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Plume-related sedimentary basins in East Asia during the Cretaceous
Palaeogeography, Palaeoclimatology, Palaeoecology 150 (1999) 1–11
Plume-related sedimentary basins in East Asia during the Cretaceous Hakuyu Okada * Oyo Corporation, 2-21-36 Ijiri, Minami-ku, Fukuoka 811-1302, Japan Received 3 March 1997; revised version received 20 January 1998; accepted 13 October 1998
Abstract Many Cretaceous sedimentary basins of various sizes occur in East Asia, which are classified into four geotectonic settings: forearc, intra-arc, back-arc, and intra-continental basins. Among these, intra-arc and back-arc basins are characterized by rifting and pull-apart origins, and intra-continental basins by rifting related to large-scale magmatism or plume. The pull-apart basins in Southwest Japan are closely related to plate-subduction magmatism. The continental region rift basins in the Early Cretaceous, however, seem to have no direct relation to subduction-controlled magmatisms, because almost no accretionary prism was developed along the continental margin at that time. This means that plume-related magmatism played an important role in the eastern part of the Asian continent due to return flow effects from the 670 km thermal boundary layer. Such a plume magmatism explains systematically the origin and development of varieties of rift basins in East Asia in Cretaceous time. 1999 Elsevier Science B.V. All rights reserved. Keywords: pull-apart basin; rift basin; plume magmatism; strike-slip fault
1. Introduction The origin and development of sedimentary basins in East Asia have been critical to understanding the geological evolution in this region, because many huge basins that developed in the Mesozoic bear significant amounts of hydrocarbon resources (e.g., Zhang et al., 1984; Chen and Dickinson, 1986; Watson et al., 1987; Zhu, 1989; Zhou et al., 1989; Li, 1991; Okada and Sakai, 1993; Sakai and Okada, 1997). Most of the previous work on sedimentary basins in China discusses sedimentary basins in terms of plate tectonic setting, and the general conclusion of these studies is that basins are classified into rift basins or extensional basins, flexural basins and Ł Fax:
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their transitional basins (Chen and Dickinson, 1986; Watson et al., 1987) (Fig. 1). The flexural basins are characteristic of western China, while the rift basins occur in eastern China (Watson et al., 1987) (Fig. 1). Chen and Dickinson (1986) identified western basins and eastern basins, respectively, proposing a division line running from central Mongolia to Laos, which I here call the Baikal–Yunnan Line (Fig. 2). This line reflects the difference in major tectonic environments between the two regions (Chen and Dickinson, 1986; Watson et al., 1987). The concept of these basin classifications is acceptable in principle, but mechanisms for the origin of these basins have not been fully explained. Therefore, in this paper I consider the origin and development of sedimentary basins from the Asian continent to an island arc in the context of superplume activity.
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 0 3 - 6
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Fig. 1. Distribution of major sedimentary basins and their basin types in East Asia (based mainly on Watson et al., 1987) and magmatic areas in Jurassic to Early Cretaceous time.
2. Geotectonic setting of Mesozoic basins
2.1. Forearc basins
In a traverse section from the Japanese Islands arc to the East Asian continent during the late Mesozoic, four types of sedimentary basins are recognizable: forearc basins, intra-arc basins, back-arc basins, and intra-continental basins. The island arc portion consists of forearc basins, intra-arc basins and back-arc basins from the ocean side to the continental side, as is well documented in the Southwest Japan Arc (Okada and Sakai, 1993; Sakai and Okada, 1997). Among these, intra-arc and back-arc basins are characterized by rifting and pull-apart features, and the intra-continental region from the Korean Peninsula to eastern China is characterized by large-scale rifting features.
These are developed in the Japanese Islands and are typically found in the Shimanto Belt in Southwest Japan and contain the Cretaceous to Paleogene Shimanto Supergroup, which is characterized by highly deformed strata originally deposited in trench to forearc basins. They show typical features of accretionary prisms due to trench subduction (Taira et al., 1982, 1988). The Cretaceous sequence of the Shimanto Supergroup is composed mainly of alternations of sandstone turbidites and shale, and subordinately of chert, red shale, micritic limestone and pillow basalt, attaining a total thickness of more than 10,000 m (Okada, 1996).
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Fig. 2. Tectonic framework and basin arrangement in the Cretaceous Eurasian Plate.
2.2. Intra-arc basins In Southwest Japan, particularly on Kyushu and Shikoku Islands, intra-arc basins are well developed (Sakai and Okada, 1997). They are generally faultbounded trough-like depressions arranged in parallel to the Median Tectonic Line. They are scattered, small elongate basins (Fig. 3) filled by clastic sediments of non-marine, paralic to shallow-sea and deep-sea facies (Okada and Sakai, 1993; Okada, 1996; Sakai and Okada, 1997) with considerable thickness.
On Kyushu Island, small Cretaceous basins in the Kurosegawa Belt are rift basins, while those in the Axial Zone are either rift basins or pull-apart basins (Sakai and Okada, 1997). In the Axial Zone, these basins show a unique trend in timing of sedimentation in close association with magmatism (Okada, 1996), as will be discussed in detail later. 2.3. Back-arc and intra-continental basins In a broad region from the back-arc of Kyushu Island through the Korean Peninsula to eastern China,
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Fig. 3. Distribution of the main Cretaceous basins along the Axial Zone of Southwest Japan.
large-scale rift basins and relatively small-scale pullapart basins occur. Basins of the former type include the Upper Jurassic to Lower Cretaceous Tetori Basin and the Lower Cretaceous Kanmon Basin in the back-arc of Southwest Japan, the Lower Cretaceous Kyongsang Basin in South Korea, the Lower Cretaceous Songliao Basin as well as the Upper Cretaceous Bohai Bay and Yellow Sea Basins in Northeast China and the East China Sea Shelf Basin (Fig. 1). The Yongdong Basin and other small basins along the Ogchon Belt in the Korean Peninsula (Lee and Paik, 1990; Lee et al., 1991) belong to pull-apart type basins.
in a north–south compression is sharply different from that in eastern China, the Korean Peninsula and the Japanese Island arc, all of which were under east–west stress (Fig. 2).
2.4. Flexural basins
3.1. Pull-apart basins
Basins of this type occur in western China and follow an E–W trend. These basins were originally formed within microcontinental blocks in Paleozoic and early Mesozoic times, which had been subjected to north–south compression by northward subduction episodes of the Tethyan Plate (Zhang et al., 1984; Chen and Dickinson, 1986; Watson et al., 1987; Li, 1991). Basin sediments as well as surrounding mountain ranges were subject to a second phase of folding by the renewed northward collision of the Indian Plate during the mid-Tertiary. Therefore, the geotectonic setting of these basins
Pull-apart basins are generally elongated and developed mostly along large-scale strike-slip faults. Intra-arc basins in the Chichibu Belt and the Axial Zone and back-arc basins in the Japanese Islands (Okada and Sakai, 1993; Okada, 1996; Sakai and Okada, 1997), the Yongdong Basin and related basins in South Korea (Lee and Paik, 1990; Lee et al., 1991), the Yilan–Yitong Fault-related basins east of the Songliao Basin, and elongated basins along the Tanlu Fault east of the Hefei Basin, all belong to the pull-apart basins. In general, they show half-graben profiles. Miyata (1980) showed experimentally that
3. Basin development in East Asia Sedimentary basins, except for forearc basins in East Asia, are divided into two tectonic types: pullapart basins closely related to strike-slip faults and rift basins related mainly to large-scale magmatism or plume activity.
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Fig. 4. Age trend of magmatism and timing of basin formation. Data of magmatism are based on Kinoshita (1995). The starting point of distance is located at the western part of the Amakusa Islands, central Kyushu (A) and at the southern margin of the Onogawa Basin, central Kyushu (B) (see Fig. 3).
the scale of graben structures is controlled by the degree of slope angles of the main strike-slip faults. The relationship between the development of pullapart basins and magmatic events is well presented by intra-arc basins along the Median Tectonic Line in Kyushu and Shikoku Islands. In this area, small Cretaceous basins with clastic sediments of considerable thickness are exposed (Fig. 3): (1) the westernmost basin is filled by the latest Albian to earliest Turonian Goshonoura Group comprising fossil-rich shallowmarine sediments (in the lower) and brackish-water to freshwater sediments (in the upper); (2) the western central Mifune Basin composed of Cenomanian to Turonian clastics consists of brackish-water to shallow-marine sandstones with abundant molluscan fossils in the lower and lacustrine red beds in the upper; (3) the eastern central Onogawa Basin filled mainly by Turonian to Santonian turbidites; and (4) the Izumi Basin in the Shikoku Island accommodating Campanian to Maastrichtian turbidites. The Izumi Basin itself, which extends over 500 km, began its sedimentation in the Campanian in the west
and in the Maastrichtian in the east. The Onogawa and Izumi Basins are characterized by extraordinarily thick piles of turbidites (Okada, 1996). It is worthwhile to point out that the depocenters in these basins shifted gradually eastwards (Matsumoto, 1978; Okada, 1996) (Fig. 4). This diachronism in the basin formation is very similar to the age trend in magmatism in Southwest Japan (Fig. 4). The age trend in magmatism shows a rate of about 1=30 (Ma=km) (Kinoshita, 1995), which is close to that of movements of the Kula and Pacific Plates (Seno and Maruyama, 1984). Therefore, it is evident that formation of pullapart basins and subduction-related magmatic events in Southwest Japan are closely related to each other. 3.2. Rift basins Rift basins in East Asia are generally characterized by their large scale and fault-bounded structure. They are distributed in four major tectonic zones: Sichuan and Ordos Zone in the west, Hefei–Bohai
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Bay and Songliao Zone, Subei Zone and the East China Sea and Kyongsang Zone in the east (Fig. 1). Most of these intra-continental basins were superimposed on older structural trends (Chen and Dickinson, 1986; Zhu, 1989). These basins are filled by a fining-upward sequence of clastics and evaporites of fluvio-lacustrine origin. The Sichuan (about 800 km ð 500 km) and Ordos (700 km ð 500 km) Basins exhibit a westward thickened wedge of basin-fill (Wang et al., 1989, for the Sichuan Basin; Sun et al., 1989, for the Ordos Basin) that appear to have developed in the Jurassic to Early Cretaceous, with syn-sedimentary development of antithetic growth faults. The Hefei and Bohai Bay Basins are located north of the Qinglin Suture and west of the Tanlu Fault (Fig. 1). The Hefei Basin is fault-bounded by the Gushi–Shangcheng Fault on the west and by the Tanlu Fault on the east side as well as by the Tongbai– Dabie Fold Belt to the south. The Hefei Basin reached its maximum development during the Jurassic to Cretaceous with a thickness of more than 1500 m of fluvio-lacustrine clastic sediments (Han et al., 1989). The Bohai Bay Basin occupies an area as wide as 200,000 km2 between the Taihang Shan Uplift to the west and the Tanlu Fault to the east (Fig. 1). Due to extensive block-faulting activity during the Late Jurassic to Early Cretaceous, many small faulted depressions occurred in the Bohai Bay Basin area, which were filled by coal-bearing red-colored clastic sediments and varieties of volcanic rocks with a total thickness of about 4000 m (Hu et al., 1989). Principally, this basin consists of a series of faulteddepressions and faulted-uplifts. The Songliao Basin located in northeastern China is 750 km long and about 350 km wide, occupying an NNE–SSE-trending area of 260,000 km 2 . The basic structure of the basin is graben-like, controlled by faults on both sides of the basin. The basin was first filled by Upper Jurassic fluvio-lacustrine sediments associated by volcanic rock layers, and rapidly subsided during the Early Cretaceous and received deposits until Maastrichtian time with a maximum thickness of about 6700 m of petroliferous sandstone and shale of lacustrine facies (Ma et al., 1989). The largest sedimentary basin in the Korean Peninsula is the Kyongsang Basin, measuring about 250 km long and 125 km wide. The basin-fill con-
sists of the Kyongsang Supergroup, which is divided into three sequences, the Singdong Group, the Hayang Group and Yuchon Volcanic Group, in ascending order (Chang and Park, 1995). The Valanginian to Hauterivian Singdong Group, 2000 to 3000 m thick, is composed of conglomerate, sandstone and shale. The Barremian to Albian Hayang Group, 1000 to 5000 m thick, consists of sandstone and shale with a minor amount of conglomerate and marl. They are intercalated by pyroclastic layers and andesitic and basaltic rocks at some places and horizons. The Albian to Cenomanian Yuchon Group, more than 2000 m thick, is characterized by the predominance of intermediate to acidic volcanic rocks. The Kyongsang Basin shows a half-graben structure tilted eastwards (Okada and Sakai, 1993). The East China Sea Shelf Basin is characterized by composites of fault-controlled half-grabens formed in the Proterozoic basement (Zhou et al., 1989; Yu, 1991). They are narrowly elongated en echelon with a NE–SW to NNE–SSW trend, extending more than 1000 km. Most of the basins are filled by nearly 4000 m of Cretaceous, Paleocene and Eocene sediments (Zhou et al., 1989). All these half-grabens initiated in the Late Jurassic, but major subsidence took place in the Late Cretaceous.
4. Relationship between basin formation and plume magmatism 4.1. Overview It has long been advocated that Cretaceous to Paleogene magmatism migrated eastwards at a constant rate in Southwest Japan, oldest in the west and youngest in the east (Matsumoto, 1969, 1978; Kinoshita, 1995, 1997) (Figs. 3 and 4). The mechanism of this migration was attributed to plate movements because of motion rates of proto-Pacific plates of about 20 cm=year (Seno and Maruyama, 1984). Further, for vigorous magmatism, many workers have invoked ridge subduction (Uyeda and Miyashiro, 1974; Kinoshita and Ito, 1986; Kiminami et al., 1993; Kinoshita, 1995, 1997). However, the area of magmatism is too large to ascribe to ridge subduction. In addition, oblique ridge subductions can only partly explain the left-lateral faultings north of the Qinling
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Suture, but are hardly applicable to the faults south of the Qinling Suture as well as in the South China Sea region, where right-lateral faults are expected. However, almost all the major strike-slip faults in the East Asian continent are of left-lateral sense (Figs. 1 and 2). Matsumoto (1978) first pointed out that such a migration of Cretaceous magmatism must have been related to eastward-younging sedimentation in Southwest Japan. Okada and Sakai (1993), Okada (1996) and others, also confirmed this concept. The problems of these connotations are that magmatism from Jurassic to Cretaceous time was far too large to attribute this to simple subduction of oceanic plates or ridge subduction, and that Sakai and Okada (1997) have revealed a deficiency of accretionary prisms in the Outer Zone of Kyushu Island and instead, the development of a passive margin. This seems to be a general feature in Southwest Japan, as suggested by Takahashi and Ishii (1995). This fact of non-development of accretionary prisms in the Early Cretaceous is critically important to understand the significance of large-scale magmatism on the East Asian continental margin.
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4.2. Plume magmatism on the East Asian continental margin Magmatism in Southwest Japan and the East Asian continental margin was extremely active during the late Mesozoic to early Cenozoic, which produced tremendous amounts of volcanic and plutonic rock emplacements. These are distributed from the Guangxi region in Southeast China to the Chukotka Peninsula in East Russia over 7000 km along the eastern continental margin, which are in parallel to major tectonic lines (Figs. 1 and 5). Takahashi (1983) divided the igneous rock associations in the East Asian region into four types, mainly based on their chemical natures: island arc, continental margin, intra-continent and collision types. He further discriminated five chronological series of magmatism: 190–160 Ma, 160–130 Ma, 130–100 Ma, 100–70 Ma and 70–40 Ma (Fig. 5). Among these time series, the 160–130, 130–100 and 100–70 Ma magmatic events of the continental margin and intra-continent types seem to have been most active. These magmatic events are characterized by
Fig. 5. Space and time distributions of various types of magmatisms in Jurassic to Cretaceous East Asia (modified from Takahashi, 1983).
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granite, granodiorite, felsic to intermediate welded tuffs, rhyolite, andesite and basalt. Takahashi (1983) ascribed these magmatic events to the subduction of oceanic ridge and young hot oceanic lithosphere, as Uyeda and Miyashiro (1974) suggested. Basically, Kinoshita and Ito (1986, 1988) and Kinoshita (1995, 1997) followed the concept of ridge subduction as a cause of large-scale magmatic events in the late Mesozoic. On the contrary, it is argued that many intra-cratonic large basins in East China are dissected by many faults, producing intra-basinal half-grabens and grabens, as in the Subei, Bohai Bay, Sichuan and East China Sea Basins. It is generally believed that this faulting was due to the crustal stretching and thinning (Hsu, 1989) induced by the swelling of the upper mantle (Li, 1991) or by the arching force by upwelling of a thermal plume (Yano and Wu, 1995, 1997). The broad distribution and huge amount of the late Mesozoic igneous rocks on the East Asian continental margin, as revealed by Takahashi (1983), are too extensive to be regarded as the product of arcmagmatism simply related to plate subduction. The
space and time distribution of these late Mesozoic igneous rocks is comparable to that of the large igneous provinces proposed by Larson (1991a,b) and Coffin and Eldholm (1994), though these igneous rocks consist of mixtures of plutonic and volcanic rocks from acidic to basic (Takahashi, 1983). It is interesting to point out that the subduction-related magmatisms do not seem to have any relation to basin formation, as is clearly shown in Fig. 6. This is reinforced by the fact that the accretionary prism was not formed in the Outer Zone of Southwest Japan during the Early Cretaceous (Sakai and Okada, 1997), suggesting that subduction-related magmatisms at that time should have been inactive. Nevertheless, Early Cretaceous magmatic events were vigorous on the eastern margin of the Asian continent, as described above. Therefore, at least Early Cretaceous large magmatic events must have been plume-related. Yano and Wu (1995, 1997) suggested that the inclined upwelling of a superplume produced the asymmetric arching structure over a vast area as wide as 2500 km on the Eastern Asian continental margin.
Fig. 6. Timing of basin formation and its relation to subduction-controlled magmatic belts on the East Asian continental margin. Data of magmatism are based on Kinoshita (1995). The starting point of distance is located at the western tip of Honshu Island, Japan.
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Fig. 7. Development of rift basins closely related to plume activity at the East Asian continental margin (modified from Yano and Wu, 1995).
According to this explanation, the origin of most of the sedimentary basins of rifting origin must have been controlled by superplume-related upwellings, but not by subduction-related magmatism (Fig. 7). Even at the place of slab penetration due to the subduction of oceanic plates, return flows from the 670 km thermal boundary layer begin to produce plumes beneath the continental crust (Coffin and Eldholm, 1994; Larson and Kincaid, 1996). This plume magmatism must have played an important role to produce large igneous provinces at the eastern margin of the Asian continent, in addition to the normal subduction magmatism. Many of the rifting faults may have been further converted to strike-slip faults by northward movements of the Izanagi and Pacific Plates, resulting in the formation of pull-apart basins along some major strike-slip faults (Okada and Sakai, 1993; Sakai and Okada, 1997).
the Early Cretaceous. Nevertheless, huge amounts of magmatic emplacement did occur at that time in close association of large sedimentary basins of rifting origin. Thus, these magmatic events are well explained in terms of plume activity. This means that plume-related magmatism was developed in the eastern part of the Asian continent due to return flow effects from the 670 km thermal boundary layer. Such a plume magmatism may explain systematically the origin and development of varieties of rift basins in East Asia during Cretaceous time.
Acknowledgements I would like to express my sincere thanks to Dr. Niall J. Mateer of the University of California, Prof. Ki-Hong Chang of Kyungpook National University, Korea, and Dr. Takashi Sakai of Kyushu University for valuable discussions. Niall Mateer and Ki-Hong Chang critically read the manuscript.
5. Conclusion On the eastern margin of the Asian continent, subduction of oceanic plates was inactive or completely ceased during the Early Cretaceous, and instead, the passive margin prevailed. Therefore, no subduction-related magmatism should be encountered in
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Plume-related sedimentary basins in East Asia during the Cretaceous Hakuyu Okada * Oyo Corporation, 2-21-36 Ijiri, Minami-ku, Fukuoka 811-1302, Japan Received 3 March 1997; revised version received 20 January 1998; accepted 13 October 1998
Abstract Many Cretaceous sedimentary basins of various sizes occur in East Asia, which are classified into four geotectonic settings: forearc, intra-arc, back-arc, and intra-continental basins. Among these, intra-arc and back-arc basins are characterized by rifting and pull-apart origins, and intra-continental basins by rifting related to large-scale magmatism or plume. The pull-apart basins in Southwest Japan are closely related to plate-subduction magmatism. The continental region rift basins in the Early Cretaceous, however, seem to have no direct relation to subduction-controlled magmatisms, because almost no accretionary prism was developed along the continental margin at that time. This means that plume-related magmatism played an important role in the eastern part of the Asian continent due to return flow effects from the 670 km thermal boundary layer. Such a plume magmatism explains systematically the origin and development of varieties of rift basins in East Asia in Cretaceous time. 1999 Elsevier Science B.V. All rights reserved. Keywords: pull-apart basin; rift basin; plume magmatism; strike-slip fault
1. Introduction The origin and development of sedimentary basins in East Asia have been critical to understanding the geological evolution in this region, because many huge basins that developed in the Mesozoic bear significant amounts of hydrocarbon resources (e.g., Zhang et al., 1984; Chen and Dickinson, 1986; Watson et al., 1987; Zhu, 1989; Zhou et al., 1989; Li, 1991; Okada and Sakai, 1993; Sakai and Okada, 1997). Most of the previous work on sedimentary basins in China discusses sedimentary basins in terms of plate tectonic setting, and the general conclusion of these studies is that basins are classified into rift basins or extensional basins, flexural basins and Ł Fax:
C81 92 573-0240; E-mail: [email protected]
their transitional basins (Chen and Dickinson, 1986; Watson et al., 1987) (Fig. 1). The flexural basins are characteristic of western China, while the rift basins occur in eastern China (Watson et al., 1987) (Fig. 1). Chen and Dickinson (1986) identified western basins and eastern basins, respectively, proposing a division line running from central Mongolia to Laos, which I here call the Baikal–Yunnan Line (Fig. 2). This line reflects the difference in major tectonic environments between the two regions (Chen and Dickinson, 1986; Watson et al., 1987). The concept of these basin classifications is acceptable in principle, but mechanisms for the origin of these basins have not been fully explained. Therefore, in this paper I consider the origin and development of sedimentary basins from the Asian continent to an island arc in the context of superplume activity.
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 0 3 - 6
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Fig. 1. Distribution of major sedimentary basins and their basin types in East Asia (based mainly on Watson et al., 1987) and magmatic areas in Jurassic to Early Cretaceous time.
2. Geotectonic setting of Mesozoic basins
2.1. Forearc basins
In a traverse section from the Japanese Islands arc to the East Asian continent during the late Mesozoic, four types of sedimentary basins are recognizable: forearc basins, intra-arc basins, back-arc basins, and intra-continental basins. The island arc portion consists of forearc basins, intra-arc basins and back-arc basins from the ocean side to the continental side, as is well documented in the Southwest Japan Arc (Okada and Sakai, 1993; Sakai and Okada, 1997). Among these, intra-arc and back-arc basins are characterized by rifting and pull-apart features, and the intra-continental region from the Korean Peninsula to eastern China is characterized by large-scale rifting features.
These are developed in the Japanese Islands and are typically found in the Shimanto Belt in Southwest Japan and contain the Cretaceous to Paleogene Shimanto Supergroup, which is characterized by highly deformed strata originally deposited in trench to forearc basins. They show typical features of accretionary prisms due to trench subduction (Taira et al., 1982, 1988). The Cretaceous sequence of the Shimanto Supergroup is composed mainly of alternations of sandstone turbidites and shale, and subordinately of chert, red shale, micritic limestone and pillow basalt, attaining a total thickness of more than 10,000 m (Okada, 1996).
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Fig. 2. Tectonic framework and basin arrangement in the Cretaceous Eurasian Plate.
2.2. Intra-arc basins In Southwest Japan, particularly on Kyushu and Shikoku Islands, intra-arc basins are well developed (Sakai and Okada, 1997). They are generally faultbounded trough-like depressions arranged in parallel to the Median Tectonic Line. They are scattered, small elongate basins (Fig. 3) filled by clastic sediments of non-marine, paralic to shallow-sea and deep-sea facies (Okada and Sakai, 1993; Okada, 1996; Sakai and Okada, 1997) with considerable thickness.
On Kyushu Island, small Cretaceous basins in the Kurosegawa Belt are rift basins, while those in the Axial Zone are either rift basins or pull-apart basins (Sakai and Okada, 1997). In the Axial Zone, these basins show a unique trend in timing of sedimentation in close association with magmatism (Okada, 1996), as will be discussed in detail later. 2.3. Back-arc and intra-continental basins In a broad region from the back-arc of Kyushu Island through the Korean Peninsula to eastern China,
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Fig. 3. Distribution of the main Cretaceous basins along the Axial Zone of Southwest Japan.
large-scale rift basins and relatively small-scale pullapart basins occur. Basins of the former type include the Upper Jurassic to Lower Cretaceous Tetori Basin and the Lower Cretaceous Kanmon Basin in the back-arc of Southwest Japan, the Lower Cretaceous Kyongsang Basin in South Korea, the Lower Cretaceous Songliao Basin as well as the Upper Cretaceous Bohai Bay and Yellow Sea Basins in Northeast China and the East China Sea Shelf Basin (Fig. 1). The Yongdong Basin and other small basins along the Ogchon Belt in the Korean Peninsula (Lee and Paik, 1990; Lee et al., 1991) belong to pull-apart type basins.
in a north–south compression is sharply different from that in eastern China, the Korean Peninsula and the Japanese Island arc, all of which were under east–west stress (Fig. 2).
2.4. Flexural basins
3.1. Pull-apart basins
Basins of this type occur in western China and follow an E–W trend. These basins were originally formed within microcontinental blocks in Paleozoic and early Mesozoic times, which had been subjected to north–south compression by northward subduction episodes of the Tethyan Plate (Zhang et al., 1984; Chen and Dickinson, 1986; Watson et al., 1987; Li, 1991). Basin sediments as well as surrounding mountain ranges were subject to a second phase of folding by the renewed northward collision of the Indian Plate during the mid-Tertiary. Therefore, the geotectonic setting of these basins
Pull-apart basins are generally elongated and developed mostly along large-scale strike-slip faults. Intra-arc basins in the Chichibu Belt and the Axial Zone and back-arc basins in the Japanese Islands (Okada and Sakai, 1993; Okada, 1996; Sakai and Okada, 1997), the Yongdong Basin and related basins in South Korea (Lee and Paik, 1990; Lee et al., 1991), the Yilan–Yitong Fault-related basins east of the Songliao Basin, and elongated basins along the Tanlu Fault east of the Hefei Basin, all belong to the pull-apart basins. In general, they show half-graben profiles. Miyata (1980) showed experimentally that
3. Basin development in East Asia Sedimentary basins, except for forearc basins in East Asia, are divided into two tectonic types: pullapart basins closely related to strike-slip faults and rift basins related mainly to large-scale magmatism or plume activity.
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Fig. 4. Age trend of magmatism and timing of basin formation. Data of magmatism are based on Kinoshita (1995). The starting point of distance is located at the western part of the Amakusa Islands, central Kyushu (A) and at the southern margin of the Onogawa Basin, central Kyushu (B) (see Fig. 3).
the scale of graben structures is controlled by the degree of slope angles of the main strike-slip faults. The relationship between the development of pullapart basins and magmatic events is well presented by intra-arc basins along the Median Tectonic Line in Kyushu and Shikoku Islands. In this area, small Cretaceous basins with clastic sediments of considerable thickness are exposed (Fig. 3): (1) the westernmost basin is filled by the latest Albian to earliest Turonian Goshonoura Group comprising fossil-rich shallowmarine sediments (in the lower) and brackish-water to freshwater sediments (in the upper); (2) the western central Mifune Basin composed of Cenomanian to Turonian clastics consists of brackish-water to shallow-marine sandstones with abundant molluscan fossils in the lower and lacustrine red beds in the upper; (3) the eastern central Onogawa Basin filled mainly by Turonian to Santonian turbidites; and (4) the Izumi Basin in the Shikoku Island accommodating Campanian to Maastrichtian turbidites. The Izumi Basin itself, which extends over 500 km, began its sedimentation in the Campanian in the west
and in the Maastrichtian in the east. The Onogawa and Izumi Basins are characterized by extraordinarily thick piles of turbidites (Okada, 1996). It is worthwhile to point out that the depocenters in these basins shifted gradually eastwards (Matsumoto, 1978; Okada, 1996) (Fig. 4). This diachronism in the basin formation is very similar to the age trend in magmatism in Southwest Japan (Fig. 4). The age trend in magmatism shows a rate of about 1=30 (Ma=km) (Kinoshita, 1995), which is close to that of movements of the Kula and Pacific Plates (Seno and Maruyama, 1984). Therefore, it is evident that formation of pullapart basins and subduction-related magmatic events in Southwest Japan are closely related to each other. 3.2. Rift basins Rift basins in East Asia are generally characterized by their large scale and fault-bounded structure. They are distributed in four major tectonic zones: Sichuan and Ordos Zone in the west, Hefei–Bohai
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Bay and Songliao Zone, Subei Zone and the East China Sea and Kyongsang Zone in the east (Fig. 1). Most of these intra-continental basins were superimposed on older structural trends (Chen and Dickinson, 1986; Zhu, 1989). These basins are filled by a fining-upward sequence of clastics and evaporites of fluvio-lacustrine origin. The Sichuan (about 800 km ð 500 km) and Ordos (700 km ð 500 km) Basins exhibit a westward thickened wedge of basin-fill (Wang et al., 1989, for the Sichuan Basin; Sun et al., 1989, for the Ordos Basin) that appear to have developed in the Jurassic to Early Cretaceous, with syn-sedimentary development of antithetic growth faults. The Hefei and Bohai Bay Basins are located north of the Qinglin Suture and west of the Tanlu Fault (Fig. 1). The Hefei Basin is fault-bounded by the Gushi–Shangcheng Fault on the west and by the Tanlu Fault on the east side as well as by the Tongbai– Dabie Fold Belt to the south. The Hefei Basin reached its maximum development during the Jurassic to Cretaceous with a thickness of more than 1500 m of fluvio-lacustrine clastic sediments (Han et al., 1989). The Bohai Bay Basin occupies an area as wide as 200,000 km2 between the Taihang Shan Uplift to the west and the Tanlu Fault to the east (Fig. 1). Due to extensive block-faulting activity during the Late Jurassic to Early Cretaceous, many small faulted depressions occurred in the Bohai Bay Basin area, which were filled by coal-bearing red-colored clastic sediments and varieties of volcanic rocks with a total thickness of about 4000 m (Hu et al., 1989). Principally, this basin consists of a series of faulteddepressions and faulted-uplifts. The Songliao Basin located in northeastern China is 750 km long and about 350 km wide, occupying an NNE–SSE-trending area of 260,000 km 2 . The basic structure of the basin is graben-like, controlled by faults on both sides of the basin. The basin was first filled by Upper Jurassic fluvio-lacustrine sediments associated by volcanic rock layers, and rapidly subsided during the Early Cretaceous and received deposits until Maastrichtian time with a maximum thickness of about 6700 m of petroliferous sandstone and shale of lacustrine facies (Ma et al., 1989). The largest sedimentary basin in the Korean Peninsula is the Kyongsang Basin, measuring about 250 km long and 125 km wide. The basin-fill con-
sists of the Kyongsang Supergroup, which is divided into three sequences, the Singdong Group, the Hayang Group and Yuchon Volcanic Group, in ascending order (Chang and Park, 1995). The Valanginian to Hauterivian Singdong Group, 2000 to 3000 m thick, is composed of conglomerate, sandstone and shale. The Barremian to Albian Hayang Group, 1000 to 5000 m thick, consists of sandstone and shale with a minor amount of conglomerate and marl. They are intercalated by pyroclastic layers and andesitic and basaltic rocks at some places and horizons. The Albian to Cenomanian Yuchon Group, more than 2000 m thick, is characterized by the predominance of intermediate to acidic volcanic rocks. The Kyongsang Basin shows a half-graben structure tilted eastwards (Okada and Sakai, 1993). The East China Sea Shelf Basin is characterized by composites of fault-controlled half-grabens formed in the Proterozoic basement (Zhou et al., 1989; Yu, 1991). They are narrowly elongated en echelon with a NE–SW to NNE–SSW trend, extending more than 1000 km. Most of the basins are filled by nearly 4000 m of Cretaceous, Paleocene and Eocene sediments (Zhou et al., 1989). All these half-grabens initiated in the Late Jurassic, but major subsidence took place in the Late Cretaceous.
4. Relationship between basin formation and plume magmatism 4.1. Overview It has long been advocated that Cretaceous to Paleogene magmatism migrated eastwards at a constant rate in Southwest Japan, oldest in the west and youngest in the east (Matsumoto, 1969, 1978; Kinoshita, 1995, 1997) (Figs. 3 and 4). The mechanism of this migration was attributed to plate movements because of motion rates of proto-Pacific plates of about 20 cm=year (Seno and Maruyama, 1984). Further, for vigorous magmatism, many workers have invoked ridge subduction (Uyeda and Miyashiro, 1974; Kinoshita and Ito, 1986; Kiminami et al., 1993; Kinoshita, 1995, 1997). However, the area of magmatism is too large to ascribe to ridge subduction. In addition, oblique ridge subductions can only partly explain the left-lateral faultings north of the Qinling
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Suture, but are hardly applicable to the faults south of the Qinling Suture as well as in the South China Sea region, where right-lateral faults are expected. However, almost all the major strike-slip faults in the East Asian continent are of left-lateral sense (Figs. 1 and 2). Matsumoto (1978) first pointed out that such a migration of Cretaceous magmatism must have been related to eastward-younging sedimentation in Southwest Japan. Okada and Sakai (1993), Okada (1996) and others, also confirmed this concept. The problems of these connotations are that magmatism from Jurassic to Cretaceous time was far too large to attribute this to simple subduction of oceanic plates or ridge subduction, and that Sakai and Okada (1997) have revealed a deficiency of accretionary prisms in the Outer Zone of Kyushu Island and instead, the development of a passive margin. This seems to be a general feature in Southwest Japan, as suggested by Takahashi and Ishii (1995). This fact of non-development of accretionary prisms in the Early Cretaceous is critically important to understand the significance of large-scale magmatism on the East Asian continental margin.
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4.2. Plume magmatism on the East Asian continental margin Magmatism in Southwest Japan and the East Asian continental margin was extremely active during the late Mesozoic to early Cenozoic, which produced tremendous amounts of volcanic and plutonic rock emplacements. These are distributed from the Guangxi region in Southeast China to the Chukotka Peninsula in East Russia over 7000 km along the eastern continental margin, which are in parallel to major tectonic lines (Figs. 1 and 5). Takahashi (1983) divided the igneous rock associations in the East Asian region into four types, mainly based on their chemical natures: island arc, continental margin, intra-continent and collision types. He further discriminated five chronological series of magmatism: 190–160 Ma, 160–130 Ma, 130–100 Ma, 100–70 Ma and 70–40 Ma (Fig. 5). Among these time series, the 160–130, 130–100 and 100–70 Ma magmatic events of the continental margin and intra-continent types seem to have been most active. These magmatic events are characterized by
Fig. 5. Space and time distributions of various types of magmatisms in Jurassic to Cretaceous East Asia (modified from Takahashi, 1983).
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granite, granodiorite, felsic to intermediate welded tuffs, rhyolite, andesite and basalt. Takahashi (1983) ascribed these magmatic events to the subduction of oceanic ridge and young hot oceanic lithosphere, as Uyeda and Miyashiro (1974) suggested. Basically, Kinoshita and Ito (1986, 1988) and Kinoshita (1995, 1997) followed the concept of ridge subduction as a cause of large-scale magmatic events in the late Mesozoic. On the contrary, it is argued that many intra-cratonic large basins in East China are dissected by many faults, producing intra-basinal half-grabens and grabens, as in the Subei, Bohai Bay, Sichuan and East China Sea Basins. It is generally believed that this faulting was due to the crustal stretching and thinning (Hsu, 1989) induced by the swelling of the upper mantle (Li, 1991) or by the arching force by upwelling of a thermal plume (Yano and Wu, 1995, 1997). The broad distribution and huge amount of the late Mesozoic igneous rocks on the East Asian continental margin, as revealed by Takahashi (1983), are too extensive to be regarded as the product of arcmagmatism simply related to plate subduction. The
space and time distribution of these late Mesozoic igneous rocks is comparable to that of the large igneous provinces proposed by Larson (1991a,b) and Coffin and Eldholm (1994), though these igneous rocks consist of mixtures of plutonic and volcanic rocks from acidic to basic (Takahashi, 1983). It is interesting to point out that the subduction-related magmatisms do not seem to have any relation to basin formation, as is clearly shown in Fig. 6. This is reinforced by the fact that the accretionary prism was not formed in the Outer Zone of Southwest Japan during the Early Cretaceous (Sakai and Okada, 1997), suggesting that subduction-related magmatisms at that time should have been inactive. Nevertheless, Early Cretaceous magmatic events were vigorous on the eastern margin of the Asian continent, as described above. Therefore, at least Early Cretaceous large magmatic events must have been plume-related. Yano and Wu (1995, 1997) suggested that the inclined upwelling of a superplume produced the asymmetric arching structure over a vast area as wide as 2500 km on the Eastern Asian continental margin.
Fig. 6. Timing of basin formation and its relation to subduction-controlled magmatic belts on the East Asian continental margin. Data of magmatism are based on Kinoshita (1995). The starting point of distance is located at the western tip of Honshu Island, Japan.
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Fig. 7. Development of rift basins closely related to plume activity at the East Asian continental margin (modified from Yano and Wu, 1995).
According to this explanation, the origin of most of the sedimentary basins of rifting origin must have been controlled by superplume-related upwellings, but not by subduction-related magmatism (Fig. 7). Even at the place of slab penetration due to the subduction of oceanic plates, return flows from the 670 km thermal boundary layer begin to produce plumes beneath the continental crust (Coffin and Eldholm, 1994; Larson and Kincaid, 1996). This plume magmatism must have played an important role to produce large igneous provinces at the eastern margin of the Asian continent, in addition to the normal subduction magmatism. Many of the rifting faults may have been further converted to strike-slip faults by northward movements of the Izanagi and Pacific Plates, resulting in the formation of pull-apart basins along some major strike-slip faults (Okada and Sakai, 1993; Sakai and Okada, 1997).
the Early Cretaceous. Nevertheless, huge amounts of magmatic emplacement did occur at that time in close association of large sedimentary basins of rifting origin. Thus, these magmatic events are well explained in terms of plume activity. This means that plume-related magmatism was developed in the eastern part of the Asian continent due to return flow effects from the 670 km thermal boundary layer. Such a plume magmatism may explain systematically the origin and development of varieties of rift basins in East Asia during Cretaceous time.
Acknowledgements I would like to express my sincere thanks to Dr. Niall J. Mateer of the University of California, Prof. Ki-Hong Chang of Kyungpook National University, Korea, and Dr. Takashi Sakai of Kyushu University for valuable discussions. Niall Mateer and Ki-Hong Chang critically read the manuscript.
5. Conclusion On the eastern margin of the Asian continent, subduction of oceanic plates was inactive or completely ceased during the Early Cretaceous, and instead, the passive margin prevailed. Therefore, no subduction-related magmatism should be encountered in
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