Research Article Thermal ... - Osaka City University · forming the paired metamorphic belt in southwest Japan have not been understood fully (e.g. Brown 2002; Iwamori 2002), and

The Island Arc (2004) 13, 520–532

Blackwell Science, LtdOxford, UKIARThe Island Arc1038-48712004 Blackwell Publishing Asia Pty LtdDecember 2004134520532Research ArticleThermal consequences of a slab window formationT. Okudaira and Y. Yoshitake

*Correspondence.†Present address: Tokyo Computer Service Company Limited, Kyoto 604-

8811, Japan.

Received 7 April 2004; accepted for publication 27 July 2004.© 2004 Blackwell Publishing Asia Pty Ltd

Research ArticleThermal consequences of the formation of a slab window

beneath the Mid-Cretaceous southwest Japan arc:A 2-D numerical analysis

TAKAMOTO OKUDAIRA* AND YOSHIKO YOSHITAKE†

Department of Geosciences, Osaka City University, Osaka 558-8585, Japan (email: [email protected])

Abstract The development of voluminous granitic magmatism and widespread high-grademetamorphism in Mid-Cretaceous southwest Japan have been explained by the subductionof a spreading ridge (Kula–Pacific or Farallon–Izanagi plate boundaries) beneath theEurasian continent and the formation of a slab window. In the present study, the thermalconsequences of the formation of a slab window beneath a continental margin are evaluatedthrough a 2-D numerical simulation. The model results are evaluated by comparison withthe Mid-Cretaceous geology of southwest Japan. Of particular interest are the absence ofan amphibolite- to granulite-facies metamorphic belt near the Wadati–Benioff plane, andsignificant melting of the lower crustal-mafic rocks sufficient to form a large amount ofgranitic magma. Because none of the model results simultaneously satisfied these twogeological interpretations, it is suggested that subduction of plate boundaries in Mid-Cretaceous southwest Japan was not associated with the opening of a slab window. Accord-ing to previous studies, and the results of the present study, two different tectonic scenarioscould reasonably explain the geological interpretations for Mid-Cretaceous southwestJapan: (i) The spreading ridge did not subduct beneath the Eurasian continent, but waslocated off the continental margin, implying the continuous subduction of very youngoceanic lithosphere; (ii) ridge subduction beneath the continental margin occurred afteractive spreading had ceased. Consequently, in both tectonic scenarios, the subduction ofplate boundaries at the Mid-Cretaceous southwest Japan was not associated with a slabwindow, but very young (hot) oceanic lithosphere.

Key words: Mid-Cretaceous igneous activity, Ryoke–Sanyo granitoids, Sambagawa Meta-morphic Belt, slab window, spreading-ridge subduction, thermal modeling.

INTRODUCTION

It is generally accepted that high-temperature (T)metamorphic belts associated with the productionof great quantities of granites at convergent platemargins, as typified by the Circum-Pacific regionsof southwest Japan and North America in the Mid-Cretaceous and Early Cenozoic, are indicative of

the ancient subduction of ‘hot’ oceanic lithosphere(e.g. Uyeda & Miyashiro 1974). The Cretaceousand Early Cenozoic are also characterized byFarallon–Izanagi or Kula–Pacific spreading insouthwest Japan (e.g. Engebretson et al. 1985;Maruyama & Seno 1986; Fig. 1) and Pacific–Farallon spreading in North America (e.g. vanWijk et al. 2001). It is conventionally thought thatridge subduction is required to form such high-Tmetamorphic belts, because petrologic estimationof the metamorphic field gradient of high-grade(amphibolite- to granulite-facies) metamorphicbelts cannot be explained simply by the thermalstructure of the arc-trench system in ‘normal’ or

Thermal consequences of a slab window formation 521

steady-state subduction (Peacock et al. 1994;Iwamori 2000; Okudaira 2002).

Along the Mid-Cretaceous Eurasian continentalmargin, the subduction of a ridge is interpreted bymany geological observations in southwest Japan,including the voluminous along-arc activity of theRyoke–Sanyo granitoids and in-situ basalts withmid-ocean ridge basalt (MORB) affinity withinCretaceous accretionary complexes (Banno &Nakajima 1992; Kiminami et al. 1993; Kinoshita1995, 2002; Nakajima 1996; Brown 1998). Based onthese geological circumstances, Iwamori (2000)suggested that the formation of the paired meta-morphic belt of southwest Japan; that is, the high-pressure (P) Sambagawa Metamorphic Belt (oce-anic side) and the low-P Ryoke Metamorphic Belt(continental side), occurred as a result of ridgesubduction at the Cretaceous Eurasian continentalmargin. However, this model in fact represents thesubduction of a very young slab rather than anactive spreading-ridge, because the spreading rateof the ridge in his model is zero.

When a slab window opens during ridge subduc-tion, the asthenosphere is assumed to rise and fillthe space formerly occupied by the slab. If such ahigh-T section had indeed been subducted alongthe Wadati–Benioff plane, the forearc region, inparticular near the Wadati–Benioff plane, wouldexhibit extreme thermal perturbations with theresultant formation of an amphibolite- to granulite-facies metamorphic belt. However, the rocks of theSambagawa Metamorphic Belt, which is consid-

ered to be formed near the Wadati–Benioff planeduring subduction of the plate boundary beneaththe Mid-Cretaceous Eurasian continental margin,do not indicate such low-P/high-T metamorphicconditions (e.g. Banno & Nakajima 1992; Enami1998; Aoya et al. 2003). For the subduction-typeSambagawa metamorphic rocks, most of the pro-grade P–T paths proposed so far indicate thatmetamorphic pressures (reflecting the depth ofthe rocks) increase with increasing temperatures(e.g. Enami 1998; Aoya et al. 2003). Because suchP–T paths of the rocks would represent their sub-duction-stage tectonic history, it is believed thatthey were located near the Wadati–Benioff planeduring their subduction stage.

Based on the shape of metamorphic P–T pathsfor the low-P/high-T Ryoke metamorphic rocks,Brown (1998) suggested that the low-P/high-Tmetamorphism was caused by ridge subduction.His ridge subduction model for the Ryoke Meta-morphic Belt required that the juxtaposition ofthe high-P/low-T Sambagawa Metamorphic Beltagainst it was a result of terrane amalgamation,and the paired metamorphic belt of the Ryokeand Sambagawa Metamorphic Belts representslaterally contemporaneous terranes, rather thanacross-arc components of a trench/arc ‘paired’ sys-tem. If this interpretation were true, the petrolog-ically derived P–T conditions of the Sambagawametamorphic rocks could not be used to evaluatethe simulated results. However, tectonic processesforming the paired metamorphic belt in southwestJapan have not been understood fully (e.g. Brown2002; Iwamori 2002), and further geological infor-mation about them is required. In the presentstudy, we adopt the traditional view that the pairedmetamorphic belt for the Ryoke and SambagawaMetamorphic Belts is representative of the geolog-ical situation in southwest Japan during the Mid-Cretaceous (e.g. Uyeda & Miyashiro 1974; Banno& Nakajima 1992; Iwamori 2002); we adopt thisstance because lateral displacement along themajor fault (Median Tectonic Line) during theLate Cretaceous has been estimated to be~500 km, which is less than the lateral extent ofthe Ryoke Metamorphic Belt (~700 km) (e.g.Yamakita & Otoh 2000).

The thermal consequences of the formation of aslab window are not yet fully understood, althoughthe subduction of very young (hot) oceanic litho-sphere has been investigated in this regard (e.g.DeLong et al. 1979; Peacock et al. 1994; Iwamori2000; van Wijk et al. 2001; Miyazaki & Okamura2002). In the present study, we used 2-D numerical

Fig. 1 Paleogeographic reconstruction of the Eurasian continentalmargin area during the Late Cretaceous (simplified from Maruyama &Seno 1986, with permission). The Kula–Pacific ridge is located off theEurasian continental margin.

Kula Plate (85 70 Ma) 20 cm/yr

Izanagi Plate (150 80 Ma) ~23 cm/yr

Pacific Plate 10 cm/yr

Eurasia Plate

Okhotsk Plate

522 T. Okudaira and Y. Yoshitake

modeling to evaluate the thermal structurebeneath an arc-trench system that arose becauseof the subduction of a spreading ridge. The analy-sis focuses on the role of model parameters, suchas the convergence velocity of a spreading ridgeand the spreading rate at the ridge center, in thedevelopment of the thermal structure beneaththe arc-trench system. Results of the model arediscussed in terms of geological interpretationsused in southwest Japan, notably: (i) The extentof development of the amphibolite- to granulite-facies metamorphic rocks near the Wadati–Benioffplane; and (ii) the time scale of partial melting oflower crustal rocks needed to promote long-lastinggranite activity.

MODEL

The setup for the numerical model used to calcu-late the thermal structure beneath an arc-trenchsystem is shown in Figure 2. In the model, a300 ¥ 200-km finite-difference grid is set as themodel domain (subduction domain). Before sub-duction of the oceanic lithosphere in the subduc-tion domain, the thermal structure of the oceaniclithosphere is calculated independently using a

different model domain of 2000 ¥ 70 km (oceandomain). In the subduction domain, the subductionis assumed to continue for 70–100 My, using a setof boundary conditions calculated in the oceandomain. The subducting oceanic lithosphere hasbeen modeled to be in contact with the continentallithosphere and mantle asthenosphere along theWadati–Benioff plane.

The heat-transfer equation is solved to obtainthe variation in temperature (T) with distancefrom the trench (x) and depth below sea level (z),as a function of time (t):

rc (dT/dt) = k—2T - v ◊rc—T + Q (1)

in which k is thermal conductivity, r is density, c isheat capacity, v is velocity and Q is the heat pro-duction rate. On the basis of the equation and con-ditions described below, the thermal structures ofboth simulation domains are calculated using anexplicit finite difference method (Dx = Dz = 1000 m;Dt = 3.15 ¥ 109 s).

In the ocean domain, the thickness of the mantlelithosphere is set constant at 60 km, consideringthe known depth of the lithosphere–asthenosphereboundary (~65 km) and its invariance with age(Hirth & Kohlstedt 1996; Karato & Jung 1998).The initial (pre-spreading) thermal structures ofthe oceanic lithosphere are approximated using a60-km-thick plate-model with a basal temperatureof 1280∞C, equivalent to a mantle heat flow of0.058 W/m2. An adiabatic gradient of ~0.3∞C/km isincorporated into the vertical temperature struc-ture in the mantle asthenosphere. Values of ther-mal conductivity, radiogenic heat production, rockdensity, and heat capacity of the mantle litho-sphere and mantle asthenosphere are shown inTable 1.

The spreading-ridge is modeled as follows. Aregion with a constant asthenospheric tempera-ture (1280∞C) within the oceanic lithospherebeneath the ridge center is considered. The shapeof the high-temperature region is assumed to betriangular, with a basal length of 2 km and a

Fig. 2 Kinematic model used for calculation of temperature in thesubduction domain. Arrows show the direction of subduction of theoceanic lithosphere and the direction of viscous flow in the mantleasthenosphere.

T = 0 C x = 300 km

z = 200 km

Trench

Gabbroic crust (30 km)

Mantle (30 km)

Mantle asthenosphere

T calculated in "ocean domain"

Mantle (oceanic) lithosphere

Continental lithosphere (60 km)

T = 1280 C

T = 1280 C

Table 1 Physical properties used in this study

Thermalconductivity

(W/m/K)

Heatproduction

(mW/m3)

Density(kg/m3)

Heat capacity(J/m3/K)

Gabbroic crust 2.5 1.6 2950 940Mantle lithosphere 2.7 0.0 3250 1000Mantle asthenosphere 2.7 0.0 3250 1000

Data source: Turcotte and Schubert (1982) and Wang et al. (1995).


height of 53 km (‘constant asthenospheric temper-ature region’ in Fig. 3). This high-temperatureregion beneath the ridge is kept within the oceaniclithosphere during the calculation, with differentparameters set in the ocean and subductiondomains. The half-spreading rates (Vs) for theridge center are set at 10, 30 and 50 mm/year, cor-responding to slow, medium and fast rates of plateseparation at presently active oceanic spreading

centers (Lonsdale 1977). The faster (160 mm/year)and slower (60 mm/year) cases of ridge conver-gence velocity (Vc) are considered in this study.The modern convergence velocity of the Philip-pine Sea Plate to the Eurasian Plate is esti-mated to be ~60 mm/year (Seno et al. 1993),whereas that in Cretaceous southwest Japan forconvergence of the Kula plate orthogonal to thetrench axis of the Eurasian continent is consid-ered to have been ~200 mm/year (Engebretsonet al. 1985; Maruyama & Seno 1986). The kine-matic parameters used in the model are summa-rized in Table 2. For example, for a convergencerate at the ridge center of 160 mm/year and a half-spreading rate of 50 mm/year, the opposite ridgeflanks will be subducted at velocities of 210 and110 mm/year (Fig. 3b) in the subduction domain.In contrast, in the ocean domain, because thermalstructures in the opposite ridge flanks should besymmetric, and a relative ridge convergencevelocity with respect to the continental litho-sphere could be zero (Vc = 0 mm/year), thermalcalculations have been solely associated with thehalf-spreading rates (Fig. 3a). Therefore, for thecalculation in the ocean domain, the position ofthe ridge center is fixed as a boundary conditionand thermal structures within a ridge flank arecalculated. The thermal structure of the oceaniclithosphere near the spreading ridge is modifiedby diffusional heat flux from vertical thermalboundary layers. In the ocean domain, even after30 My of system evolution, the thermal structuresremain relatively unchanged and approach asteady state. The steady-state thermal structuresare used for the oceanward boundary condition inthe subduction domain. The age of oceanic litho-sphere entering the subduction domain is directlyproportional to the half-spreading rate of theridge center and the distance from the ridge in theocean domain. For example, for half-spreadingrates of 10, 30 and 50 mm/year, the oceanic litho-sphere is 30 My in age at distances of 300, 900 and1500 km away from the spreading center. In the

Fig. 3 Schematic diagram showing the geometry of the spreading-center within the oceanic lithosphere. (a) Ocean domain, (b) subductiondomain.

Ridge center

Subducting oceanic lithosphere

2 km

not to scale

7 km


Constant asthenospheric temperature region

Vc+Vs

Vc Vs

b) Subduction domain

Ridge center

2 km

not to scale

7 km


Constant asthenospheric temperature region

VsVs

a) Ocean domain

Continental lithosphere or mantle asthenosphere

Simulated domain

Simulated domain

Table 2 Kinematic parameters used in this study

Velocity

Ridge convergence velocity Vc (mm/year)Slow 60Fast 160

Half-spreading velocity Vs (mm/year)Slow 10Medium 30Fast 50


present study, the oceanic ridge axis approachesthe trench parallel to the trench axis. In fact, theconsistent Th–U total Pb ages on monazite in boththe west and east of the Ryoke Metamorphic Beltare believed to show that intrusion of the grani-toids occurred synchronously along the length ofthe belt (Brown 1998; Suzuki & Adachi 1998), sug-gesting that subduction of the spreading axis ofan oceanic ridge should be almost parallel to thetrench axis.

In the subduction domain (Fig. 2), the continen-tal lithosphere is fixed in space and the velocity ofthe subducting lithosphere is given by v (Vc + Vs

for the leading slab and Vc – Vs for the followingone) in Equation 1. We have assumed that theopposite ridge flanks subducted at different veloc-ities, and then a ‘thermal’ slab window, defined bythe region with a temperature of ~1280∞C, couldbe formed during the ridge subduction in the sub-duction domain. The initial (pre-subduction) ther-mal structures of the continental lithosphere areapproximated using a 60-km-thick plate-modelwith a basal temperature of 1280∞C. An adiabaticgradient of ~0.3∞C/km is incorporated into the ver-tical temperature structure in the mantle astheno-sphere. After onset of subduction of the oceaniclithosphere, the thermal structure in the mantleasthenosphere caused by viscous flow is calculatedusing the analytical solution of McKenzie (1969).On the landward boundary, the temperature inthe mantle asthenosphere above the point at whichthe flow direction of mantle convection changes inthe wedge is calculated according to an adiabaticgradient and fixed, whereas that below the pointis calculated based on the stream function ofMcKenzie (1969). The angle of the subductingoceanic lithosphere is set at 26.6∞C, approximatingthe average dip of modern subduction zones (e.g.Central Chile, Peru and Colombia) associated witha young slab with high convergence rate for depthsof 0–100 km (Jarrard 1986). In the present study,frictional heating is not considered, because pre-vious studies have shown that the temperatureincrease caused by frictional heating is very smallin many subduction zones (Furukawa & Uyeda1989; Wang et al. 1995). Hence, the third term onthe right-hand side of Equation 1 solely representsradiogenic heat production. The values of physicalproperties of the gabbroic crust, mantle litho-sphere and mantle asthenosphere used for model-ing are shown in Table 1.

Before 30 My, at which time the thermal struc-tures in the ocean domain begin to approach asteady state, the initial (pre-spreading) thermal

structure of the oceanic lithosphere is used forthe oceanward boundary condition. After 30 My,the thermal structures in the subduction domainremain relatively unchanged and also approach asteady state. After 30 My, for each successive timestep, the vertical temperature distribution in thedowngoing oceanic lithosphere on the oceanwardboundary was calculated in the ocean domain. Thethermal structures of the subduction domain at30 My of system evolution are shown in Figure 4for ridge convergence velocities of 60 and 160 mm/year at a half-spreading velocity of 10 mm/year. Atthe onset of the subduction, continental litho-sphere and mantle asthenosphere cool as a resultof heat conduction out of the subducting oceaniclithosphere. In contrast, the upper and lower partsof the subducting lithosphere are heated, prima-rily because of conduction of heat from the sur-rounding mantle. As shown in Figure 4, thethermal structures of these two scenarios areslightly different. In the case of high convergencevelocity, advective heat transfer resulting from the

Fig. 4 Temperature profiles in the subduction domain after 30 My ofsystem evolution for ridge convergence velocities (Vc) of (a) 60 and (b)160 mm/year at half-spreading velocity (Vs) of 10 mm/year.

0 200 400 600 800 1000 1200 1280

Temperature scale ( C)

0 50 100 150 200 300250 0

50

100

150

200

x (km)

z (km)

(a) Vc = 60 mm/yr Vs = 10 mm/yr

(b) Vc = 160 mm/yr Vs = 10 mm/yr


subducting ‘cool’ oceanic lithosphere occurs muchfaster than conductive heat transfer from thesurrounding higher-T continental lithosphere andmantle asthenosphere. Hence, the thermal struc-ture of the subduction zone in a higher conver-gence velocity regime is much cooler than underlower-velocity conditions. In contrast, becauseviscous flow in the mantle wedge is proportionalto the convergence velocity, faster convergenceresults in higher temperatures in the mantlewedge.

RESULTS

After 30 My of system evolution, the temperatureprofile of the subducting slab at the oceanwardboundary in the subduction domain has changed.The boundary thermal profiles were calculated inthe ocean domain for a range of half-spreadingrates (10, 30 and 50 mm/year) and the ridge con-vergence velocities were set at 60 and 160 mm/year in the subduction domain: Six examples areprovided in Figure 5. In Figure 5, in order to com-pare the thermal consequences of spreading-ridgesubduction, the thermal structure for each case isrepresented in terms of the position of the ridgecenter away from the trench. With the ridge centerat –100 km from the trench, there is significantvariation in the thermal structure of the slab, as aresult of different spreading rates. The thermalstructure of the subducting oceanic lithosphereunder a fast-spreading regime (50 mm/year) ismuch hotter than at slower spreading rates(10 mm/year). The age of the oceanic lithosphereat the trench differs according to the distancebetween the ridge center and the trench (e.g.10 My at Vs = 10 mm/year, 3.3 My at Vs = 30 mm/year, and 2 My at Vs = 50 mm/year), following fromthe differences in half-spreading rate.

As shown in Figure 5, a ‘thermal’ slab windowopens when the ridge center moves below the depthof the continental lithosphere–asthenosphereboundary, and with continued subduction theslab window expands. The extent of a slab windowis significantly different for the different parame-ter sets. The temperature structure of the oceaniclithosphere near the ridge differs significantlydepending on the convergence and spreadingrates, which govern the extent and the duration ofa ‘thermal’ slab window at a particular location.Opening is initiated at slightly different timesaccording to the convergence velocity. Theseresults suggest that in the case of slow conver-

gence and fast-spreading rate, the effect of heatfrom the surrounding higher-temperature mantleasthenosphere is sufficient for a slab window toform (Fig. 5c,f ). In previous works (e.g. Peacocket al. 1994; Iwamori 2000; Miyazaki & Okamura2002), the initial thermal structure of the oceaniclithosphere before subduction was calculated byusing an analytical solution based on a steady-state geotherm that is dependent on the distancefrom the spreading center (i.e. age of the oceaniclithosphere). Because the half-spreading rate is setat 0 mm/year, a slab window cannot open duringsubduction in these studies, and the thermal struc-ture around the ridge center within the subductingslab could not change substantially. In contrast,the present study demonstrates that the spreadingcenter is still likely to be opening during subduc-tion, which would allow the formation of a regionwith an asthenospheric temperature (~1280∞C)within the slab.

For the period that the spreading ridge islocated beneath the continental lithosphere (5 Myfor Vc = 60 mm/year; 1.9 My for Vc = 160 mm/year), the thermal structure near the Wadati–Benioff plane will vary dramatically depending onthe convergence rate. That is, the temperature ofthe lithosphere in models with lower convergencerates will be much higher than that in higher-ratemodels. As the ridge center is subducted, a ther-mal anomaly is created, causing the isotherms torise, such that the 1000∞C isotherm intersectsthe Wadati–Benioff plane (Fig. 5). In the slow-spreading cases, an overturned isotherm of 200∞Cappears in the forearc region. Overall, the influ-ence of the subduction of a spreading ridge on thethermal structure of the crust becomes greaterwith increasing half-spreading rate and decreasingridge-convergence rate.

The results of these models contradict those ofIwamori (2000). In his model, higher temperatureconditions are predicted around the ridge centerwithin the faster subducting slab, rather thanwhere slower subduction occurs. The difference inthermal structure around the ridge center resultsmainly from the difference in the mantle-wedgetemperature near the interface between the slaband the wedge. The stream function formulated byMcKenzie (1969), which was also incorporated inthe model of Iwamori (2000), is strongly dependanton the velocity of the subducting slab: Faster sub-duction will cause faster convection in the mantlewedge and result in higher temperatures in themantle wedge. In contrast, in the model presentedin the present study, thermal structures around


Fig. 5 Thermal structure of the subduction domain in terms of distance between the ridge center and trench for various convergence velocity (Vc) andspreading rate (Vs) scenarios.

(a) Vc = 60 mm/yr, Vs = 10 mm/yr (c) Vc = 60 mm/yr, Vs = 50 mm/yr(b) Vc = 60 mm/yr, Vs = 30 mm/yr

100 km

0 km

50 km

100 km

200 km

300 km

400 km

100 km

0 km

50 km

100 km

200 km

300 km

400 km

100 km

0 km

50 km

100 km

200 km

300 km

400 km


Fig. 5 Continued

(f) Vc = 160 mm/yr, Vs = 50 mm/yr(e) Vc = 160 mm/yr, Vs = 30 mm/yr(d) Vc = 160 mm/yr, Vs = 10 mm/yr

100 km

0 km

50 km

100 km

200 km

300 km

400 km

100 km

0 km

50 km

100 km

200 km

300 km

400 km

100 km

0 km

50 km

100 km

200 km

300 km

400 km


the ridge center are mainly controlled by the half-spreading velocities that were not incorporated inthe model of Iwamori (2000).

DISCUSSION

IS THE SPREADING RIDGE MODEL APPLICABLE TO MID-CRETACEOUS SOUTHWEST JAPAN?

Figure 6 shows the distribution of metamorphicfacies in the crust of the forearc region. This dis-tribution is based on the highest temperature–pressure (depth) relationships of the crust after asteady geotherm has been reached before subduc-tion of the ridge, because the P–T conditions ofmetamorphic rocks are most likely to record whenthe rocks reached the highest-T conditions duringmetamorphism. The boundaries of the metamor-phic faces follow those of Spear (1993). Iwamori(2000) indicated that the supply and presence ofwater is essential to drive the metamorphism, yetthe ridge subduction can provide water only in alimited period and to a limited space, because it istoo hot. Because the model presented here doesnot incorporate the parameters associated withwater production, we cannot evaluate the effect ofwater on the model results. As shown in Figure 6,in the case of slow convergence rate and fast half-spreading rate, the region of high-grade facies(amphibolite and granulite facies) is enlarged,particularly in the vicinity of the Wadati–Benioffplane. The thermal effect caused by the ridge sub-duction is limited in the region near the Wadati–Benioff plane, and thermal structures in theregion far from the plane do not change signifi-

cantly. Only in the case of the fastest convergencerate and slowest spreading rate, high-grade meta-morphism does not develop in the vicinity of theWadati–Benioff plane, and metamorphic conse-quences range from the zeolite facies, throughthe prehnite–pumpellyite and greenschist facies,to the epidote–amphibolite facies. In this model,because we assumed a crust with a thickness of30 km, the eclogite facies metamorphic rockscould not form. The metamorphic facies of theSambagawa Belt range from the prehnite–pumpellyite facies, through the greenschist faciesand the blueschist or epidote–amphibolite facies,to the eclogite facies (e.g. Banno & Nakajima1992; Enami 1998; Aoya et al. 2003). This suggeststhat the formation of the high-P/T Sambagawametamorphic rocks near the Wadati–Benioff planerequires the subduction of a spreading ridge at afast convergence rate; results associated with thecases of slow convergence rate are not consistentwith this geological interpretation.

The Sambagawa Metamorphic Belt might havebeen formed near the Wadati–Benioff plane,simultaneously with the Mid-Cretaceous igneousactivity; however, the precise timing of the beltformation and igneous activity is a very criticalissue. Because of the very high temperature gra-dient near the Wadati–Benioff plane, only a slightoff-set in the timing of belt formation and igneousactivity could produce significantly lower temper-ature conditions, which might explain the Sam-bagawa metamorphic conditions (DeLong et al.1979; Iwamori 2000, 2002). Unfortunately, we donot know how far the Sambagawa metamorphicrocks were located from the Wadati–Benioffplane when the rocks were metamorphosed under

Fig. 6 Distribution of metamorphic facies in the crust of the forearc region under various scenarios of convergence velocity and spreading rate. Dashedline in (a) indicates vertical section representing geotherms in Fig. 7.

Zeolite facies Prehnite-pumpellyite facies

Granulite faciesAmphibolite facies

Blueschist faciesGreenschist facies

Epidote-amphibolite facies

(a) Vc = 60 mm/yr, Vs = 10 mm/yr (c) Vc = 60 mm/yr, Vs = 50 mm/yr(b) Vc = 60 mm/yr, Vs = 30 mm/yr150 km

(f) Vc = 160 mm/yr, Vs = 50 mm/yr(e) Vc = 160 mm/yr, Vs = 30 mm/yr(d) Vc = 160 mm/yr, Vs = 10 mm/yr

30 km


the highest-T condition. Iwamori (2002) discussesthis issue in terms of calculated pressure–temperature–time (P–T–t) paths of the rocks in aforearc wedge when the ridge axis approached.Iwamori (2002) illustrated preliminary results ofthe P–T–t path calculation and stated that thetiming of the temperature elevation depends onthe position of the rock when the ridge axisapproaches. Because P–T–t paths for the Sam-bagawa metamorphic rocks have been petrologi-cally proposed by many researchers (e.g. Enami1998; Aoya et al. 2003), there is a possibility thatsuch numerical analysis would clarify the accurateposition of the Sambagawa metamorphic rockswhen the ridge or slab window subducted. How-ever, further research in both the petrologic andnumerical approaches is needed.

Almost all of the Mid-Cretaceous Ryoke–Sanyogranitoids are I-type granitoids, with majorsources being mafic-magmatic rock or metamor-phosed equivalents (Kagami et al. 1992; Nakajima1996; Kutsukake 2002). Kutsukake (2002) reportedthe rare earth element (REE) patterns of theseI-type granitoids and suggested that they mighthave been generated by dehydration melting ofamphibolite or hydrous melting of tholeiite atpressures of 1 GPa or higher. Therefore, one ofthe geological evaluations of the present model iswhether the melting of wet tholeiite was sufficientto form a large amount of granitic magma.Because it is difficult to estimate an accurate vol-ume for the granitic magma, based on the surfacegeology in southwest Japan, we evaluated themodel results by using periods of the Mid-Cretaceous granitic magmatism. According to

Suzuki and Adachi (1998), granitic activity in theRyoke–Sanyo zone occurred from 100 Ma to70 Ma, with the main period of magmatism occur-ring over ~10 My. Figure 7 shows the geothermsin the crust that lies 50 km from the trench, alongwith the solidus of dry tholeiite and 5 wt% H2Otholeiite (Wyllie 1971; Green 1982), and the phaseboundaries of aluminosilicates (Spear 1993). Twocases are presented in this figure: The case withthe largest thermal effect (Vc = 60 mm/year andVs = 50 mm/year) and the case with the smallestthermal effect (Vc = 160 mm/year and Vs = 10 mm/year). When the ridge center is located at a dis-tance of 50 km from the trench, the temperaturesin the lower part of the crust reach hydrous-tholeiitic solidus temperatures under both scenar-ios. When the ridge center is located 100 km fromthe trench, the temperatures in the case with thelargest thermal effect remain above the hydrous-tholeiitic solidus, whereas temperatures dropbelow the wet tholeiite solidus in the case with asmaller thermal effect. In the case with the largestthermal effect, temperatures remain above thehydrous-tholeiitic solidus when the ridge centeris located 400 km away from the trench (Fig. 7b).These results indicate that the thermal anomalyis restricted in time to less than 1 My for the caseof fast convergence rate (160 mm/year) and slowspreading rate (10 mm/year), but might extend formore than 10 My for the case of a slow conver-gence rate (60 mm/year) and a fast spreading rate(50 mm/year). Even in the case of Vc = 60 mm/yearand Vs = 30 mm/year, temperatures in the crustexceed the hydrous-tholeiitic solidus for less than4 My. Therefore, in contrast to the interpretation

Fig. 7 Geotherms in the crust 50 kmfrom the trench (dashed line in Fig. 6a)under various scenarios of convergencevelocity (Vc) and spreading rate (Vs) interms of the distances between the ridgecenter and the trench (0 km, 50 km,100 km, 200 km, 300 km and 400 km).Dry tholeiite and wet (5 wt% H2O) tholei-ite solidi (Wyllie 1971; Green 1982) andaluminosilicate phase boundaries (Spear1993) are also shown. Temperature ( C)

Dep

th (

km)

And

SilKy

0 200 400 600 800 1000 12000

5

10

15

20

25

30

Solidus (wet tholeiite)

a) Vc = 160 mm/yr, Vs = 10 mm/yr

0 k

m

100 k

m

200 k

m

300 k

m

400 k

m

Temperature ( C)

Dep

th (

km)

And

Sil

0200 400 600 800 1000 1200

0

5

10

15

20

25

30b) Vc = 60 mm/yr, Vs = 50 mm/yr

0 k

m

100 k

m

200 k

m

300 k

m

400 k

m

50 k

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Solidus (dry tholeiite)

Solidus (wet tholeiite)

Solidus (dry tholeiite)

50 k

m


of the development of metamorphic facies in thecrust, the geological observations implied by Mid-Cretaceous granitic activity requires the subduc-tion of a spreading ridge with a fast spreadingrate (50 mm/year) in a slow-convergence regime(60 mm/year). Consequently, in the model of sub-duction of a spreading ridge studied in the presentstudy, no parameter set can suitably satisfy thetwo geological interpretations simultaneously,indicating that a slab window did not open duringridge subduction in Mid-Cretaceous southwestJapan.

POSSIBILITY OF SLAB MELTING

In a subduction zone, the geothermal gradientalong the slab–mantle interface remains low(=10∞C/km). Under such thermal conditions, thesubducted slab is metamorphosed and progres-sively dehydrates in such a way that the dehydra-tion curves intersect with the oceanic lithospheresurface before reaching the solidus at 2 GPa, withthe result that the surface of the oceanic litho-sphere (i.e. oceanic crust) should be almost com-pletely dehydrated. Melting of such dry tholeiitecannot occur at low temperatures; in fact, temper-atures exceeding 1200∞C are required at 2 GPa.However, if the subducted oceanic crust reaches650–700∞C before becoming dehydrated, theremnant water will allow dehydration melting tooccur, producing adakitic magmas in the sub-ducted oceanic crust (e.g. Drummond & Defant1990; Peacock et al. 1994; Martin 1999; Iwamori2000).

In the present study, for the case ofVc = 160 mm/year and Vs = 50 mm/year, the devel-opment of high-grade metamorphic rocks alongthe Wadati–Benioff plane is restricted in space(Fig. 6f). With the passage of the ridge center, athermal anomaly is created and isotherms rise insuch a way that the 1000∞C isotherm intersects theWadati–Benioff plane (Fig. 5f). Because this ther-mal anomaly can easily exceed the solidus temper-ature of tholeiite with 5 wt% H2O, slab melting ispossible under these conditions. Therefore, for thecase of fast convergence rate and fast spreadingrate, if slab melting occurs, adakitic magmas willbe formed without the development of high-grademetamorphic rocks near the Wadati–Benioffplane. However, as the thermal anomaly drivingthe formation of adakitic magmas is restricted inboth time (<4 My) and space, the activity of adak-itic magma caused by slab melting is too short togenerate the extended (>10 My) granitic activity

indicated in southwest Japan. In fact, the adakiteactivity in the Ryoke–Sanyo zone is very restricted(Iizumi et al. 2000; Kinoshita 2002), suggestingthat the formation of the adakitic magma mightnot have occurred extensively beneath Mid-Cretaceous southwest Japan.

Iwamori (2000) suggested quantitatively that anormal melt (not adakitic melt) could be producedby slab melting, due to the subduction of a youngoceanic slab with a high subduction rate (~250 mm/year). If the slab melting occurs at pressures lowerthan 1 GPa (~40 km in depth), the melt producedis not adakitic, but normal granite. This shallowslab melting could result in a large amount ofgranitic magmatism in the forearc region and/ornear trench (Iwamori 2000). However, there is nosuitably aged granitic activity in the SambagawaMetamorphic Belt and the Jurassic–Cretaceousaccretionary terranes (Chichibu and CretaceousShimanto terranes) in the outer zone of southwestJapan, suggesting that the shallow slab meltingdid not occur in the Mid-Cretaceous.

WHAT IS A PLAUSIBLE TECTONIC SCENARIO?

As described above, two geological constraintson the tectonics for Mid-Cretaceous southwestJapan are: (i) The absence of amphibolite- togranulite-facies metamorphic rocks near theWadati–Benioff plane; and (ii) significant meltingof lower crustal-mafic rocks sufficient to form along-lasting granitic magmatism. In the model ofsubduction of an active spreading ridge examinedin the present study, no parameter set will satisfythese two geological interpretations simulta-neously, which suggests that the ridge subductionthat occurred in Mid-Cretaceous southwestJapan did not result in the opening of a slab win-dow. In contrast, the subduction of a very youngslab is required to form the high-grade metamor-phic belts associated with huge amounts of gra-nitic rock, because the petrologically determinedmetamorphic field gradient of the high-grademetamorphic belts cannot be explained simply bythe thermal structure of the arc-trench systemin ‘normal’ or steady-state subduction (Uyeda& Miyashiro 1974; Peacock et al. 1994; Iwamori2000; Okudaira 2002). According to this sugges-tion, and the results of this study, two differentplausible tectonic scenarios could be suggested tosatisfy the geological interpretations for Mid-Cretaceous southwest Japan. The first scenario isthat the spreading ridge at the Kula–Pacific (orFarallon–Izanagi) plate boundary, located off the


Eurasian continental margin, did not subductbeneath the continent. In this scenario, veryyoung oceanic lithosphere will be continuouslysubducted, and the thermal consequences mightcorrespond to those suggested by Aoya et al.(2003). As a spreading ridge approaches a trench,the buoyancy of the subducted slab increases.Relative to the asthenosphere, oceanic litho-sphere older than ~10 Ma is negatively buoyantand inherently likely to be subducted, whereaslithosphere younger than ~10 Ma is positivelybuoyant (Cloos 1993). Therefore, in some cases,subduction of a spreading ridge might be stag-nant before the ridge intersects the trench. Thissituation appears to have developed along partsof the East Pacific Rise where it approachedNorth America in the middle Tertiary (Thorkel-son 1996). For the ridge subduction to occur, thecombined forces of ridge push and slab pull mustexceed the buoyant force exerted by the ridge-proximal part of the plate, and the strength of theplate must be sufficient for the slab to subductwithout fragmentation (van den Beukel 1990;Larter & Barker 1991). In the second scenario,the spreading ridge associated with the plateboundary ceased spreading before being sub-ducted beneath the Cretaceous continental margin.This scenario is associated with the subductionof very young oceanic lithosphere, similar to themodels of Iwamori (2000) and Peacock et al. (1994).Consequently, in both the tectonic scenarios,the subduction of plate boundaries at the Mid-Cretaceous southwest Japan was not associatedwith a slab window, but very young (hot) oceaniclithosphere. However, because we have not evalu-ated thermal consequences in an arc-trench sys-tem caused by the subduction of a very youngoceanic lithosphere, we cannot state quantita-tively that the tectonic scenarios presented hereare suitable for Mid-Cretaceous southwest Japan.Further numerical analyses are required for bothtectonic scenarios.

ACKNOWLEDGEMENTS

The authors thank S. Maruyama and M. Toriumifor their constructive suggestions. N. Aikawa,A. Chemenda, M. Inui and S. R. Wallis are alsothanked for their helpful comments on an earlyversion of the manuscript. The paper benefitedsubstantially from detailed reviews by H. Iwamoriand K. Miyazaki. T. Ikeda and S. R. Wallis arethanked for their editorial skills and advice.

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Research Article Thermal ... - Osaka City University · forming the paired metamorphic belt in southwest Japan have not been understood fully (e.g. Brown 2002; Iwamori 2002), and