Tectonic processes, from rifting to collision via subduction, in SE Asia and the western Pacific: A key to understanding the architecture of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (or Altaids, 600–250 Ma) is a ca. 1.0 Ga–250 Ma tectonic collage of oceanic and continental fragments that includes ophiolites, island arcs, Andean-type magmatic arcs, subduction-accretion complexes, passive margins, seamounts, and microcontinents (Şengör and Natal’in, 1996; Jahn, 2004; Kröner et al., 2007; Windley, 2007; Wilhem et al., 2012). It also contains high-pressure (HP) blueschists and eclogites, high-temperature (HT) metamorphic rocks, postaccretionary granitic-alkaline-peralkaline rocks, and important mineral deposits of, for example, Au and porphyry Cu. The Central Asian Orogenic Belt (COAB) is widely regarded to have evolved in a tectonic setting like that of modern Indonesia (e.g., Xiao et al., 2010); however, because of the late HT overprint and the old age of the Central Asian Orogenic Belt’s construction, the original paleogeographic architecture and tectonic evolution are still poorly known.

SE Asia and the western Pacific are no doubt the best world laboratory for understanding the geological and tectonic processes that take place in the oceans and during accretion (Fig. 1) before any continent-continent collision has occurred (Lallemand et al., 2001; Hall, 2002, 2010; Pubellier et al., 2005; Pubellier, 2008; Metcalfe, 2010; Yin, 2010). In order to help unravel the complex accretionary framework and evolution of the Central Asian Orogenic Belt, we review the modern analogue of the Phanerozoic geology of SE Asia and the western Pacific (Fig. 2). This paper relates the main tectonic processes that have taken place in the western Pacific through Phanerozoic time, from ocean formation and arc creation to subduction-accretion and the formation of suture zones, with those that probably took place in the Central Asian Orogenic Belt during the early Paleozoic. Hence, the examples selected hereafter are located between the Central Asian Orogenic Belt and the present-day plate boundary (Sunda Trench and India-Eurasia collision zone).

Microcontinents, detached from Gondwana, moved northward to collide with the margins of the Asian continent, which became larger with time (Metcalfe, 2010). The early history of the breakup of Gondwana in the Devonian and the start of the Paleo-Tethys Ocean are recorded, for example, in Chiang Dao, north Thailand (Hara et al., 2010), where anoxic sediments were deposited in confined basins along the Gondwana margin (Fig. 3). Once the continental fragments detached and drifted away from Gondwana, oxidized seawater flowed into the basins, enabling growth of small organisms such as plankton, radiolarian, and pelagic foraminifera.

The rifting of an ocean is traditionally considered to be triggered by the subduction of an existing ocean. For example, Neo-Tethys opened in response to the subduction of Paleo-Tethys (Şengör, 1987). In modern examples, the trench pull is responsible for the formation of back-arc basins floored with a large oceanic crust in the upper plate (Karig and Sharman, 1975), or in the downgoing plate as observed for the South China Sea (Bénard et al., 1990; Pubellier et al., 2003). According to Stampfli (2000) and Wang et al. (2010), the opening of Paleo-Tethys took place in the Late Ordovician and Silurian. On the other hand, Metcalfe (2010) suggested that several Chinese continental blocks, including Indochina, separated in the Late Devonian from Gondwana. The history of the Paleo-Tethys Ocean from birth to death is recognizable in the pelagic and hemiplegic sequences of ocean plate stratigraphy, which record the ridge-trench transition from the Devonian to the Triassic (Wakita and Metcalfe, 2005). The Cimmerian continent including the Sibumasu block collided with Indochina in Triassic time. However, the age of opening of Paleo-Tethys was not clear until Wonganan and Caridroit (2005) revealed the presence of a continuous ocean-floor sequence from the Lower Devonian to Lower Carboniferous in northern Thailand.

The Thailand sequence is composed of Lower Devonian black shale and younger cherts (Fig. 3). The black shale contains Early Devonian graptolites. In ascending order, the sequence consists of thinly bedded black shale, thinly bedded black siliceous shale, interbeds of black siliceous shale and white chert, and bedded white chert; the overall change from black shale to chert is gradual. The black shales decrease in thickness toward the stratigraphic top, as do the numbers of interbeds of black shale in bedded chert. This lithologic change from black shale to white bedded chert represents the biological recovery from anoxic to oxic conditions in the opening basin. Paleo-Tethys started to open, not in the Late Devonian, but in the Early Devonian. The sequence from northern Thailand to Peninsular Malaysia records the early stage of opening of Paleo-Tethys (Fig. 3), and the Lower Devonian–Late Triassic stratigraphy represents the history of Paleo-Tethys from its opening to closure.

Ocean plate stratigraphy records the travel history of an oceanic plate from its “birth” at a mid-oceanic ridge to its “death” at a trench (Matsuda and Isozaki, 1991; Wakita and Metcalfe, 2005). Ocean plate stratigraphy is an idealized stratigraphic succession of an ocean-floor trench, obtained from protoliths in ancient accretionary complexes. However, in some cases in SE Asia, the generally dismembered ophiolites and their overlying stratigraphic oceanic series may represent relicts of back-arc basins as demonstrated by their geochemical signature (Pubellier et al., 2004). In the Paleozoic, it is usually reconstructed by means of biostratigraphic relations of radiolaria, conodonts, and fusulinid microfossils. Radiolaria are the most useful because they occur throughout the Phanerozoic record and in various lithologies from argillaceous to siliceous and calcareous. Ocean plate stratigraphy commonly is composed of an upward tectonic succession of pillow basalt, limestone, chert, siliceous shale, and detrital turbidite (Fig. 4). Late Paleozoic basalt of a Jurassic accretionary complex in Japan was considered as normal mid-ocean-ridge basalt (N-MORB; Sano and Tazaki, 1989; Sano et al., 2000) or enriched (E) MORB (Jones et al., 1993). Recently, Ichiyama and Ishiwatari (2005) and Ichiyama et al. (2006, 2008) indicated that the basalt has high-μ basalt (HIMU)–like and oceanic plateau–like geochemical signatures. On the other hand, Triassic to Jurassic basalts in Jurassic accretionary complexes in Japan are alkali basalts of oceanic island basalt (OIB) type (Hattori and Yoshimura, 1982; Wakita, 1984).

The age range of chert within an ocean plate stratigraphy sequence documents the age, duration, and history of the ocean floor from ridge to the trench (Osozawa, 1994; Xenophontos and Osozawa, 2004). Volcanic islands, created near shallow oceanic ridges just after the oceanic plate was born, may be covered by reef limestone following subsidence below sea level as the plate spreads away from the ridge; this produces basalts overlain by reef limestones in the lower part of the succession (Sano and Kanmera, 1988). Siliceous mud overlying radiolarian cherts is deposited in the hemipelagic region just before the oceanic plate arrives at the trench. The oceanic plate, covered by radiolarian chert and siliceous mud, is then covered by trench-fill clastic sediments on its arrival in the trench. However, the complete oceanic crustal section is rarely preserved in accretionary wedges, and thus in accretionary orogens, because the ultramafic rocks, gabbros, sheeted dikes, and most of the basalts are typically subducted, and only the upper basalts and the sediments are accreted (Kimura and Ludden, 1995). Moreover, the upper parts of the ocean plate stratigraphy are often scraped off at the toe of the accretionary wedge and stacked tectonically into the accretionary wedge; thus, the lower parts of the ocean plate stratigraphy are accreted into the accretionary wedge. Also, when seamounts arrive at a subduction zone, their upper volcanic rocks and sediments are commonly scraped off along deep-seated décollements. Many accretionary complexes in orogenic belts are composed of tectonically disrupted units of ocean plate stratigraphy, and their metamorphosed products. However, in many cases, relicts of small continental ribbons (typically 50 × 250 km in the case of the Sulu Ridge, the NW arm of Sulawesi, the Sula block, and the South Java block) are suspected and have been partly subducted, like in the Molucca Sea or Sumatra. Therefore, the ocean plate stratigraphy has to integrate small basins opened within the upper plate instead of a single subducted ocean.

The accretionary complexes of Japan illustrate what happened during the long-lasting subduction-accretionary processes in the past. The first evidence of accretion in Japan was at ca. 500 Ma (Isozaki et al., 2010). Accretionary complexes that developed in the Permian, Jurassic, Cretaceous to Paleogene, and Neogene have a common structure that is younger oceanward and/or structurally downward. Detailed biostratigraphy and structural analyses have demonstrated that this structure was created by the tectonic stacking of ocean plate stratigraphy. Each packet of ocean plate stratigraphy consists of an upward lithologic sequence: pillow basalt, limestone, radiolarian chert, siliceous shale, and turbidite. The approximate thickness of each rock unit in the Jurassic accretionary complexes of Japan is 500 m for basalt, 500 m for limestone, 100 m for chert, 100 m for siliceous shale, and several hundreds of meters for turbidite. The age of the basal basalts and lowermost limestones and cherts indicates the approximate time of accretion at a mid-oceanic ridge, and the age of the uppermost clastic sediments (i.e., sandstone) indicates the time of arrival of the ocean plate stratigraphy in the trench and thus the age of accretion. The age ranges of accretion indicated by Japanese trench sediments are ca. 100 Ma (Late Triassic to Early Cretaceous) in Jurassic accretionary complexes, and ca. 70 Ma for Cretaceous accretionary complexes (Wakita, 1988). Radiolarian chert of the Jurassic accretionary complexes of Japan ranges in age from Middle Permian to Early Jurassic and indicates a long travel time of the oceanic plate in a pelagic environment of more than 100 m.y. (Wakita, 1988). Thus, the age span of the ocean plate stratigraphy provides critical evidence of the history and time span of the relevant ocean. The imbricated units of ocean plate stratigraphy form detached slices of oceanic crust in the trench, which are added to the base of a growing accretionary wedge, where they may be further dismembered by, for example, strike-slip transpressional deformation (Wakita, 1988, 2000a; Şengör and Natal’in, 1996; Isozaki, 1997).

The accreted sediments are normally detached along a décollement that develops on rheologically weak, stratigraphic boundaries. Disruption and mixing occur first along the décollement and next along successively developed out-of-sequence thrusts. The sediments in the trench are saturated with cold water with an excess pore-fluid pressure that reduces friction, with the result that they slide along thrusts on bedding planes, and in consequence are hardly recognizable in the field. Mélanges form by several processes, such as debris flows on steep trench walls, in imbricated duplexes, and in sedimentary diapirs that interact with synsedimentary thrusts (Wakabayashi and Dilek, 2011). Similar accretionary complexes are widespread along the W/SW Pacific margins, e.g., the Philippines (Tsumanda, 1994; Zamoras and Matsuoka, 2001), and eastern Indonesia (Wakita et al., 1994a, 1994b, 1996, 1998; Wakita, 2000b). These complexes are more than 500 km long and 200 km wide, although they have been divided into several tectonic blocks by later tectonic events. Each ocean plate stratigraphic section of these complexes is several hundred meters in thickness, which is the same size as the Japanese example.

Seamounts or crustal fragments, up to hundreds of kilometers long, may be subducted without changing the plate convergence, so creating a subduction jump (Cadet et al., 1985; Ranero et al., 1997), but they may cause a break in the evolution of the wedge. Sandbox analogs (Dominguez et al., 2000) have shown how a wedge becomes the backstop of a new wedge. In nature, this is illustrated by an unconformity sealing low-angle faults, a good example being on Nias Island in offshore Sumatra, where a Neogene wedge was deformed mostly prior to the late Miocene, meaning before the present-day accretionary wedge of the Sunda Trench (Pubellier et al., 1992), without any trace of the subducting asperity.

Accretionary wedges do not remain in a steady state for long. They are easily destabilized by a variety of factors, such as material cohesion, subduction angle, velocity of the subduction, and basal friction. Commonly, the roughness (rugosity) of the downgoing plate is responsible for formation of unconformities and breaks in the development of the wedge. However, although a volcano can create a bulge in a wedge and a re-entrant of the trench, it stills goes into the subduction zone and does not create the unconformities and breaks (Ranero et al., 1997; Ranero and Reston, 1999). On the other hand, a major topographic morphostructure such as a crustal rift or a large intra-oceanic island arc may be responsible for moderate deformation in the accreting wedge, but such deformation is usually short-lived and lasts only for the duration of a subduction jump (Pubellier et al., 2003).

Along a trench, an ocean plate subducts beneath a continental margin or an island arc (Karig and Sharman, 1975). Depending on geodynamic and rheological parameters (e.g., convergence velocity and obliquity, plate rugosity, subduction angle, etc.), oceanic plate subduction causes two major types of tectonic regime: accretion and erosion, which also depend on sediment supply, among other parameters (von Huene and Scholl, 1991). If sediment supply is sufficient and the ocean plate retreats oceanward, sediment accretion occurs in the trench. If sediment supply is poor and the continental margin shifts toward the continent, tectonic erosion may occur along the trench. Scholl and von Huene (2007) showed the distribution of sediment accretion and tectonic erosion along the trenches of the circum-Pacific. However, in the western Pacific, the margins of the South China and Sunda plates underwent significant extension in the Tertiary, resulting in the opening of deep rift basins floored with oceanic crust. The opening of these basins has extended the margin considerably toward the south and the southeast. Interestingly, it was the same subduction zone that later was responsible for closure of the basins and created margin shortening and mountain building. Recently, an event of nonaccretion without tectonic erosion was also reported in the Franciscan complex (Dumitru et al., 2010; Wakabayashi, 2011, 2012). In the Franciscan complex, Grove et al. (2008) explained the removal of a significant (>100 km) width of the forearc region by subduction erosion. Yanai et al. (2010) estimated the amount of tectonic erosion of the forearc region of NE Japan as ∼14 million km³, which is much more than the estimation of von Huene and Scholl (1991). On the other hand, nonaccretion without tectonic erosion was also proposed in the Franciscan complex based on the age gap between the initiation of subduction estimated by metamorphic rocks and the oldest accreted sediments (Dumitru et al., 2010; Wakabayashi, 2011, 2012).

The accretion of sediments does not always lead to growth of the upper plate, because the basal part of that plate may be removed by tectonic erosion (von Huene and Lallemand, 1990; Lallemand et al., 2001; von Huene et al., 2004). The Japanese Islands were considered to have grown by continuous accretion along the Asian continental margin during the Phanerozoic. However, it is now known that whole arcs and accretionary complexes have been removed by tectonic erosion up to six times since 520 Ma (Isozaki et al., 2010; Suzuki et al., 2010). The most remarkable example of tectonic erosion is in NE Japan, where the Japan Trench cuts the trend of ancient geologic structures such as Jurassic accretionary complexes and Cretaceous volcanic fronts. Also, in Sumatra, the present-day volcanic arc lies anomalously on top of a former back-arc area (Lallemand et al., 2001), and on Sumba, Eocene–Oligocene volcanic occurrences were transferred northward onto the island of Flores in the late Miocene.

Strike-slip truncation, proposed by Karig (1980) as an alternative to subduction erosion, has probably also occurred along many paleo-convergent margins. This is a possible mechanism to remove forearc and accretionary complexes in orogenic belts, particularly in orogens that include a history of ridge subduction. The collapse of the Philippine arc in Taiwan is a much debated example. Some ridge subduction events may have a “California-style” signature (Atwater, 1970), wherein subduction of the ridge results in a transition to a transform plate boundary, because the seaward side of the subducted ridge has a motion parallel/subparallel to the plate boundary. Shear partitioning along subduction zones has been widely documented, particularly in SE Asia (Fitch, 1972; Jarrard, 1986; McCaffrey, 1996). It is illustrated by transcurrent faults exceeding 1000 km, such as the Sumatra fault (2 cm/yr), the Sagaing fault in Myanmar (1.8 cm/yr), and the Philippine fault (3.8 cm/yr).

If ocean plate stratigraphy does record the transition of an ocean plate from a ridge to a trench, then sooner or later a mid-oceanic ridge will reach a trench and be subducted on one side of the ocean basin. Evidence of ridge subduction was first discovered in Japan (e.g., Osozawa, 1992; Kiminami et al., 1994) from the decrease in age of offscraped oceanic crust toward the age of the ridge basalt, and its role in producing an orogeny was presented by Osozawa (1998). In Japan, biostratigraphic data indicate that ridge subduction has taken place every 100 m.y. during the past 450 m.y. The effect of putting a hot ridge under an accreted complex is to provide extra heat that helps to partially melt the lower crust, producing HT crustal melt granites, peralkaline granites, and sanukitoids (Maruyama, 1997). It also leads to sharp juxtaposition of low-grade against high-grade metamorphic rocks (Iwamori, 2000), such as in the Hidaka magmatic zone in Hokkaido (Maeda and Kagami, 1996), to emplacement of distinctive Alaska-type zoned mafic-ultramafic complexes, to suprasubduction ophiolites in ridge-affected forearcs (as opposed to back arcs), to progressively migrating igneous activity (Kinoshita, 1995), and to orogenic gold deposits that probably form above a slab window created by a subducting ridge (Sisson et al., 2003). This HT metamorphism occurred at 56–50 Ma (Owada et al., 1991, 1997) and created a belt of HT metamorphic rocks that is 150 km long and 20 km wide in central Hokkaido. All these effects can be looked for in old ridge-affected accretionary orogens like the Central Asian Orogenic Belt (Windley, 2007). However, ridge subduction is not the only process that can increase the temperature. The subduction of a crustal morphostructure associated with a simple subduction jump may either lead to slab breakoff (Nur and Ben-Avraham, 1982) or mantle delamination (Pubellier and Meresse, 2013).

Blueschists and eclogites provide evidence of subduction to HP depths in subduction zones and exhumation in extruded wedges or channels, and they are distinctive components of subduction-accretion complexes. The youngest western Pacific blueschists occur in the active Mariana forearc, having been brought up in serpentine mud volcanoes of Pleistocene age from a serpentinite seamount onto the ocean floor (Maekawa et al., 1993). Older blueschists in East Asia and the western Pacific occur in Sakhalin, Russia; Hokkaido, Kyushu, Honshu, and Shikoku in Japan; Taiwan; the Philippines; Java, Kalimantan, Sulawesi, Timor, and Tanimbar in Indonesia; and New Caledonia (Maruyama et al., 1996). Blueschists were exhumed at 5.4 Ma (Berry and McDougall, 1986) following subduction of the Australian continent beneath the East Indonesian Islands (Kaneko et al., 2007), and the Cretaceous exhumation of blueschists and eclogites (113–132 Ma) in Sulawesi, Indonesia, was caused by the buoyancy of microcontinents (Parkinson, 1998; Wakita, 2000b), or stacking of slices beneath a décollement (Konstantinovskaya and Malavieille, 2011). The metamorphic belt in Timor is ∼300 km long, while the Cretaceous metamorphic belt of central and southwest Sulawesi is ∼800 km long.

The Sanbagawa metamorphic belt in Japan is divisible into two, i.e., Sambagawa metamorphic belt of 110–120 Ma, and Shimanto metamorphic belt of 60–70 Ma (Aoki et al., 2008). Both belts extend for 800 km from Okinawa to the Kanto region. The Sanbagawa thrust sheet in Japan contains retrogressed eclogites in amphibolites, which were derived from a subducted oceanic plateau (Okamoto et al., 2004). When the extruded wedge reaches a high crustal level, it bends to become a subhorizontal tectonic sheet with a thrust on its lower boundary and an extensional fault on its upper boundary, as in the HP Sanbagawa belt (Okamoto et al., 2000). Apparent cross-sectional extrusion geometry, i.e., highest-pressure rocks structurally interleaved between lower-pressure units, is recognized in many western Pacific and SE Asian HP belts, as documented by Maruyama et al. (1996), who also pointed out that similar structures with HP metamorphic rocks occur in various localities of the world (reaffirmed by Agard et al., 2009). The HP rocks in SE Asia are mostly pre-Tertiary and are exposed over 5000 km² in central Sulawesi, and they may be correlated with HP rocks in central Borneo and the north arm of Sulawesi (Parkinson et al., 2004), but the geodynamic framework of the emplacement of these rocks is unclear and predates the intense Neogene tectonics. The Mutis complex of Seram is only a few kilometers in size, but the larger Papua New Guinea (PNG) ophiolite, underlain by HT metamorphic rocks is tectonically in contact with the HP Emo metamorphic rocks, and overlies greenschist facies rocks (Lus et al., 2004). Although, the PNG ophiolite is one of the best examples of HT on HP rocks in the West Pacific, its mode of emplacement is much debated; derivation from the Pacific Ocean or from a Mesozoic back-arc basin (Monnier et al., 2000).

Subduction jumps and reversals have been long described as a way to accrete slivers of crust or volcanic belts (several tens of kilometers by several hundreds of kilometers), and continue the subduction process, so that only the location of the megathrust actually changes. The time frame for the process of subduction blocking and shift of the megathrust is very short of the order of one to two million years as shown in the Philippines or Western Papua. The Sumba-Timor Islands (Fig. 5) illustrate the beginning of wedge shortening (Pubellier and Meresse, 2013). The northern margin of the Australian continent is being subducted beneath the Sunda arc. The Australian continent, being buoyant, moved upward after the subducted ocean plate was detached from the slab of the Australian continent. Following the arrival of the NW Australian continental margin at the trench, a collision zone developed and underwent a short period of intense shortening via thrusting, giving rise to an imbricated pile of sediments in a fold-and-thrust belt duplex on the Australian margin. The resulting accretionary wedge has incorporated fragments of the subduction backstop and back-thrusted sediments of the Australian shelf (e.g., Harris et al., 1998). In Sumba Island (Fleury et al., 2009), the shortening began only 1 m.y. ago, when a distal rifted block of the Australian margin entered the subduction zone. The consequence was doming of the forearc basin and an intense mass wasting at the receding side of the asperity.

Taiwan is another example of arc-continent collision in East Asia (Huang et al., 2000; Lallemand et al., 2001). The collision occurred in Taiwan between the Eurasian continent and the Luzon volcanic arc a few millions years ago and is still active (Lin, 2002). Near Taiwan, the Philippine Sea plate is subducting beneath the Eurasian continent along the Ryukyu Trench, while the Eurasian plate is subducting beneath the Philippine Sea plate along the Manila Trench. Taiwan is an active orogenic belt where a preexisting accretionary complex was deformed and metamorphosed under HP conditions.

Along the western margin of the Philippine arc, the South China Sea is being subducted beneath the arc, and the Palawan block is colliding with the Philippine arc (Zamoras et al., 2008; Yumul et al., 2003, 2009). The Palawan block is composed of Mesozoic accretionary complexes and a Cretaceous ophiolite. The collision has caused a volcanic arc gap in the Philippine arc and ophiolite emplacement in both the forearc and back arc (Yumul et al., 2009). The Palawan block, derived from the Asian continental margin, detached from mainland China during opening of the South China Sea. This is a collision zone between a volcanic arc and a detached continental margin, rather than a separate microcontinent.

In Japan, many arc-continent collisions are well documented (Isozaki et al., 2010). The collision between the Chishima Arc and NE Japan occurred in Hokkaido in Miocene time when North Japan was the eastern margin of the Eurasian continent (Komatsu et al., 1994). Delamination split the Chishima arc into two parts. The upper part was thrusted over the Honshu arc, and high-grade and HT metamorphic rocks from the deep arc crust were exhumed to the surface in the Hidaka belt, which today has high topography created by the arc-arc collision.

The Izu-Bonin arc collided into the Honshu arc and created the major curvature of the latter seen today (Ogawa et al., 1985; Arai et al., 2009). During the formation of the Philippine Sea plate, the Izu-Bonin arc and the Kyushu Palau Ridge split apart as a result of back-arc opening. First, the Izu-Bonin arc shifted from west to east, and then it collided with the Honshu arc, which was deformed by folding and thrusting. The collision zone was uplifted and eroded to supply sediments into the trench, which are a major component of the Holocene accretionary wedge of the Nankai Trough.

Many marginal basins formed and are still open in Indonesia, but in the 50 m.y. history of Japan, there have been only a few marginal basins of minor importance (Maruyama, 1997). When an upper plate enters a compressional regime, shortening is best accommodated by subduction of a former back-arc basin. One of the best-documented back-arc basins is the Eocene Celebes Sea (Silver and Rangin, 1990), which was originally square shaped, although almost half of it has been subducted beneath its southern margin (Fig. 6). The northern arm of Sulawesi Island (Fig. 6) has rotated clockwise since ca. 8 Ma (Otofuji et al., 1981) and is still moving at a rate approaching 4.5 cm/yr along the Palu Koro fault (Walpersdorf et al., 1998).

Accretionary wedges are well known for their current seismic activity, which usually involves subduction of the early deep-sea sediments from the former oceanic plate. As a continental margin enters the subduction zone, increasing amounts of proximal sediments and even continental shelf sediments are imbricated in the accretionary wedge. Finally, in the late stages of basin closure, crustal rocks may also be stacked into the wedge, thus exhuming metamorphosed basement rocks of the downgoing plate, and metamorphic sediments from the deep accretionary wedge. When highly metamorphosed, these sediments, which represent the “cratonized rocks” of “orogenic wedges,” may be difficult to distinguish from the old metasediments in the surrounding craton.

In the continental Sunda plate, there is a record of complete closure of the proto–South China Sea, which was a Late Cretaceous basin that had completely disappeared by mid-Miocene time; it is only represented today by a thick accretionary wedge that has been thrust over the opposite continental margin, part of which has been uplifted and currently constitutes the Rajang-Crocker Mountains of Borneo (Bénard et al., 1990). Ophiolites derived from the disappeared marginal basin crop out north of Sabah in Malaysia and on Palawan Island in the Philippines. The relicts of the proto–South China Sea do not contain any HP metamorphic rocks; if they formed, they must be still buried under the thick accretionary wedge.

The Palawan (or Luconia) microcontinent collided with Sundaland during the formation of an accretionary complex along the northwestern margin of Borneo. The opening of the proto–South China Sea caused the subduction zone along the western margin of Borneo and the Philippines to create the accretionary wedge of the Rajang-Crocker Mountains. At the same time, the Luconia microcontinent collided with Borneo (Hutchison, 2010; Metcalfe, 2010).

The microcontinents of Indonesia may have separated by back-arc opening from the Eurasia/Sunda plate, or they may have been detached from Gondwana prior to collision along the convergent margin of Sundaland. If an exotic or rifted microcontinent is forced down a subduction zone, subsequent exhumation in an extrusion channel may expose ultrahigh-pressure to HP metamorphic rocks of continental origin (Wakita, 2000b). The HP rocks are typically peraluminous in composition and may include conglomerates. A classical example of such HP rocks is in central Sulawesi. The East Sulawesi block docked against the Sunda margin in the early Miocene, and this created ophiolite obduction and HP metamorphism (Parkinson, 1998).

Lateral displacements on strike-slip faults are famous in the accretionary terranes in the North American Cordillera, where the translation of slivers within the upper plate along the active margin has been documented since Fitch (1972) and Jarrard (1986). In Indonesia, the best known examples are the Sumatra fault (McCaffrey, 1996) and the Philippine fault (Aurelio et al., 1991), which are associated with strain partitioning. In Japan, major lateral displacements took place in late Mesozoic time, along the Median tectonic line and the Tan-Lu fault. These trench-parallel displacements, which were of the order of several centimeters per year (e.g., the Philippine fault with 3.8 cm/yr), may create difficulties when trying to unravel past accretion and tectonic erosion.

The Central Asian Orogenic Belt was as wide as it was long, lasted for some 750 m.y., and evolved from the closure of the paleo–Asian Ocean (Mossakovsky et al., 1993; Jahn, 2004; Kröner et al., 2007; Wilhem et al., 2012). The framework of this huge accretionary orogen has been worked out in terms of its interpreted plate-tectonic components, such as island and Andean-type arcs, ophiolites, subduction-accretion complexes, passive margins, seamounts, oceanic islands, and microcontinents (e.g., Şengör et al., 1993; Badarch et al., 2002; Buslov et al., 2004; Windley, 2007; Wilhem et al., 2012). Specific features of the Central Asian Orogenic Belt, which have been widely reported, are: HP blueschists and eclogites (e.g., Volkova and Sklyarov, 2007; Hegner et al., 2010; Štípská et al., 2010; Qu et al., 2011), arc-continent collisions (e.g., Xiao et al., 2004, 2010), granitic magmatism and its sources (e.g., Jahn, 2004; Kröner et al., 2007), ultrahigh-temperature metamorphism (e.g., Jiang et al., 2010), sedimentary basins in different tectonic settings (e.g., Carroll et al., 1995), strike-slip displacements of accretionary belts (e.g., Şengör and Natal’in, 1996), and mineralization (e.g., Xiao et al., 2009; Wan et al., 2011). The tectonic evolution of the CAOB has been constrained against paleomagnetically determined paleo-geography of the western Pacific through the Paleozoic (Cocks and Torsvik, 2012).

There has been little or no discussion on subjects such as: sediment accretion versus tectonic erosion, structural variations in different accretionary wedges, subduction of marginal basins, or subduction jumps or flips; such debates will inevitably arise when more is known about this vast accretionary orogen. Although island arcs and microcontinents are common and typical of the many terranes in the Central Asian Orogenic Belt, we concentrate here on evidence for ocean plate stratigraphy and ridge subduction as two of the components most relevant for this review.

Recognition of ocean plate stratigraphy has had a profound influence on plate-tectonic interpretations and evolution of large parts of Central Asia. Although Şengör et al. (1993) and Zorin (1999) well interpreted the Khangai-Khentey region of central Mongolia as containing a prominent accretionary wedge (but provided no field data to support the idea), different interpretations were made by Gordienko (2006; a back-arc basin), Ruzhentsev and Mossakovsky (1996; a postorogenic successor basin on top of crystalline basement), and Badarch et al. (2002; coherent turbidites overlying cratonic basement). However, detailed field studies by Kurihara et al. (2009) revealed that extensive regions south of the capital Ulaanbaatar are occupied by repeated, imbricated ocean plate stratigraphy, which means that these sediments cannot be underlain by continental basement. Specifically, the Gorki Formation extends for more than 200 km along strike and has a present tectonic-stacking thickness of >20 km; the original stratigraphy is only retained in thin fault-bound slices several tens of meters thick. OIB-type basaltic lavas several meters thick and hyaloclastites and mafic tuffs ∼20 m thick are overlain successively by Devonian radiolarian red cherts up to 30 m thick, siliceous shales (up to 20 m), sandstone-shale turbidites (up to 20 m), and massive sandstones (up to 15 m). Kuzmichev et al. (2007) suggested that the Oka belt of southern Siberia and northern Mongolia contains similar lithological components of ocean plate stratigraphy as the Japanese Shimanto belt.

A major contribution was made by Safonova (2009), who demonstrated that in the Gorny Altai and Altai-Sayan regions of Siberia and the Dzhida region of Mongolia, OIB-type alkali basalts that originally formed in oceanic islands, seamounts, and plateaus occur at the base of all ocean plate stratigraphy units. They are variably overlain by bedded and massive limestones (carbonate reef caps), and other ocean plate stratigraphy sediments such as siliceous and calcareous mudstones, micritic limestones, lime-volcaniclastics (hemipelagic slope facies), bedded/ribbon/radiolarian cherts (pelagic facies), and terrigenous-clastic trench sediments. Individual units of ocean plate stratigraphy in different areas of the Central Asian Orogenic Belt range in age from Late Neoproterozoic to Early Silurian, but there are insufficient data to determine the age ranges of individual slices or imbricates. Safonova (2009) noted that the Neoproterozoic–Paleozoic ocean plate stratigraphy units of the Central Asian Orogenic Belt are remarkably similar to equivalent Mesozoic ocean plate stratigraphy units in Japan. So far, no detailed studies have been made to assess the relative roles of nonaccretion versus subduction erosion in the Central Asian Orogenic Belt. Clearly, substantial evidence has recently accrued on the considerable importance of subduction erosion in formation of accretionary complexes in the western Pacific, such as the Japanese Islands (e.g., Isozaki et al., 2010, 2011), and such an approach will in the future contribute to a better understanding of the rather more-complicated and far-less-studied huge Central Asian Orogenic Belt. It is suspected that detailed analysis of ocean plate stratigraphy will help to revolutionize understanding of the geology and tectonic evolution of the Central Asian Orogenic Belt.

If subduction-accretion has been a major factor in controlling the growth of the Central Asian Orogenic Belt, then the subduction of one or more mid-oceanic ridges is likely to have taken place (Sisson et al., 2003), especially in such a wide and long-lived accretionary orogen. In addition, subduction-accretion of small-scale crustal blocks may not leave much trace of deformation if subduction jump has taken place (Pubellier and Meresse, 2013). In the past few years, evidence has accrued for such ridge subduction, in particular from the associated diagnostic magmatism and metamorphism. Western Junggar in Xinjiang, China, is the most outstanding region where the effects of ridge subduction have been documented.

From geochemical and isotopic evidence from different areas, Geng et al. (2009), Tang et al. (2010), and Zhang et al. (2011) suggested that coeval, high-Mg diorites, Nb-enriched basalts, adakites, and alkali-feldspar granites were most likely produced by an enhanced heat flux and asthenospheric upwelling through a slab window created by ridge subduction at ca. 305 Ma. Ma et al. (2012) reported from Western Junggar a swarm of high-Mg diorite dikes, comparable in composition to sanukitoids. To account for the high temperature required to generate such a HT magma, the authors proposed that asthenosphere upwelled through a slab window created by interaction with a ridge of a subducted slab in the Pennsylvanian, and the swarm of hot-magma diorite dikes was intruded through the slab window.

The processes of plate accretion, subduction, tectonic erosion, arc collision, and microcontinent collision take place from the birth of an ocean to its closure and formation of a suture zone. The cycle has run to completion in Central Asia but is ongoing in the Pacific.

The tectonic processes documented in this paper result from the work of many earth scientists. We thank Wenjiao Xiao, John Wakabayashi, Robert Hall, and Bor-ming Jahn for the invitation to write this paper, and the reviewers for their comments on the manuscript.