Structural evolution of an inner accretionary wedge and forearc basin initiation, Nankai margin, Japan

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<ul><li><p>on</p><p>. M. H</p><p>875b Center for Integrative Geoscience, University of Connecticut, Storrs, CT, USAc National Oceanography Centre Southampton, University of Southampton, Southampton, UKd</p><p>g Queensland University of Technology, Brisbane, Austra</p><p>a r t i c l e i n f o</p><p>seismogenic slip over space and time (Wells et al., 2003; Song and</p><p>ismic</p><p>llentation,as a</p><p>tic ocean drilling, including the current NanTroSEIZE complex</p><p>Contents lists available at SciVerse ScienceDirect</p><p>w.e</p><p>Earth and Planetar</p><p>Earth and Planetary Science Letters 353-354 (2012) 163172Expedition 319 Scientists, 2009). During Expedition 319, we wereE-mail address: (N.W. Hayman).Simons, 2003; Wang and Hu, 2006; Simpson, 2010). However, the drilling program of the IODP (Tobin and Kinoshita, 2006). As aninitial step to access the inner wedge, IODP Expedition 319recovered core from beneath the Kumano basin (at Site C0009)using the D/V Chikyu riser technology (Saffer et al., 2009;</p><p>0012-821X/$ - see front matter &amp; 2012 Elsevier B.V. All rights reserved.</p><p></p><p>n Corresponding author.thrust ridges. The mechanics of prism, slope, and forearc basindevelopment are linked, and can correlate with the distribution of</p><p>history of large earthquakes, and a rich and growing database ofseismic imaging, earthquake and geodetic monitoring, and scien-off a subducting oceanic plate along an outer wedge, in manyplaces accompanied by forearc basin development over an innerwedge. Though in many places covered by only a thin veneer ofsediment on the surface slope, some accretionary wedges alsodevelop a slope apron and/or local piggy-back basins between</p><p>relative timing of structures otherwise inferred from seimaging.</p><p>The Nankai margin of southwestern Japan is an excetesting ground for linkages between inner wedge deformforearc basin development, and seismicity. The margin hwith a change in the Philippine Sea plate convergence direction. Forearc basin development therefore</p><p>postdates a protracted geologic evolution of shear-zone development, tectonic rotations, and inner-</p><p>wedge development, the last of which coincides with a rheological evolution toward localized frictional</p><p>faulting.</p><p>&amp; 2012 Elsevier B.V. All rights reserved.</p><p>1. Introduction</p><p>Accretionary wedges develop as marine sediment is scraped</p><p>geological evolution of inner accretionary wedges along modernmargins has not been documented in detail largely due to the lackof coring, which provides valuable information on the nature andKeywords:</p><p>accretionary prisms</p><p>fault mechanics</p><p>Integrated Ocean Drilling Program</p><p>D/V Chikyu</p><p>Expedition 319</p><p>Site C0009faults which are the youngest structures in the core and highly localized. Microstructural analyses of</p><p>the shear zones suggest that they formed via multiple increments of shear localization and a mixedArticle history:</p><p>Received 28 March 2012</p><p>Received in revised form</p><p>27 July 2012</p><p>Accepted 28 July 2012</p><p>Editor: P. ShearerAvailable online 11 September 2012Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan</p><p>versity of Michigan, Ann Arbor, MI, USA</p><p>lia</p><p>a b s t r a c t</p><p>Core recovered during Integrated Ocean Drilling Program (IODP) Expedition 319 from below the</p><p>Kumano forearc basin of Japans Nankai margin provides some of the only in situ samples from an inner</p><p>accretionary wedge, and sheds light on the tectonic history of a seismically hazardous region. The 84 m</p><p>of core comprises Miocene-age well-bedded muds, silts, and volcaniclastic sediments. Beds increase in</p><p>dip with depth, and are cut by (i) soft-sediment deformation bands (vein structures), (ii) 1-cm thickshear zones within 10-cm thick regions of high shear strain, and (iii) o1-mm thick slickensided</p><p>granular and cataclastic ow. Kinematic analysis of slip indicators in shear zones further reveals that</p><p>they formed via northsouth shortening. In contrast, the faults cut the shear zones with mixed slip</p><p>kinematics, and accommodated northwestsoutheast shortening, roughly parallel to the modern</p><p>shortening direction. The entire section was also rotated 151 counterclockwise about a roughlyvertical axis. Therefore the principle strain axes and stratigraphic section rotated during or postdating</p><p>development of the major sub-basin (5.63.8 Ma) unconformity, a time that generally coincidesDepartment of Earth Sciences, Chiba University, Chiba, Japane IFREE/Subduction Geodynamics Research Team, Japanf Department of Earth and Environmental Sciences, UniStructural evolution of an inner accretiinitiation, Nankai margin, Japan</p><p>Nicholas W. Hayman a,n, Timothy B. Byrneb, Lisa CCassandra M. Brownea, Anja M. Schleicherf, Gary Ja Institute for Geophysics, University of Texas, 10100 Burnet Rd., ROC 196, Austin, TX 7</p><p>journal homepage: wwary wedge and forearc basin</p><p>cNeillc, Kyuichi Kanagawad, Toshiya Kanamatsue,uftileg</p><p>8-4445, USA</p><p></p><p>y Science Letters</p></li><li><p>able to document a range of sedimentary and deformationalstructures in core recovered from below the forearc basin,and supplement these geologic data with inferences fromX-ray computerized tomography (CT) scans of core, Formation</p><p>Micro-Imager (FMI) logging of the borehole, and paleomagneticand microstructural data. We present these results below fol-lowed by a discussion of how they constrain the geologicalevolution of the uppermost inner wedge.</p><p>2. Background and methods</p><p>Results presented here come from Site C0009, located within theKumano forearc basin, in 2054m water depth (Figs. 1 and 2).Expedition 319 employed the D/V Chikyu to drill 1600 m belowseaoor (mbsf) at C0009, using riser-drilling technology from 700 to1600 mbsf. Coring operations are the central focus of this study butExpedition 319 had several objectives, partly surrounding the initialuse of riser drilling that is anticipated to access deeper parts of thesubduction zone in the future (Tobin and Kinoshita, 2006). Addi-tional operations included the use of cuttings to determine thebiostratigraphy of the Kumano basin (Saffer et al., 2009), thedocumentation of in situ stress conditions (e.g., Lin et al., 2010)and variations in permeability (e.g., Boutt et al., in press).</p><p>During riser operations, the section below the forearc basinwas studied in cuttings from 1287.7 to 1603.7 mbsf and core wasacquired from 1509.7 to 1593.9 mbsf. Overall core recoverywas 69% (Saffer et al., 2009), though this was largely due topoor recovery during initiation of coring; recovery was in general76100%.</p><p>km</p><p>33</p><p>34</p><p>35N</p><p>135E 136 137 138 139</p><p>Kii Peninsula</p><p>0 50</p><p>SiteC0002</p><p>SiteC0009</p><p>1944</p><p>1946</p><p>~4.1-6.5 cm/y</p><p>KumanoBasin</p><p>accretionar</p><p>y prism</p><p>Iz uIz uIz u</p><p>30</p><p>140 150</p><p>40N</p><p>130E</p><p>PhilippineSea Plate</p><p>EurasianPlate</p><p>Fig. 1. After Saffer et al. (2009). Location of the NanTroSEIZE complex drillingprogram, and Site C0009. Stars indicate the epicenters of the 1944 and 1946 M48earthquakes, black line and box represent the 2D and 3D seismic surveys of Park</p><p>et al. (2002) and Moore et al. (2007). Green diamonds are two NanTroSEIZE sites.</p><p>(For interpretation of the references to color in this gure legend, the reader is</p><p>referred to the web version of this article.)</p><p>Hole C 0009A Hole C 0002A 2000</p><p>2500</p><p> 3000</p><p> 3500</p><p> 4000</p><p>Dep</p><p>th (m</p><p>bsl)</p><p>NW SE</p><p>S1</p><p>S2</p><p>UC1</p><p>UC2</p><p>S-A</p><p>S-B</p><p>cored interval</p><p>I</p><p>II</p><p>IIIA</p><p>IIIB</p><p>IV</p><p>I</p><p>II</p><p>IIIIV</p><p>Units</p><p>20 40 60 801520</p><p>1540</p><p>1560</p><p>1580</p><p>1600</p><p>0 20 40 60 80</p><p>Dip</p><p>Dep</p><p>th (m</p><p>bsf)</p><p>3650 mbsl</p><p>Bedding(From Core)</p><p>Bedding(From FMI) Shear Zones</p><p>9 an</p><p>e de</p><p>n-h</p><p>the</p><p>2 (U</p><p>N.W. Hayman et al. / Earth and Planetary Science Letters 353-354 (2012) 163172164Fig. 2. Seismic prole across the Kumano basin (see Fig. 1 for location of Sites C0002007; Expedition 315 Scientists, 2009). The unconformities and associated ages wer</p><p>core and cuttings (Saffer et al., 2009). The cored interval is indicated in red, and dow</p><p>(and in the case of bedding, FMI) observations in Supplementary materials 1. On</p><p>cuttings; cuttings were sampled every 5 m during drilling and unconformities 1 andto color in this gure legend, the reader is referred to the web version of this article.)d C0002 in map view), extracted from the Kuman3D seismic volume (Moore et al.,</p><p>termined with biostratigraphy and sedimentological characterization of recovered</p><p>ole plots of bedding, shear-zone, and fault dips are plotted against depth from core</p><p>right is a down-hole plot of vein structures observed per 20 randomly selected</p><p>C1 and UC2) are plotted for a depth reference. (For interpretation of the referencesUnit I &amp; II: Upper Kumano Basin, &lt; 0.9 MaUnit III: Lower Kumano Basin, 3.8-1.34 MaUnit IV: Sub-Basin, &gt;5.6 Ma</p><p>20 40 60 80 1100</p><p>1200</p><p>1300</p><p>1400</p><p>1500</p><p>0 5 10 15 20 25</p><p>Number</p><p>Faults Vein Structures</p><p>UC1</p><p>UC2</p></li><li><p>younger (Pliocene) section in Hole C0002A on the seaward side of</p><p>N.W. Hayman et al. / Earth and Planetary Science Letters 353-354 (2012) 163172 165Once recovered, core was scanned with a CT-scanner. Theintensity of any pixel within a CT image is a function of the X-rayattenuation of the material, and therefore, in any 2D or 3Drendering of multiple images, brighter regions reect generallydenser material and can highlight key structural and sedimento-logical features (Expedition 315 Scientists, 2009). We used the CTimages to guide our subsampling of the core for bulk-rockdestructive analyses (10 cm of core-length for every 1.5 mof core), and then split the core down the long (z) axis. One half ofthe core was then used to document the depositional anddeformational structures in the core.</p><p>Because the core is only oriented vertically, structures weremeasured in an arbitrary core reference frame, including theirorientation and interpretation of their kinematic history. Many ofthe structures are difcult to observe, and we routinely usedbinocular microscopes for inspection, and compared notesbetween three observers, eliminating any ambiguous kinematicinterpretations from our compilation. The reference frame waslater reoriented into geographic coordinates using additionalinformation from FMI logs and paleomagnetic results. FMI logs generated by resistivity contrast in the borehole walls wereparticularly useful for providing clear visualization of beddingorientations. Paleomagnetic samples were collected from at leasteach core section, depending on the coherency of the coredsection. Measurements of magnetization were made using ahorizontal 2-G Enterprises cryogenic magnetometer. Naturalremanent magnetization (NRM) and demagnetization were rou-tinely measured at several alternating eld levels. Characteristicremanent directions were determined using the principal-com-ponent analysis for demagnetization diagrams.</p><p>Reorientations of remanent magnetic directions were achievedby tting CT images of the core to the FMI borehole images by themethod described in MacLeod et al., (1994). Bedding dip direc-tions observed in the CT images, in which paleomagnetic sampleswere taken, were tted to a mean dip direction from boreholeimages. Then, remanent magnetic directions were tilt-correctedfrom in situ directions. Corrected bedding orientations werecalculated on the assumption that magnetization was lockedwhen bedding planes were horizontal. Mean directions withconcentration parameters in each attitude were then calculated.Using the paleomagnetic and FMI constraints, we were able toreorient sufcient numbers of structures to generate a straininversion using software FaultKin (Allmendinger et al., 2012).</p><p>The data used in the reorientation and strain analysis areindicated in the data table in Supplementary materials 1, and theerror associated with these values is within the normal range ofstructural geology measurements (e.g., individual measurementsshould be treated as 751). The geological measurements carrythe largest analytical uncertainty, though there is clearly a spreadof the paleomagnetic and strain-analysis values as presented inSupplementary materials 2 and the following sections.</p><p>Thin sections from selected samples were prepared for opticaland scanning electron microscopy (SEM) for detailed microstruc-tural analysis. One of the thin sections was chosen for quantita-tive microstructural analysis using the digital image analysisfreeware, Jmicrovision (Roduit, 2008). A graphical example ofthe method is provided by Milliken and Reed (2010) and ispresented below (in Section 3). The analysis method requiresthe observer to manually trace a population of (many hundredsof) grains, in this case 42 mm, which are then subject tostatistical analyses of, in this case, grain size and orientation.Both microstructural measurements approximate each grain as anellipse and extract the long axis for calculations of grain diameterand orientation. Though this dataset is limited to several areas inone thin section for this effort, the results are from a shear zone</p><p>similar to the 166 shear zones we documented and are consistentthe Kumano basin (Expedition 315 Scientists, 2009) (Fig. 2). Notethat vein-structures are not veins in the traditional sense, butrather are o1-mm-wide bands lled with densely packed grains(Fig. 3) (see also Expedition 319 Scientists). Previous workers havewith results from other, more extensive microstructural studiesfrom convergent margins (e.g., Byrne et al., 1993b; Vannucchi andLeoni, 2007; Milliken and Reed, 2010).</p><p>3. Results</p><p>3.1. Stratigraphic and structural overview</p><p>Hole C0009A, like Hole C0002A further seaward within theforearc basin (Araki et al., 2009; Kinoshita et al., 2009), penetratedfour major lithostratigraphic units dened by core and cuttingscharacterization, biostratigraphy, and properties resolved inlogging data (Saffer et al., 2009). Units 1 through 3 make up thebasin-ll deposited during Plio-Pleistocene Kumano forearc basinsubsidence. Units 1 and 2 are younger than 0.9 Ma based on thepresence of R. Asanoi, and Unit 3 was deposited between 3.8 and1.34 Ma based upon the presence of R. pseudoumbilicus andH. Sellii, respectively (Fig. 2) (see Figure F46 in Expedition 319Scientists (2010)). At least 4 major unconformities span the entireforearc basin throughout these units (Fig. 2). Lateral thicknessvariations and changes in stratal orientation illustrate the variablesubsidence pattern across the basin over time, much of whichis likely linked to activity on the megasplay fault since1.31.95 Ma (Strasser et al., 2009; Gulick et al., 2010).</p><p>Our focus here is Unit 4, for which biostratigraphic ages rangefrom 5.6 to 7.9 Ma. The unconformity between Units 3 and 4 there-fore reects 1.8 myr of missing section, and is here referred to asthe sub-basin unconformity. Similarly, because of the age gap andresults presented below, one cannot consider Unit 4 part of theKumano basin, but rather as the uppermost inner wedge or slopebasin immediately below the forearc basin. Unit 4 comprises well-bedded silty mudstones with minor silt interbeds and volcanics,but is generally ner grained, more devoid of organics, and morelithied than the overlying units. Unit 4 is also highly deformedwith respect to overlying units, which resulted in a drop inborehole conditions, and an increase in bedding dips from o201to as much as 470...</p></li></ul>


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