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Time:May 25 09:00-09:15

Early stages of Philippine Sea Plate birth, growth and migration : a tribute to Anne Deschamps
LALLEMAND, Serge1∗ ; DESCHAMPS, Anne2
1

Geosciences Montpellier, Montpellier University, CNRS, France, 2 IUEM, Brest, France

The early stages of the Philippine Sea Plate (PSP) are debated among the scientific community. At least one fourth of the PSP
area has disappeared by subduction along the Ryukyu and Philippine trenches, obliterating the relics of its northern and westernboundaries, and by subduction erosion along the estern Izu-Bonin-Mariana (IBM) margin. Furthermore, small parts might havealso disappeared by intraplate local thrusting and/or subduction along the Gagua or Daito ridges. Another intriguing observation
concerns the origin of the ”proto-PSP” as seen in the Daito area, the southernmost West Philippine Basin, the Huatung Basin oreven within the internal structure of the Izu-Bonin Arc. Indeed, Early Cretaceous ages have been discovered in most of theseregions, i.e., much older than the commonly accepted ages of the PSP entrapment ranging from Early to Late Eocene . The
so-called ”forearc basalts” or ”boninites” are often supposed to represent typical magmatism marking subduction initiation along
a transform fault. A detailed examination of the geodynamic context of their emplacement allows to suggest another origin forthese rocks. Even the kinematic evolution of the PSP does not make any consensus.
Based on the tremendous work that has been done by many teams since the 80’s, providing key data inserting into the PSP
puzzle, and including the important contribution of Anne Deschamps in the late 90’s and early 20’s, we can deliver an updated
picture of the early stages of the PSP tectonic evolution.

Keywords: Philippine Sea Plate, tectonic evolution

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SIT07-02

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Time:May 25 09:15-09:30

Rethinking spreading model of the northwestern West Philippine Basin (WPB): 39Ar/40Ar
dating and geochemical constraints
SHINJO, Ryuichi1∗ ; ISHIZUKA, Osamu2
1

Univ. Ryukyus, 2 Geological Survey of Japan, AIST

We report 40Ar/39Ar dating results of basalts from the northwestern West Philippine Basin (around the Urdaneta Plateau area),
with petrological/geochemical characteristics. Our samples are mostly from ROV KAIKO dives conducted in 2003.
Our dating results range from ˜40 to ˜32 Ma. These ages are younger than previously estimated seafloor ages based on studies
for topography and magnetic anomaly lineations. In addition, geographical distribution of obtained ages appears to be incompatible with previous models. Therefore, if our dating results are correct, it requests rethinking about the spreading history ofnorthwestern part of WPB.
Geochemical data revealed that basalts around the Urdaneta Plateau have OIB-like characteristics (as also reported in Ishizuka
et al., 2013, Geology), confirming that plume activity involved in the spreading process of this part of basin.

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SIT07-03

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Time:May 25 09:30-09:45

Exhumation history of Taiwan and Mindoro orogenic belts and its implication to platemotion of the Philippine Sea plate
LEE, Yuan hsi1∗ ; SHIU, Chung-wei1 ; RAU, Reuy-juin2 ; BACOLCOL, Toto3
1

1. Department of Earth and Environmental Sciences, National Chung-Cheng University, Taiwan, 2 Department of earth sciences,
National Cheng Kung University, Taiwan, 3 Philippine Institute of Volcanology and Seismology, Philippines
Both Taiwan and Mindoro orogenic belts form as the collision between the Eurasia continental margin and Luzon arc. We
conduct the low temperature thermochronology analysis to reveal the exhumation history in both orogenic belts.
In northern Mindoro the Mesozoic green schist facies strata show the oldest zircon fission track reset ages of ca.4-6 Ma,
which is consistent with the Pliocene Punso conglomerate deposition in southern Mindoro Island and therefore indicative of the
beginning of collision at ca.5-6 Ma.
Meanwhile the zircon fission track ages also imply similar exhumation history in Taiwan orogenic belt. The oldest reset zircon
fission track ages are also 5-6 Ma and are consistent with deposit ages of the foreland basin. In addition, the Philippine trenchand Okinawa trough started to develop from ca. 5-6 Ma.
Our new data support the model of the Philippine Sea plate has changed its moving direction to NW around 5-6 Ma, which
had been suggested by Hall (2002).

Keywords: Taiwan orogenic belt, Mindoro orogenic belt, Philippine Sea plate, exhumation histroy, Luzon arc

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SIT07-04

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Time:May 25 09:45-10:00

Temporal constraints for the tectonic development of the Philippine ophiolite belts fromnew zircon U-Pb ages
TANI, Kenichiro1∗ ; GABO, Jillian aira S.2 ; HORIE, Kenji3 ; ISHIZUKA, Osamu4 ; PADRONES, Jenielyn5 ;PAYOT, Betchaida6 ; TEJADA, Maria luisa7 ; FAUSTINO-ESLAVA, Decibel V.8 ; IMAI, Akira5 ;ARAI, Shoji9 ; HOKADA, Tomokazu3 ; YUMUL JR., Graciano P.10 ; DIMALANTA, Carla B.6
1

National Museum of Nature and Science, 2 Kyushu University, 3 National Institute of Polar Research, 4 Geological Survey ofJapan/AIST, 5 Akita University, 6 University of the Philippines, Diliman, 7 Japan Agency for Marine-Earth Science and Technology, 8 University of the Philippines, Los Banos, 9 Kanazawa University, 10 Apex Mining Co. Inc.
The tectonic framework of the Philippine Islands is important in understanding how the western Pacific margin developed
along the Eurasian and the Pacific Plates since the Eocene.
The basement rocks of the Philippine Islands are characterized by the presence of ophiolitic complexes exposed among theislands. Yumul (2007, Island Arc) defined four belts in the Philippine ophiolites and proposed that they progressively become
younger towards west, from Early ? Late Cretaceous at the easternmost belt to Eocene - Oligocene in the west. However, mostof the ophiolitic complexes have been dated by radiolarians and foraminifera in the overlying sediments and lacked reliable radiometric ages from the igneous rocks.
To precisely determine the igneous ages of the Philippine ophiolites, we have conducted SHRIMP zircon U-Pb dating of thegabbroic and leucocratic rocks collected from the ophiolitic complexes in the Philippine Islands, including those from Luzon(Zambales, Isabela, and Lagonoy ophiolites), Masbate (Balud ophiolite), Tablas (Sibuyan ophiolite), Dinagat, and Cebu.
New zircon ages show that most of the ages obtained from the northern ophiolite belts are Eocene in age, from 52 Ma to 41
Ma. These ages coincide well with the opening of the West Philippine Basin (49 ? 33 Ma, Taylor and Goodliffe, 2004, GRL),
which is a backarc basin formed behind the incipient Izu-Bonin-Mariana Arc. Furthermore, geochemical data available from theigneous rocks in the eastern ophiolite belts show backarc basin basalt-like geochemical affinities (e.g. Yumul, 2007), suggestingthat these ophiolites are genetically associated with the West Philippine Basin.
On the contrary, southern ophiolites are significantly older, gabbroic and leucocratic rocks that are associated with the ophiolitic complex in Cebu and gabbroic rocks in Lagonoy and Dinagat ophiolites are Jurassic to Late Cretaceous (200 - 90 Ma) in
age. Similar Mesozoic arc and ophiolitic rocks have recently discovered in the Daito Ridges, currently located north of the West
Philippine Basin. Such Mesozoic terranes in the Philippine Sea Plate may potentially be correlated to the Mesozoic ophiolites inthe southern Philippines, before the opening of West Philippine Basin in the Eocene.

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SIT07-05

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Time:May 25 10:00-10:30

Geologic and geochronological constraints on the Philippines Sea tectonics around 50
Ma
ISHIZUKA, Osamu1∗ ; TANI, Kenichiro2 ; HARIGANE, Yumiko1 ; YAMAZAKI, Toshitsugu3 ; OHARA, Yasuhiko4 ;KUSANO, Yuki5 ; EXP. 351, Scientific party6
1

GSJ/AIST, 2 National Museum of Nature and Science, 3 AORI, Univ. Tokyo, 4 HODJ, 5 Kanazawa University, 6 IODP

Recent geological and geophysical survey cruises as well as IODP drilling expeditions in the Philippine Sea are providing new
geologic and geochronological information useful for tectonic reconstruction of the Philippine Sea plate. Especially, tectonic
reconstruction around 50 - 55 Ma, i.e., the period immediately before and after the subduction initiation to form Izu-BoninMariana (IBM) arc is of particular interest, because it is critical for understanding the possible tectonic processes to enablesubduction initiation.
In this contribution, the following three sets of data recently published or obtained from this region will be introduced, andpossible tectonic models which could consistently explain the new data will be discussed.
1) OIB-like magmatism in the Philippine Sea: 40 Ar/39 Ar ages from a 1000km NW-SE line of oceanic plateaus in and around
the West Philippine Basin (WPB) demonstrates that they formed as a time-progressive volcanic chain mirrored either side of the
WPB backarc spreading center (Ishizuka et al., 2013). In the north this chain is bounded by a Mesozoic remnant arc (i.e., Daito
Ridge group). Geochemically these oceanic plateaus have an EM-2 ocean island basalt (OIB) signature matching the older 45-51
Ma volcanic edifices discovered overlapping the remnant arc and intervening basins. The wide distribution of these edifices
could mark the first arrival of upwelling mantle. These features are consistent with the extension and splitting of the Mesozoic
arc terrane, driven by regional upwelling centered on the impact of the mantle plume at c. 51 Ma.
2) YK10-14 and YK13-08 cruises by R/V Yokosuka investigated Palau Basin (southernmost part of the Philippine Sea plate)and southern part of WPB obtained some crucial geological and geophysical data (Sasaki et al., 2014) for reconstruction of thePhilippine Sea Plate. 40 Ar/39 Ar dating of dolerite sample from the Palau Basin crust exposed at the Mindanao Fracture Zone
gave a plateau age of 40.4 Ma. This result indicates that the spreading of the Palau Basin was still going on at 40.4 Ma. This age
is within an age range of active spreading of the WPB (e.g., Okino and Fujioka, 2002). This strongly implies that contrary to theprevious hypotheses, the Palau Basin formed while the WPB was still opening, and the Palau Basin is not the oldest part of the
WPB.
3) IODP Exp.351, targeted evidence for the earliest evolution of the Izu-Bonin-Mariana arc following inception, was conductedin Amami Sankaku Basin (ASB), west of the Kyushu-Palau Ridge (KPR), i.e., paleo-IBM arc. Igneous basement of this site,which presumably corresponds to the basement of the IBM arc, was successfully recovered. On-board preliminary data suggeststhat basaltic basement is early-middle Eocene (or older) and geochemically similar to forearc basalts from IBM forearc. Thisimplies that the area of initial seafloor spreading associated with subduction initiation covers an area from forearc to reararc of
future IBM arc.

Keywords: Philippines Sea plate, Izu-Bonin-Mariana arc, OIB, West Philippine Basin

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SIT07-06

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Time:May 25 10:30-10:45

Upper-mantle shear-wave structure under Asia from Automated Multimode Inversion of
waveforms
LEGENDRE, Cedric1∗ ; ZHAO, Li1 ; CHEN, Qi-fu2
1

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, 2 Key Laboratory of Earth and Planetary Physics, Institute ofGeology and Geophysics, Chinese Academ
An automated multimode inversion technique of partial waveform was applied to available data of broadband stations in Asiaand surrounding regions.
It performs a fitting of the complete waveform starting from the S-wave onset to the surface wave. Assuming the location andfocal mechanism of a considered earthquake as known, the first basic step is to consider each available seismogram separately
and to find the velocity perturbations that can explain the filtered seismogram
best. In a second step, each velocity perturbations serves as a linear constraint in an inversion for a 3D S-wave velocity modelof the upper mantle.
We collected data for the years from 1977 to 2012 from all permanent stations as well as temporary experiments for whichdata were available.
In this way, a huge data set of about 12 million seismograms came about from which about 2.3 million seismograms provide
8 million linear constrains for the resulting 3D model.
The frequency content of the data associated with the sensitivity kernels as well as the path density in the considered region
allows us to perform a high resolution tomography at a continental scale.
The resulting models exhibit an overwhelming detail in relation to the size of the region considered in the inversion. They are
to our knowledge the most detailed models of shear wave velocity currently available for Asia and surroundings. Most prominent
features are a details on the morpholoogy of the Tibetan Plateau and its extent at depth that are properly mapped.
The vertical extent of its lithosphere as well as other cratonic lithosphere could be imaged at depths greater than 200 km.
Orogenic volcanism is almost exclusively found in regions of shallow asthenosphere and in areas where the depth of the LABchanges significantly, along the passive margins.
Keywords: Tomography, Asia, Surface-waves

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Time:May 25 11:00-11:30

Subducted-slab constraints on late Cenozoic motion of the Philippine Sea plate and its
collision with continental Asia
SUPPE, John1∗ ; WU, Jonny1
1

Geosciences, National Taiwan University

The geology of the Eurasian margin near Taiwan and Japan contains a record of Philippine Sea plate tectonics from the Miocene
to present-day. We present a detailed reconstruction of late Cenozoic Philippine Sea plate motions, for which we emphasize theimplications for the geology of Taiwan, Japan, and East China Sea. The new Philippine Sea plate reconstruction is based on 3D
mapping and unfolding of subducted slabs from seismic tomography of East Asia.
Using mapped slab constraints, we show that:[1] The Pacific subduction zone near the Marianas and southern Izu-Bonin has remained +/‒ 250 km near its present-day
position since at least ∼45 Ma, which provides an eastern limit to the Philippine Sea plate.
[2] A major 8000 km x 2500 km ocean once existed in the area now occupied by the Philippine Sea plate and is imaged as aset of flat slabs in the upper part of the lower Mantle. This now-vanished ocean, which we call the East Asian Sea stretched frompresent-day Taiwan and the Ryukyus to a southern limit near northern Australia and New Zealand.
[3] The subducted Philippine Sea plate near the Ryukyus has a maximum ∼1000 km NS length and a western NS edge of∼1600km. Therefore the northern edge of the Philippine Sea plate was far from continental Asia in early and middle Cenozoic
when it was near the Equator, based on paleomagnetism.
[4] The Eurasian slab has a NS edge of ∼3000km against which the northern Philippine Sea plate fit prior to initiation of theManila trench subduction, providing a western limit to the Philippine Sea plate.
[5] These slab constraints severely limit the longitudinal position of the Philippine Sea plate and rule out models involving
large late Cenozoic rotations. Geologic constraints near the continental Asian margin and paleomagnetism help fix the latitudinal
position of the Philippine Sea plate.
Our slab-constrained plate tectonic model implies that from the Eocene to Miocene, the East Asian Sea progressively subducted southwards, overrun by a northward moving Philippine Sea plate that originated as part of the Indo-Australian Ocean. In
middle Miocene, the arc that formed at the pre-subduction northern margin of the Philippine Sea plate collided with the Ryukyu
and SW Japanese continental margin, with deformation penetrating deeply into the East China Sea. Erosion of the collisional
mountain belt fed turbidite fans that extended ∼1000km south onto the Shikoku basin of the Philippine Sea plate. This arccontinent collision was followed by subduction of Philippine Sea lithosphere under Eurasia and opening of the Okinawa troughbackarc basin. Between 1-2 Ma the Philippine Sea plate motions changed from NNW to its present-day Pacific-like WNW motions, based on the limited ∼450 km extent of the Eurasian/South China Sea slab near Taiwan.
Keywords: Philippine Sea plate, arc-continent collision, Shikoku basin turbidites, Taiwan tectonics, southwest Japan tectonics,
East China Sea tectonics

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SIT07-08

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Time:May 25 11:30-11:45

Accretion vs. tectonic erosion in Cenozoic margin of southwest Japan
KIMURA, Gaku1∗
1

Dept. Earth and Planetary Science, The University of Tokyo

Accretionary and erosive margins provide tectonic end-members within our understanding of the subduction zone setting, andhow these tectonic processes might be recorded and recognizable in ancient subduction complexes remains a challenging issue.
Tectonic erosion includes sediment subduction and basal erosion along the plate boundary megathrust which drags down the crustof the upper plate into the mantle. This process means that the ”evidence” for such erosion is commonly based on lost geologicaltectono-stratigraphic data, i.e. gaps in the record, and must be speculated from indirect phenomena such as subsidence of theforearc slopes. A topographically rough surface to the subducting oceanic plate, such as that provided by seamounts, has beensuggested to work like an erosive saw carving into the upper plate. Another mechanism of basal erosion has been suggested tobe hydrofracturing of upper plate materials due to dehydration-induced fluid pressures, resulting in entrainment of upper platematerials into the basal decollement. Considering the interaction between the ˜30 km thick crust of the upper plate and subducting
oceanic plate, a subduction dip angle of ˜15 ⁰ , and convergent rate of ˜10 cm/year, at least ˜1 Ma of continuous basal erosion isnecessary to induce clear subsidence of the forearc because the width of plate interface between the upper crustal and subducting
plates is about 115 km (30/cos15 ⁰ ). In several examples of subduction zones, for example the Japan Trench and the Middle
America Trench off Costa Rica, the subsidence of a few thousand metres of the forearc, combined with a lack of accretionary
prism over a period of several million years, suggest that the erosive condition needs to be maintained for several to tens of
million years (e.g. von Huene & Lallemand 1990) .
Such an age gap in the ancient accretionary complex would be one of the signals for tectonic erosion in the past. Recently,
a hypothesis that a tremendous amount of tectonic erosion has taken place during Early Cretaceous and middle to late Miocene
time has proposed based on age gaps in the accretionary complex. Such age gaps in the accretionary complex, however, do not
automatically imply that tectonic erosion has taken place, as other interpretations such as no accretion, cessation of subduction,
and/or later tectonic modification, are also possible. In the case of the middle to late Miocene period, for example, a drastic
tectonic change after the opening of the Japan Sea and clockwise rotation of southwest Japan may be linked to ridge subduction
or a switch in subduction from the Pacific to Philippine Sea Plate. Recent drilling in the forearc of the Nankai Trough suggeststhat the accretion was renewed at ˜6 Ma after igneous activity intruding the early Miocene accretionary prism. Kimura et al.
(2014) interpreted that the subduction ceased between ˜12 Ma to ˜8 Ma due to the transference of subduction from the PacificPlate to the Philippine Sea Plate, as opposed to the ”continuous subduction with subduction erosion” viewpoint. These different
scenarios need to be tested in the future.
Referencesvon Huene, R. & Lallemand, S. 1990. Tectonic erosion along the Japan and Peru convergent margins. Geological Society ofAmerica Bulletin, 102, 704-720.
Kimura, G., Hashimoto,Y., Kitamura,Y., Yamaguchi,A. & Koge, S. 2014. Middle Miocene swift migration of the TTT triple
junction and rapid crustal growth in southwest Japan ? A review, Tectonics 33, 1219?1238, doi:10.1002/2014TC003531.

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Time:May 25 11:45-12:00

Dynamic evolution of accretionary prism in the Nankai Trough
TSUJI, Takeshi1∗ ; ASHI, Juichiro2 ; KIMURA, Gaku2
1

I2CNER, Kyushu University, 2 The University of Tokyo

To evaluate evolution of accretionary prism as well as forearc basin, we analyzed seismic reflection data acquired in the
Nankai Trough. On the seismic profiles, the volcanic layer can be identified as strong reflection at the surface of accretionary
prism beneath landward edge of the Kumano forearc basin. The accretionary prism beneath the strong reflection was constructed
before the deposition of volcanic layer (14-15Ma). The undeformed basin sediment above the volcanic layer indicates that theold accretionary prism has not been much deformed after construction of the forearc basin. The accretionary prism trench-wardof the old prism was constructed at ˜6Ma. The age gap of accretionary prism indicates that plate subduction (or accretion process)was halted from 15Ma to 6Ma. Because the trench-ward younger accretionary prism is much deformed, the undeformed older
accretionary prism beneath the volcanic layer could work as backstop. We further find a continuous ridge system beneath the
Kumano forearc basin sediment and interpret that it was generated by activities of previous splay fault. The previous splay
fault develops parallel to the backstop interface as well as the coastline of the Kii peninsula. These observations demonstratethat the evolution of accretion prism including backstop interface is not stable process, but it is strongly influenced by the plateconvergence rate and direction.
Keywords: seismic data, accretionary prism, nankai trough, dynamic evolution, backstop

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SIT07-10

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Time:May 25 12:00-12:15

New constraints on the volcanic and tectonic evolution of SW Japan since 15 Ma fromIODP EXP. 322 in the Shikoku Basin
NAKAJIMA, Takeshi1∗ ; DANHARA, Tohru2 ; YAMASHITA, Tohru2 ; IWANO, Hideki2 ; ODA, Hirokuni1 ;KANAMATSU, Toshiya3 ; ZHAO, Xixi4
1

Geological Survey of Japan, AIST, 2 Kyoto Fission-Track Co.Ltd., 3 JAMSTEC, 4 University of California Santa Cruz

Tectonic model and origin of volcanism for Southwest Japan and the Philippine Sea Plate (PSP) since Middle Miocene havebeen controversial between “fixed TTT junction hypothesis” and “migrating TTT junction hypothesis” (Kimura et al., 2014).
This study aims to provide new constraints on the volcanic and tectonic evolution of Southwest Japan since 15 Ma based on
occurrence, mineral composition and fission-track dating of tuff beds and sandstone within two drill holes in the Shikoku Basin
during EXP.322, IODP NanTroSEIZE seismogenic zone experiments. Large amount of tuffs with broad mineral composition had
accumulated during 15-14.5 Ma, possibly because of swift subduction of the PSP. During 14.5-10.5 Ma, tuff deposition attenuatedwhile large submarine fan sandstones had deposited in the Shikoku Basin until 12 Ma. Mineralogy and detrital fission-track
dating of sandstone indicated that sources of sandstone were the Kumano Acidic Rocks and other parts of SW Japan. During10.5-8 Ma, tuff and turbidite deposition ceased while rate of hemipelagic sedimentation decreased, suggesting quiet volcanism
and tectonics in SW Japan. During 8-7 Ma, andesitic tuffs and scoria possibly derived from Izu-Bonin Arc temporarily increased
with contemporaneous deposition of channelized volcaniclastic sandstone and sheet-like tuffaceous sandstone. After the second
quiet period of volcanism during 7-5.5 Ma, tuff intercalation increased at 5.5 Ma. The present results contradict the “migrating
TTT junction hypothesis” and support the “fixed TTT junction hypothesis” that Izu-Bonin Arc had collided with central Japanby 15 Ma with swift subduction of young Shikoku Basin, which resulted in Middle Miocene outer arc volcanism in SW Japan.
This study also implies cessation of subduction of the PSP during 10.5-6 Ma, followed by resumption of subduction.
Keywords: SW Japan, Philippine Sea Plate, tectonics, volcanism, IODP

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SIT07-11

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Time:May 25 12:15-12:30

Present day comparison to the past convergent margin tectonics in Western Pacific bysubmersible dives
OGAWA, Yujiro1∗ ; KAWAMURA, Kiichiro2
1

No affiliation, 2 Yamaguchi University

Although some people believe that catastrophism overcomes uniformitarianism in the geological history, yet the concept of
The present is a key to the past is adoptable and adaptable to the general understanding in a generic space and time dimension.
As for the convergent margin geology and tectonics, the present is observable mostly under the sea by coring and submersible
research, while the past is on land. Through various expeditions to the Japanese trenches, including Japan trench, Boso triple
junction area, Sagami trough and Nankai trough in addition to some Izu-Bonin-Mariana trenches (Ogawa et al. (eds.) 2011
Springer Solid Earth Series 8), we conclude the best way of classifying these types of margins is into three categories: 1) erosion
and collapsing, 2) partial accretion, and 3) full accretion. The Japan trench is known as a typical erosional type although a small
scale accretionary prism has developed in its toe, partly during the 2011 Tohoku earthquake (Kodaira et al., 2012). However, thisprism may be a temporary feature that will be recycled by large scale erosion and collapse associated with huge scale landsliding(Kawamura et al., 2012; Strasser et al., 2013). The landward side of the Sanriku Escarpment is largely underlain by Miocene
diatomaceous, calcite-cemented breccias that show tectonic erosion and collapse has occurred during the past 10 Myrs (Ogawa,2011). The Boso TTT triple junction is another example of a large collapsed margin, in which under the thin cover of Pleistocene
are recycled Miocene diatomaceous sediments of 15 Ma that can be mapped from the Bonin (Ogasawara) trench to the south,
north to Boso on land (Ogawa and Yanagisawa, 2011). Most of the on land Miocene to Pleistocene accretionary prisms in MiuraBoso peninsular areas (Emi, Miura and Chikura prisms) are the result of accretion from the Izu forearc side to Honshu, andthe trench sedimentation and accretion style is well correlatable to the present Nankai trough section (Kobayashi, 2002). Thicktrench fills form fold-and-thrust belts in which methane-supported chemosynthetic bio-communities with calcareous concretions
are developed on the splay faults between the forearc basin with many debris flows and mud diapirs. The structural styles in
the present Nankai prism documented by the Shinkai 6500 JAMSTEC expeditions (Kawamura et al., 2009, 2011; Anma et al.,2011 and others) are almost equivalent to the on land observations in the Miura-Boso areas as many duplexes, thrust faults,
and isoclinal folds, commonly with layer parallel faults (Hanamura and Ogawa, 1993; Yamamoto et al., 2005; Michiguchi andOgawa, 2009, 2011; Muraoka and Ogawa, 2011). Only the Pliocene to Pleistocene Chikura prism was developed from fully
ponded trench fill sediments, which are equivalent to the present Sagami Bay of the northern Sagami trough (Ogawa et al., 1989).
The type of margin, the erosional to collapsed type margin (Japan trench, Boso triple junction), a stepwise sector development ofprism (Nankai prism), or a ponded trench fill prism (Sagami Bay) might be due to the type of trench-fill sediments; without, or
partly filled, or fully sedimented.
(References) Anma et al. 2011 Springer Solid Earth 8, 169-196; Kawamura et al. 2009 GSAB 121, 1629-1646 doi: 10.1130/B26219.1;
Kawamura et al., 2012 GRL 39, L05308, doi:10.1029/2011GL050661, 2014, Marine Geology 356, 44?49; Kobayashi, 2002 Marine Geology 187, 3-30; Kodaira et al., 2011 Nature Geosci. doi: 10.1038; Hanamura & Ogawa 1993 Island Arc 3, 126-141;
Michiguchi & Ogawa 2009; GSASP480 249?262, oi:10.1130/2011.2480(12); 2011 Springer Solid Earth 8, 229-246; Muraoka& Ogawa, 2011GSASP 480, 233?247, doi:10.1130/2011.2480(11); Ogawa, 1989 Tectonophysics 160, 135-150; Ogawa, 2011
Springer Solid Earth 8, 39-52,; Ogawa & Yanagisawa 2011 Springer Solid Earth 8, 53-73; Strasser et al. 2013 Geology 41,935?938 doi:10.1130/G34477.1; Yamamoto et al. 2005 Tectonics, 24, TC5008, doi:10.1029/2005TC001823.

Keywords: subduction zone, accretionary prism, erosional margin, collapse, submersible

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Time:May 25 14:15-14:30

Where have all the slabs gone in the Northwestern Pacific subduction zones?
KAWAKATSU, Hitoshi1∗ ; OBAYASHI, Masayuki2 ; HONDA, Satoru1
1

Earthquake Research Institute, University of Tokyo, 2 Institute of Research on Earth Evolution, Japan Agency for Marine-Earth
Science and Technology
Based on the new high-resolution seismic image of the mantle beneath the northwestern Pacific subduction zones (Obayashi
et al., 2015, in preparation), we discuss the fate of subducted Pacific slabs in the region, as well as their possible relation with the
opening of Japan Sea and the widespread off-arc Cenozoic volcanism in Northeast China.
It is well known that the slab subducting from the Japan Trench (Honshu slab) is stagnant in the mantle transition zone. The
stagnant slab extends to the west and reaches to northeast China where several late Cenozoic volcanoes exist. These volcanoes
are located far from the volcanic front of the Honshu slab subduction and some study proposed the relation between the off-arc
volcanoes and the stagnant Honshu slab. The active magmatism started about 25Ma and followed by opening of the Japan Sea
back-arc basin that separated Japan away from Eurasia continent. It is suggested that the both surface phenomena were caused
by upwelling asthenosphere from petrological viewpoint.
The new tomographic P-wave velocity model with focus on the northwestern Pacific is obtained by combing P data fromNECESSArray with global data. Differential travel times between any two stations as a function of frequency are inverted bytaking finite frequency effect into account to improve resolution through the upper mantle. The result indicates, to our surprise,
the absence of long-tailed stagnant Honshu slab in the mantle transition zone beneath NE China, which was earlier interpreted
by Tang et al. (2014, Nature Geoscience) as a gap of the stagnant slab for a deep upwelling in the region that may feed theChangbaishan volcanism. With a help of geodynamic modeling (Honda, 2014, G-cubed), we hope to revisit this issue, as well as
the geodynamics of the Northwestern Pacific subduction zones to understand where all the slabs have gone.

Keywords: slab, Japan sea

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Time:May 25 14:30-14:45

Plate tectonic reconstruction of the northwest Pacific stagnant flat slabs under Japan, Korea and NE China
WU, Jonny1∗ ; SUPPE, John1 ; KANDA, Ravi v.s.2
1

Dept. of Geoscience, National Taiwan University, 2 Utah State University

An extensive swath of stagnant flat slab tomographic anomalies have been documented within the 410 to 660 km mantle transition zone under Japan, Korea and NE China, downdip of the northwest Pacific trench Benoiff zones (e.g. Fukao et al., 1992;
Zhao et al., 2015). In this study we mapped these slab anomalies in 3D from MITP08 global seismic tomography (Li et al., 2008)
and used these slabs to constrain a new regional NW Pacific plate tectonic reconstruction in the Cenozoic.
Our mapped slabs included the Izu-Bonin, Kuril, and Kamchatka slabs, their associated stagnant flat slabs, and other regionalslabs. The mapped slabs were then unfolded (i.e. structurally restored) to a spherical Earth surface to assess their pre-subduction
geometries. The unfolded slab constraints were input into plate tectonic reconstructions using Gplates software.
When unfolded the western Pacific stagnant slabs have a minimum 2000 km east-west length. The mapped northern stagnant
slab edge near the western Aleutians and the southern edge at the southernmost Izu-Bonin trench are roughly east-west andconsistent with transform directions predicted by mean Pacific motions since 47.5 Ma.
Using the slab constraints in a quantitative plate reconstruction, we show it is unlikely the stagnant slabs were subducted bysimple eastward retreat of Eurasia in the Cenozoic relative to a mantle reference. Instead, we show that the stagnant slabs wereformed by ˜3500 km of Eocene to present-day fast western Pacific subduction at the eastern margin of a slow-moving Eurasia.
After subduction, the Pacific slabs advanced westward >2000 km under NE Asia within the mantle, pushed westward along the
mantle transition zone by the fast Pacific subduction. We discuss the fit of this plate reconstruction against the timings of Japan
Sea, Kuril Basin and Okhotsk Sea opening and NE China intraplate volcanism. We show possible reasons why the stagnant flatslabs did not penetrate the lower mantle. Today the western Pacific stagnant flat slabs form a barrier that has prevented other NE
Asia slabs from penetrating the lower mantle.

Keywords: western Pacific, plate tectonic reconstruction, stagnant slab, Izu-Bonin, Kuril-Kamchatka, seismic tomography

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SIT07-14

Room:A03

Time:May 25 14:45-15:00

Combining tomographic results, geologic history and geodynamic modeling in the subduction zones around Japanese Islands
HONDA, Satoru1∗
1

Satoru Honda

In this presentation, I show the slab feature estimated from the seismic tomography result combined with the recent estimate ofd(ln Vp)/dT [Karato, 2008] and the upper limit of potential temperature estimated from the deep earthquakes and the theoreticalmodeling of the subducting slab [Emmerson and McKenzie, 2007]. Obtained results show a significant gap and/or the diluted
slab in the topmost lower mantle just below the stagnated slab in the transition zone, that is consistent with the previous view ofthe flushing or avalanche of the mantle flow caused by the interaction between the mantle convection and the endothermic phasetransition at the 660 km depth. However, such intermittent flow may not occur, if the strongly temperature-dependent viscosity istaken into account. It is also known that the slab morphology, especially, the stagnation of slab, is controlled by the movement of
surface plate. Thus, to understand the slab morphology in the subduction zone around the Japanese Islands, I construct a simple
2D model of subduction zone which takes into account the geologic factors such as the opening of the Japan Sea. I find two
possibilities of the cause of the existence of such gap and diluted slab: the opening of the Japan Sea and the subduction of oldridge.
Keywords: subduction zone, seismic tomograohy, geologic history, geodunamic modeling, Japanes Islands

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