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Journal of Asian Earth Sciences 86 (2014) 1–11
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes
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, France
Associated 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 collision
Middle crust
Intra-oceanic slivering
Mantle decoupling
Forearc subduction
Luzon arc
IBM 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 collisional settings leading to arc material subduction, delamination and/or accretion. The study focusses
on two well-documented colliding island arcs: the Izu–Bonin–Mariana (IBM) arc in Japan and the Luzon
arc in Taiwan, both carried by the Philippine Sea plate. Firstly, there is a body of evidence that both the
IBM and the Luzon arcs were built on the same Late Jurassic to Early Cretaceous ‘‘proto-Philippine Sea
Plate’’ crust. Their internal structure is thus more heterogeneous than expected from Paleogene or Neogene supposedly ‘‘intra-oceanic’’ island arcs. Secondly, those arc systems and proximal ‘‘back-arcs’’ have
similar seismic characteristics attesting either for the presence of a middle crust with continental velocities and/or serpentinized uppermost mantle that facilitate crustal shortening/slivering and subsequent
decoupling 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 its
strength if slab-derived hydration occur. Decoupling then occurs below the Moho. Arc delamination
likely 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 geodynamic configuration. Most of the accreted material is probably underplated rather than frontally
accreted.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Modern collisions between a volcanic arc and a continent are
common in southeast Asia: Luzon arc in Taiwan, Izu–Bonin and
Kurile 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 indenter
pushing forward the continental upper crustal layers (e.g., Suppe,
1981; Wu et al., 1997; Malavieille and Trullenque, 2009). Some
proportion of arc material may be scraped off the subducting plate
and 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 or
colliding island arc from their initiation to the ultimate delamination/peeling or subduction. I do not describe onland outcrops of
island arc slivers accreted to the overriding plate but rather
examine the timing of the various deformation phases and the
ingredients that control the localization of the arc deformation.
E-mail address: [email protected]
1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jseaes.2013.07.043
Well-constrained examples of such processes are found on both
eastern and western borders of the Philippine Sea Plate (PSP).
The collision of the Izu–Bonin–Mariana (IBM) Arc with central
Japan has been carefully studied since the early eighties (e.g., Ogawa, 1983; Huchon and Kitazato, 1984; Soh et al., 1991; Taira et al.,
1998; Mazzotti et al., 1999) but a new set of studies these last
years has provided determining constraints on ongoing deep processes (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 Taiwan. Studies there were achieved later but again, efforts have been
done these last two decades to better characterize the collision
process, especially offshore (e.g., Wu et al., 1997; Teng et al.,
2000; Lallemand et al., 2001b; Malavieille et al., 2002; Theunissen
et al., 2012). Even if the geodynamic context differs from that in Japan, many similarities exist in these two situations and one may
use the better knowledge of the IBM – Honshu collision to address
questions in Taiwan, and vice versa.
The two cartoons in Fig. 1 illustrate the common points and
main differences between the geodynamic contexts in Taiwan
and in Japan. In Taiwan, the Miocene to present Luzon volcanic
arc results from the subduction of the South China Sea (SCS) oceanic lithosphere, which belongs to the Eurasia plate (EP), beneath
the PSP. That subduction system ends at the latitude of (northern)
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
Taiwan and is relayed by an orthogonal subduction of the PSP
beneath EP, so that the Luzon arc that overrides the EP in the
southern part of Taiwan, subducts beneath the same EP east of
northern Taiwan. To summarize the tectonic situation, one may
say that the Luzon arc first collides with the Taiwan orogen as a result of the continental nature of the subducting EP at the latitude of
Taiwan north of the SCS, and then collides with and subducts beneath the EP east of northern Taiwan. The convergence rate between both plates averages 8 to 9 cm/yr in the collision area and
the convergence azimuth is oblique to both plate boundaries (Seno
et al., 1993). The emerging part of the deformed Luzon arc forms
the Coastal Range east of Taiwan (about 150 10 km). In Japan,
the Eocene to present IBM arc results from the subduction of the
Pacific plate (PAC) oceanic lithosphere beneath the PSP. The IBM
arc is carried down the Nankai subduction zone at a rate of about
4 cm/yr as it is part of the subducting PSP. The emerging part of the
deformed IBM arc forms the Izu Peninsula and collision zone (ICZ)
along the southern coast of central Honshu. It covers an area of
about 100 40 km.
Despite variations in maturity, convergence rates and geometry
of plate boundaries of both arc collision zones, we will see that
their behavior is often similar in terms of deformation modes.
2. Recent advances in understanding the IBM arc and the ICZ in
Japan
2.1. IBM arc origin and age
Based on studies of forearc oceanic rocks, supposed to have
formed as a result of subduction initiation stage along a former
fracture zone (Stern and Bloomer, 1992), the inception of the
IBM arc has been dated at 51–52 Ma (Ishizuka et al., 2011). Lallemand (1998) argued that an Early Eocene age is a minimum since
the forearc rocks that have been sampled might have formed after
subduction began, particularly if they did not formed in the former
forearc (Deschamps and Lallemand, 2003). Indeed, IBM is an
erosional margin with rates of forearc consumption of several kilometers per million years (von Huene and Scholl, 1991; Lallemand,
1995), and it may be that the oldest arc rocks have been consumed
by subduction. Furthermore, the presence of Mesozoic continental
crust in the Mariana forearc basement was suspected by Azéma
and Blanchet (1982) after leg DSDP 60 when they discovered Late
Jurassic–Early Cretaceous reworked pebbles in a volcanic matrix.
More recently, Ishizuka et al. (2012) have described Jurassic basaltic pillow lavas with Indian Ocean MORB affinities in the Bonin
forearc suggesting that Mesozoic crust constitutes the basement
of the IBM arc. To summarize, most of the arc consists in a volcanic
ridge that began to form in Early Eocene or earlier but there are
striking evidences that part of the arc basement is inherited from
a ‘‘proto-PSP’’ that is composed of older (Jurassic to Cretaceous
detrital zircons), non-oceanic, possibly continental crust (Tani
et 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 rhyolitic
rocks are exposed both in the forearc of the main actual IBM arc
(sometimes in association with boninites) and along the PalauKyushu Ridge (Figs. 1 and 2) which is the remnant part of the
pre-Miocene IBM arc rifted during the Miocene spreading of the
Shikoku and Parece-Vela basins (e.g., Karig, 1971). This Eo-Oligocene arc was emplaced over a zone about 200 km wide across
the present IBM arc (Suyehiro et al., 1996) to which the PalauKyushu Ridge must be added to restore the original arc. Based on
seismic velocities, the crustal thickness varies from 20 to 30 km beneath 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 collision
zones. 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.
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
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2.3. IBM ‘‘rear-arc’’ – transition between arc and back-arc
The Izu–Bonin arc presents a singularity with respect to ‘‘classical’’ arcs. Mio-Pliocene volcanism occurred at distances up to
150 km back of the present volcanic front along en-échelon northeast-southwest trending ridges called ‘‘across-arc seamount
chains’’ (Ishizuka et al., 2003; Fig. 2). These volcanoes first erupted
contemporaneously with the last spreading episodes of the
Shikoku Basin. Ishizuka et al. (2003) have demonstrated that the
volcanism has migrated from west to east between 17 and 3 Ma.
This migration coincides with the increasing dip of the Pacific slab
caused by the motion change from trench rollback to advance
around 8–5 Ma (Faccenna et al., 2009). The internal seismic structure of that region, called ‘‘rear-arc’’ by the authors, is similar to
that of the main arc (Fig. 2). Indeed, it has been considered by
Kodaira et al. (2008) as a paleoarc. Crustal thickness can reach
25–30 km beneath the rear-arc (Kodaira et al., 2007) but the thickest crust is not observed beneath volcanoes. For Kodaira et al.
(2008), this finding suggests that the Mio-Pliocene volcanism
may have been superimposed onto a crust formed before the opening of the Shikoku Basin. Rear-arc volcanic products are exposed in
the Izu Peninsula. They consist of upper Miocene to Plio-Quaternary andesites and dacites (Tani et al., 2011).
The most-striking across-arc seamount chain is the Zenisu
Ridge which aligns parallel to the Nankai Trough south of the Izu
Peninsula (Fig. 2; Lallemant et al., 1989). A small part of the ridge
is subaerial (Zenisu Rocks outcrop of andesitic lava flows dated
2–3 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 pillow basalts were collected along a fresh scarp at its southwest termination (Henry et al., 1997).
A
B
Fig. 2. Top: Northern part of the IBM arc and collision area with central Japan (DEM
and active volcanoes were extracted using SubMap tool http://submap.fr/) Dark
triangles represent active volcanoes. The A–B profile of the bottom section and
those of Figs. 3 and 4 are shown on the map. Bottom: Interpreted wide-angle
velocity profile A–B after Takahashi et al. (2009). MC = Middle-crust.
100 km (Kodaira et al., 2007). Variations in crustal thickness
mainly come from variations in thickness of the middle crust
and, in a lesser extent, the lower crust (Takahashi et al., 2009).
Middle-crust exhibits typically ‘‘continental crust’’ P-waves velocities VP between 5.7 and 6.8 km at depths ranging from 5 to
12 km (see section on Fig. 2; Kodaira et al., 2007). Such velocities
commonly correspond to granites, diorites or tonalites (Tamura
et al., 2010). Some authors like Tatsumi et al. (2008) considered
that layer as an unsubductable nucleus of ‘‘continental’’ crust.
The lower crust (6.7 < VP < 7.4 km/s at depths generally ranging
from 10 to 20 km) is more mafic. It is likely composed of hornblende gabbros (Kitamura et al., 2003) or granulites derived from
the melting and differentiation of underplated gabbros by comparison with the fossil Kohistan arc rocks sequence (Dhuime et al.,
2009). Crustal materials are generally denser in the oldest Eocene
arc than in the current volcanic arc (Takahashi et al., 2009). The
upper crust (4.5 < VP < 6.0 km/s) consists of basalts, andesites and
intrusives (Taira et al., 1998; Takahashi et al., 2007).
2.4. Focus on the Zenisu Ridge
Before being described as an ‘‘across-arc seamount chain’’, the
Zenisu Ridge was interpreted as an intra-oceanic sliver (Le Pichon
et al., 1987b). The pillow basalts sampled at its southwestern edge
thus represent the top layer of an uplifted oceanic crust sequence
of the Shikoku Basin (Lallemant et al., 1989). Chamot-Rooke and
Le Pichon (1989) have proposed a model of plate buckling and failure along a lithospheric thrust localized along the magmatic ridge,
propagating further southwest into the oceanic crust, and caused
by the compressive stress generated by the arc-continent collision.
Nakanishi et al. (1998, 2002) found a 5 km vertical offset in the
Moho beneath the south flank of the ridge supporting the lithospheric fault hypothesis. Given the dip angle of the thrusts imaged
in reflection seismics, such offset should result in a 10 km throw
along the main thrust which is not observed at the surface.
Mazzotti et al. (2002) thus proposed a model of conjugated lowangle thrusts within the crust distributing the slip over a wide area.
They also proposed that the serpentinized mantle–crust transition
acts as a decoupling level above which distributed shortening may
occur. Based on the deformation observed in the accretionary
wedge landward of the ridge, Lallemand et al. (1992) have suggested that earlier oceanic slivers were formed, subducted and
then potentially accreted to the margin. Le Pichon et al. (1996),
Park et al. (2003) or Kimura et al. (2011) have confirmed, after a
magnetic and a detailed seismic surveys, the presence in the subducting 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 full
waveform inversion from dense OBS data, Kodaira et al. (2004)
and Dessa et al. (2004) have proposed a tectonic interpretation of
the thrusts that affect the ‘‘paleo-Zenisu’’ ridge. Knowing today
that most of the Zenisu ridge has a magmatic origin with middle
crust velocities, we have adapted the tectonic model of the
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
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 survey
published 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 Mazzotti
et 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; and
LC = Lower crust.
previous authors onto the velocity profile of Park et al. (2003)
assuming that most of the low-angle thrusts localize within the
crust, and that the uppermost mantle, or the mantle–crust transition, 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 between the IBM arc and Honshu arc might have begun as early as
15 Ma ago. Plates reconstructions (Sdrolias and Müller, 2006)
rather predict a collision further west around 15 Ma near the Shikoku island and a beginning of collision with central Honshu
around 10 Ma in better agreement with the end of Japan Sea opening. During its subduction, the IBM arc is supposed to have crept
eastward along the Nankai Trough until about 5–8 Ma and then
westward until present (Faccenna et al., 2009) so that the ICZ
was confined to central Honshu, with some sinistral and dextral
motion component, probably during the last 10 million years.
Successive slivers of IBM arc material accreted onto Honshu since
the beginning of the collision. The Tonoki–Aikawa Tectonic Line
(TATL) is considered as the tectonic boundary between the original
Honshu crust (Kanto Mountains) to the north and the accreted slivers (Tanzawa mountains, Izu Peninsula, Zenisu Ridge) to the south
(e.g., Arai et al., 2009). Some other authors include another unit to
the north delimited by the Mineoka Tectonic Line (Taira et al.,
1998; Soh et al., 1998). Fig. 4 shows a simplified along-strike section across – from south to north – the undeformed IBM arc, the
ICZ including the accreted IBM-derived units and the shortened
crust of Honshu (Tamura et al., 2010). Two main Miocene batholiths are observed in the ICZ: the Tanzawa tonalites and the Kofu
Granitic Complex. Based on their composition, Tamura et al.
(2010) consider that they are derived from syn-collisional partially
melted Oligocene middle crust. Based on an intensive seismic
experiment, it has been possible to trace the TATL down to about
18 km beneath the Kanto mountains (Arai et al., 2009). From the
geometry of reflectors and the seismic activity, the Tanzawa block,
with VP typical of upper to middle crust of the IBM arc, appears delaminated from the subducting slab forming a wedge-like body
which is inserted, through low-angle thrusts, between the upper
and lower crust of Honshu. These results strongly suggest that
the middle crust of the IBM arc is weak and localizes the intraarc deformation, allowing the lower crust and the mantle part of
the arc to subduct with the rest of the PSP. Such process is in agreement with observations of arc-continent or continent–continent
collisions in other contexts like the fossil Kohistan arc in eastern
Himalaya (Burg et al., 2005) or the Alps (Schmid et al., 1996).
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 the
Tanzawa tonalites were emplaced during the Miocene within the ICZ. TATL = Tonoki–Aikawa Tectonic Line. Closed and open-triangles show respectively basalt-dominant and
rhyolite-dominant volcanoes.
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
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 South
China Sea oceanic lithosphere beneath the PSP since mid-Miocene
right after the cessation of back-arc spreading of the South China
Sea (Defant et al., 1989; Briais et al., 1993). Late Oligocene to Early
Miocene calc-alkaline volcanism is only found in northwest Luzon
island and Dalupiri island (Yang et al., 1996) but these occurrences
have been attributed by Polvé et al. (2007) to a short-lived episode
of westward subduction of the PSP lithosphere beneath Luzon.
After a quiescent period, magmatic activity resumed around
15 Ma in relation with the start of eastward subduction of the
SCS off Luzon island. Geochemical patterns observed in these
calc-alkaline lavas from 15 to 1 Ma reflect the progressive addition
to the Luzon arc mantle wedge of SCS sediment (Polvé et al., 2007).
Manila Trench extends from west of Luzon to the latitude of
Taiwan, which probably did not exist yet in Miocene. Indeed, many
authors consider that the orogen of Taiwan results from an arccontinent collision starting between 6.5 Ma (Huang et al., 1997)
and 3 or even 2 Ma (Malavieille et al., 2002). However, Lu and
Hsu (1992) have proposed a two-stage collision at 12 Ma and
3 Ma. Most absolute ages obtained on lavas from volcanoes situated north of Luzon Island are younger than 10 Ma (Defant et al.,
1989; Yang et al., 1996). Closer to Taiwan, ages obtained on Lutao
and Lanyu andesites cover a period between 3.9 and 5.5 Ma
(McDermott et al., 1993). There is still an isolated oldest age
around 16 Ma obtained on lavas from the Chimei complex which
outcrops in the northern part of the Coastal Range of Taiwan (Juang
and Bellon, 1984; Lo et al., 1994) that led Yang et al. (1995) to conclude that the volcanic activity of the arc segment forming the
present Coastal Range of Taiwan extended from 16 to 2 Ma. The
cessation of magmatism in the northern part of the Luzon arc coincides with the start of the collision between the arc and the Chinese continental platform.
An interesting point is that recent radiolarian biostratigraphic
results provide evidence for the existence of a Mesozoic substratum upon which Luzon and the neighboring regions within the
Philippine archipelago were built (Queano et al., 2013). Similar
ages were found by Deschamps et al. (2000) on gabbros dredged
in the Huatung Basin east of the Taiwan, from radiolarian assemblages collected on Lanyu island (Yeh and Cheng, 2001), in eastern
Indonesia (Ali et al., 2001) as well as in the basement of the IBM
ridge (Ishizuka et al., 2012) or the Amami Plateau (Hickey-Vargas,
2005) and Oki-Daito ridges (Ishizuka et al., 2011) suggesting a
common provenance.
5
trapped piece of an old oceanic basin (Deschamps et al., 2000), a
composite back-arc basin with an old (Mesozoic ?) part in the
south and an Eocene part in the north (Sibuet et al., 2002) or an Oligo-Miocene oceanic basin (Kuo et al., 2009). Since Hickey-Vargas
et al. (2008) found an Indian MORB-OIB Hf–Nd isotopic signature
and Pb isotope ratios intermediate between Indian and Pacific
MORB in the mantle source of the gabbros dredged in the Huatung
Basin, it likely corresponds to a trapped piece of early Cretaceous
oceanic crust produced at a ridge overlying an Indian-type mantle
as proposed by Deschamps et al. (2000). This old oceanic lithosphere might have been rejuvenated later in order to reconcile
its geochronological and ‘‘isotopic’’ age with its ‘‘seismic’’ and
‘‘gravity’’ age (Kuo et al., 2009). Indeed, the basement of the basin
is 400 m shallower than the theoretical depth according to normal
thermal subsidence (Deschamps et al., 2000). Furthermore, the
crust 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 proposed
including the influence of a plume that would have thickened the
crust or a rejuvenation caused by convective cells activated from
the Manila subduction ‘‘back-arc’’ mantle dynamics, but since the
subducting crust and the subduction itself are both young, it seems
unlikely (Arcay, personal communication). Based on two onlandoffshore integrated seismic experiments (TAICRUST and TAIGER
projects), McIntosh et al. (2005) and Kuo-Chen et al. (2012) have
built velocity models across southern Taiwan (see location on
Fig. 5). Despite some global agreement between the two models,
they obtain quite different pictures along sections distant by only
50 km. Along the TAICRUST lines 29–33 at the latitude of Lanyu,
McIntosh et al. (2005) have imaged a 30 km thick crust below
the forearc region which was supposed to underthrust the 6 km
crustal layer of the arc as a result of the collision with the Chinese
platform. Further north at the latitude of Lutao (Fig. 5), Kuo-Chen
3.2. Luzon arc and ‘‘back-arc*’’ seismic and petrological structure
The seismic structure of the Luzon arc is less known than that of
the IBM arc except in its northern part near Taiwan where the Luzon arc collides with the Chinese margin. The width of the arc drastically reduces between northern Luzon island and southern
Taiwan from 200 to 50 km even if, as mentioned above, early
Cretaceous radiolarian cherts were sampled in both places (Luzon
and Lanyu) indicating that the same Mesozoic substratum probably lies underneath the entire volcanic arc. Regarding the origin
of the Huatung Basin situated in the backside of the arc, many
interpretations have been given including an Eocene part of the
West Philippine Basin (Hilde and Lee, 1984), an Early Cretaceous
⁄
The term ‘‘back-arc’’ here refers to the Huatung Basin which is today located back
of the Luzon arc. It does not mean that the basin opened in a ‘‘back-arc’’ position as
the result of a trench rollback.
Fig. 5. Map of the Luzon arc (dark triangles = active volcanoes) with location of
seismic profiles discussed in the text. S.O.T. = Southern Okinawa Trough;
SRA = Southern Ryukyu Arc; PSP = Philippine Sea Plate; and EP = Eurasian Plate.
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
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.
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. Three
points merit attention along this section. First, the rise of high
velocities (HVZ) beneath the Longitudinal Valley (noted ‘‘suture’’
on Fig. 6) and the deepening of the Moho between the orogen
and the arc support the ‘‘forearc subduction’’ model of Chemenda
et al. (1997). Second, as in the IBM arc, a thick (10 km) sequence
with ‘‘middle-crust’’ velocities is observed right beneath the
extinct Lutao volcano (see MC on Fig. 6). Third, well-resolved low
velocities (LVZ) are observed in the Huatung Basin, presumably
within the uppermost mantle, down to 25 km (Fig. 6; Kuo-Chen
et al., 2012).
3.3. Deformation mode in the Luzon arc and ‘‘proximal back-arc ’’
In terms of deformation mode, the situation differs from the
IBM 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 with
respect to the PSP and more specifically with respect to the northern Luzon volcanic arc including the Coastal Range of Taiwan,
north of 22°N (see Fig. 1). On another hand, the ‘‘indented’’ northern Luzon arc, acts itself as an indenter north of 23°N, with respect
to the EP under which it subducts. Onshore and offshore geological
and geophysical records and observations east of Taiwan show that
the arc and ‘‘back-arc’’ strain is dominated by the collision with the
Chinese platform.
3.3.1. Effects of the collision between the Luzon arc and the Chinese
platform
The main feature observed south of Taiwan is the underthrusting of the whole forearc basement beneath the arc within less than
200 km along strike (Fig. 6; Chemenda et al., 1997; Malavieille
et al., 2002; Shyu et al., 2011). The width of the forearc basement
does not exceed 60 km at the latitude of the Batan islands south
of Taiwan so that, if we consider that the forearc block was fully
locked with the subducting plate (extreme case) and a splay (out
of sequence) fault accommodated its underthrusting, about
1 m.y. is enough for the forearc to totally disappear from the
surface. If the convergence partitioned equally between frontal
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.
subduction and the splay fault, then 2 m.y. are needed for full subduction of the forearc block which is still plausible with respect to
convergence rates. The forearc block, about 45 km long and
25 km thick, is well identified on the velocity profile at the latitude of Lutao (Fig. 6). It appears tilted clockwise as it underthrusts
the arc. The narrow Coastal Range is mostly made of forearc and
intra-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). Based
on the Moho depth extracted from two different 3D-tomographic
models, Kuo-Chen et al. (2012) and Theunissen et al. (2012) have
shown that the root axis of the volcanic arc aligns about
10 ± 5 km offshore the east coast. They also noticed that the Coastal Range was underlain by a narrow (10 km wide) high-velocity
zone (HVZ) that could be interpreted as relics of the forearc basement squeezed in the suture zone (Figs. 6 and 7a). Moreover, there
are seismic evidences that the arc itself is decoupled from the PSP
all along the Coastal Range along west-dipping thrusts (Lallemand
et al., 1999; Malavieille et al., 2002). Both tomography and seismicity confirm the previous hypothesis of Chemenda et al. (1997) that
incipient subduction of the PSP occurs off the east coast of Taiwan
north of 23°300 N. Despite the fact that part of the arc overthrusts
the Taiwan orogen along the Longitudinal Valley separating the
Coastal from the Central ranges, the arc is shortened and underthrust beneath the Central Range north of the Coastal Range
(Fig. 7a; Lallemand et al., 2013). Upper crustal slices may ultimately be accreted to EP, thanks to decoupling levels allowing
the Huatung Basin lithosphere to subduct beneath the arc. Only
relics of the forearc are detected between the orogen and the arc
up to the section illustrated on Fig. 7a (see HVZ), suggesting that
the main part of the forearc has been subducted to larger depths.
3.3.2. Effects of the collision between the Luzon arc and the South
Ryukyu margin
Active and passive seismic experiments were conducted in
2008 and 2009 (RATS-TAIGER project) with a dense OBS network
in the southern Ryukyu forearc area where the arc-continent
collision culminates as attested by the seismic deformation (Klingelhoefer et al., 2012; Theunissen et al., 2012). The new 3-D velocity model, as well as the improved resolution of earthquakes
distribution, allowed Lallemand et al. (2013) to revisit the PSP
deformation in its subducted part below the Southern Ryukyu
arc (Fig. 7b). The two orthogonal velocity profiles shown on
Fig. 7 (see location on Fig. 5) were interpreted using constraints
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
7
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.
from adjacent lines, seismicity distribution, focal mechanisms of
earthquakes, upper plate deformation and any available data in
the survey area. Thrusting mainly develops along NNE-SSW faults
as a result of the collision between the arc and the Taiwan orogen
even in the subducted part of the PSP (Fig. 7a). The thrusting is
likely responsible for local thickening of the oceanic crust. The
short wavelength of the oceanic slivers indicates a shortening
mainly confined within crustal levels rather than lithospheric
buckling. In a direction normal to the Ryukyu subduction zone
(Fig. 7b), no compressional feature is observed within the subducting plate. On the contrary, the slab appears down-faulted along a
major shear zone which has been interpreted by Lallemand et al.
(2013) as an incipient sinistral tear accommodating the differential
stress between E–W compression in the south and free subduction
in the north (Lallemand et al., 1997). As observed by Kuo-Chen
et al. (2012) and Klingelhoefer et al. (2012), velocities are rather
low in the mantle beneath the Huatung Basin crust (see low velocity zones (LVZ) in Fig. 7). These low velocities were attributed by
Klingelhoefer et al. to mantle serpentinization. On the contrary,
some ‘‘relatively’’ high velocity regions were mapped in the basement of the Southern Ryukyu arc right above the ramp in the subducting PSP (Fig. 7b). These velocities are similar to those observed
below the Coastal Range and Longitudinal Valley. It may thus represent relics of the subducting Luzon arc or forearc basement
squeezed or accreted in the collision zone. This would be the only
expression of a ‘‘collision’’ between the subducting Luzon arc and
the Southern Ryukyu arc.
4. Comparison of the deformation styles and possible analogies
4.1. Crustal distinctive characteristics and rheology of an island arc
and its proximal ‘‘back-arc’’
We have shown that based on seismic velocities, both arc and
proximal ‘‘back-arc’’ (called ‘‘rear-arc’’ by Japanese authors) crusts
show seismic and petrological characteristics contrasting from a
regular oceanic crust. Thick sections characterized by intermediate velocities between 5.7 and 6.8 km/s are interpreted as the
presence of middle crust in the IBM arc and rear-arc by analogy
with the rocks exposed in the Izu Peninsula (Tani et al., 2011).
Burg et al. (2005) have described in detail the processes of shear
strain localization from uppermost mantle to middle-crust levels
in the exceptional outcrops of the fossil Kohistan island arc in
Pakistan. They observed that the plutonic lower crust of the arc
is strongly affected by sub-horizontal, syn-magmatic shear zones,
probably consistent with the bulk flow direction of the paleo-subduction zone. Whatever the origin of this middle crust is in the
IBM arc: continental or differentiated from underplated gabbros,
the rocks should be weak and prone to shear. One may notice that
the same seismic structure in the IBM arc and rear-arc lead to different levels of decoupling. It either occurs within the weak middle crust in the arc probably because it is heated by the active
volcanic activity, or below the Moho in the rear-arc. In the last
case, the lithosphere is certainly colder because volcanism is no
more active but the uppermost mantle might have been
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S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
serpentinized by dewatering from the subducting Pacific slab.
Rocks of the middle and lower crust of the Luzon arc do not outcrop even though the crust exhibits similar seismic characteristics
to that of the IBM arc. Along the velocity section shown in Fig. 7a,
the Taiwan orogen is backthrust onto the Luzon arc but the arc
itself decouples from the adjacent Huatung Basin and could still
potentially be accreted to the orogen. The proximal Huatung Basin oceanic crust, although much older than the Shikoku Basin,
shows anomalous low velocities (LVZ) down to a depth of about
25 km (Fig. 6) or even 50 km (Fig. 7). Three non-exclusive explanations can been proposed:
– Since the Huatung Basin is sliced by numerous north–south
trending fracture zones (Deschamps et al., 1998) and east–west
trending neo-formed normal faults caused by plate bending
before subduction (e.g., Schnürle et al., 1998), seawater can
penetrate to great depths and serpentinize the oceanic crust
and even the uppermost lithospheric mantle (e.g., Ranero
et al., 2003).
– The western part of the Huatung basin lies above the subducting EP slab at least south of 23°N today (see Fig. 6). Considering
the thermal state of the subducting oceanic crust, slab dehydration can occur down to 150 km depth and cause serpentinization of the top lithospheric mantle (Arcay et al., 2005).
– The crust has been thickened and rejuvenated when passing
above a plume in Eo-Oligocene time (Deschamps and Lallemand, 2003).
The first two processes produce serpentinites that favor shear
strain localization and low-angle thrusting within the crust and
the uppermost mantle if compression occurs. The last process
rather produces OIB but differentiation may also occur in case of
thick accumulation of oceanic material.
4.2. Crustal versus lithospheric deformation
Based on seismic imagery, the ICZ appears to be mainly constructed by the stacking of upper and mid-crustal slivers scraped
off the subducting IBM arc (Fig. 4; Arai et al., 2009; Tamura et al.,
2010). First interpretations, which proposed that thrusting along
the Zenisu Ridge might cut through the entire lithosphere, should
result in successive seaward jumps of the subduction interface
associated with duplication of the slab thickness in the subduction
zone, which is not observed. Data re-examination and new data
acquisition show that compressive features like the Zenisu Ridge
or the accreted imbricates of the ICZ only affect the crustal levels.
Decoupling occurs in the middle crust along the main ridge and beneath the Moho in the uppermost mantle in the ‘‘rear-arc’’ (Fig. 8).
Because of the special configuration of the arc-continent collision in Taiwan, i.e., the main collision involves an arc system on
the upper plate and a subducting ocean-continent transition zone,
part of the deformation occurs at a lithospheric scale (forearc subduction) and part at a crustal scale (arc and proximal ‘‘back-arc’’
crustal shortening). Two mechanical explanations can be invoked
for the subduction of the Luzon forearc:
– forearc buckling and horizontal compression caused by the subducting buoyant Chinese continental platform (Chemenda et al.,
2001; Tang et al., 2002) and/or
– increasing friction between the two converging plates caused
by the cooling of the forearc as a result of the decreasing convergence rate (Arcay et al., 2007).
The high velocities observed at depth between the arc and the
Taiwan orogen (HVZ in Figs. 6 and 7a) might attest for the presence
of cool forearc basement material (Kuo-Chen et al., 2012; Lallemand 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. For
Taiwan, only the collision between the Luzon arc and the Chinese continental platform has been sketched because the other arc collision with the overriding southern Ryukyu
arc 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 SRA
except that the vergence of the thrusts in the Huatung Basin are orthogonal to the trench. COB = Continent-ocean boundary; PBA = Proximal back-arc; AW = accretionary
wedge; CP = Chinese platform; ICZ = Izu collision zone; IBM = Izu–Bonin–Mariana arc; T.C. = Tenryu Canyon; and M = Moho.
Author's personal copy
S. Lallemand / Journal of Asian Earth Sciences 86 (2014) 1–11
by the northward decrease in subduction rate as a result of continental subduction (Arcay et al., 2007). Lithospheric mantle weakening may thus occur right above the slab dehydration domain,
i.e., in the arc and proximal ‘‘back-arc’’. One may also expect that
water impregnation of the mantle favors melting, but no volcanic
activity was mentioned in the Huatung Basin except one controversed report off the Coastal Range in 1853 (Chen and Shen,
2005). Such uppermost mantle weakening combined with crustal
hydrothermalism and eventually the presence of ‘‘middle crust’’
in the arc favor the development of crustal and uppermost mantle
shortening whereas the lower lithospheric PSP mantle subducts
beneath the Taiwan orogen.
4.3. Arc (and ‘‘back-arc’’) accretion in the collision zone
Arc and ‘‘rear-arc’’ crustal accretion has been demonstrated by
many 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 itself
and may involve the uppermost lithospheric mantle in the ‘‘reararc’’ domain. Present observations in the Nankai margin do show
deformation in the overriding wedge like ridges controlled by
out-of-sequence thrusts (Lallemand et al., 1992; Lallemant et al.,
1995; Dessa et al., 2004) or the presence of a transfer zone, which
surface expression is the Tenryu Canyon in the prolongation of the
Akaishi Tectonic Line, accommodating the differential motion between the normal subduction to the west and shortening above
the rear-arc domain (Le Pichon et al., 1987a, 1996, see T.C. in
Fig. 8). Despite no direct observations, Shikoku Basin crustal accretion beneath the accretionary wedge, thanks to a decoupling level
located in the uppermost mantle, is suspected (Fig. 8).
East of northern Taiwan, the slivering of the crustal part of the
arc and proximal ‘‘back-arc’’ is reflected in the shallower bathymetry of the Southern Ryukyu forearc basement which is uplifted
with respect of the rest of the forearc (Font et al., 2001; Lallemand
et al., 2013). As a consequence, the crust is probably decoupled
from the PSP lithospheric mantle. A décollement level in the uppermost mantle of the Huatung Basin is required to accommodate the
shortening of the crust and the westward subduction of the lower
part of the Huatung lithosphere (see LVZ in Figs. 7 and 8). The crustal wedge is underthrusted beneath the Central Range north of
24°N (Fig. 7a, Lallemand et al., 2013) and could potentially be accreted to the orogen. With respect to the northward subduction
beneath the Southern Ryukyu arc, rather than massive accretion
of the crustal wedge, little evidences of arc or forearc material were
found in the deeper parts of the SRA basement based on seismic
velocities (HVZ in Fig. 7b).
5. Conclusions
The comparative examination of the deformation modes within
two colliding island arcs of the PSP lead us to a series of observations that illustrate original mechanical processes of arc-continent
interaction in collision zones.
The Luzon and IBM arcs were both emplaced on the same Late
Jurassic–Early Cretaceous ‘‘proto-PSP’’ crust.
The studied island arcs have distinctive petrological and rheological characteristics that facilitate strain localization.
Forearc lithospheric subduction is favored when a continent
subducts beneath a volcanic arc because of the combination of
increasing compression due to continental crust buoyancy and
increasing friction due to forearc cooling as a result of subduction slowing.
The oceanic crust and uppermost mantle adjacent with the arc
on the back-arc side is still under the influence of the
9
subducting slab as attested either by ancient magmatism
(northern IBM rear-arc) or by a higher hydration (Huatung
Basin). Slab hydration is amplified if subduction slows and slab
dip increases.
Accretion of arc material onto the upper plate is not systematic.
It strongly depends on the characteristics of the arc itself and
the geodynamic configuration.
When accretion occurs, like in the Izu collision zone, only upper
crustal levels are involved. The main part of the lithosphere still
subducts. In Taiwan, similar conclusions may be reached except
that no surface outcrop attests yet for arc crustal accretion. Only
seismic evidences might indicate that arc and/or forearc material may be underplated beneath the upper plate.
In both cases, decoupling levels allow the delamination
between upper and lower lithospheric levels. Such decoupling
levels can be either facilitated by the presence of ‘‘weak and
hot middle crust’’ or serpentinized uppermost mantle.
Acknowledgements
I acknowledge Michel Faure and Yan Chen who invited me to
present a keynote at the International Conference ‘‘Tectonics of
Asia’’ hold in Orléans in November 2012 in honour of Jacques Charvet, with whom I shared memorable discussions about the IBM
subduction in Japan at the end of the eighties and early nineties.
It was a unique opportunity for me to reconcile some early work
in Japan with present studies in Taiwan. Anne Delplanque has done
a tremendous work in drawing most of the illustrations. I have
tested some suggested processes of lithospheric weakening
through ‘‘recurrent’’ discussions with Diane Arcay. I also deeply
thank Ken Tani, Jin-Oh Park, Hao Kuo-Chen, Stéphane Mazzotti,
Stéphane Dominguez, Y. Font, Thomas Theunissen, Jacques Malavieille, René Maury, Hervé Bellon, Jean-Louis Bodinier, Fleurice Parat,
Adolphe Nicolas, Françoise Boudier and Fred Gueydan for the scientific discussions or help. The revised manuscript has benefited
from two anomymous reviews.
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