thrust Zone; BSR is the Bottom-Simulating Reflector. Major observations pertinent to subduction of the oceanic plate and development of the décollement are shown in the figure. Seismic reflections show a polarity reversal at the décollement layer. As the décollement approaches the updip limit of the seismogenic zone, reflection amplitudes from the décollement decline. Top of oceanic basement, acoustically transparent zone between the top of oceanic basement and the décollement layer, and stepdown of the décollement layer are also depicted in the figure. 1 1173 808 ACORK 700 1200 m 15 Ma Moore et al., 2001 2 Mikada et al., 2002a 40 Mikada et al., 2002a 7 km Mikada et al., 2002a 1174 808 Tsuji et al., 2005 Saffer 2003
3 Boulange et al. 2003 808 a b mbsf a 0.30 0.35 0.23 0.35 b Fig. 3 Normal a and fracture b porosities estimated in and around the décollement layer at Site 808 Figure from Bourlange et al., 2003. Vertical axis denotes depth in meters below seafloor mbsf. Solid curve in a depicts both density- and resistivity-derived porosities. Resistivity-derived porosity varies between 0.23 and 0.35, while density-derived porosity varies between 0.30 and 0.35. Discrepancies between resistivity-derived and density-derived porosities are attributed to fracture porosities, which have a strong influence on permeability and on change of permeability due to fluid pressure. Negative values in b are caused by estimation errors. Bourlange et al. 2003 808 3 Stauffer and Bekins 2001 Newberry et al. 2004 1 1173 1174 1176 1177 1176 1177 1173 1174 400 500 m 1173 1174 1176 48 73 Thermococcus Kormas et al., 2003 1176 1 12
Kormas et al., 2003 196 Logging While Drilling 808 McNeill et al., 2004 1173 Ienaga et al., 2006 808 McNeill et al., 2004 III Moore and Silver, 2002 25 10 19 10 18 m 2 Chloride Kastner et al., 1993; Saffer and Bekins, 1998 Moore and Silver, 2002; 4 Saffer and Bekins 1998 1990 808 10 16 m 2 80 160 10 13 m 2 20 Bourlange et al. 2003 Saffer and Bekins 1998 10 12 m 2 100 Saffer and Bekins 1998 Bourlange et al. 2003 Jouniaux et al., 1994 Morgan and Ask 2004
4 Moore and Silver 2002 Fig. 4 Chloride anomalies observed at Costa Rica, Barbados, and Nankai subduction zones Figure from Moore and Silver, 2002. Contrary to chloride anomalies limited to the vicinity of décollement layers at the former two subduction zones, a wide depth zone of anomalous chlorinity was observed at the décollement layer off Muroto. This observation recalled discussions on a relationship between the difference in chlorinity and that of fluid migrations. 2.8 Saffer and Bekins, 1998 IV Hyndman et al. 1995
100 150 Steurer and Underwood 2003 1500 25 Moore and Saffer 2001 30 50 100 150 Brown et al. 2001 Henry and Bourlange 2004 Saffer and Bekins 1998 2 Moore et al. 2005 Moore and Silver 2002 Ge and Screaton 2005 10 km ACORK 5 Hyndman et al. 1995 Vrolijk,
5 808 A 1173 B ACORK Mikada et al. 2002b 808 1173 808 1173 1173 Fig. 5 Schematic figures for ACORK systems installed at Sites 808 A and 1173 B, off Muroto Modified from Mikada et al., 2002b. Locations of packers and screens for pressure measurements are also shown. They measure fluid pressures on the seafloor through downhole tubing. The locations of the screens are chosen at depths of décollement and stratigraphic equivalent of décollement layers. A strong seismic reflector at Site 808 indicates the location of the décollement layer, which disappears between the two sites. At Site 1173, the diagenetic boundary of a quartz-cristobalite phase transition is a strong seismic reflector. Pressure gauges are installed to detect fluid migration at the locations of proto-thrust fault, décollement layer, diagenetic boundary, and inside the oceanic basement. 1990 V 196 808 1173 808 1173 McNeill et al. 2004 Ienaga et al. 2006
McNeill et al. 2004 808 1173 Henry et al. 2003 190 1174 1174 Ienaga et al. 2006 Yamada et al. 2005 6 1 808 1173 Bangs et al. 2004 Saffer 2003 1 Stauffer and Bekins 2001 Ienaga et al. 2006 Davis et al. 2006 ACORK 11 km 10 6 Davis et al. 2004 Park et al. 2002 2 Out of Sequence Thrust Zone Deep Seismic Reflector 15 Tsuru et al. 2005 P 10 15 m 2 1.6 2.2 5.6 MPa VI
6 Ienaga, et al. 2006 808 1173 40 45 A 808 1173 Fig. 6 Schematic stress diagram demonstrating the relationship with observed fracture dips at Sites 808 and 1173 where low and high angle dips were observed, respectively Modified from Ienaga, et al., 2006. Friction angle of the sediments was assumed to be 40 45 degrees. Arrows indicate the direction of fracture and slip to be developed by slanted principal stress. Due to heavy mass loading of the prism, the vertical normal trend of compaction to the sediments is distorted towards the prism. It is qualitatively indicated that thrust type and horizontal fracturing tend to be formed above and in the décollement, respectively. A blow-up image of the décollement layer at the bottom implies that tensile fractures could be developed at the top and at the base of the décollement layer. The area indicated by A denotes a possible zone of tensile field due to spatial changes in principal stress directions. It is necessary to consider possible stress distributions for fluid migrations., Zhang et al., 2000; Bernabe et al., 2003 K. Becker ACORK ACORK
7 MT Fig. 7 Schematic structure around seismogenic zone using seismic reflection and magnetotelluric profiles. A decline of seismic reflections is found in the vicinity of the décollement step-down and the updip limit of the seismogenic zone, while a zone of low re-sistivity and microtremors are found near the downdip limit. Both phenomena may be explained by the existence of interstitial fluids. However, in-situ data should be provided for further discussions of fluid circulation on a spatial scale including both accretionary prism and seismogenic zone. ; 2 Obara, 2002 7 VII Moore and Silver 2002 Moore et al., 2005
Vrolijk, 1990; Hyndman et al., 1995 Steurer and Underwood, 2003; Moore et al., 2005 190 196 Logging-While-Drilling In- Situ Mikada et al., 2002a McNeill et al., 2004; Ienaga et al., 2006 Newberry et al., 2004 Kormas et al., 2003 1500 Moore et al., 2005 Davis et al., 2006 Clavier et al., 1984 Pore Bourlange et al., 2003 McNeill et al., 2004; Ienaga et al., 2006 ACORK 1,500 1944 1946 Kasaya et al. 2005 ACORK
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