SDScientific DrillingSDSci. Dril.1816-3459Copernicus PublicationsGöttingen, Germany10.5194/sd-24-93-2018IODP workshop: Core-Log Seismic Investigation at Sea – Integrating legacy
data to address outstanding research questions in the Nankai Trough
Seismogenic Zone ExperimentCore-Log Seismic Investigation at SeaCerchiariAnnaFukuchiRinahttps://orcid.org/0000-0001-9953-2047GaoBaiyuanHsiungKan-Hsihttps://orcid.org/0000-0002-6575-954XJaegerDominikKanekiShunyaKellerJonasKimuraGakuKuoSzu-Tinghttps://orcid.org/0000-0001-7479-7719LymerGaëlg.lymer@bham.ac.ukhttps://orcid.org/0000-0002-0836-2925MaisonTatianaMotohashiGintahttps://orcid.org/0000-0002-7727-2843RegallaChristineSingletonDrakeYabeSuguruDepartment of Chemical and Geological Sciences, University of
Modena and Reggio Emilia, Modena, 41125, ItalyAtmosphere and Ocean Research Institute, The University of Tokyo,
Chiba, 277-8564, JapanDepartment of Geological Sciences,
Institute for Geophysics, The University of Texas at Austin,
Austin, TX 78712, USAResearch and Development Center for Ocean
Drilling Science, Japan Agency for Marine-Earth Science and Technology,
Yokohama, 236-0001, JapanDepartment of Geology, University of
Innsbruck, Innsbruck, 6020, AustriaDepartment of Earth and Space
Sciences, Osaka University, Osaka, 560-0043, JapanDepartment of
Marine Environment and Resources, Tokyo University of Marine Science and
Technology, Tokyo, 108-8477, JapanDepartment of Geology and
Geophysics, Texas A&M University, TX 77843, USAEarth Sciences Research Group, University of Birmingham, Birmingham, B15 2TT, UKUMR 2018.C100 Basins-Reservoirs-Resources (B2R), UniLaSalle, UPJV,
Beauvais, 60026, FranceGraduate School of Life and Environmental
Sciences, University of Tsukuba, Ibaraki, 305-0006, JapanDepartment of Earth and Environment, Boston University, Boston, MA
02215, USADepartment of Geological Sciences, San Diego State
University, San Diego, CA 92182, USADepartment of Solid Earth
Geochemistry, Japan Agency for Marine-Earth Science and Technology, Kanagawa,
237-0061, JapanGaël Lymer (g.lymer@bham.ac.uk)22October2018249310715May201821August201810September2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://sd.copernicus.org/articles/24/93/2018/sd-24-93-2018.htmlThe full text article is available as a PDF file from https://sd.copernicus.org/articles/24/93/2018/sd-24-93-2018.pdf
The first International Ocean Discovery Program (IODP) Core-Log-Seismic
Integration at Sea (CLSI@Sea) workshop, held in January–February 2018,
brought together an international, multidisciplinary team of 14 early-career
scientists and a group of scientific mentors specialized in subduction zone
processes at the Nankai Trough, one of the Earth's most active
plate-subduction zones located off the southwestern coast of Japan. The goal
of the workshop was to leverage existing core, log, and seismic data
previously acquired during the IODP's Nankai Trough Seismogenic Zone
Experiment (NanTroSEIZE), to address the role of the deformation front of the
Nankai accretionary prism in tsunamigenic earthquakes and slow slip in the
shallow portion of the subduction interface. The CLSI@Sea workshop was
organized onboard the D/V Chikyu concurrently with IODP Expedition
380, allowing workshop participants to interact with expedition scientists
installing a long-term borehole monitoring system (LTBMS) at a site where the
workshop's research was focused. Sedimentary cores from across the
deformation front were brought onboard Chikyu, where they were made
available for new description, sampling, and analysis. Logging data, drilling
parameters, and seismic data were also available for investigation by
workshop participants, who were granted access to Chikyu laboratory
facilities and software to perform analyses at sea.
Multi-thematic presentations facilitated knowledge transfer between the
participants across field areas, and highlighted the value of
multi-disciplinary collaboration that integrates processes across different
spatiotemporal scales. The workshop resulted in the synthesis of existing
geophysical, geologic, and geochemical data spanning IODP Sites C0006,
C0007, C0011 and C0012 in the NanTroSEIZE area, the identification of key
outstanding research questions in the field of shallow subduction zone
seismogenesis, and fostered collaborative and individual research plans
integrating new data analysis techniques and multidisciplinary approaches.
NameSpecialityInstitutionEmailAnna CerchiariStructural geologyUniv. of Modena (It.)anna.cerchiari@gmail.comRina FukuchiSedimentology, structural geologyJAMSTEC (Jap.)rfukuchi@jamstec.go.jpBaiyuan GaoGeomechanics, physical propertiesUniv. of Texas (USA)baiyuan@utexas.eduKan-Hsi HsiungSedimentology, structural geologyJAMSTEC (Jap.)hsiung@jamstec.go.jpDominik JaegerLithostratigraphy, sedimentologyUniv. of Innsbruck (AUT)Fabian.Jaeger@student.uibk.ac.atShunya KanekiGeochemistry, physical propertiesOsaka Univ. (Jap.)skaneki@ess.sci.osaka-u.ac.jpJonas KellerLithostratigraphy, sedimentologyUniv. of Innsbruck (AUT)Jonas.Keller@student.uibk.ac.atSzu-Ting KuoGeomechanics, physical propertiesTexas A&M Univ. (USA)teddythebest@tamu.eduGaël LymerGeophysics, structural geologyUniv. of Birmingham (UK)g.lymer@bham.ac.ukTatiana MaisonSedimentology, structural geologyUniLaSalle (Fr.)Tatiana.Maison@unilasalle.frGinta MotohashiPhysical propertiesUniv. of Tsukuba (Jap.)ginta_m@geol.tsukuba.ac.jpChristine RegallaStructural geologyBoston Univ. (USA)cregalla@bu.eduDrake SingletonGeophysics, sedimentologySan Diego Univ. (USA)dsinglet@ucsd.eduSuguru YabeGeophysics, seismologyJAMSTEC (Jap.)syabe@jamstec.go.jpCLSI@Sea mentors Keir BeckerIODP Exp. 380 Co-ChiefUniv. of Miami (USA)k.becker@miami.eduKyuichi KanagawaStructural geologyChiba Univ. (Jap.)kyu_kanagawa@faculty.chiba-u.jpGaku KimuraStructural geologyTokyo Univ. of Marinegkimur0@kaiyodai.ac.jpScience and TechnologyMasa KinoshitaIODP Exp. 380 Co-ChiefUniv. Of Tokyomasa@eri.u-tokyo.ac.jpGregory MooreGeophysicsUniv. of Hawaii (USA)gmoore@hawaii.eduDemian SafferPhysical propertiesPenn. State Univ. (USA)dms45@psu.eduMichael StrasserSedimentologyUniv. of Innsbruck (AUT)Michael.Strasser@uibk.ac.atKiyoshi SuyehiroSeismologyJAMSTEC (Jap.)suyehiro@jamstec.go.jpMichael UnderwoodLithostratigraphyUniv. Missouri (USA)underwoodm@missouri.eduStaff scientists Yukari KidoLogging scientistJAMSTEC (Jap.)ykido@jamstec.go.jpLena MaedaLab managerJAMSTEC (Jap.)maedal@jamstec.go.jpYoshinori SanadaLogging scientistJAMSTEC (Jap.)sanada@jamstec.go.jpSean ToczkoIODP Exp. 380 project managerJAMSTEC (Jap.)sean@jamstec.go.jpIntroduction
Subduction zones account for 90 % of global seismic moment release and
generate damaging earthquakes and tsunamis with potentially disastrous
effects on heavily populated coastal areas (e.g., Lay et al., 2005; Moreno et
al., 2010; Simons et al., 2011). Seismologic, geodetic, and borehole
observatory data from subduction zones throughout the globe indicate that the
shallow portion of the subduction zone may accumulate and release strain
through a variety of deformation mechanisms (seismogenic slip, creep, slow
slip, tremor) over a range of timescales (seconds, weeks, months, years; Peng
and Gomberg, 2010).
The Core-Log-Seismic Integration at Sea (CLSI@Sea) workshop was held from 12
January to 7 February 2018 onboard D/V Chikyu in the Nankai Trough
subduction zone, off southwestern Japan. This workshop was developed to
enhance multidisciplinary research to address the role of accretionary prism
frontal deformation in tsunamigenic earthquakes and slow slip in the shallow
portion of the subduction interface. A singular aspect of CLSI@Sea lay in the
fact that it was designed to leverage existing archives of IODP cores and
logging data and associated seismic datasets previously acquired as part of
the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) program. Recent
findings from the 2011 Tohoku-Oki mega earthquake in the Japan Trench indeed
provided evidence that tsunamigenic slips can be continuous from the locked
portion of the subduction plate boundary all the way out to the trench
(Chester et al., 2013). Similar behavior has been previously documented in
the shallow portion of the Nankai subduction zone (Kinoshita et al., 2009;
Sakaguchi et al., 2011; Ito et al., 2013), within the NanTroSEIZE study area,
thus highlighting the interest in re-investigating archived NanTroSEIZE data
to characterize the nature of fault slip and strain accumulation, fault
architecture, and state variables throughout subduction plate boundary
systems.
The CLSI@Sea workshop represented an original approach in several aspects.
First, CLSI@Sea was an unprecedented opportunity to examine legacy data from
multiple former expeditions in the context of a mission not dedicated to core
recovery. CLSI@Sea workshop was the first of its kind organized concurrently
with an International Ocean Drilling Program (IODP) expedition. Expedition
380 installed a long-term borehole monitoring system (LTBMS) at Site C0006,
above the deformation front of the Nankai accretionary prism, thus allowing
workshop participants to investigate archived data from the site where the
IODP expedition was focused and interact with the expedition science party.
Second, a challenging aspect of the workshop was to connect a group of
science mentors with extensive experience in the Nankai margin with
early-career researchers from diverse research backgrounds (Table 1), to work
on common research questions. Third, workshop participants were given the
opportunity to pursue new research while onboard Chikyu, thanks to
full access to shipboard laboratory facilities to re-investigate the IODP
archived data. Well-preserved sedimentary cores were brought onboard
Chikyu from the Kochi Core Center and made available for laboratory
analyses. Seismic reflection, log, and drilling parameter data were also
kindly provided to all workshop participants onboard.
The international team of workshop participants was then able to develop
interdisciplinary discussions and organize both individual and collaborative
research plans to address outstanding questions regarding seismogenic-,
tsunamigenic- and slow-slip processes in the Nankai subduction zone, a
crucial topic for the regional tectonics, but also a fundamental aspect of
the Earth's geodynamics.
Background and geological setting
The Nankai Trough is formed by the subduction of the Philippine Sea Plate
beneath the Eurasian Plate, forming the Nankai prism by the accretion of the
Shikoku Basin oceanic plate sediments (Fig. 1). The complex geodynamic
evolution of the Shikoku Basin, including the migration of the boundaries of
the Amurian, Pacific and Philippine plates over time (e.g., Moore et al.,
2015), has resulted in large lateral variations in basement relief, with
associated variations in the nature and thickness of sediments, resulting in
structures highly variable laterally within the accretionary prism (Fig. 1).
In cross section (Fig. 2), the deformation front is located at the toe of the
prism, at the boundary with the deepest part of the trough. Upslope from the
prism toe (Fig. 2), several landward-dipping imbricated thrusts and
associated anticlines and back-thrusting branches form together the Imbricate
Thrust Zone (ITZ; Park et al., 2002; Moore et al., 2009). The thrusts of the
ITZ sole into the basal décollement, corresponding to a strong
continuous positive polarity reflection on the seismic reflection data. A
potential inactive décollement occurs ∼500 m above the
active one in a zone of the low P-wave velocity zone that may correspond to
underplated hemipelagic material (Kamei et al., 2012). Beneath the upper
slope, two distinctive branches of a first-order thrust correspond to the
Mega-Splay Fault (MF; Park et al., 2002, 2010). The MF cuts across the older
part of the accretionary prism and can be traced deep (∼10 km) toward
the top of the subducting plate (Fig. 2). The MF extends more than 120 km
along strike (Fig. 1; Moore et al., 2007; Park et al., 2010) and is strongly
suspected to be one of the primary coseismic faults that may have contributed
to generating devastating historic earthquakes and tsunamis (Tanioka and
Satake, 2001; Kikuchi et al., 2003; Wang and Hu, 2006). Landward of the MF,
the oldest parts of the prism are overlain by marine sediments accumulated in
the Kumano forearc basin, bounded to the southeast by a topographic valley at
the limit between the basin and the upper part of the trench slope (Fig. 2).
This valley corresponds to the Kumano Basin edge fault zone (KBEFZ) that may
have a combination of normal and strike–slip faults (Martin et al., 2010).
Synthesis map of the Nankai Trough offshore southwestern Japan. The
area of investigation of the CLSI@Sea workshop corresponds to the black
frame, including Site C0006, the target of IODP Expedition 380. The colors
show the stage of evolution of the deformation within the Nankai Trough. The
two sections across the Kumano and Muroto transects are shown in Fig. 2.
Compiled and modified from Kimura et al. (2014) and Moore et al. (2001,
2009).
Synthesis structural cross sections across the Nankai accretionary
prism along the Kumano and Muroto transects. See location in Fig. 1. The
colors are the same as Fig. 1 and refer to the stage of evolution of the
deformation within the Nankai Trough, namely, blue: non-accreted sediments;
red: accreted sediments less than 2 My ago; green: accreted sediments more
than 2 My ago. Modified from Moore et al. (2001) and (2009).
The Nankai subduction zone has a 1300-year historical record of recurring
tsunamigenic earthquakes, including the 1944 Tonankai Mw 8.2 and
1946 Nankai Mw 8.3 earthquakes (Fig. 1), and has been the focus
of worldwide marine scientific surveys, including the NanTroSEIZE drilling
program. NanTroSEIZE focuses on the Kumano transect of the Nankai
accretionary prism (Fig. 2). To date, the program has involved 11 oceanic
expeditions, 13 sites of coring and logging, collection of 2-D and 3-D
seismic data, and installation of a network of LTBMS' recording in situ
data within the Nankai accretionary prism. Collectively, these data provide
an unprecedented record of the margin's stratigraphic and structural
evolution. Within the NanTroSEIZE study area, Sites C0006 and C0007 (IODP
Expeditions 314 and 316; Kimura et al., 2008; Kinoshita et al., 2008 and
2009) located at the deformation front of the Nankai accretionary prism
(Fig. 1), and Sites C0011 and C0012 (IODP Expeditions 322, 333, and 338;
Underwood et al., 2010; Henry et al., 2011; Strasser et al., 2014), located
oceanward of the deformation front, were key targets for the CLSI@Sea
workshop to investigate the most recent deformations within the prism toe. In
addition, Site C0006 is considered to be outside of the major earthquakes'
nucleation zone (Hyndman and Wang, 1993; Oleskevich et al., 1999). However,
normal and slow earthquakes (low-frequency tremors, very low-frequency
earthquakes, and slow slip events) all seem to possibly occur inside the core
sample zone of Sites C0006 and C0007. The installation of the LTBMS during
Expedition 380 provides the opportunity to collect data to facilitate
understanding of how the seismic slips are controlled at this particular zone
of the deformation front, possibly one of the most studied areas on Earth in
the quest to better understand tsunamigenic earthquake processes. Equally,
the purpose of the CLSI@Sea program is to enhance understanding of this area
by re-visiting previously acquired data and core material from the region.
Workshop organization
The CLSI@Sea workshop brought together a team of international scientists,
mentors and early-career researchers (Table 1) to address outstanding
questions on tsunamigenic earthquake processes at plate subduction zones,
specifically investigating the Nankai Trough (Fig. 1). A fundamental
characteristic of the workshop was the multi-disciplinary expertise of the
participants, selected on the basis of their research proposals aiming to
investigate different aspects of subduction mechanisms and related
processes. Scientific self-introductions during the first days of the
workshop were essential to identify the research interests of each
participant (Table 1) and develop collaborative research plans based on
multi-disciplinary teamwork. Scientific talks given by the mentors and IODP
Expedition 380 Co-Chiefs (Table 1) provided an overview of the geological
processes identified over the last 10 years at the Nankai subduction zone,
at different levels and at different scales of the NanTroSEIZE area, from
the seafloor to the top of the subducting plate. Presentations by both
mentors and workshop participants helped to trigger discussions and
facilitate knowledge transfer between groups, which represented crucial
points to identify the key research questions and develop both individual
and collaborative research strategies.
Lithological section across the Nankai frontal prism (Sites C0006
and C0007) and the input sites (C0011 and C0012). The columns at each site
show core recovery, lithologic units, sedimentary age, and lithology
distribution modified from Kinoshita et al. (2009), Strasser et al. (2014),
and references therein.
Workshop participants developed efficient self-organization, supported by
personal initiatives contributing to the CLSI@Sea community research
objectives. The participant group self-organized, supported by individual
research plans that had been proposed to contribute to the CLSI@Sea program
research objectives. As is always the case with highly multi-disciplinary
collaborative projects such as IODP, a degree of flexibility and compromise
was also required, with initial research plans being modified and adjusted to
avoid duplication of effort and to ensure the best fit with the over-arching
program objectives. In parallel to personal research plans, group research
was undertaken by small teams of participants focused on generating a
comprehensive synthesis of previous Nankai Trough research arising from IODP
expeditions (see Sect. 4), to help identify any outstanding scientific
questions. In addition, this collaborative work extended to the development
of a database of the scientific literature generated by previous NanTroSEIZE
projects and the submission of this workshop report to promote the
international, multi-disciplinary CLSI@Sea initiative.
Personal research projects centred on the analysis of the seismic reflection
data, logging data and sedimentary cores from Site C0006 (Holes C, D, E, F),
Site C0007 (Holes A, B, C, D) and Site C0012 (Hole A) (see Fig. 1 for
location). In particular, Sites C0006 and C0007 provided the opportunity to
examine cores from the Pliocene and Miocene sections down to the
décollement (Figs. 4 and 5) at the deformation front. In line
with IODP protocols, sample requests had to be submitted and approval
received prior to any sampling and analysis of the core material. Software
and data handling training was also provided by laboratory technicians to
participants to improve efficiency in executing their research projects while
onboard Chikyu. These individual and collaborative efforts were
supported by daily meetings in which the mentoring team provided workshop
participants with input and guidance for their ongoing work. The research
projects undertaken by the participants included (Sects. 4 and 5)
investigations on an accurate age model for the accretionary prism's frontal
thrust development; seafloor/sub-bottom morphological response to subduction
along the Nankai Trough; P-wave velocity–porosity relationship in the
deformation front area; and multi-dimensional analysis of plate boundary
faults.
Finally, CLSI@Sea participants were invited to closely follow the progress of
Expedition 380, especially in terms of drilling processes and day-to-day
technical challenges occurring during IODP expeditions.
Integration of legacy IODP data
The coordination of the synthesis of previous IODP NanTroSEIZE projects made
to facilitate the organization of future individual and collaborative
research plans resulted in the formation of research teams focused on three
main sub-topics: lithostratigraphy, tectonic structures of the Nankai prism
and physical properties of the deformation front. The goal of these research
teams was to build a compilation of the formerly published results in the
different sub-topics, and across the different IODP sites investigated during
this workshop. This compilation was a fundamental aspect of the workshop, to
create a state-of-the-art of the existing results and bring up new scientific
questions, but also to set up a dynamic work collaboration between the
participants.
Lithostratigraphy of the frontal prism and incoming sediments
The workshop resulted in a new synthesis of the lithostratigraphy and
chronostratigraphy for reference Sites C0011 and C0012 and frontal thrust
zone Sites C0006 and C0007 (Fig. 3). Reconstruction of the regional
sedimentation history and the plate boundary evolution was considered through
lithological and stratigraphic analysis of core and data from the incoming
Philippine Sea Plate. The original intent of coring ahead of the prism (Sites
C0011 and C0012, Fig. 1) was to provide a largely undisturbed record of input
sediment cover that could be used to decipher the timing of the formation of
the structures at the deformation front (Sites C0006 and C0007, Underwood et
al., 2010; Henry et al., 2011; Strasser et al., 2014). The onset of
deformation was identified based on the lateral changes in thickness of the
upper wedge deposits (lithologic Units I and II in C0006 and C0007, Fig. 3),
a key target for scientific questions. During CLSI@Sea, the temporal
resolution of stratigraphic units was investigated, and the efficiency of new
crystallographic analytical tools to better understand accretionary wedge
deformation evolution was tested. Below we briefly summarize the correlated
stratigraphy at each site and variations in bulk mineralogical content.
Sites C0011 and C0012 transect the entire incoming sediment sequence of the
Shikoku Basin to basaltic ocean crust (Fig. 3) (Underwood et al., 2010;
Strasser et al., 2014). Unit I corresponds to late Pleistocene to late
Miocene Upper Shikoku Basin deposits, and contains silty clay with minor ash.
Unit II is marked by the occurrence of volcaniclastic sandstones in silty
claystone and corresponds to late Miocene Middle Shikoku Basin deposits.
Units III to V correspond to late to middle Miocene Lower Shikoku Basin
deposits. Unit III is a succession of uniform silty claystone and lime
mudstone; Unit IV contains silty claystone/clayey siltstone comprising
fine-grained, turbiditic sand layers; Unit V contains tuffaceous sandy
siltstone with some silty claystone and tuff. The oldest units recovered
include calcareous mudstone (Unit IV) overlying oceanic basement basalts
(Unit VII). The dominant mineral assemblages are quartz, feldspar, clay
minerals and calcite. No major trends in mineral content are observed within
Units I and V, but Unit VI has an overall higher clay content and lower
quartz and feldspar contents compared to overlying units.
Sedimentary, logging and seismic data at the deformation front of
the Nankai prism (Sites C0006 and C0007). Dark blue markers (strips, dots)
are additions provided by the CLSI@Sea workshop. The figure shows (from left
to right) the logging data at Hole C0006B, the interpreted seismic section
(IL 2435) across the Nankai 3-D volume illustrating the structures at the
deformation front, and the lithological units and the logging units
identified at both Sites C0006 and C0007. Some structures (dark blue) have
been identified by integration of the Core-Logging-Seismic data. Figure
compiled and modified from Kimura et al. (2008) and Kinoshita et al. (2008,
2009).
(a–e) Density, CT number, porosity, P-wave velocity
(VP), and rate of penetration (ROP) along depth at Site C0012.
Density, porosity and VP data are from discrete samples of Holes
C0012A, C0012C, and C0012G. ROP is measured at Holes C0012B and C0012H. CT
measurements on cores are from Holes C0012A, C, D, E, F, and G.
(f) The correlation between CT value and density value at C0006,
C0007, C0011, and C0012. (g) The correlation of porosity and
VP at Sites C0011, C0012, 1173, and 1177.
Sites C0006 and C0007 transect accreted Shikoku basin deposits and overlying
wedge slope deposits (Kinoshita et al., 2009). Unit I contains unconsolidated
hemipelagic mud and turbidites deposited in a wedge slope environment
(Kinoshita et al., 2009). Unit II contains alternating sequences of
Pleistocene hemipelagic muds and turbidites deposited in a transitional
trench–wedge environment. Unit II, which directly overlies the
décollement at Sites C0006 and C0007, is made up of accreted
sediments correlated with the Pliocene to Upper Miocene in the Shikoku Basin.
Unit IV, recovered below the décollement at Site C0007, consists
of a small (∼15 cm) section of dark, medium to coarse-grained sands.
As in Site C0012, the dominant mineral assemblage is feldspar, clay minerals
and calcite, and does not display major trends in mineral content.
New investigation and sampling of the sedimentary cores from Sites C0006,
C0007 and C0012 during the CLSI@Sea workshop involve the following methods:
new tephrachronology/geochronology, clay mineral composition analysis, and
macroscopic and XCT re-observation of sedimentary structure. These methods
will provide data that will help address the following outstanding questions.
What is the provenance of accretionary prism body, slope, and fault
sediments? How well can we correlate incoming Shikoku basin stratigraphy
with off scraped sediments in the frontal prism?
Is it possible to link the origin of the Nankai prism sediments with the
migration of the Philippine Sea–Pacific plate boundary over the Miocene to
the present?
How well can we reconstruct the development and the evolution of the prism
from the tectono-stratigraphic framework of the Kumano Basin?
Tectonic structure of the frontal thrust
The workshop results in a new synthesis of structural features observed in
core, log, and seismic data from the incoming plate and frontal wedge of the
Nankai prism, including identification of new faulted intervals in the formal
prism. The structures of the Nankai prism at the deformation front, where the
thrust–fault activity is the youngest (Fig. 1) and the compressional
deformation propagates oceanward (Moore et al., 2009; Underwood and Moore,
2012), were examined. Proposed research projects arising from CLSI@Sea and
focusing on the structures of the subduction zone included scientific
questions related to the timing, amplitude and mechanisms of fault slips, the
relationships between deformations and shallow sedimentary processes and
their link with potentially tsunamigenic events, especially earthquakes and
generation of mass transport deposits. Specifically, the integration and
compilation of structural and lithologic data (Fig. 4) across multiple IODP
reports and publications allow for the refinement of the presence, extent,
and slip history of faults at the deformation front of the prism. Below we
briefly summarize structural data from core and log data at Sites C0012,
C0006 and C0007 before discussing new observations made during the workshop.
Site C0012 is located seaward of the deformation front. Bedding orientations
at this site are dominantly subhorizontal, with intervals of higher angle
bedding resulting from gravitational slumping, and high angle fractures
resulting from subvertical compaction. Sites C0006 and C0007 penetrated the
first two imbricate thrusts at the deformation front, including the main
frontal thrust at ∼700 m LWD depth below seafloor (LSF) (Figs. 2, 3
and 5; Kinoshita et al., 2008, 2009). Bedding at Site C0007 is dominantly
subhorizontal, but a major lithologic inversion at ∼400–450 m b.s.f.
places moderately consolidated hemipelagic mudstones over poorly consolidated
trench turbidite sands. Site C0006 can be divided into four log units
corresponding to distinct structural domains (Fig. 4). Unit I
(0–100 m LSF) has generally west-dipping bedding resulting from
northwestward tilting driven by plate convergence and southwestward tilting
driven by gravitational slumping. Unit II (100–220 m LSF) is a thrust zone
that contains several faults identified in core, log and seismic data
(Fig. 5). Units III and IV (below 220 m LSF) contain northwestward-dipping
beds and fractures, consistent with north-northwestward-directed shortening
driven by plate convergence.
In addition to previously reported faults, workshop work resulted in the
observation of new faults correlated thanks to the integration of core, log
and seismic data (blue strips in Fig. 4). Cores from Sites C0006 and C0007
intersect several major faults, including the plate boundary interface
intersected at Site C0006 at ∼700 m LSF and Site C0007 at ∼400 m LSF (Kinoshita et al., 2008, 2009). In the cores, faults largely
occur as breccias, gouges, and zones with striated fractures, in which the
sense of displacement is often difficult to observe directly. A black
gouge-bearing fault zone recovered in the core from Site C0007 at 438 m LSF
exhibits a vitrinite reflectance anomaly interpreted to reflect shear heating
during past seismic slip to the trench (Sakaguchi et al., 2011). In log data,
faults can be identified by simultaneous decreasing of the gamma ray and
resistivity values. Cross-comparison of core and log data with 3-D seismic
reflection data across the Kumano transect allowed observation of the
persistent lateral continuity of the main thrusts across the seismic volume,
whereas secondary and tertiary branches of the thrusts show important lateral
variations in three dimensions.
Continued structural analysis of the site C0006 and C0007 data involving
resistivity log fracture analysis, thin section observations, high-resolution
CT scans, re-interpretation of the NanTroSEIZE 3-D volume and geochemical
analysis across fault zones will be used to address the following questions.
What is the temporal history of slip along frontal prism faults? Which
sedimentary horizons are offset by thrusts? Do any thrusts breach the
seafloor?
Which structural features exist in the region where seismic tremors have
been identified, and which may be genetically linked to tremorgenic
processes?
What are the distribution, origin and timing of the mass transport
deposits along the Kumano transect and are they temporally linked with fault
activity?
What is the 3-D architecture of the prism? How strong are the lateral
variations of the structures within the prism?
Physical properties in the frontal thrust and incoming plate
The workshop also led to new compilations and analysis of log, computed
tomography (CT), and rate of penetration (ROP) data that will promote
research on the physical properties (PPs) of the sedimentary units and faults
recovered from the cores. These include establishing a relationship between
the P-wave velocity (VP) and the porosity within the sedimentary
layers; mapping the physical properties within the prism; estimating stress
within the prism; quantifying the frictional heat generated during fault
slips, particularly at the décollement; investigation of the
slip properties along the faults in order to understand the slip behavior
from temperatures; and mapping the porosity distribution. PPs of drilled
cores are measured onboard Chikyu using a Multi-Sensor Core Logger
(MSCL) system, discrete-sample measurements, and CT images. Each of these
measurements has limitations and was not previously well integrated. For
example, the MSCL system can measure PPs of intact and split cores
non-destructively, but the measurement is sometimes scattered if the
recovered cores are fractured. PPs can also be measured precisely using
discrete samples; however, the results from discrete samples are conducted
intermittently and only represent a portion of the recovered cores.
Concerning CT images, they are taken as a first step before core splitting
and are thus able to reveal the continuous internal structure of a drilled
core without any destruction. Such images therefore represent a high
potential for future studies investigating, e.g., 3-D PPs distributions or
fracture mechanics considering small-scale structure around the fault.
Compilation and integration of various types of data including CT scan
analysis can then provide continuous and more accurate PPs for whole drilled
cores. Below we summarize new synthesis and analyses completed during the
CLSI@Sea workshop.
Investigations on PPs included analysis on density, porosity, and
compressional P-wave velocity of the incoming plate sites (Sites C0011 and
C0012, Fig. 1) and the frontal thrust sites (Sites C0006 and C0007). CT
values are converted into density using the following equation (Kinoshita et
al., 2009):
CTvalue=fmaterial-fwaterfwater×1000,
where fwater and fmaterial are, respectively, the
linear attenuation coefficients of water and the measured material. The
attenuation coefficient is a function of the chemical composition and the
density of the material. We find that the relation between CT value and
density for input sites (C0011 and C0012) lies well on a straight line
(Fig. 5), but there is a slight difference from the relation obtained for
sites in the accretionary prism (Conin et al., 2014). Although the dependency
on lithology and chemical compositions has been investigated in relation to
onboard visual core descriptions and X-ray fluorescence analysis, the cause
has not been clarified yet. The porosity–depth and porosity–VP
relationships in the outboard Sites C0011 and C0012 follow a similar trend to
site 1173 (Muroto transect, Fig. 1). Hoffman and Tobin (2004) have shown an
empirical relationship of porosity and VP based on the Muroto
transect. Here we incorporate the NanTroSEIZE data (C0011, C0012) and define
a modified empirical relationship based on Hoffman and Tobin (2004) (Fig. 5).
C0006 and C0007 have limited and scattered discrete core data; thus, they are
not included to constrain the porosity–VP trend.
ROP is one of the key parameters to assess the drilling efficiency and can be
used to estimate the sliding friction coefficient (Pessier and Fear, 1992).
We compile ROP at Sites C0006B and C0012H where drilling parameters are
available and compare the trend to the core-scale physical properties. The
first 100 m of a dataset may not be appropriate for further calculation as
there is obvious fluctuation. Future work includes investigation of the
impacts of different drilling methods and comparison of the mechanical
properties of different lithological units.
The spatial distribution of PPs (especially porosity) contains key
information of the degree of compaction and stress states around faults and
décollement. As an example of CLSI@Sea using compiled PPs, we
estimated the porosity distribution along the NanTroSEIZE transect. The
VP dataset (Moore et al., 2007; Park et al., 2010) covers the
locations from the inner wedge (near Site C0009) to the deformation front
(Sites C0006 and C0007) and to the seaward sites (C0011, C0012). We are then
able to apply the empirical VP–porosity relationship to estimate
the porosity distribution. From the preliminary results, we clearly see that
there is a high-porosity zone along the décollement associated
with the low-velocity zone (Park et al., 2010). This high-porosity zone
indicates under-compaction and may suggest high excess pressure during
tectonic loading. Future work will include (i) mapping the porosity
distribution more precisely by integrating the data in the inner wedge (e.g.,
Site C0002, Fig. 1), and (ii) estimating the stress and pressure state using
the improved porosity distribution results.
Additional post-cruise research involving data compilation, consolidation
tests, frictional heating experiments, refinements to velocity structure
models and log–seismic correlations will be used to address the following
research questions.
What is the ancient and modern thermal state in the frontal prism, and how
is it modified by frictional heating and fluid flows?
What is the porosity and pore fluid pressure evolution in the accretionary
prism, basal décollement, and under-thrust sediments?
What are the roles of faults and fractures in fluid circulation within the
frontal prism and deformation zones?
Bulk density distributions converted from the CT value for section
C0006E31X05 110–128 cm. Two cross sections through the centre of the core
are presented. A 3-D distribution of this section is presented as a movie in
the Supplement.
New findings and opportunitiesCore-log-seismic integration at the deformation front of the Nankai
accretionary prism
One of the main objectives of the CLSI@Sea workshop was to provide
participants with the opportunity to undertake research based on legacy IODP
data acquired in the frontal thrust (Fig. 1, Sites C0006, Holes C, D, E, F,
and C0007, Holes A, B, C, D) and outboard regions (Site C0012, Hole A) of the
Nankai prism. Through individual and collaborative research, CLSI@Sea
participants conducted preliminary analysis during the workshop, using
integration of core, logging and seismic data. The cores were brought onboard
Chikyu for the workshop and made available for sampling, allowing
the participants to obtain a total of 519 core samples to analyze the
lithological properties, micro-fabrics, grain size distribution, and fracture
characteristics.
An example of core-log-seismic integration concerns investigations of the
fault zone at Site C0006 (Fig. 4). Although several fault zones are reported
in the expedition reports of this site (see Sect. 4, Kimura et al., 2008, and
Kinoshita et al., 2008, for more details), studies of fault zones so far have
mainly concentrated on the Mega-Splay Fault (Fig. 1) and the toe of the
décollement (Site C0007, Fig. 4). As a first step of the
investigation, seismic reflection data have been checked to identify major
fault zones with sufficient displacement. The depths of identified faults
have then been estimated by studying the logging resistivity images and core
CT images. Based on the estimation of the depth of the faults, stored cores
have been thoroughly observed, allowing identification of several faults,
which were consequently sampled for further analysis.
During CLSI@Sea, we recognized the importance of quantitatively integrating
CT images into other data; CT values are indeed meaningful since they reflect
the physical and chemical properties of the material as addressed in
Sect. 4.3. Despite the great non-destructive advantage of CT images, only a
few studies have used the information of CT values in the NanTroSEIZE project
so far (e.g., Conin et al., 2014). Figure 6 (and the Supplement) shows the
3-D bulk-density distribution of one identified fault with a proper color
scale, which is usually with grey scale in the original CT images (see
Fig. 24 of the expedition report by Kinoshita et al., 2009). A 3-D image of
CT-value-derived bulk density with millimeter-scale resolution was thus very
powerful in identifying potential faults.
Post-cruise studies
The time spent onboard Chikyu during IODP Expedition 380 was the
first step of a longer-term renewal of the NanTroSEIZE research program
resulting from the CLSI@Sea workshop investigations. Once back in their home
institutions (Table 1), CLSI@Sea participants will make progress with their
individual and collaborative research projects that were initiated onboard
Chikyu. The targets of these studies fall into three main scientific
fields: (i) lithology and stratigraphy (paleo-temperature, frontal prism and
incoming plate stratigraphy, turbidites, sedimentary flow paths, and clay
minerals); (ii) tectonic structures (along- and across-strike geometry of
thrusts, faults and relationship with the décollement and
shallow sedimentary processes, age and evolution of the deformation, and
fracture distribution); (iii) physical properties (porosity, stress, heat,
shear deformation, P-wave velocity, friction, fault slip behavior, and
sedimentary consolidation). The future laboratory analysis and experiments on
the sedimentary cores sampled onboard Chikyu will include optical
microscope observation, scanning electron microscope (SEM) observation,
energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) analysis,
X-ray CT, triaxial tests, friction experiments, radiogenic decay, and
vitrinite reflectance measurements. The logging, seismic reflection, and
velocity data will be analyzed using geophysical interpretation tools, and
will eventually be correlated with the new results. The successful
installation of the LTBMS on Site C0006 during Expedition 380 will also
provide additional in situ data for NanTroSEIZE investigators, including
real-time pressure, strain, and seismological data. Connection of the Site
C0006 LTBMS to the Dense Oceanfloor Network System for Earthquakes and
Tsunamis (DONET) cabled network managed by the Japanese Agency for
Marine-Earth Science and Technology (JAMSTEC) will allow provision of crucial
day-to-day global records of the Nankai prism activity to understand the
evolution of the subduction zone.
Discussing all together the different open targets raised within each
research sub-group, workshop participants agreed that their correlation of
former results (Sect. 4) and future analysis would allow them to answer key
questions that can be summarized as follows.
What governs subduction zone seismogenic fault locking vs. stable-slip and/or
transitional fault behavior? Is there an “updip limit” to the seismogenic
zone, and what controls its spatio-temporal evolution? What governs tsunami
generation characteristics for a given great earthquake?
What are the geologic, physical, and chemical signatures of slow slip,
tremor, and seismogenesis? What are the deformation mechanisms recorded in
the frontal prism and how do the length scales and timescales of their
formation relate to the observed length scales and timescales of strain
accumulation and release?
Does fault state evolve during interseismic and pre-seismic periods? If so,
how?
What do fault zone structure and compositions reveal about slip mechanisms
in tsunamigenic frontal ramp thrusts? What are the implications of variable
properties for the fault zones and their architecture, evolution, and slip
behavior?
What are the geomechanical, frictional and physical properties of the fault
zones and wall rock in the overriding and subducted plate? How do these rock
properties vary spatially in the volume of rock away from the borehole, as
sampled by 3-D seismic data? How are these properties related to the in situ stress
state and strain accumulation?
What is the ancient and modern thermal state in the frontal prism, and how
is it modified by frictional heating and fluid flow?
What is the porosity and pore fluid pressure evolution in the accretionary
prism, basal décollement, and under thrust sediments? What are
the roles of faults and fractures in fluid circulation within the frontal
prism and deformation zones?
What are the distribution, origin and timing of mass transport deposits and
how may these relate to past seismogenic (tsunamigenic) slip, eustacy, and
climatic variations?
What is the sediment provenance of Nankai sediments? What can this tell us
about forearc evolution and plate boundary migration over the Miocene to
present?
What is the sequence stratigraphic and tectono-stratigraphic framework of
the Kumano basin and forearc? What is the relationship between basin
development, uplift, eustasy, or tectonism in the accretionary wedge and
trench?
What is the 3-D spatio-temporal evolution of accretion and fault slip in the
frontal prism? How do the timing and rate of slip on prism faults relate to
seismogenesis? How does prism evolution respond to changing subduction
parameters?
The CLSI@Sea workshop has demonstrated the value of re-investigating archive
data. The results arising from post-cruise research undertaken by the
participants has the potential to further improve our understanding of the
Nankai Trough and subduction processes.
Lessons learned and suggestions for future workshops
The CLSI@Sea workshop was the first of its kind to run alongside an IODP
expedition. The initiative was motivated by a need to re-investigate legacy
data, and to bring “new eyes” to the NanTroSEIZE program. Some suggestions
arising from this new initiative were the following.
Multi-disciplinary collaboration. One of the keys in the efficacy and the success of the CLSI@Sea workshop
was the multi-disciplinarity of the participant pool (Table 1), resulting in
a complementary approach to the scientific questions, which demonstrates the
value of group collaboration in tandem with individual research projects.
Results from studying the evolution of the Nankai Trough highlighted the
multi-parameter factors of the complex processes occurring at this plate
boundary, and the necessity to address subduction and earthquake mechanisms
with multiple methodologies that integrate across different scales of space
and time. Similar initiatives on specific research thematics or regional
problematics, including data and core material from former IODP expeditions,
could represent an efficient mechanism by which research value could be added
to the IODP legacy.
Communication and focussing. Connecting specialist mentors and early-career scientists from different
backgrounds successfully resulted in a transfer of knowledge between both
parties. Lively discussions in a cordial atmosphere and teamwork between
mentors, participants and IODP staff in the Chikyu focused
environment were key components in the development of scientific questions
and efficient formulation of both individual and collective research plans.
Face-to-face (or “face-to-direct-live-screen”) interactions during talks
given by the mentors were crucial to confirm the current state-of-the-art and
to avoid duplication of efforts. Self-introduction of workshop participants
was also a fundamental point to highlight individual research interests and
build research teams and collaborations. Pre-workshop communication
(mentors–participants, participants–participants, collection of
publications and list of available data during the workshop) represents
efficiency in identification of research interests, organization of
collaborations, and improvement of individual knowledge and research
strategy.
Data availability and sampling. Training on data analytical techniques, school sampling and software
support provided by scientific staff was crucial to facilitate access to and
work on the data. During CLSI@Sea, workshop participants had the opportunity
to access public data (IODP archives) and restricted data (3-D seismic
reflection data) kindly provided for investigations. A major issue concerning
the archived sedimentary cores is the limited amount of data due to the
nature of the cores themselves, by definition limited in terms of quantity of
available material. Thus, complete missing sections of cores previously
already sampled (fault planes particularly) represented important issues to
the participants aiming to analyze the sedimentary records of the
deformation, as well as for future researchers interested in investigating
sedimentary archives theoretically available for the entire scientific
community. This issue could be resolved at long term with the development of
non-destructive analysis, such as X-ray CT (see Sects. 4.3 and 5.1).
Motivation and self-organization. The motivation, initiative, and self-organization of the workshop
participants were essential keys to the program's success. Most of the ideas
on the organization of the research aspect of the workshop (see Sect. 3) came
from the participants, who were free to organize their research and expose
their ideas to the other participants, supported by the staff scientists and
the mentors. This freedom allowed the ideas to arise easily from the
discussion sessions and individual thinking, and were a fundamental aspect of
the success of the CLSI@Sea workshop.
Summary
The Nankai Trough is one of the most active plate margins of the planet, and
one of the most studied, being the focus of many IODP expeditions and related
surveys that collectively acquired a huge quantity of data over the last
decades, thus being a key area for the understanding of tsunamigenic
earthquakes. The CLSI@Sea workshop demonstrated the value of revisiting IODP
archives and integration of different types of data, core, logging and
seismic, providing indications of several processes acting on different
scales. The success and the productivity of CLSI@Sea thus demonstrated the
interest in organizing such a workshop for any scientific discipline. The
workshop participants formulated scientific questions to better understand
the sedimentary, structural and physical aspects of subduction processes.
Analyses initiated onboard Chikyu are now continuing at
participants' home institutions, following the research plans elaborated
during the workshop. CLSI@Sea participants plan to meet in the year following
the workshop, in order to discuss and correlate their results, confident that
international collaboration will lead to high-impact outcomes in the global
understanding of inter-disciplinary processes in subduction zones.
All the data used during this workshop and presented in
this report are archived IODP data and therefore publicly accessible, or
accessible on request, from the IODP website
(https://www.iodp.org/resources/access-data-and-samples, IODP Science
support office, 2018).
Information about the Supplement
Supplementary movie. Three-dimensional distributions of bulk density
converted from the CT value presented in Fig. 6. In Supplement movies, the
same section (C0006E31X05 110–128 cm) is rotated horizontally. The color
scale is the same as the one presented in Fig. 6.
The supplement related to this article is available online at: https://doi.org/10.5194/sd-24-93-2018-supplement.
RF, BG, GL, TM, CR and SY were the main contributors to this report.
All authors participated in the discussions, data synthesis and writing. GL
suggested writing and publishing this report and coordinated the writing work
during and after the expedition.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank IODP, CDEX, JAMSTEC, MWJ, and MQJ for generously providing
logistical and financial support for the CLSI@Sea workshop and event
attendees. We thank all the mentors for their support, their availability
resulting in constructive discussions during the workshop. A special mention
is for Gaku Kimura, at the origin of the idea of this workshop. We
acknowledge the staff from JAMSTEC, Sean Tockzo, Lena Maeda, Yukari Kido, and
Yoshinori Sanada, for the logistic organization of the workshop and their
valuable advice onboard Chikyu. We also thank the Expedition 380
science party for their good mood and their availability to discuss and
present their tasks, despite the incredible amount of work they had to
manage. The whole crew and staff of D/V Chikyu, from the sailors to
the stewards, the kitchen staff and the laboratory staff, and all the other
teams we cannot list here, deserve special thanks for their hospitality and
for doing a superb job, taking care of the workshop participants, and
providing them with a fantastic environment to focus only on scientific
questions. Finally, thanks go to all workshop attendees and participating
scientists for their motivation and enthusiastic contributions, during and
after the event, thereby helping to develop this report as a first concrete
outcome from the CLSI@Sea workshop.
All investigators, workshop attendees, the Exp. 380 Science party and daily
reports can be found on the NanTroSEIZE Expedition 380 project website at
https://www.jamstec.go.jp/chikyu/e/nantroseize/expedition_380.html
(last access: 22 September 2018). Edited by:
Ulrich Harms Reviewed by: Sally Morgan, Michael Riedel,
and one anonymous referee
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