SUBDUCTION ZONES OF SOUTHEAST ASIA: MAIN TYPES, SEISMICITY AND MINERALIZATION

¹YU.G. GATINSKY, ²G.L. VLADOVA

¹Vernadsky State Geological Museum, Mokhovaya ul.
11, korp. 2, Moskva, 125009 Russia, e-mail: yug@sgm.ru;
²International Institute of Earthquake Forecast and Mathematic Geophysics,
Profsoyuznaya ul. 84/32, Moskva, 117997 Russia, e-mail: vladova@mitp.ru

Abstract: S. Uyeda [12] was one of the first who took attention to the subduction zone heterogeneity. Basing on the difference in the age and density of the subducted oceanic lithosphere he distinguished two main types of these zones within the Pacific Ring: the Marianna type in the west with the steep dipping Benioff Zone (BZ), prevailing of extension and massive copper mineralization, and the Chilean one in the east with gently sloping BZ, prevailing of compression and porphyry copper deposits. But the more detailed analysis of active margins in various ring segments shows more complicated relations between both types [4-6]. Some segments in the west of the ring must be attributed to the Chilean type by: 1) The prevalence of compression; 2) Subduction of the relatively young oceanic lithosphere; 3) Gently sloping BZ; 4) Development of the porphyry copper and accompanying mineralization. In SE Asia following segments can be mentioned as examples: the south part of the Japanese Arc and north Ryukyu, north Sulawesi, the west part of the Sunda Arc (Sumatra, Java), which is adjacent to the Pacific Ring in the Indian Ocean, and perhaps the north Philippines. Other segments in the west of the ring belong to the Marianna type. They are characterized by: 1) Extension with oceanic or suboceanic crust in the back-arc area; 2) As a rule the highest subduction velocity (as much as 9.5-10 cm/y); 3) Sinking of the oldest oceanic crust; 4) Maximal values of BZ dip angle (as much as 50-85⁰) and depth of its penetration (up to 650-700 km); 5) Appearance of some gaps and discontinuities along BZ dipping sometimes with slab break-off; 6) The most intensive discharge of the seismic energy above BZ. Typical examples in SE Asia are: south Ryukyu and the south Philippines, Izu-Bonin and Marianna arcs, the east Sunda Arc. The transition from one type to another is accompanied by changes of the predominant mineralization. Porphyry copper, molybdenum, hydrothermal gold and base metal deposits prevail in the west Sunda Arc and north Philippines, but massive copper and copper - base metal deposits developped in the Izu-Bonin Arc, south Philippines and in the east Sunda Arc.

I. INTRODUCTION

Subduction zones are situated in areas of the lithosphere plate interaction in conditions of their convergence. Such interaction takes place in active margins of continents or island arcs and evokes the formation of some Earth's crust structural elements and specific mineral deposits. The following segments of active margins can be distinguished in SE Asia within boundaries between Eurasian, Pacific, Philippines and Australian plates (Fig. 1): I) South Japan - north Ryukyu; II) Izu-Bonin, III) South Ryukyu (Okinawa); IV) North Philippines; V) Marianna; VI) South Philippines; VII) North Sulawesi; VIII) West Sunda; and IX) East Sunda. In this work we try to establish as far as the active margins and subduction zones that are homogeneous along their strike and how they diverge in their geological-geophysical and metallogenic characteristics.

Figure 1. Segmentation of active margins and subduction zones in SE Asia: 1-2) Continental crust (1- Continents and islands, 2- Shelf); 3) Oceanic crust; 4-6) Faults (4- In continents, 5- Spreading axis and transform in oceans; 6- Surface trace of BZ on convergent plate boundaries); 7) Limits of segments. Born: Borneo (Kalimantan); Bu: Indo-Burman Ranges; GD: Ganges (Bengal) Deep; Ind: Indochina Pen.; Jap: Japan; Ko: Korean Pen.; Mal: Malay Peninsula; Mol: Moluccas; NG: New Guinea; Sch: south China; Sul: Sulawesi; Sum: Sumatra. Roman numerals: numbers of segments (see text).

II. TYPES OF SUBDUCTION ZONE AND PRINCIPLES OF THEIR SEGMENTATION

Three main criterions determine the deep structure of the subduction zone, the type of magmatic activity in the volcanic arc and tension intensity in back-arc as follows: 1) Convergence velocity; 2) Age of subducted lithosphere and its buoyancy; 3) Direction of displacement of each interacting plates [14]. Different types of subduction zones are distinguished by these criterions. Two of them, which were for the first time established by S. Uyeda [12], are the most developed. The type I (Marianna or west Pacific) is characterized by relatively low velocity of convergence and the old cool (and heavier in consequence of that) lithosphere of subducted plate. A steep dipping of BZ prevails as well as weak cohesion between oceanic and continental plates and tension in their margins. The island arc basalt and boninite is the main products of activity in volcanic arcs. Back-arc basins with oceanic or suboceanic crust are widely developed and characterized by a high heat-flow, sometimes by bimodal basalt - rhyolite series. The massive sulfide mineralization prevails.

The type II (Chilean or east Pacific) is marked by a high convergence velocity, relatively young age of weakly cooled subducted lithosphere and a contrary displacement of both plates with the quick thrusting of a continental plate over oceanic one. Well developed back-arcs are absent and the contraction prevails. The magma in melt spots under arc havent time for essential contamination by the crust material because of the fast convergence. As a result of that latite-andesite and granite-granodiorite series are developed with copper porphyry and copper-molybdenum porphyry mineralization. Some gold, silver, plumb and zinc deposits are also common.

S. Uyeda emphasized the location of above mentioned types of subduction zones mainly in opposite sides of Pacific. But, more detailed analysis of the structure and geodynamics in different parts of the Pacific Ring and in SE Asia showed more complex correlation [14-16]. It turned out that some likely indivisible active margins and subduction zones are disintegrated at separate segments, which remarkably distinguish by the age of subducted lithosphere, intensity of seismicity, BZ dipping and other characteristics as well as by the type of mineralization. Such large arcs as Kurile-Kamchatka, Japan and Sunda can be mentioned as examples. Using different geological-geophysical indexes we divided active margins and subduction zones of the Pacific Ring and adjacent part of the Indian Ocean into 28 segments. Four of them are characterized in this work (see Table 1). The seismic energy (E_s) is calculated by the formula from the paper [17] as follows:

log₁₀E_s = a . M_s + b, where a = 1.5, b = 11.8

The magnitude (M_s) of surface waves is taken from the NEIC catalog beginning from 1966 (http://earthquake.usgs.gov//regional/neic) without aftershocks exception.

In the segment I (see Fig. 1), the relatively young lithosphere of the Philippines Plate (LMA 6-22) subsides under the continental lithosphere of SW Japan and north Ryukyu. Here a minimal volume of the seismic energy escapes in comparison with other segments (see table) and there isnt any evidences of considerable extension in the back-arc. The depth of BZ subsidence under this segment reaches only 250-300 km (Fig. 2). So we can see here the type II of the subduction zone. The narrow Okinawa Trough is developed in the back-arc of south Ryukyu (III in Fig. 1). It is characterized by suboceanic crust (16-21 km), reduction of the lithosphere thickness up to 35-40 km and abnormal high heat flow up to 600-1500 mWt/m² [9].

The quantity of seismic energy and earthquake intensity increase southward to Taiwan, where the collision of the Philippines Arc and Eurasia took place [8]. The opening of the Okinawa Trough began not long ago (LMA 3-2). The older oceanic crust subsided under this segment in comparison with the segment I and the angle of dip reaches 84. Hence we can state that the transition of type I to type II takes place from north to south along the strike of active margin. Evidently the subduction zone of the Marianna Type has only begun to be born in the segment III. Near Taiwan the angle of dip again decreases up to 25-5 due to the oblique subduction and appearance of continental crust in the back-arc [13].

The Izu-Bonin Segment (II in Fig. 1) is the more advanced variant of the Marianna Type. All main characteristics of the type are developed here (see Table 1), and among them one must be mentioned. A seismic gap exists within BZ in the depth of 170-220 km and an intensive seismic bunch in 300-500 km, corresponding to the slab detachment (Fig. 3). Seismotomographic data confirm the slab existence and its spreading in the depth of about 650 km [1].

Table 1. Segmentation of some convergent plate boundaries in SE Asia

№ in Fig.1	Name of segment and type of subduction zone (M-Marianna, Ch-Chilean)	Length of segment (km) above subduction zone (L)	Type of the crust and its thickness (km) in the arc / back-arc area (C: continental, SO: suboceanic, O: oceanic)	Subduction velocity (V) (cm/y)	Age of the lithosphere of the subducted plate / numbers of LMA	Angle of dip (a) and depth (km) of subsidence (H) of the seismofocal plane (BZ)	Seismic gap within the seismofocal plane (BZ) and their depth (km)	Preliminary appraisal of the whole seismic energy of the segment (E) (erg10^19)	Main metallogenic specialization of the segment in the Cenozoic (symbols indicate ore elements, m - massive sulfide, p - porphyry ores)
I	South Japan (Ch)	875	C - 30 / C - 25-30	6.2	Paleocene - middle Miocene / 22-6	H = 300		5867	veined Sb-Hg, Au-Ag
II	Izu-Bonin (M)	1380	O - 11.7-16 / O - 6.7-7.1	4.7-7.7 (increasing northward)	Late Jurassic - early Cretaceous / M21-M5	a = 50-77^o, H = 450-637 (both increasing southward)	Gap at 170-220, bunch at 300-500 (slab detachment)	20657	mCu, mCu-Zn-Pb-Au
VIII	West Sunda (Ch)	3000	C - 25-30 / C - 25-30	6.0-6.7	Early Cretaceous - Eocene/ M4-M20	a = 30-45^o, H = 200-600 (both increasing eastward)		52705	pCu-Mo, pCu-Au, veined Cu-Pb-Zn, Au-Ag
IX	East Sunda (M)	2250	SO - 15-20 / O - 5-9	7.6-8.0	Late Jurassic / M26-M16	a = 45-72^o (increasing eastward), H = 660	Gap at 400-450, bunch at 470-600 (slab detachment)	137782	mCu, mCu-Zn

Figure 2. Seismic energy distribution along the dip of BZ in the segment I
(south Japan - north Ryukyu).

Now well examine active margins at the boundary of Eurasia and the Indian Ocean in SE Asia. Two segments can be distinguished within the Sunda Arc (Fig. 4). The west Sunda Segment (VIII) is characterized by relatively gently sloping BZ (30-45), the depth of which is of 150-200 km in the west and little by little increases up to 500-650 km in the extreme east of the segment. The arc and back-arc are composed by rocks of the continental crust (25-30 km). According to focal mechanism solution the compression and wrench-faults predominate in the arc and back-arc, the share of extension increases eastward [11; http://earthquake.usgs.gov//regional/neic]. So, this segment belongs most probably to the Chilean Type with the gradual transition in the east of the Marianna Type.

The latter begins to prevail east of Bali Island, where east Sunda Segment (IX) is established. The angle of the BZ dip increases up to 70-72, the depth of its penetration reaches 700 km, and a trough of the Banda Sea appears in the back-arc. The trough depth in some areas is more than 5 km and the thickness of the oceanic crust is 5-9 km. This segment is characterized by the maximal volume of the seismic energy (see Table 1). The oceanic crust of different ages subsides under the Sunda Arc (see Fig. 4): late Cretaceous - Paleogene in the west (LMA 20-33) up to the east end of Sumatra, early Cretaceous (LMA M0-M4) under Java and the most ancient Jurassic (LMA M16-M26) under the eastern segment. So, the segmentation here is also connected with different ages of subducted lithosphere. We havent any reliable data on the heat flow from the Banda Sea deep-water trough, but there are strong negative up to slightly positive free-air anomalies there. Such geophysical characteristics allow to suppose the existence of the back-arc plume in the trough [4]. The development of the potassic alkaline volcanics in east Java and Sumbava Island conforms to this supposition.

Fig. 3. Seismic energy distribution along the dip of BZ in the segment III (Izu-Bonin).

Other segments shown in Fig. 4 belong also to different types of subduction zone: north Sulawesi (VII) to the Chilean type, south Philippines (VI) to the Marianna type. So, we can summarize following characteristics of both types in SE Asia, which often replace each other along their strike.

On the whole, the Marianna Type is characterized by: 1) Subduction of the oldest oceanic lithosphere in the given zone; 2) The maximal velocity of plate convergence (up to 7.7-8.0 cm/year); 3) The development of the oceanic or suboceanic crust in the back-arc; 4) Maximal angle of dip (60-84) and depth of the BZ penetration (600-637 km); 5) Appearance of gaps within bazan, sometimes with slab detachment; 6) Maximal escape of seismic energy (see Table 1). The Chilean Type on the contrary is characterized by: 1) Subduction of the relatively younger lithosphere; 2) The smaller velocity of convergence (6.0-6.7 cm/year); 3) The development of the continental crust in the back-arc; 4) Smaller angle of dip (35-60) and depth of the BZ penetration (200-500 km); 5) Essentially smaller escaping of the seismic energy. The segment of south Ryukyu (IV in Fig. 1) can be considered as transitional between both mentioned types. It is characterized, on the one hand, by steep dipping of the sinking plate, which is composed of relatively old oceanic lithosphere, and, on the other hand, of a rather small quantity of escaping seismic energy. The deep-water trough (the Okinawa Deep) only begins to be developed in its back-arc.

Figure 4. Segments of active margins and subduction zones at the boundaries of Australian, Philippines, and Eurasian lithosphere plates: 1) Surface trace of BZ on convergent plate boundaries; 2) Isolines of BZ depth (km); 3) Contours of deep-water troughs in marginal seas; 4) Foot of the continental slope in the Australian passive margin; 5) Faults; 6) Established and supposed wrench-faults; 7-10) late Cenozoic mineralization occurrences and deposits (7- Exhalite; 8- Hydrothermal and metasomatic; 9- Sedimentary and supergene, 10- Massive sulfide); 11) Outcrops of late Cenozoic alkaline volcanics; 12) Segment boundaries. Numbers of segments see in Fig. 1. Dash shading corresponds to the Indian Ocean crust of different ages (in million years).

III. METALLOGENIC SPECIALIZATION OF SEGMENTS

As a rule, a hydrothermal vein with Au-Ag, Sb, and Pb-Zn mineralizations (http://www.mmaj.go.jp/mric_web/deposit/map.htm) prevails in the south Japan Segment (I in Fig. 1), a relatively young lithosphere of the north part of the Philippines Plate sinks under which. On the contrary, a massive sulfide mineralization of the Kuroko Type is established in the Okinawa Trough [9], and gold-bearing copper massive sulfide deposits were not long ago discovered in islands of the Izu-Bonin Arc [10]. So, different segments of west Pacific active margins are characterized by specific mineralizations, which evidently depends of the subduction zone type.

The same regularity can be seen for active margins of SE Asia in the boundary with the Indian Ocean (Fig. 4). A copper porphyry and polymetallic mineralization bearing gold and silver is typical for the west Sunda (VIII) and north Sulawesi (VII) segments [2, 5]. At the same time copper and copper-zinc massive sulfide, sedimentary and supergene manganese deposits are developed in the east Sunda Segment (IX) above the steeply sinking BZ. Therefore the change of segments geodynamics always is accompanied by the change of the metallogenic specialization. This gives as a chance to suppose any possible mineralization of segments, which are insufficiently studied in connection with the metallogeny on the basis of such geodynamic peculiarities as parameters of BZ and age of sinking lithosphere. In particular, copper porphyry deposits predominate in the north part of the Philippines Arc (IV in Fig. 1), that does not contain a common idea of S. Uyeda [12] on the metallogeny of the Pacific Ring. But, that consists of gently sloping BZ from the South China Sea and relatively young age of its oceanic lithosphere (32-17 Ma.).

ACKNOWLEDGEMENT

This work is fulfilled with the support of the Earth Sciences Department RAS (Program No. 6 Geodynamics and deformation mechanism of lithosphere) and RFBR (grant No. 06-05-64866). The authors are grateful to Prof. Cao Dinh Trieu for kind useful remarks and advices during the preparation of the paper.

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