Borehole image analysis of the Nankai Accretionary Wedge, ODP Leg 196: Structural and stress studies
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indicated that the maximum horizontal compressive stress (SHmax) axes have an azimuth of 303, and analyses of breakout data
Tectonophysics 426 (2006) Corresponding author. Fax: +81 75 753 4776.from RAB images indicated an azimuth of 310. These azimuths approximate the convergence direction of the Philippine Sea platetowards the Eurasian plate. The frontal thrust at Site 808 was encountered at about 389 mbsf. Density, porosity, resistivity, andgamma ray data change across the frontal thrust. The dcollement zone at the deformation front was identified between 937 and965 mbsf. The base of the dcollement is sharply defined as the maximum extent of conductive fracturing and is marked by abruptchanges in physical properties [Mikada, H., Becker, K., Moore, J.C., Klaus, A., Austin, G.L., Bangs, N.L., Bourlange, S.,Broilliard, J., Brckmann, W., Corn, E.R., Davis, E.E., Flemings, P.B., Goldberg, D.B., Gulick, S.S., Hansen, M.B., Hayward, N.,Hills, D.J., Hunze, S., Ienaga, M., Ishiguro, H., Kinoshita, M., Macdonald, R.D., McNeill, L., Obana, S., Hong, O.S., Peacock, S.,Pettigrew, T.L., Saito, S., Sawa, T., Thaiprasert, N., Tobin, H.J., Tsurumi, H., 2002. Proc. ODP, Initial Rep., 196, College Station,TX, (Ocean Drilling Program)]. The upper boundary of the dcollement is marked by several sets of conductive fractures and byhigh variability in physical properties. The dcollement zone is characterized by intense brittle fracturing. These fractures areconsidered to be the consequence of cyclic stresses and high fluid pressures in this zone. We analyzed fracture dips and theirorientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites. 2006 Elsevier B.V. All rights reserved.
Keywords: Dcollement; Borehole breakout; Nankai Trough; LWD; RAB; ODPODP Leg 196: Structural and stress studies
Masanori Ienaga a,1, Lisa C. McNeill b, Hitoshi Mikada c,, Saneatsu Saito d,David Goldberg e, J. Casey Moore f
a Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 104-0045, Japanb Southampton Oceanography Centre, Southampton SO14 3ZH, UK
c Department Civil and Earth Resources Eng., Kyoto University, Yoshida-hommachi, Sakyo-ku, Kyoto 606-8501, Japand IODP-MI Sapporo Office, CRIS, Hokkaido University, N21W10 Kita-ku, Sapporo 001-0021, Japan
e Borehole Research Group, Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, USAf Department of Earth Sciences, University of California, Santa Cruz, CA 95064, USA
Accepted 7 February 2006Available online 30 June 2006
Electrical images recorded with Resistivity-At-Bit (RAB) from two sites drilled during Ocean Drilling Program (ODP) Leg 196were analyzed to study the effects of subduction at the Nankai margin. For the first time in the history of scientific deep-sea drillingin ODP, in situ complete borehole images of the dcollement zone were obtained. Analyses of all drilling-induced fracture dataBorehole image analysis of the Nankai Accretionary Wedge,E-mail address: email@example.com (H. Mikada).1 Present address: SciMarkJ Inc., Kawasaki-shi, Kanagawa, 210-
0040-1951/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2006.02.018207220www.elsevier.com/locate/tectoThe active margin offshore Japan (Fig. 1a) hasperiodically generated large subduction earthquakes of
ophys208 M. Ienaga et al. / Tectonthe order of magnitude 8 (e.g., in 1605, 1707, 1854,1946; Ando, 1975). The Nankai Trough is one of thebest-studied convergent plate boundaries. Subductioncomplexes provide an opportunity to examine theinitiation of dcollement zones, i.e., detachment planesthat separate accreted from subducted sediments andruptures during subduction earthquakes. Recent studieson subduction zones have established that fluids play amajor role in their physical and chemical evolution andseismogenic activities (Hyndman et al., 1995). Theoverall structure of the prism and the general nature ofthe deformation processes are now reasonably wellknown. However, many questions remain and thedistribution, nature, shape, and origin of the dcollementzone at the base of the prism are still poorly understood.
The objective of Ocean Drilling Program (ODP) Leg196 was to clarify the nature of deformation and fluidflow in the Nankai accretionary prism. Leg 196 was the
Fig. 1. (a) Location map shows tectonic setting of the Nankai AccretionFsc=Fossil Spreading Center; IBT=Izu Bonin Trench. (b) Seismic reflectionthrust faults, Dcollement zone, and top of Lower Shikoku Basin sedimentscomplex structure on the left side of figure. A strong seismic reflector perceiveShikoku Basin sediments where a number of normal fault type discontinuitiics 426 (2006) 2072205th deep-sea drilling investigation at the Nankai Trough,following DSDP (Deep-Sea Drilling Project) Legs 31and 87 and ODP 131 and 190. Logging-while-drilling(LWD) was carried out during Leg 196 to measure thephysical properties of the dcollement zone andoverlying prism. A logging tool string was located justabove the drill bit to measure resistivity, density,porosity, and sonic velocity. In Leg 196, two siteswere drilled to obtain physical properties: one is the Site808 in the proto-thrust zone of the prism and the otherSite 1173 about 9 km seaward from the Nankai Trough.The dcollement and the seaward stratigraphic equiva-lent of the dcollement (referred to as pre-dcollementhere) zones were penetrated. Sites 808 and 1173 wereselected for investigation of the early stages ofdcollement evolution. The dcollement zone wascharacterized seismically as a high-amplitude re-versed-polarity reflection (Fig. 1b), which might suggest
ary Prism. PSP=Philippine Sea Plate; KPR=Kyushu Palau Ridge;profile across the Muroto transect (Moore et al., 1990). Recognizableare shown in broken lines. Thrusts and their conjugate faults form ad as Dcollement zone (left half of figure) jumps to the top of the Loweres are observed below the reflector.
ophysa high-porosity and low-density layer in the dcolle-ment. However, core samples recovered during Leg 131suggest low-porosity and high-density in the dcolle-ment zone. Core recovery within the dcollement zonewas 20% or less (Taira et al., 1991), and continuous andsequential data obtained by logging were required toreconstruct the structure and properties of the dcolle-ment zone. This included resistivity borehole images ofthe dcollement zone (Mikada et al., 2002) using LWD,which had a resolution of 10 in the horizontal aperture.
Many of the initial observations from the RAB dataset were first described by Mikada et al. (2002). Initialresults and interpretations were presented by McNeill etal. (2004) for Site 808. Here, we expand theirdiscussions and present to interpret the most significantstructural features observed in the RAB images for bothSites 808 and 1173. Two methods were used to analyzethe borehole images: characterization of fracture dip andbedding directions, and determination of in situ stressdirections from borehole breakouts (Peka and Zoback,1995). Finally, based on the results from these analyses,we introduce discussions on the role of fluid pressure inthe propagation of the dcollement zone in the stressenvironment that we suggest based on results from asandbox experiment by Yamada et al. (2006). Weconclude that all observed fracture data are consistentwith a unique stress field model and that thedevelopment of the prism might have controlled thestress orientation and dip, which caused the dcollementzone to advance seaward parallel to the plate motiondirection.
2. Nankai accretionary prism
The Nankai Trough marks the subducting plateboundary between the Shikoku Basin, i.e., a part of thePhilippine Sea plate, and the southwest Japan arc, i.e., apart of the Eurasian plate. At this boundary, thePhilippine Sea plate is subducting to the northwestbeneath the Eurasian plate at a rate of 0.020.04 m/year(Seno, 1977; Karig and Angevine, 1986), oblique to theplate margin. The convergent margin of southwesternJapan has a geological record of accretion of deep-seadeposits extending to at least the Cretaceous (Taira et al.,1988). In the area drilled in Legs 131, 190, and 196, i.e.,the Muroto Transect (Fig. 1a), the basin-to-margintransition can be divided into the undeformed ShikokuBasin and overlying trench fill, the proto-thrust zone,the imbricate thrust zone, the frontal out-of-sequencethrust zone, the large thrust slice zone, and thelandward-dipping reflector zone (Moore et al., 2001a,
M. Ienaga et al. / Tectonb). The Philippine Sea Plate entering the Nankai Troughalong the Muroto Transect is close to the axis of anextinct spreading center marked by the Kinan Sea-mounts (Okino et al., 1999). Along the Muroto transect,the prism toe has a taper angle of 45 (Moore et al.,2001b). The low taper angle along this transect has beeninferred to represent either high dcollement porepressures or low internal sediment strength (Screatonet al., 2002). The dcollement zone is characterizedseismically as a high-amplitude and reversed-polarityreflection (Fig. 1b), which is consistent with observa-tions of high pore pressures in the sediments below thedcollement layer (Moore and Shipley, 1993).
3. Characterization of fracture and bedding
Electrical images were obtained with logging-while-drilling technology using the Resistivity-at-Bit (RAB)tool at shallow, medium, and deep depths of investiga-tion. The penetration of each depth was 0.025, 0.076,and 0.13 m, respectively. These images can be displayedusing either static or dynamic normalizations. The staticnormalization color range covers all resistivity valuesfor the logged interval, whereas the dynamic normali-zation is limited to the resistivity values over a specifieddepth interval. These two normalizations providecomplementary images of geological structures. Dy-namic normalization is used for detailed comparisons ofsedimentary structures, while static normalization ispreferred for correlating lithological or facies changes.In RAB images, bedding, fractures, and faults appear asconductive or resistive anomalies with a sinusoidalshape (Fig. 2). Fractures from RAB images can beclassified into either conductive or resistive features thatcross bedding planes. The conductive fractures wereinterpreted as fractures filled with conductive fluids.Resistive fractures are filled with non-conductive claygouge or minerals causing relative reduction in porosity.Mechanically, drilling can induce fractures by thedrilling process or by a hydraulic fracture. Consideringsmooth profiles of illitesmectite composition in theformations (Shipboard Scientific Party, 2001, 2002a,b),we think drilling induced fractures appear as conductivefractures which may reflect the present day leastprincipal stress direction (Zoback et al., 1985).
3.2. Site 1173 (reference site)
3.2.1. BeddingBedding data interpreted from the RAB images at
209ics 426 (2006) 207220Holes 1173B and 1173C were compared with seismic
ophys210 M. Ienaga et al. / Tectonreflection data (Fig. 3). Hole 1173C was drilled downto 175 mbsl to acquire sediment physical propertiesusing LWD. Data from the overlapped depth zone(124175 mbsf) between the two holes was takenfrom Hole 1173B. Bedding dips are relatively lowthroughout the hole, with predominantly 05,although two sets of dips ranging from 5 to 35were identified.
The very well laminated bedding of trench turbiditesis in contrast to the heterogeneous feature of bedding,i.e., varied dip angles, at the top of the Upper ShikokuBasin facies, and clearly shows a change in thesedimentation process. These bedding planes dip from0 to 20 with the majority at 05 (depths 50170 mbsf). Bedding dip direction is to the north.Bedding dips in the Upper Shikoku Basin facies andLower Shikoku Basin facies are primarily sub-horizon-tal between 170 and 725 mbsf, although bedding dipsexceeding 5 occur at 250 mbsf, 300590 mbsf, and700725 mbsf. Seismic depth sections suggested an
Fig. 2. Example of the fractures and normal fault at Site 1173 for a 10 m int(referred as pre-dcollement here). Figure shows uninterpreted image (left), inics 426 (2006) 207220increase in bedding dips at 300500 mbsf, therefore inagreement with RAB image interpretation. Althoughbedding dip direction could not be estimated accuratelybecause of the low dip values, the average dip directionof southwest to northwest shows that the entire sequenceis tilted westward (
Fig. 3. Bedding dips measured from resistivity-at-the-bit (RAB) data with depth-converted seismic reflection data, core-based lithology, and facies interpretation at Sites 808 and 1173. Holes A to G atSite 808 were drilled during Leg 131 (Taira et al., 1991), while Hole A at Site 1173 during Leg 190 (Moore et al., 2001a). It is well recognized that beddings for both sites are all close to horizontal.
Fig. 4. (a) Fracture dip and direction from resistivity-at-the-bit image interpretation at Sites 808 and 1173. (b) Stereographic projection of poles ofbedding, fractures, and faults at Sites 808 and 1173. Fractures have high dip angles around 60 but low dip angle fractures appearing below dips areobserved gradually lighter as depth at Site 808 while the orientations are always bi-modal in parallel to plate motion direction. Fracture density at Site1173 is lighter than at Site 808.
212 M. Ienaga et al. / Tectonophysics 426 (2006) 207220
majority dipping at 3065. Fracture dip directionsshow orientations mainly ranging 150360 for the top600 m and 270360 for the bottom of the drilled hole,respectively.
3.3. Site 808 (deformation front)
3.3.1. BeddingBedding was discernible from RAB images at Hole
808I on Leg 196. Here, the variations in bedding dip anddirection with depth are shown in Fig. 3. Hole 808Isuccessfully penetrated both the frontal thrust zone andthe dcollement zone in the toe of the accretionaryprism, allowing detailed structural analysis of thesezones and the state of deformation between them.Bedding dip angles are low throughout the hole, withmost at 05. Bedding direction appears to be fairlyrandom throughout the hole, although three discrete setsof bedding planes were identified according to theiraverage dip and orientation.
Bedding dip above 400 mbsf (Unit A) with the verywell laminated bedding of trench turbidities ranged from0 to 25, with the majority showing a dip of 05. Instructural Unit B at depths of 400800 mbsf, thebedding dip ranged from 0 to 20, with the majorityshowing a value of 05, which is almost sub-horizontal. A major change in the feature of the beddingoccurs below 800 mbsf within structural Unit C.Bedding dips are generally sub-horizontal, but increasesin the dip angle to 525 were detected. The dipdirection was widely scattered in structural Unit C.
3.3.2. Natural fracturesIncreases in deformation and fracturing were
detected at Site 808 (Fig. 4) as compared with referenceSite 1173. Fracture frequen...