Kimura et al. Nankai_EPSL-2007

14 Pages·2007·1.33 MB·English

EarthandPlanetaryScienceLetters255(2007)471–484
www.elsevier.com/locate/epsl
Transition of accretionary wedge structures around the up-dip
limit of the seismogenic subduction zone
⁎
Gaku Kimura a,b, , Yujin Kitamura a, Yoshitaka Hashimoto c, Asuka Yamaguchi a,
Tadahiro Shibata c, Kohtaro Ujiie b, Shin'ya Okamoto a
a DepartmentofEarthandPlanetaryScience,GraduateSchoolofScience,TheUniversityofTokyo,
7-3-1Hongo,Bunkyo-ku,Tokyo113-0033,Japan
bInstituteforResearchonEarthEvolution,JapanAgencyforMarine-EarthScienceandTechnology,
2-15Natsushima-cho,Yokosuka,Kanagawa237-0061,Japan
c DepartmentofNaturalEnvironmentScience,FacultyofScience,KochiUniversity,2-5-1Akebono-cho,Kochi,Kochi780-8520,Japan
Received14March2006;receivedinrevisedform2January2007;accepted2January2007
Availableonline10January2007
Editor:M.L.Delaney
Abstract
TheNankaiaccretionaryprismisdividedintothreesegments:outerandinnerwedgesandtheirtransitionzone.Thesewedges
reflectdifferentaspectsofwedgetaper,internaldeformation,andbasalplateboundaryfault.Theouterwedgeischaracterizedby
narrowcriticaltaper,internaldeformationbyin-sequence-fold-and-thrustandaseismicdécollement.Theinnerwedgerepresentsa
stablenarrowtaper,weaklydeformedinternalstructureandseismogenicplateboundaryfaultalongitsbase.Thetransitionzone
betweenthetwowedgesshowslargecriticaltaperwithsteepsurfaceslope,internalstructureofout-of-sequencethrust,andstep-
downofdécollementontothesediment–oceanicbasementinterface.Thetrenchslopebreakandoceanwardmarginofforearcbasin
is located around the landward edge of this transition zone. These common aspects might be related to the lithification of both
accreted and underthrust sediments and the resultant switch of the plate boundary fault. Deformation and lithification process
recorded in exhumed on-land mélange of accretionary complexes suggest that the step-down of the plate boundary décollement
occursaroundthe up-dip limit of seismogenic subduction zone.
©2007Elsevier B.V. All rightsreserved.
Keywords:NankaiTrough;accretionaryprism;Coulombwedge;seismogeniczone;mélange;trenchslopebreak
1. Introduction zone, which is defined as earthquake generating
rupture fault zone. Several hypotheses have been
One of the unsolved problemsin subduction zones proposed; 1) a change in frictional behavior from
is what controls the up-dip limit of the seismogenic stable to unstable slip of clay minerals caused by
thermally controlled transformation [1–4], 2) an
increase in effective shear strength due to reduction
⁎
Correspondingauthor.DepartmentofEarthandPlanetaryScience, of fluid pressure and change in frictional behavior of
GraduateSchoolofScience,TheUniversityofTokyo,7-3-1Hongo,
sedimentary rocks caused by diagenesis [5], 3) a
Bunkyo-ku,Tokyo113-0033,Japan.Tel.:[phone];fax:+813
changeinlocationoftheplateboundaryfaultintothe
58418378.
E-mailaddress:[email protected](G.Kimura). basement basalts due to lithification and hardening of
0012-821X/$-seefrontmatter©2007ElsevierB.V.Allrightsreserved.
doi:10.1016/j.epsl.2007.01.005
472 G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484
the aseismic décollement together with underthrust 2.1. Change in slope and wedge taper angles
sediments [6], and 4) reactivation of a roof thrust,
whichisonceabandonedduringunderplating[7].The Forearc slope angle varies with distance from the
former two hypotheses implicitly consider that the deformationfront(Fig.1).TheMurototransecthasbeen
frictional behavior of the same fault zone changes at studied as a typical accretionary prism [5,10,12,13,16].
theup-diplimit,whereasthelattertwoemphasizethat This part of the subducting Philippine Sea Plate dips
the location of the plate boundary fault changes northward at an angle of about 1° over several tens of
around the up-dip limit with change in their friction kilometers from the deformation front (Fig. 1). Associ-
behaviors. ated with the change in surface slope the wedge taper
Relating to the onset location of the seismogenic angle also varies. The frontal part of the wedge in the
plate boundary, many geological and geophysical Murotoregionwithinadistanceofabout0–20kmfrom
aspects appear to change around the up-dip limit of thedeformationfronthasataperangleofabout2.1°in
the seismogenic zone: 1) the onset of a trench slope average, whereas from about 20 to 42 km, the angle
break (or outer arc high) which coincides with the becomes larger, about 8.5° (Fig. 1). Landward of this
trenchwardedgeofforearcbasin;2)achangeinwedge part,thesurfaceslopeandtaperanglesoverthatdistance
taper and a change in the thickening mode of are again gentle and forearc deposits overlie the
accretionary prism from in-sequence thrusting to out- accretionary prism. Thus the accretionary wedge is
of-sequence thrusting; 3) a step-down of the aseismic divided into three segments: outer wedge, transition
décollement; and 4) ramping up of low angle, out-of- zoneandinnerwedge(Fig.1).Upperslopeinflectionis
sequences thrust above the underplated complex. How definedtraditionallyasatrenchslopebreakorouterarc
these aspects are intimately linked to the onset of the high [17].
seismogenic zone is quite important to address the TheforearcslopeandwedgeintheAshizuriandKii
questions above. regions are also divided into three segments which
Inthispaper,wereviewthesefeatures,especiallyin nevertheless show different features from the Muroto
the Nankai Trough, and discuss the onset of the region(Fig.1).ThePhilippineSeaPlatedipsatabout6°
seismogenic zone, taking recent studies of on-land in these regions (Fig. 1). The outer wedge near the
ancientaccretionary complexinto account. trenchshowsataperangleofabout7.8°and4.6°,inthe
Kii and Ashizuri regions, respectively. The taper angle
2. Morphological and geological aspects of the ofthetransitionzoneincreasesto16.5°inKiiand7.9°
modern Nankai accretionary prism in Ashizuri regions, respectively (Fig. 1). The zone of
steep surface slope, i.e. the transition zone, is narrower
In the Nankai Trough, detailed bathymetric intheKiiregionthanintheAshizuriregion.Athickpile
depth-sounding survey data have been published offorearcdeposits covers theaccretionaryprismofthe
(Japan Oceanographic Data Center, Online publica- innerwedgeintheKiiregion,whereasonlyathinlayer
tion) and many multichannel seismic reflection of sediments overlies the prism in the Ashizuri forearc
surveys have been conducted, including a 3D survey (Fig. 1). The most drastic change in slope angle in the
(e.g. [8–13]). Simplified profiles are shown in Fig. 1 Nankai Trough is observedin theKii region(Fig. 1).
with the recently inferred up-dip limit of the rupture Theupperinflectionpointfromsteeptogentleslopes
zone of the 1944 Tonankai and 1946 Nanakai between transition and inner wedge is defined as the
earthquakes [12,14,15] although there are uncertain- trench slope break or outer-arc high [17]. The up-dip
ties in the observationally constrained up-dip limit limit of the seismogenic zone inferred from seismic
of the seismogenic zone. The profiles represent andtsunamiinversion intheKii andMurotoregionsis
several morphological and geological aspects as located somewhere beneath the transition zone
follows: [9,11,14,15]. The up-dip limit of the seismogenic and/
Fig.1.(a)Locationsofseismicprofilein(b).BathymetricdatawasobtainedthroughtheJapanOceanographicDataCenter.(b)Simplifiedprofilesof
theNankaiTrough.TheprofileoftheAshizuiriKR9801-01isfromParketal.[12].KR-9806-02offKiiisfromParketal.[11].141-2Dprofileoff
MurotoisfromMooreetal[9].Alltheprofilesarearrangedinthesamescalewithoutverticalexaggeration.Interpretationsforeachprofilearefrom
eachpaperwithsimplification.Numbersabovetheprofilewithsmalltriangleindicateaveragetaperangleinadistanceshownbythearrow.Notethat
taperanglesofthetransitionzonearerelativelylargeincomparisonwithgentleonesoftheouterandinnerwedges.Dottedpatternrepresentsyounger
slopesediments.OOST:out-of-sequencethrust,DSR:deepstrongreflectorzone,LAR:lowamplitudereflectorzonefromParketal.[12].Contours
(2minterval)intheforarcisruptureareaandestimatedslipalongtheplateboundarybySagiyaandThatcher[78].
G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484 473
474 G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484
Fig.2.Generalizedaspectsaroundtheup-diplimitoftheseismogeniczoneinferredfrommodernexamplesoftheNankaiTrough.Topographicand
intra-prismstructuresarefromFig.1andthermalstructureisfromK.Wang,personalcommunication,2003.Notethatasignificanttransitionalregion
isrecognizedbetweentheup-diplimitofthelowangleOOSTandtheplaceofstep-downoftheaseismicdécollement,wheretheclearchangein
wedgetaperwouldtakeplace.NumbersinwhitecirclesshowthelocationsofsimplifiedsectioninFig.8.Lowerdiagramshowsapproximate
temperatureofthefault.Notethatthetransitionzoneislocatedinarangefromabout100°Cto180°C.
or tsunamigenic zone in the Ashizuri region is also duringthe1944TonankaiEarthquake[11]andthe1946
situated beneath the transition zone [18]. Nankai earthquake,respectively[12].
2.2. In-sequence fold and thrusting to out-of-sequence 2.3. Change in aspects of plate boundary fault
thrusting
IntheseismicprofilesintheNankaiTrough,wecan
The internal structure of the accretionary prism is observe that the basal décollement, which is present
well documented by seismic reflection profiles [9– beneath the outer wedge, steps down to reach the
13,19].Tairaetal.[20],AshiandTaira[21],andMoore sediment–basement interface or the basement [9,11–
etal.[9]dividedtheinternalstructureoftheprisminto 13]. The step-down takes place beneath the transition
proto-thrust zone, imbricate thrust zone, frontal out-of- zone in the Kii region [6,11]. The décollement beneath
sequencethrustzone,andlargethrustslicezone,mainly the outer wedge is mostly represented by a negative
on the basis of the Muroto transect. The former two polarity reflector, whereas the plate boundary after the
zones coincide with the outer wedge and the latter two step-downischaracterizedbyvariousaspects:negative,
zonesaredevelopedinwhatwedefinedasthetransition non-reflective or ambiguous positive polarities. In the
zone in this paper. Wang and Hu [22] put the name of western Nankai Trough off Ashizuri, a deep strong
outerwedgetoallofthemalthoughwedividethatinto reflector(DSR)withnegativepolarityappearsatseveral
two as defined above. This division indicates that tens kilometers landward behind the step-down part
accretionaryprocesses changefromin-sequencethrust- [12]. The reflector is located above the underthrust
ing to out-of-sequence thrusting, which is commonly complex andisinterpretedasaroof thrustfor thestep-
observedinotherregionsoftheNankaiTrough[11,12]. down of the décollement [11,12]. The region between
In-sequence thrusts are well developed at a distance of thestep-downofthedécollementandthetrenchwardtip
about 20–25 km from the deformation front in the Kii ofthestrongreflectorwasdescribedbyParketal.[12]
andtheAshizuriregionsandabout30kmintheMuroto asalowamplitudereflectorzone(LAR)andinterpreted
region(Fig.1).TheplaceofinitiationofOOST(out-of- thatzoneasanunderplatingdominatedzone.Deforma-
sequencethrust),whichcontributetothickentheprism, tion in the accretionary prism above the LAR is
coincideswiththeonsetofthesteepslopeonthesurface characterized by the development of OOSTs described
and widened tapers described above. OOSTs in the Kii above (Fig. 1). The root of OOSTs appears to be
and Muroto regions are considered to have slipped connected with the strong reflector and the OOSTs
G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484 475
appear to ramp up from the low angle reflector, cut outer wedge and transition zone is in critical regime,
whole prism, and emergeon the seafloor (Fig.2). following Davis et al. [25] and Dahlen [23,24]. The
surface slope (α) and dip angle of the plate boundary
2.4. Interpretation in terms of Coulomb–Mohr critical fault (β) for the outer wedge and transition zone are
taper theory plotted on diagrams of different effective basal friction
(μ =(1−λ )/(1−λ⁎))inthreeregions,theKii,Muroto
b b
Geometric changes in wedge taper and inferred andAshizuriregionsdescribedabove(Fig.3).Twocases
internaldeformationoftheprismarewellexplainedby of wedge internal friction coefficient, μ=0.85 and
Coulomb–Mohr wedge theory [23–26] although such μ=0.6areassumed.Variousfluidpressureratiosinthe
originalstudiesconsideredanuniqueandhomogeneous wedge are assumed, from hydrostatic condition of
prism,ignoringlocalchangesinwedgetapersuchasthe (normalized pore pressure ratio; k⁎ ¼ Pf−Ph, where P,
Pl−Ph f
trench slope break. Recently Lohrmann et al. [27] P , and P are fluid pressure, hydrostatic pressure and
h l
explainedthechangeintaperwithinasingleaccretion- lithostatic pressure, respectively) to over-pressured
ary prism in terms of the change in internal friction of condition of λ⁎=0.17 and 0.33, neglecting the water
thewedgeon thebasis sand-analogueexperiments, but column above the submarine wedge. For the case of
basal friction was kept constant in their experiment. hydrostaticfluidpressureinthebasaldécollementofthe
WangandHu[22]proposedanewinterpretationforthe wedge, the friction coefficient of the décollement
wedgestructureusingthedynamiccriticaltapertheory, coincides with internal friction coefficient of the
based on the difference in basal friction between wedge. μ=0.6 is common for rock fracture in low
coseismic andinter-seismicperiod. confiningpressureandsand,andμ=0.85isthesameas
AllthecrosssectionsoftheNankaiTroughprismare Byrlee'scoefficientfortheuppercrust[28].Sedimentary
segmented into three domains with different tapers as rocks of sandstone and siltstone from the accretionary
described. To explain the evolution from outer to inner prismonlandshowarangeofinternalfrictioncoefficient
wedge,wefirstmaketheassumptionthatthroughoutthe as0.6–0.9[29],thereforetheassumptionisreasonable.
Fig.3.ChangeintaperangleofouterwedgeandtransitionzoneoftheNankaiTroughintheKii,Muroto,andAshizur(cid:1)iregio(cid:3)nsandestimated
1 sina 1
effectiv(cid:1)e basa(cid:3)l friction coefficient. Equations for critical line calculation are α+β=ψ −ψ , w ¼ arcsin − a;andw ¼
1 sinu 1 b 0 0 2 sinu 2 b
arcsin b − u ;whereψ istheintersectionanglebetweentheslopesurfaceandmaximumprincipalaxisofstressinthewedge,ψ is
2 sinu 2 b 0 b
theintersectionanglebetweenthebasalthrustandmaximumprincipalaxisofstress,andφandφ isinternalfrictionangleofthewedge(forinternal
0
friction coefficient μ) and angle for friction coefficient for basal thrust (friction coefficient μ), respectively [23,24]. All the parameters are of
b
effectivepressureincorporatingfluidpressure.Normalizedporepressureratiointhewedgeisexpressedasλ⁎.
476 G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484
Theouterwedgecharacterizedbyin-sequencethrusts friction determine the slope over numerous earthquake
showsgentleslope surface andnarrow taper andmight cycles. This means that the fault beneath the outer
beexplainedintermsoflowbasalfrictionassuggested wedge and transition zone has a velocity- (or slip-)
bySafferandBekins[30]andLohrmannetal.[27].The strengtheningbehavior.Incontrast,thefaultbeneaththe
narrowtaperangleespeciallyintheMurotoregionmay inner wedge, in addition to being normally weak,
result from its basal low friction due to unconsolidated becomesevenweakerduringearthquakesbecauseofits
and mud-dominated composition [9,20] in comparison velocity- (or slip-) weakening (i.e., seismogenic)
withthoseinAshizuriandKii,whereturbiditicsandsare behaviour, and that is why the inner wedge is stable.
much more abundant [9]. The upwardconvexshape of Insummary,theentiresubductionfaultisweak,butthe
that segment can be explained by landward increase of seismogeniczonebecomesweakerandtheup-dipzone
internal friction of theprism [26] and hardening due to becomesstrongerduring great earthquakes.
diffusive deformation [31,32], together with landward
rotationofthethrusts[27]. 3. Deformation mechanism observed in on-land
Adrasticchangeintaperangleinitiatesattheplaceof accretionary prism
onset of the OOST in the transition zone in all regions
described above [9,11,12]. The onset of the OOST Thestrainhistoryofafrontallyaccretedprismmight
indicatesthattheaccretionaryprismiscriticallydeformed be different from that of the underthrust sediments
by newly formed thrusts fractured in the orientation [31,32,36]. The upper part of the trench-filling sedi-
controlled by internal friction coefficient [27,33]. The ments, composed dominantly of turbidites, is scraped
basal décollement is strengthened and its shear strength off at the deformation front and starts to be internally
approachestothevalueofinternalfrictionoftheprismas deformed due to horizontal shortening [9]. The sedi-
estimated. Sandbox experiments for the change in basal ments are initially unconsolidated sand and mud with
friction show that the friction significantly controls the porosity of over 50% [9,20,36] but finally become
shapeofthewedgeandunderplating[34,35]. consolidated to sandstone and shale with porosity less
Wang andHu [22] explain that theouter wedge and than several % as observed in on-land accretionary
transitionzonehasasteeperslopenotbecauseitsbasal prism [36,37]. Tectonic diagenesis, which means that
friction isalwaysgreater,butbecause thebasalfriction lithification operated concurrently with deformation,
is greater during great earthquakes. They suggest that progressesduringtheaccretionandunderthrusting.First
the plate boundary fault is weak, much weaker than sediments are deformed by particulate flow with po-
whatissuggestedbythevaluesintransitionzoneshown rosity reduction [31,32]. During compaction, pore
inFig.3.Theonlywaytoreconciletheweakfaultand reduction is enhanced by compaction cataclasis ob-
thehighslopeoftheouterwedgeandtransitionzoneis servedasubiquitousdevelopmentof“web”structurein
to recognize that the normally weak fault can suddenly sandstone [38–40]. Experiments [41–44] suggest that
and very briefly become strong during great earth- compactioncataclasisisenhancedwithstressatporosity
quakes. Repeated coseismic pulses of higher basal largerthanabout15%.Therealignmentofclayminerals
Fig.4.GeologicalmapandprofileoftheMugiarea,Shikoku,SWJapan(modifiedfromShibataetal.,personalcommunication).
G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484 477
The on-land accretionary prism is cut by localized
discrete cataclastic thrust. Maximum temperature of
hangingwallofthethrustatburialdepth,detectedfrom
vitrinite reflectance and fluid inclusion analyses, is
higher than that of foot wall. The discrete thrust is
recognizedas exhumed ancientOOSTs[46–50].
3.1. Diffusive deformation to localized shear observed
in ancient plate boundary rocks
Thestudyofancientaccretionarycomplexesonland
provides clues to deformation processes of the basal
Fig.5.Geologicsectionsofthrustsheetsunit1to6andhorizonof plate boundary fault and underthrust sediments, and to
datumof vitrinitesample[52],veinfor fluidinclusionanalysis[6], theonset ofthe seismogenic zone.
U–Pbdatingfromzircons(Shibataetal.,personalcommunication),
Onishi and Kimura [51], Matsumura et al. [6],
andchemistryofmatrixshale[45].
Ikesawa et al. [52] and Kitamura et al. [7] studied an
upperCretaceoustolowerTertiarycomplexoftectonic
to a preferred orientation is a significant component of mélange, the Mugi Mélange, composed of basalt and
initial mechanical diagenesis for muddy sediment. sandstoneblocksinashalematrix(Fig.4).Thrustpiles
Chemical diagenesis is also important. Dehydration of the Mugi Mélange are characterized by downward
duetothetransformationofopalCTtoquartz,smectite youngingages[Shibataetal.,personalcommunication]
to illite and hydrocarbon generation are important andaninversethermalstructure[52]withafaultedgap
processes that lithify the mud to shale [5]. Chemical betweentheupperandlowersections(Figs.4,5and6).
diagenesisisdependentmainlyonthethermalcondition Themélangerecordstheprocessesofunderthrustingto
in the range of 100 °C–150 °C [5]. Such a process is underplating under maximum P–Tconditions of 120–
well recorded in deeply buried accretionary prisms on 220 °C and 6–7 km depth (Fig. 6). Such setting is
land [40,45]. similar to that around the proposed up-dip limit of the
Fig.6.Asynthesizeddiagramofage,P–TconditionandvolumechangeofshalefortheMugiMélange.DatasetsarefromMatsumuraetal.[6],
Ikesawaetal.[52],Shibataetal.(personalcommunication),andKawabataetal.[45],normalizedtothesamethicknessforeachunitinFig.5.White
andsoliddotsintherightdiagramshowvolumedecreaseandincrease,respectively.NumberofunitisshowninFigs.4and5.Notethedifferences
betweentheupperandlowersections.
478 G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484
seismogeniczone[1–3]andtheplateboundarybeneath carbonate veins suggests inflow of exotic metamorphic
the transition zone of the wedge of the modern Nankai wateralongtheshearzonefromdepth[55].Geological
Trough described above (Figs. 2 and 4). The present and structural aspects of the shear zones suggest
physical properties of the mélange show porosities episodic fluid flow in association with rapid fluidized
lower than a few percent in sandstone and several cataclasticslipalongthefault,whichmightberelatedto
percent in shale, even under atmospheric conditions seismicslip[55].
[34].Permeabilitiesdecreasefrom10−12∼−15 m2 under
atmospheric condition to less than 10−19∼−20 m2 [53] 3.2. Interpretation of on-land mélange
under confining pressure of 120 MPa corresponding to
6–7 km depth. Present pores might therefore be Pressure–temperature conditions, physical proper-
composed mostly of micro-cracks opened during ties, state, and geologic and deformational features of
exhumation to the surface but that were not present at the mélange suggest that it was located around the up-
depth. dip limit of the seismogenic zone and at the interface
The deformation mechanisms in the mélange shale between underthrust sediments and basaltic basement.
matrix are dominated by preferred alignment of illites, Most of the deformation mechanisms for the mélange
diffusive mass transfer due to pressure solution, and are pressure solution or narrow-spaced compaction
micro-cracking, with the cracks now filled with quartz cataclasis within sandstone blocks. These mechanisms
and/orcalciteformingveins[7,51].Chemicalanalysisby might notbe seismicity-related.
using the “isocon method” [54] for the shale suggests a Thebestcandidateforseismicity-relateddeformationis
fewtensofpercentofvolumelossduetomasstransferby alocalizedcataclasticfaultboundingthebasalticlayerand
pressure solution and increase of grain density (Fig. 6: mélange [52,55]. This geological feature of the mélange
[45]).Sandstoneblocksinthemélangearealsostrongly suggeststhatunderthrustsedimentsarefirstdeformedby
deformedandshowboudinagedstructures[6,7,39,51,52]. downward shear propagation into the underthrust sedi-
The initial deformation of the sandstone is independent mentsfromtheinitialdécollement,buttheirdeformationis
particulate flow, which is documented as ubiquitous dominantlydiffusiveandaseismic(Fig.8).Thedeforma-
obliteration of primary sedimentary textures of the tionpropagationmightbeduetodeformationhardeningas
turbidites. Second phase of deformation is cataclastic suggestedbyMooreandByrne[56].Asthedécollement
breakagerepresentedby“web”structure[38].The“web”
cataclastic deformation progressed until porosities de-
creasedtoabout15%[40].Thirdphaseofdeformationis
pressure solution represented by concave–convex cou-
plingofsandgrainswithathinfilmofsolutionresidue,
andprecipitationofcementscomposedofcalcite,quartz
andauthigenicclayminerals[40].Thefinaldeformation
is again brittle breakage and necking resulting in the
formationofboudinagedblocksinthemélange.Tension
crack-fillingquartzandcalciteprecipitatedinthecracks
around the neck of boudins, trapping H O–CH fluid
2 4
inclusions that indicate temperatures and pressures of
120–220 °C and 120–200 MPa, respectively (Fig. 6;
[6,55]). The lowest temperatures estimated by fluid
inclusion analysis are consistent with those estimated
fromvitrinitereflectance(Fig.6).
The Mugi mélange includes basaltic layers peeled
from oceanic basement [6,7,51,52]. The boundaries
between the basalt and mélange are cataclastic shear
zonescontainingabundantprecipitatedandcrack-filling
veins of calcite, quartz and zeolites [6,52]. The
temperature of the deformation of the boundary shear
Fig. 7. Deformation mechanisms and porosity reduction before the
zoneestimatedfromfluidinclusionsishigherthanthat
onsetofseismogeniczoneinferredfromthedeformationoftheMugi
in the boudin-neck veins of sandstone lenses in the
mélange (synthesized from Hashimoto et al., [40], Kawabata et al.,
mélange (Fig. 6, [6]) and stable isotope character of [45]).
G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484 479
Fig.8.Simplifiedaspectofplateboundaryfaultandgeologicalsectionsofunderthrustsedimentswithbasementinsubductionzone,inferredfromon-
landunderplatedcomplex[6,7,40,52,58].NumberofthesectionshowslocationinFig.2.Darkareaconnectingeachsectionindicatesplateboundary
shearzone.Steps1–2appeartobeaseismicbutthefaultatsteps3–4mightbeseismogenic.
arrivesat the basementof oceanic crustas a result ofits At the deformation front, the décollement initiates
downwardpropagation,theplateboundaryfaultcutsinto at some stratigraphic horizon below the trench-filling
theoceanicbasementprobablyduetoloweffectiveshear sedimentasobservedinthemodernNankaiTrough[9]
strength within the basement rocks caused by high pore (Figs. 8, 1). The thickness of underthrust sediments is
pressure [57] or due to weak lithofacies dominated by a few hundred meters. The décollement would
clayey hyaloclastite [52]. This breakage appears to be propagate downward due to deformation hardening
seismicbecausethecataclastictoultracataclasticfragmen- of the décollement caused by porosity reduction and
tationisubiquitousinthebasalt[52]andthefaultappears effectiveporefluidescape(Fig.8,2).Thepropagation
tobefluidized[55]. of the shear zone promotes the formation of tectonic
Previously, pseudotachylytes have been found from mélange and finally arrives at oceanic basement
four locations [7,58–60] in the Shimanto Belt. All of (Figs. 8, 3). The décollement cuts and peels off the
themarefromdiscretefaultsratherthanfrommélange- top of the oceanic crust from the basement. The shear
likediffuseshearzones.Theirsettingisdividedintotwo is localized and would be seismic. Repeated shear
kinds in relation to the hottest thermal structure along the interface between the underthrust sediment
reconstructed from vitrinite reflectance. Two of them and basement makes a pile of mélange, including thin
[7,58] are at a roof fault of mélange and do not cut the slabs of oceanic crust due to duplexing (Fig. 8, 4).
thermalstructureoftheaccretionaryprism,whereasthe Downward younging age-systematics of the pile is
other two settings are at faults that cut the thermal attained by long term underplating for several to ten
structure [59,60]. The latter setting is defined as an million years continuous subduction. Later on, thrust-
OOSTas recognized by Sakaguchi [48], Ohmori et al. ing resulted in the exhumation of this deeper
[49] and Kondo et al. [50]. Interpretation of the former underplated complex and to the formation of the
settingisaplateboundaryfaultparalleltotheisotherms inverse thermal structure.
atdepth[7,58].Theplateboundaryfaultappearstobea
localized and reactivated décollement above the under- 4. Discussion
platedcomplexoftheunderthrustsedimentsasobserved
as DSR in the seismic profiles of the modern Nankai On the basis of the above observations and inter-
Trough(Fig.1)ThepseudotachylytefoundintheMugi pretations of the modern Nankai accretionary prism
mélange belongs to the second-type, i.e. a plate andon-landunderplatedcomplexoftheMugimélange
boundary fault. oftheShimantoBelt,wediscussthefollowingpoints:
Figs.7and8presentaninferredevolutionoftheplate internal friction vs. basal friction, stable vs. unstable
boundaryfromtheouterwedgetotheup-diplimitofthe slip, and the step-down of the plate boundary
seismogenic zone on the basis of the interpretation of décollement around the up-dip limit of the seismo-
geologicalfeaturesoftheMugimélangedescribedabove. genic zone.
480 G.Kimuraetal./EarthandPlanetaryScienceLetters255(2007)471–484
4.1.Internalfrictionvs.basalfrictionandunderplating faultascalculatedbycriticaltapertheory.Suchachange
in frictional strength of the basal décollement might
A drastic change in wedge taper takes place in the cause the décollement and underthrust sediments to
transitionzonebetweentheouterandinnerwedges.The stick to the upper accretionary prism, which defines
outerwedgeandtransitionzoneareinternallydeformed underplating.
byin-sequencethrustingandout-of-sequencethrusting,
respectively.Suchinternaldeformationsuggeststhatthe 4.2. Stable slip vs. unstable slip
wedgeisgrowingundercriticaltapercondition.Alarger
critical angle of taper implies smaller internal effective Seismogenic faults are characterized by slip or
frictionor/andlargerbasaleffectivefriction(Fig.3).In- velocity weakening that can be described using rate-
sequence thrusting of the outer wedge first starts in and-state friction laws [64,65]. Several dynamic
unconsolidatedsedimentsandgraduallyrotatestosteep weakening mechanisms have been proposed, such as
dipanglesasdocumentedinseismicprofiles[13].Inthe frictional melting [66], thermal pressurization of pore
initial stage of thrusting, thrust orientation isconsistent fluid [67–71], acoustic fluidization [72,73], elastohy-
with internal friction coefficient smaller ∼ 0.6 (angle drodynamic lubrication [74], or silica gel generation
with σ1 is about 30°) and gradually departs from that due to comminution [75] for unstable slip of earth-
orientation due to rotation. The deformation of uncon- quakes. Clay mineral transformation from smectite to
solidated sediments might be first independent partic- illite was also assumed to be a cause from stable to
ulate flow with grain breakage, promoting the unstable [1] but experimental studies did not docu-
compaction, which in turn result in material hardening ment such an unstable frictional behavior of illite
asdocumentedbyexperiments[42,43].Thus,deforma- [76].
tion and rotation of the frontal segment might increase The up-dip limit of the seismogenic zone might be
theinternalfrictionuptothevalueofconsolidatedhard controlled by a transition from stable to unstable slip
rockofμ=0.85.Realignmentofgrainsmayenhancethe along the plate boundary fault. Recently, several
permeability parallel to the foliation, which in turn, seismogenicfaultcandidateswerereportedfromancient
might reduce the fluid pore pressure and increase the accretionary complexes on land [7,55,58,60,77].
effectivestrength.Finallythein-sequencethrustsmight Among these, fluidized ultracataclasites were found
becomelocked. from a boundary fault between underthrust and de-
The basal décollement beneath the frontal segment formedsediments,andbasalticbasement,bothofwhich
initiatesatsomestratigraphichorizonwhosemechanical wereaccretedinlateCretaceousandearlyTertiarytime
strength is weak due to weak clays and/or high pore [55,58]. Others include pseudotachylyte with fluidized
pressure [9,61]. Seismic reflection profiles and direct ultracataclasiteandimplosionbreccia[60,77].TheP–T
hydrological properties determined by drilling docu- conditionsofthesefaultrocks(c.a.220°C–250°C,5–
mentthatthedécollementisafaultandthat porosity is 10 km) indicate that they were located at seismogenic
larger in the foot wall than in the hanging wall. Such a depthinthepast.Geologicalevidencesuggeststhatnot
hydrologicalinverseismanifestasanegativereflection only one mechanism mentioned above operates at the
coefficient of the décollement [9]. Super-hydrostatic fault, but several mechanisms might be combined and
fluid pressure in this horizon makes the décollement promote thedynamic weakening.
weakandthedécollementeasilyandintermittentlyslips The critical wedge model uses Coulomb-plastic
asaseismicand/ortsunamigenicplateboundary[18,62]. rheology, in which case frictional strength for the slip
At the initial stage of décollement formation, the pore isthesameasthatofpeakstrength [27].WangandHu
fluid originates from sea water but is soon mixed with [22], however, point out that shear strength of the
fluiddehydratedfromclayeysediments.Suchdehydra- seismogenic fault drops to smaller values due to
tion reaction is controlled mainly by temperature [5]. velocity (or slip) weakening. On the other hand the
Thedehydration ratedecreasesbeyondtemperatures of faults beneath the outer wedge and transition zone are
about 100–150 °C [5], which rapidly decreases fluid stable-slip faults, which are characterized by velocity
pressure/lithostatic pressure ratio [30,63] and results in (or slip) strengthening in association with coseismic
an increase of effective shear strength of the décolle- slip. The different slip behaviour between seismogenic
ment. The wedge taper of the transition zone suggests faults beneath the inner wedge and aseismic faults
thattheeffectiveshearstrengthofthebasaldécollement beneath the transition and outer wedge is therefore the
approaches the strength of internal friction due to a most likely explanation for the differences in wedge
decreaseoffluidpressureratioalongtheplateboundary taper.