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Strain modes within the forearc, arc and back-arc domains in the Izu(Japan) and Taiwan arc-continent collisional settings

Serge Lallemand ⇑Géosciences Montpellier Laboratory, CNRS, Montpellier 2 University, CC.60, Place E. Bataillon, 34095 Montpellier, FranceAssociated International Laboratory LIA ADEPT between NSC (Taiwan) and CNRS, France

a r t i c l e i n f o

Article history:Available online 20 August 2013

Keywords:Arc-continent collisionMiddle crustIntra-oceanic sliveringMantle decouplingForearc subductionLuzon arcIBM arc

a b s t r a c t

In this study, I examine the strain modes of the forearc, arc and back-arc domains in arc-continent col-lisional settings leading to arc material subduction, delamination and/or accretion. The study focusseson two well-documented colliding island arcs: the Izu–Bonin–Mariana (IBM) arc in Japan and the Luzonarc in Taiwan, both carried by the Philippine Sea plate. Firstly, there is a body of evidence that both theIBM and the Luzon arcs were built on the same Late Jurassic to Early Cretaceous ‘‘proto-Philippine SeaPlate’’ crust. Their internal structure is thus more heterogeneous than expected from Paleogene or Neo-gene supposedly ‘‘intra-oceanic’’ island arcs. Secondly, those arc systems and proximal ‘‘back-arcs’’ havesimilar seismic characteristics attesting either for the presence of a middle crust with continental veloc-ities and/or serpentinized uppermost mantle that facilitate crustal shortening/slivering and subsequentdecoupling from the rest of the subducting plate. It is shown that the proximal back-arc domain (called‘‘rear-arc’’ in case of paleoarc activity), overlying the mantle wedge and the subducting slab, may lose itsstrength if slab-derived hydration occur. Decoupling then occurs below the Moho. Arc delaminationlikely occurs in mid-crustal levels because middle-crust, heated by nearby magmatism, becomes weak.Accretion of arc material onto the upper plate depends on the characteristics of the arc itself and the geo-dynamic configuration. Most of the accreted material is probably underplated rather than frontallyaccreted.

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1. Introduction

Modern collisions between a volcanic arc and a continent arecommon in southeast Asia: Luzon arc in Taiwan, Izu–Bonin andKurile arcs in Japan, Sulu and Halmahera arcs in the Philippines,Sunda arc in Indonesia or Melanesian arc in Papua – New Guinea(Lallemand et al., 2001a). Many authors have examined the ‘‘land’’expression of such collisions by studying the associated orogens(e.g., Brown and Huang, 2009; Brown et al., 2011; Mann et al.,2011), the arc being often considered as a (semi-)rigid indenterpushing forward the continental upper crustal layers (e.g., Suppe,1981; Wu et al., 1997; Malavieille and Trullenque, 2009). Someproportion of arc material may be scraped off the subducting plateand add to the growing orogen (e.g., Taira et al., 1998; Arai et al.,2009).

In this study, I focus on the deformation of the subducting orcolliding island arc from their initiation to the ultimate delamina-tion/peeling or subduction. I do not describe onland outcrops ofisland arc slivers accreted to the overriding plate but ratherexamine the timing of the various deformation phases and theingredients that control the localization of the arc deformation.

Well-constrained examples of such processes are found on botheastern and western borders of the Philippine Sea Plate (PSP).

The collision of the Izu–Bonin–Mariana (IBM) Arc with centralJapan has been carefully studied since the early eighties (e.g., Oga-wa, 1983; Huchon and Kitazato, 1984; Soh et al., 1991; Taira et al.,1998; Mazzotti et al., 1999) but a new set of studies these lastyears has provided determining constraints on ongoing deep pro-cesses (e.g., Arai et al., 2009; Tamura et al., 2010; Tani et al.,2011). On the other side of the PSP, the Luzon Arc collides with Tai-wan. Studies there were achieved later but again, efforts have beendone these last two decades to better characterize the collisionprocess, especially offshore (e.g., Wu et al., 1997; Teng et al.,2000; Lallemand et al., 2001b; Malavieille et al., 2002; Theunissenet al., 2012). Even if the geodynamic context differs from that in Ja-pan, many similarities exist in these two situations and one mayuse the better knowledge of the IBM – Honshu collision to addressquestions in Taiwan, and vice versa.

The two cartoons in Fig. 1 illustrate the common points andmain differences between the geodynamic contexts in Taiwanand in Japan. In Taiwan, the Miocene to present Luzon volcanicarc results from the subduction of the South China Sea (SCS) oce-anic lithosphere, which belongs to the Eurasia plate (EP), beneaththe PSP. That subduction system ends at the latitude of (northern)

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Taiwan and is relayed by an orthogonal subduction of the PSPbeneath EP, so that the Luzon arc that overrides the EP in thesouthern part of Taiwan, subducts beneath the same EP east ofnorthern Taiwan. To summarize the tectonic situation, one maysay that the Luzon arc first collides with the Taiwan orogen as a re-sult of the continental nature of the subducting EP at the latitude ofTaiwan north of the SCS, and then collides with and subducts be-neath the EP east of northern Taiwan. The convergence rate be-tween both plates averages 8 to 9 cm/yr in the collision area andthe convergence azimuth is oblique to both plate boundaries (Senoet al., 1993). The emerging part of the deformed Luzon arc formsthe Coastal Range east of Taiwan (about 150 � 10 km). In Japan,the Eocene to present IBM arc results from the subduction of thePacific plate (PAC) oceanic lithosphere beneath the PSP. The IBMarc is carried down the Nankai subduction zone at a rate of about4 cm/yr as it is part of the subducting PSP. The emerging part of thedeformed IBM arc forms the Izu Peninsula and collision zone (ICZ)along the southern coast of central Honshu. It covers an area ofabout 100 � 40 km.

Despite variations in maturity, convergence rates and geometryof plate boundaries of both arc collision zones, we will see thattheir behavior is often similar in terms of deformation modes.

2. Recent advances in understanding the IBM arc and the ICZ inJapan

2.1. IBM arc origin and age

Based on studies of forearc oceanic rocks, supposed to haveformed as a result of subduction initiation stage along a formerfracture zone (Stern and Bloomer, 1992), the inception of theIBM arc has been dated at 51–52 Ma (Ishizuka et al., 2011). Lalle-mand (1998) argued that an Early Eocene age is a minimum since

the forearc rocks that have been sampled might have formed aftersubduction began, particularly if they did not formed in the formerforearc (Deschamps and Lallemand, 2003). Indeed, IBM is anerosional margin with rates of forearc consumption of several kilo-meters per million years (von Huene and Scholl, 1991; Lallemand,1995), and it may be that the oldest arc rocks have been consumedby subduction. Furthermore, the presence of Mesozoic continentalcrust in the Mariana forearc basement was suspected by Azémaand Blanchet (1982) after leg DSDP 60 when they discovered LateJurassic–Early Cretaceous reworked pebbles in a volcanic matrix.More recently, Ishizuka et al. (2012) have described Jurassic basal-tic pillow lavas with Indian Ocean MORB affinities in the Boninforearc suggesting that Mesozoic crust constitutes the basementof the IBM arc. To summarize, most of the arc consists in a volcanicridge that began to form in Early Eocene or earlier but there arestriking evidences that part of the arc basement is inherited froma ‘‘proto-PSP’’ that is composed of older (Jurassic to Cretaceousdetrital zircons), non-oceanic, possibly continental crust (Taniet al., 2012).

2.2. IBM arc seismic and petrological structure

The oldest arc sequences mostly consist of tholeiitic basalts(Tamura et al., 2010). These Eo-Oligocene basaltic and rhyoliticrocks are exposed both in the forearc of the main actual IBM arc(sometimes in association with boninites) and along the Palau-Kyushu Ridge (Figs. 1 and 2) which is the remnant part of thepre-Miocene IBM arc rifted during the Miocene spreading of theShikoku and Parece-Vela basins (e.g., Karig, 1971). This Eo-Oligo-cene arc was emplaced over a zone about 200 km wide acrossthe present IBM arc (Suyehiro et al., 1996) to which the Palau-Kyushu Ridge must be added to restore the original arc. Based onseismic velocities, the crustal thickness varies from 20 to 30 km be-neath the arc showing undulations with wavelengths of about

Fig. 1. Map showing the northern part of the Philippine Sea Plate with main features and location of the two perspective views of the Luzon and Izu–Bonin arc collisionzones. Note that the scales of each diagram is different even if horizontal and vertical proportions are similar. EP = Eurasia Plate; PSP = Philippine Sea Plate; PAC = Pacific Plate;Ky. = Kyushu; Sh. = Shikoku; and Hok. = Hokkaido.

2 S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11


100 km (Kodaira et al., 2007). Variations in crustal thicknessmainly come from variations in thickness of the middle crustand, in a lesser extent, the lower crust (Takahashi et al., 2009).Middle-crust exhibits typically ‘‘continental crust’’ P-waves veloc-ities VP between 5.7 and 6.8 km at depths ranging from 5 to12 km (see section on Fig. 2; Kodaira et al., 2007). Such velocitiescommonly correspond to granites, diorites or tonalites (Tamuraet al., 2010). Some authors like Tatsumi et al. (2008) consideredthat layer as an unsubductable nucleus of ‘‘continental’’ crust.The lower crust (6.7 < VP < 7.4 km/s at depths generally rangingfrom 10 to 20 km) is more mafic. It is likely composed of horn-blende gabbros (Kitamura et al., 2003) or granulites derived fromthe melting and differentiation of underplated gabbros by compar-ison with the fossil Kohistan arc rocks sequence (Dhuime et al.,2009). Crustal materials are generally denser in the oldest Eocenearc than in the current volcanic arc (Takahashi et al., 2009). Theupper crust (4.5 < VP < 6.0 km/s) consists of basalts, andesites andintrusives (Taira et al., 1998; Takahashi et al., 2007).

2.3. IBM ‘‘rear-arc’’ – transition between arc and back-arc

The Izu–Bonin arc presents a singularity with respect to ‘‘classi-cal’’ arcs. Mio-Pliocene volcanism occurred at distances up to150 km back of the present volcanic front along en-échelon north-east-southwest trending ridges called ‘‘across-arc seamountchains’’ (Ishizuka et al., 2003; Fig. 2). These volcanoes first eruptedcontemporaneously with the last spreading episodes of theShikoku Basin. Ishizuka et al. (2003) have demonstrated that thevolcanism has migrated from west to east between 17 and 3 Ma.This migration coincides with the increasing dip of the Pacific slabcaused by the motion change from trench rollback to advancearound 8–5 Ma (Faccenna et al., 2009). The internal seismic struc-ture of that region, called ‘‘rear-arc’’ by the authors, is similar tothat of the main arc (Fig. 2). Indeed, it has been considered byKodaira et al. (2008) as a paleoarc. Crustal thickness can reach25–30 km beneath the rear-arc (Kodaira et al., 2007) but the thick-est crust is not observed beneath volcanoes. For Kodaira et al.(2008), this finding suggests that the Mio-Pliocene volcanismmay have been superimposed onto a crust formed before the open-ing of the Shikoku Basin. Rear-arc volcanic products are exposed inthe Izu Peninsula. They consist of upper Miocene to Plio-Quater-nary andesites and dacites (Tani et al., 2011).

The most-striking across-arc seamount chain is the ZenisuRidge which aligns parallel to the Nankai Trough south of the IzuPeninsula (Fig. 2; Lallemant et al., 1989). A small part of the ridgeis subaerial (Zenisu Rocks outcrop of andesitic lava flows dated2–3 Ma) but most of the ridge is submarine and older (Tani et al.,2011). A 6.4 Ma andesite was sampled in its central section and pil-low basalts were collected along a fresh scarp at its southwest ter-mination (Henry et al., 1997).

2.4. Focus on the Zenisu Ridge

Before being described as an ‘‘across-arc seamount chain’’, theZenisu Ridge was interpreted as an intra-oceanic sliver (Le Pichonet al., 1987b). The pillow basalts sampled at its southwestern edgethus represent the top layer of an uplifted oceanic crust sequenceof the Shikoku Basin (Lallemant et al., 1989). Chamot-Rooke andLe Pichon (1989) have proposed a model of plate buckling and fail-ure along a lithospheric thrust localized along the magmatic ridge,propagating further southwest into the oceanic crust, and causedby the compressive stress generated by the arc-continent collision.Nakanishi et al. (1998, 2002) found a 5 km vertical offset in theMoho beneath the south flank of the ridge supporting the litho-spheric fault hypothesis. Given the dip angle of the thrusts imagedin reflection seismics, such offset should result in a �10 km throwalong the main thrust which is not observed at the surface.Mazzotti et al. (2002) thus proposed a model of conjugated low-angle thrusts within the crust distributing the slip over a wide area.They also proposed that the serpentinized mantle–crust transitionacts as a decoupling level above which distributed shortening mayoccur. Based on the deformation observed in the accretionarywedge landward of the ridge, Lallemand et al. (1992) have sug-gested that earlier oceanic slivers were formed, subducted andthen potentially accreted to the margin. Le Pichon et al. (1996),Park et al. (2003) or Kimura et al. (2011) have confirmed, after amagnetic and a detailed seismic surveys, the presence in the sub-ducting oceanic crust of two ridges, called ‘‘paleo-Zenisu’’ and‘‘deeper paleo-Zenisu’’, elongated parallel to the Zenisu ridge(Fig. 3). Based on pre-stack MCS depth migration and fullwaveform inversion from dense OBS data, Kodaira et al. (2004)and Dessa et al. (2004) have proposed a tectonic interpretation ofthe thrusts that affect the ‘‘paleo-Zenisu’’ ridge. Knowing todaythat most of the Zenisu ridge has a magmatic origin with middlecrust velocities, we have adapted the tectonic model of the

A B

Fig. 2. Top: Northern part of the IBM arc and collision area with central Japan (DEMand active volcanoes were extracted using SubMap tool http://submap.fr/) Darktriangles represent active volcanoes. The A–B profile of the bottom section andthose of Figs. 3 and 4 are shown on the map. Bottom: Interpreted wide-anglevelocity profile A–B after Takahashi et al. (2009). MC = Middle-crust.

S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11 3


previous authors onto the velocity profile of Park et al. (2003)assuming that most of the low-angle thrusts localize within thecrust, and that the uppermost mantle, or the mantle–crust transi-tion, acts as a decoupling layer (Fig. 3).

2.5. Deformation mode in the Izu Collision Zone (ICZ)

According to Taira et al. (1989) or Soh et al. (1991), collision be-tween the IBM arc and Honshu arc might have begun as early as15 Ma ago. Plates reconstructions (Sdrolias and Müller, 2006)rather predict a collision further west around 15 Ma near the Shi-koku island and a beginning of collision with central Honshuaround 10 Ma in better agreement with the end of Japan Sea open-ing. During its subduction, the IBM arc is supposed to have crepteastward along the Nankai Trough until about 5–8 Ma and thenwestward until present (Faccenna et al., 2009) so that the ICZwas confined to central Honshu, with some sinistral and dextralmotion component, probably during the last �10 million years.Successive slivers of IBM arc material accreted onto Honshu sincethe beginning of the collision. The Tonoki–Aikawa Tectonic Line(TATL) is considered as the tectonic boundary between the originalHonshu crust (Kanto Mountains) to the north and the accreted sliv-ers (Tanzawa mountains, Izu Peninsula, Zenisu Ridge) to the south

(e.g., Arai et al., 2009). Some other authors include another unit tothe north delimited by the Mineoka Tectonic Line (Taira et al.,1998; Soh et al., 1998). Fig. 4 shows a simplified along-strike sec-tion across – from south to north – the undeformed IBM arc, theICZ including the accreted IBM-derived units and the shortenedcrust of Honshu (Tamura et al., 2010). Two main Miocene batho-liths are observed in the ICZ: the Tanzawa tonalites and the KofuGranitic Complex. Based on their composition, Tamura et al.(2010) consider that they are derived from syn-collisional partiallymelted Oligocene middle crust. Based on an intensive seismicexperiment, it has been possible to trace the TATL down to about18 km beneath the Kanto mountains (Arai et al., 2009). From thegeometry of reflectors and the seismic activity, the Tanzawa block,with VP typical of upper to middle crust of the IBM arc, appears del-aminated from the subducting slab forming a wedge-like bodywhich is inserted, through low-angle thrusts, between the upperand lower crust of Honshu. These results strongly suggest thatthe middle crust of the IBM arc is weak and localizes the intra-arc deformation, allowing the lower crust and the mantle part ofthe arc to subduct with the rest of the PSP. Such process is in agree-ment with observations of arc-continent or continent–continentcollisions in other contexts like the fossil Kohistan arc in easternHimalaya (Burg et al., 2005) or the Alps (Schmid et al., 1996).

Fig. 3. Free tectonic interpretation of the section TKY1 (see Fig. 2 for location of the profile) superimposed on the velocity profile derived from a wide-angle dense OBS surveypublished by Park et al. (2003). The top of the subducting basement is those proposed by Park et al., whereas the major thrusts are inspired by the line drawings of Mazzottiet al. (2002), Kodaira et al. (2004) and Dessa et al. (2004). The presence of middle crust is inspired by Takahashi et al. (2009). UC = Upper crust; MC = Middle crust; andLC = Lower crust.

Fig. 4. Schematic cross-section of the Izu–Bonin arc and the Honshu arc along a profile located on Fig. 2 after Tamura et al. (2010). The Kofu Granitic Complex and theTanzawa tonalites were emplaced during the Miocene within the ICZ. TATL = Tonoki–Aikawa Tectonic Line. Closed and open-triangles show respectively basalt-dominant andrhyolite-dominant volcanoes.



3. Recent advances in understanding the Taiwan collision zone(TCZ)

3.1. Luzon arc origin and age

The Luzon volcanic arc results from the subduction of the SouthChina Sea oceanic lithosphere beneath the PSP since mid-Mioceneright after the cessation of back-arc spreading of the South ChinaSea (Defant et al., 1989; Briais et al., 1993). Late Oligocene to EarlyMiocene calc-alkaline volcanism is only found in northwest Luzonisland and Dalupiri island (Yang et al., 1996) but these occurrenceshave been attributed by Polvé et al. (2007) to a short-lived episodeof westward subduction of the PSP lithosphere beneath Luzon.After a quiescent period, magmatic activity resumed around15 Ma in relation with the start of eastward subduction of theSCS off Luzon island. Geochemical patterns observed in thesecalc-alkaline lavas from 15 to 1 Ma reflect the progressive additionto the Luzon arc mantle wedge of SCS sediment (Polvé et al., 2007).Manila Trench extends from west of Luzon to the latitude ofTaiwan, which probably did not exist yet in Miocene. Indeed, manyauthors consider that the orogen of Taiwan results from an arc-continent collision starting between 6.5 Ma (Huang et al., 1997)and 3 or even 2 Ma (Malavieille et al., 2002). However, Lu andHsu (1992) have proposed a two-stage collision at 12 Ma and3 Ma. Most absolute ages obtained on lavas from volcanoes situ-ated north of Luzon Island are younger than 10 Ma (Defant et al.,1989; Yang et al., 1996). Closer to Taiwan, ages obtained on Lutaoand Lanyu andesites cover a period between 3.9 and 5.5 Ma(McDermott et al., 1993). There is still an isolated oldest agearound 16 Ma obtained on lavas from the Chimei complex whichoutcrops in the northern part of the Coastal Range of Taiwan (Juangand Bellon, 1984; Lo et al., 1994) that led Yang et al. (1995) to con-clude that the volcanic activity of the arc segment forming thepresent Coastal Range of Taiwan extended from 16 to 2 Ma. Thecessation of magmatism in the northern part of the Luzon arc coin-cides with the start of the collision between the arc and the Chi-nese continental platform.

An interesting point is that recent radiolarian biostratigraphicresults provide evidence for the existence of a Mesozoic substra-tum upon which Luzon and the neighboring regions within thePhilippine archipelago were built (Queano et al., 2013). Similarages were found by Deschamps et al. (2000) on gabbros dredgedin the Huatung Basin east of the Taiwan, from radiolarian assem-blages collected on Lanyu island (Yeh and Cheng, 2001), in easternIndonesia (Ali et al., 2001) as well as in the basement of the IBMridge (Ishizuka et al., 2012) or the Amami Plateau (Hickey-Vargas,2005) and Oki-Daito ridges (Ishizuka et al., 2011) suggesting acommon provenance.

3.2. Luzon arc and ‘‘back-arc*’’ seismic and petrological structure

The seismic structure of the Luzon arc is less known than that ofthe IBM arc except in its northern part near Taiwan where the Lu-zon arc collides with the Chinese margin. The width of the arc dras-tically reduces between northern Luzon island and southernTaiwan from �200 to �50 km even if, as mentioned above, earlyCretaceous radiolarian cherts were sampled in both places (Luzonand Lanyu) indicating that the same Mesozoic substratum proba-bly lies underneath the entire volcanic arc. Regarding the originof the Huatung Basin situated in the backside of the arc, manyinterpretations have been given including an Eocene part of theWest Philippine Basin (Hilde and Lee, 1984), an Early Cretaceous

trapped piece of an old oceanic basin (Deschamps et al., 2000), acomposite back-arc basin with an old (Mesozoic ?) part in thesouth and an Eocene part in the north (Sibuet et al., 2002) or an Oli-go-Miocene oceanic basin (Kuo et al., 2009). Since Hickey-Vargaset al. (2008) found an Indian MORB-OIB Hf–Nd isotopic signatureand Pb isotope ratios intermediate between Indian and PacificMORB in the mantle source of the gabbros dredged in the HuatungBasin, it likely corresponds to a trapped piece of early Cretaceousoceanic crust produced at a ridge overlying an Indian-type mantleas proposed by Deschamps et al. (2000). This old oceanic litho-sphere might have been rejuvenated later in order to reconcileits geochronological and ‘‘isotopic’’ age with its ‘‘seismic’’ and‘‘gravity’’ age (Kuo et al., 2009). Indeed, the basement of the basinis 400 m shallower than the theoretical depth according to normalthermal subsidence (Deschamps et al., 2000). Furthermore, thecrust of the Huatung Basin is abnormally thick in some locations:up to �10 km (Yang and Wang, 1998) or even up to �16 km(McIntosh and Nakamura, 1998). Various scenarii can be proposedincluding the influence of a plume that would have thickened thecrust or a rejuvenation caused by convective cells activated fromthe Manila subduction ‘‘back-arc’’ mantle dynamics, but since thesubducting crust and the subduction itself are both young, it seemsunlikely (Arcay, personal communication). Based on two onland-offshore integrated seismic experiments (TAICRUST and TAIGERprojects), McIntosh et al. (2005) and Kuo-Chen et al. (2012) havebuilt velocity models across southern Taiwan (see location onFig. 5). Despite some global agreement between the two models,they obtain quite different pictures along sections distant by only50 km. Along the TAICRUST lines 29–33 at the latitude of Lanyu,McIntosh et al. (2005) have imaged a �30 km thick crust belowthe forearc region which was supposed to underthrust the �6 kmcrustal layer of the arc as a result of the collision with the Chineseplatform. Further north at the latitude of Lutao (Fig. 5), Kuo-Chen

Fig. 5. Map of the Luzon arc (dark triangles = active volcanoes) with location ofseismic profiles discussed in the text. S.O.T. = Southern Okinawa Trough;SRA = Southern Ryukyu Arc; PSP = Philippine Sea Plate; and EP = Eurasian Plate.

⁄ The term ‘‘back-arc’’ here refers to the Huatung Basin which is today located backof the Luzon arc. It does not mean that the basin opened in a ‘‘back-arc’’ position asthe result of a trench rollback.



et al. (2012) found a Moho beneath the PSP at variable depths(Fig. 6): 27 km beneath the Coastal Range and the arc (Lutao),40 km in between and 12 km beneath the Huatung Basin. Threepoints merit attention along this section. First, the rise of highvelocities (HVZ) beneath the Longitudinal Valley (noted ‘‘suture’’on Fig. 6) and the deepening of the Moho between the orogenand the arc support the ‘‘forearc subduction’’ model of Chemendaet al. (1997). Second, as in the IBM arc, a thick (�10 km) sequencewith ‘‘middle-crust’’ velocities is observed right beneath theextinct Lutao volcano (see MC on Fig. 6). Third, well-resolved lowvelocities (LVZ) are observed in the Huatung Basin, presumablywithin the uppermost mantle, down to �25 km (Fig. 6; Kuo-Chenet al., 2012).

3.3. Deformation mode in the Luzon arc and ‘‘proximal back-arc�’’

In terms of deformation mode, the situation differs from theIBM case, not only because of the ‘‘maturity’’ of the island arc(Miocene vs Eocene), but also because of the nature of the indenter.Indeed on one hand, the Chinese platform acts as an indenter withrespect to the PSP and more specifically with respect to the north-ern Luzon volcanic arc including the Coastal Range of Taiwan,north of 22�N (see Fig. 1). On another hand, the ‘‘indented’’ north-ern Luzon arc, acts itself as an indenter north of 23�N, with respectto the EP under which it subducts. Onshore and offshore geologicaland geophysical records and observations east of Taiwan show thatthe arc and ‘‘back-arc’’ strain is dominated by the collision with theChinese platform.

3.3.1. Effects of the collision between the Luzon arc and the Chineseplatform

The main feature observed south of Taiwan is the underthrust-ing of the whole forearc basement beneath the arc within less than200 km along strike (Fig. 6; Chemenda et al., 1997; Malavieilleet al., 2002; Shyu et al., 2011). The width of the forearc basementdoes not exceed 60 km at the latitude of the Batan islands southof Taiwan so that, if we consider that the forearc block was fullylocked with the subducting plate (extreme case) and a splay (outof sequence) fault accommodated its underthrusting, about1 m.y. is enough for the forearc to totally disappear from thesurface. If the convergence partitioned equally between frontal

subduction and the splay fault, then 2 m.y. are needed for full sub-duction of the forearc block which is still plausible with respect toconvergence rates. The forearc block, about �45 km long and�25 km thick, is well identified on the velocity profile at the lati-tude of Lutao (Fig. 6). It appears tilted clockwise as it underthruststhe arc. The narrow Coastal Range is mostly made of forearc andintra-arc sedimentary material including an ophiolitic mélange(Lichi), detrital and volcanoclastic series and limited volcanic rocks(Chimei complex) in the northern part (Huang et al., 1995). Basedon the Moho depth extracted from two different 3D-tomographicmodels, Kuo-Chen et al. (2012) and Theunissen et al. (2012) haveshown that the root axis of the volcanic arc aligns about10 ± 5 km offshore the east coast. They also noticed that the Coast-al Range was underlain by a narrow (�10 km wide) high-velocityzone (HVZ) that could be interpreted as relics of the forearc base-ment squeezed in the suture zone (Figs. 6 and 7a). Moreover, thereare seismic evidences that the arc itself is decoupled from the PSPall along the Coastal Range along west-dipping thrusts (Lallemandet al., 1999; Malavieille et al., 2002). Both tomography and seismic-ity confirm the previous hypothesis of Chemenda et al. (1997) thatincipient subduction of the PSP occurs off the east coast of Taiwannorth of 23�300N. Despite the fact that part of the arc overthruststhe Taiwan orogen along the Longitudinal Valley separating theCoastal from the Central ranges, the arc is shortened and under-thrust beneath the Central Range north of the Coastal Range(Fig. 7a; Lallemand et al., 2013). Upper crustal slices may ulti-mately be accreted to EP, thanks to decoupling levels allowingthe Huatung Basin lithosphere to subduct beneath the arc. Onlyrelics of the forearc are detected between the orogen and the arcup to the section illustrated on Fig. 7a (see HVZ), suggesting thatthe main part of the forearc has been subducted to larger depths.

3.3.2. Effects of the collision between the Luzon arc and the SouthRyukyu margin

Active and passive seismic experiments were conducted in2008 and 2009 (RATS-TAIGER project) with a dense OBS networkin the southern Ryukyu forearc area where the arc-continentcollision culminates as attested by the seismic deformation (Klin-gelhoefer et al., 2012; Theunissen et al., 2012). The new 3-D veloc-ity model, as well as the improved resolution of earthquakesdistribution, allowed Lallemand et al. (2013) to revisit the PSPdeformation in its subducted part below the Southern Ryukyuarc (Fig. 7b). The two orthogonal velocity profiles shown onFig. 7 (see location on Fig. 5) were interpreted using constraints

Fig. 6. Free tectonic interpretation of a velocity section obtained by Kuo-Chen et al. (2012) as a result of the TAIGER experiment. EP = Eurasia Plate; PSP = Philippine Sea Plate;HVZ = High Velocity Zone; LVZ = Low Velocity Zone; and MC = Middle Crust. See Fig. 5 for profile location.

� The ‘‘proximal back-arc’’ refers to the domain of a back-arc that is close to the arc.When it is associated with arc magmatism, it is called ‘‘rear-arc’’ like in IBM.



from adjacent lines, seismicity distribution, focal mechanisms ofearthquakes, upper plate deformation and any available data inthe survey area. Thrusting mainly develops along NNE-SSW faultsas a result of the collision between the arc and the Taiwan orogeneven in the subducted part of the PSP (Fig. 7a). The thrusting islikely responsible for local thickening of the oceanic crust. Theshort wavelength of the oceanic slivers indicates a shorteningmainly confined within crustal levels rather than lithosphericbuckling. In a direction normal to the Ryukyu subduction zone(Fig. 7b), no compressional feature is observed within the subduct-ing plate. On the contrary, the slab appears down-faulted along amajor shear zone which has been interpreted by Lallemand et al.(2013) as an incipient sinistral tear accommodating the differentialstress between E–W compression in the south and free subductionin the north (Lallemand et al., 1997). As observed by Kuo-Chenet al. (2012) and Klingelhoefer et al. (2012), velocities are ratherlow in the mantle beneath the Huatung Basin crust (see low veloc-ity zones (LVZ) in Fig. 7). These low velocities were attributed byKlingelhoefer et al. to mantle serpentinization. On the contrary,some ‘‘relatively’’ high velocity regions were mapped in the base-ment of the Southern Ryukyu arc right above the ramp in the sub-ducting PSP (Fig. 7b). These velocities are similar to those observedbelow the Coastal Range and Longitudinal Valley. It may thus rep-resent relics of the subducting Luzon arc or forearc basementsqueezed or accreted in the collision zone. This would be the onlyexpression of a ‘‘collision’’ between the subducting Luzon arc andthe Southern Ryukyu arc.

4. Comparison of the deformation styles and possible analogies

4.1. Crustal distinctive characteristics and rheology of an island arcand its proximal ‘‘back-arc’’

We have shown that based on seismic velocities, both arc andproximal ‘‘back-arc’’ (called ‘‘rear-arc’’ by Japanese authors) crustsshow seismic and petrological characteristics contrasting from aregular oceanic crust. Thick sections characterized by intermedi-ate velocities between 5.7 and 6.8 km/s are interpreted as thepresence of middle crust in the IBM arc and rear-arc by analogywith the rocks exposed in the Izu Peninsula (Tani et al., 2011).Burg et al. (2005) have described in detail the processes of shearstrain localization from uppermost mantle to middle-crust levelsin the exceptional outcrops of the fossil Kohistan island arc inPakistan. They observed that the plutonic lower crust of the arcis strongly affected by sub-horizontal, syn-magmatic shear zones,probably consistent with the bulk flow direction of the paleo-sub-duction zone. Whatever the origin of this middle crust is in theIBM arc: continental or differentiated from underplated gabbros,the rocks should be weak and prone to shear. One may notice thatthe same seismic structure in the IBM arc and rear-arc lead to dif-ferent levels of decoupling. It either occurs within the weak mid-dle crust in the arc probably because it is heated by the activevolcanic activity, or below the Moho in the rear-arc. In the lastcase, the lithosphere is certainly colder because volcanism is nomore active but the uppermost mantle might have been

Fig. 7. Two orthogonal velocity sections obtained after the RATS experiment in the southernmost Ryukyu forearc (Lallemand et al., 2013). Sections are located in Fig. 5.EP = Eurasia Plate; PSP = Philippine Sea Plate; SRA = Southern Ryukyu arc; MC = Middle crust; LVZ = Low velocity zone; HVZ = High velocity zone; A.W. = accretionary wedge;and S.O.T. = Southern Okinawa Trough. Grey dots are M > 3 relocated 1992–2008 earthquakes.



serpentinized by dewatering from the subducting Pacific slab.Rocks of the middle and lower crust of the Luzon arc do not out-crop even though the crust exhibits similar seismic characteristicsto that of the IBM arc. Along the velocity section shown in Fig. 7a,the Taiwan orogen is backthrust onto the Luzon arc but the arcitself decouples from the adjacent Huatung Basin and could stillpotentially be accreted to the orogen. The proximal Huatung Ba-sin oceanic crust, although much older than the Shikoku Basin,shows anomalous low velocities (LVZ) down to a depth of about25 km (Fig. 6) or even 50 km (Fig. 7). Three non-exclusive expla-nations can been proposed:

– Since the Huatung Basin is sliced by numerous north–southtrending fracture zones (Deschamps et al., 1998) and east–westtrending neo-formed normal faults caused by plate bendingbefore subduction (e.g., Schnürle et al., 1998), seawater canpenetrate to great depths and serpentinize the oceanic crustand even the uppermost lithospheric mantle (e.g., Raneroet al., 2003).

– The western part of the Huatung basin lies above the subduct-ing EP slab at least south of 23�N today (see Fig. 6). Consideringthe thermal state of the subducting oceanic crust, slab dehydra-tion can occur down to 150 km depth and cause serpentiniza-tion of the top lithospheric mantle (Arcay et al., 2005).

– The crust has been thickened and rejuvenated when passingabove a plume in Eo-Oligocene time (Deschamps and Lalle-mand, 2003).

The first two processes produce serpentinites that favor shearstrain localization and low-angle thrusting within the crust andthe uppermost mantle if compression occurs. The last processrather produces OIB but differentiation may also occur in case ofthick accumulation of oceanic material.

4.2. Crustal versus lithospheric deformation

Based on seismic imagery, the ICZ appears to be mainly con-structed by the stacking of upper and mid-crustal slivers scrapedoff the subducting IBM arc (Fig. 4; Arai et al., 2009; Tamura et al.,2010). First interpretations, which proposed that thrusting alongthe Zenisu Ridge might cut through the entire lithosphere, shouldresult in successive seaward jumps of the subduction interfaceassociated with duplication of the slab thickness in the subductionzone, which is not observed. Data re-examination and new dataacquisition show that compressive features like the Zenisu Ridgeor the accreted imbricates of the ICZ only affect the crustal levels.Decoupling occurs in the middle crust along the main ridge and be-neath the Moho in the uppermost mantle in the ‘‘rear-arc’’ (Fig. 8).

Because of the special configuration of the arc-continent colli-sion in Taiwan, i.e., the main collision involves an arc system onthe upper plate and a subducting ocean-continent transition zone,part of the deformation occurs at a lithospheric scale (forearc sub-duction) and part at a crustal scale (arc and proximal ‘‘back-arc’’crustal shortening). Two mechanical explanations can be invokedfor the subduction of the Luzon forearc:

– forearc buckling and horizontal compression caused by the sub-ducting buoyant Chinese continental platform (Chemenda et al.,2001; Tang et al., 2002) and/or

– increasing friction between the two converging plates causedby the cooling of the forearc as a result of the decreasing con-vergence rate (Arcay et al., 2007).

The high velocities observed at depth between the arc and theTaiwan orogen (HVZ in Figs. 6 and 7a) might attest for the presenceof cool forearc basement material (Kuo-Chen et al., 2012; Lalle-mand et al., 2013). Moreover slab dehydration should be amplified

Fig. 8. Simplified cartoons of Izu and Taiwan collision zones with three schematic sections illustrating the two main levels of decoupling during arc-continent collision. ForTaiwan, only the collision between the Luzon arc and the Chinese continental platform has been sketched because the other arc collision with the overriding southern Ryukyuarc is probably much less important than the previous one. Section BB’ across the Nankai Trough might also illustrate the case of the Luzon arc subducting beneath the SRAexcept that the vergence of the thrusts in the Huatung Basin are orthogonal to the trench. COB = Continent-ocean boundary; PBA = Proximal back-arc; AW = accretionarywedge; CP = Chinese platform; ICZ = Izu collision zone; IBM = Izu–Bonin–Mariana arc; T.C. = Tenryu Canyon; and M = Moho.



by the northward decrease in subduction rate as a result of conti-nental subduction (Arcay et al., 2007). Lithospheric mantle weak-ening may thus occur right above the slab dehydration domain,i.e., in the arc and proximal ‘‘back-arc’’. One may also expect thatwater impregnation of the mantle favors melting, but no volcanicactivity was mentioned in the Huatung Basin except one contro-versed report off the Coastal Range in 1853 (Chen and Shen,2005). Such uppermost mantle weakening combined with crustalhydrothermalism and eventually the presence of ‘‘middle crust’’in the arc favor the development of crustal and uppermost mantleshortening whereas the lower lithospheric PSP mantle subductsbeneath the Taiwan orogen.

4.3. Arc (and ‘‘back-arc’’) accretion in the collision zone

Arc and ‘‘rear-arc’’ crustal accretion has been demonstrated bymany authors in the ICZ (e.g., Taira et al., 1998; Tani et al., 2011).Accretion is restricted to the upper crustal levels in the arc itselfand may involve the uppermost lithospheric mantle in the ‘‘rear-arc’’ domain. Present observations in the Nankai margin do showdeformation in the overriding wedge like ridges controlled byout-of-sequence thrusts (Lallemand et al., 1992; Lallemant et al.,1995; Dessa et al., 2004) or the presence of a transfer zone, whichsurface expression is the Tenryu Canyon in the prolongation of theAkaishi Tectonic Line, accommodating the differential motion be-tween the normal subduction to the west and shortening abovethe rear-arc domain (Le Pichon et al., 1987a, 1996, see T.C. inFig. 8). Despite no direct observations, Shikoku Basin crustal accre-tion beneath the accretionary wedge, thanks to a decoupling levellocated in the uppermost mantle, is suspected (Fig. 8).

East of northern Taiwan, the slivering of the crustal part of thearc and proximal ‘‘back-arc’’ is reflected in the shallower bathym-etry of the Southern Ryukyu forearc basement which is upliftedwith respect of the rest of the forearc (Font et al., 2001; Lallemandet al., 2013). As a consequence, the crust is probably decoupledfrom the PSP lithospheric mantle. A décollement level in the upper-most mantle of the Huatung Basin is required to accommodate theshortening of the crust and the westward subduction of the lowerpart of the Huatung lithosphere (see LVZ in Figs. 7 and 8). The crus-tal wedge is underthrusted beneath the Central Range north of24�N (Fig. 7a, Lallemand et al., 2013) and could potentially be ac-creted to the orogen. With respect to the northward subductionbeneath the Southern Ryukyu arc, rather than massive accretionof the crustal wedge, little evidences of arc or forearc material werefound in the deeper parts of the SRA basement based on seismicvelocities (HVZ in Fig. 7b).

5. Conclusions

The comparative examination of the deformation modes withintwo colliding island arcs of the PSP lead us to a series of observa-tions that illustrate original mechanical processes of arc-continentinteraction in collision zones.

� The Luzon and IBM arcs were both emplaced on the same LateJurassic–Early Cretaceous ‘‘proto-PSP’’ crust.� The studied island arcs have distinctive petrological and rheo-

logical characteristics that facilitate strain localization.� Forearc lithospheric subduction is favored when a continent

subducts beneath a volcanic arc because of the combination ofincreasing compression due to continental crust buoyancy andincreasing friction due to forearc cooling as a result of subduc-tion slowing.� The oceanic crust and uppermost mantle adjacent with the arc

on the back-arc side is still under the influence of the

subducting slab as attested either by ancient magmatism(northern IBM rear-arc) or by a higher hydration (HuatungBasin). Slab hydration is amplified if subduction slows and slabdip increases.� Accretion of arc material onto the upper plate is not systematic.

It strongly depends on the characteristics of the arc itself andthe geodynamic configuration.� When accretion occurs, like in the Izu collision zone, only upper

crustal levels are involved. The main part of the lithosphere stillsubducts. In Taiwan, similar conclusions may be reached exceptthat no surface outcrop attests yet for arc crustal accretion. Onlyseismic evidences might indicate that arc and/or forearc mate-rial may be underplated beneath the upper plate.� In both cases, decoupling levels allow the delamination

between upper and lower lithospheric levels. Such decouplinglevels can be either facilitated by the presence of ‘‘weak andhot middle crust’’ or serpentinized uppermost mantle.

Acknowledgements

I acknowledge Michel Faure and Yan Chen who invited me topresent a keynote at the International Conference ‘‘Tectonics ofAsia’’ hold in Orléans in November 2012 in honour of Jacques Char-vet, with whom I shared memorable discussions about the IBMsubduction in Japan at the end of the eighties and early nineties.It was a unique opportunity for me to reconcile some early workin Japan with present studies in Taiwan. Anne Delplanque has donea tremendous work in drawing most of the illustrations. I havetested some suggested processes of lithospheric weakeningthrough ‘‘recurrent’’ discussions with Diane Arcay. I also deeplythank Ken Tani, Jin-Oh Park, Hao Kuo-Chen, Stéphane Mazzotti,Stéphane Dominguez, Y. Font, Thomas Theunissen, Jacques Malavi-eille, René Maury, Hervé Bellon, Jean-Louis Bodinier, Fleurice Parat,Adolphe Nicolas, Françoise Boudier and Fred Gueydan for the sci-entific discussions or help. The revised manuscript has benefitedfrom two anomymous reviews.

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Author's personal copy - Géosciences MontpellierAuthor's personal copy 100 km (Kodaira et al., 2007). Variations in crustal thickness mainly come from variations in thickness of the