Thematic Article - Department of Earth Science and...

The Island Arc (1997) 6, 25-51

Thematic Article Jurassic accretion tectonics of Japan

YUKIO ISOZAKI Department of E a r t h and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152, Japan

Abstract The Jurassic accretionary complex and coeval granites in Japan represent remnants of the Jurassic arc-trench system developed between the Asian continent and Pacific Ocean. The Jurassic accretionary complex occurs as a large-scale nappe that is tectonically sandwiched between the overlying pre-Jurassic nappes and underlying post-Jurassic nappes. By virtue of new research styles (microfossil mapping and chronometric mapping) the following new views of the Jurassic accretionary complex in Japan, that suggest those for on-land exposed ancient accretionary complexes in general, have been obtained: (i) the accretion age of the Jurassic accretionary complex ranges over - 80 million years from the latest Triassic to earliest Cretaceous according to a reconstructed stratigraphy of component rocks (oceanic plate stratigraphy); (ii) the accretionary complex is subdivided into several nappe units, each characterized by unique oceanic plate stratigraphy; (iii) a tectonically downward-younging polarity is observed in the piled nappes; (iv) the Jurassic accretionary complex is composed of coherent-type and chaotic-type units, the former retaining the primary accretionary structures while the latter are characterized by collapsed and secondarily mixed materialslfabrics derived from the former; (v) the chaotic-type units predomi- nate in volume over the coherent-type units; (vi) the accretionary complex suffered from a regional low-grade metamorphism (up to the lower greenschist facies) within -10-20 million years after the accretion timing; and (vii) the lateral extent of the Jurassic accretionary complex in East Asia is intermittently traced from the Koryak mountains in Russia to North Palawan in the west Philippines for -6000 km. Discussion focuses on (i) the low preservation ratio of the coherent-type units to the chaotic-type units with respect to frequent subduction erosion by seamount subduction; (ii) absence of the Franciscan-type melange, suggesting sedimentary mixing origin for the chaotic- type unit; (iii) a growth rate of the Jurassic accretionary complex compatible to modern analogues; and (iv) the total volume of the Jurassic accretionary complex in Japan with respect to the most likely terrigeiious elastics source along the 250 Ma continent-continent collision suture in central China (between the Sino-Korean and Yangtze blocks).

Key words: accretion, Asia, Japan, Jurassic, nappe, oceanic plate stratigraphy, orogeny, radiolaria, subduction, tectonic erosion.

INTRODUCTION

Studies of modern accretionary prisms clarified many new aspects 'of the active subduction zone during the last two decades. Various internal structures of the frontal part of accretion prisms, and growth meachanisms such as offscraping, underplating, role of decollement etc., have been revealed through numerous deep-sea drilling experi- ments coupled with regional mapping by hi-tech seismic reflection techniques in several active

Accepted for publication May 1996

trenches (e.g., Barbados Ridge in the Caribbean and Nankai Trough off Southwest Japan; Moore et al. 1988; Taira et al. 1992). On the other hand, on-land geological research on ancient accretionary complexes (AC) also provided valuable information on much deeper parts of the accretionary wedge (in Alaska and Japan; Sample & Moore 1987; Taira et al. 1988; Isozaki et al. 1990b).

Studies of the Late Paleozoic t o Mesozoic AC in Japan, in particular those in Southwest Japan in the 1980s (Yao et al. 1980; Kanmera & Nishi

26 Y. Isoxaki

1983; Taira et al. 1988; Ichikawa et al. 1990; Isozaki & Maruyama 1991; Matsuda & Isozaki 1991), have established new research styles (microfossil mapping and chronometric mapping) for ancient AC (see historical review in Isozaki 1996) in which detailed field mapping on a 1/5000 scale is combined with high-resolution microfossil (conodont and radiolaria) and/or radiometric dating methods in a layer-by-layer or block-by-block manner. These new mapping schemes have brought unique and significant insights to ancient AC in orogenic belts (Isozaki 1996; Maruyama 1997). In terms of high-resolution mapping, good fossil and radiometric age control, and fertile petrologic and/or geochemical analyses, the Jurassic AC in Japan may represent one of the largest and best analyzed examples of on-land exposed ancient AC in the world.

In this article, the latest aspects of the Jurassic AC in Japan and in East Asia are reviewed through regional compilation. Characteristics of component rocks, internal and external structures, and regional low-grade metamorphism are briefly described, and on the basis of these new observations, their tectonic implications are discussed. Some newly recognized aspects require consider- able changes in general understanding of ancient AC exposed on land: (i) distinction of the coherent part and chaotic part within the same AC; (ii) a subhorizontal piled nappe structure with a

downward-younging polarity; and (iii) a 10-20 million-year gap between accretion and high-P/T metamorphism. The spatial extent of the Jurassic AC in East Asia is also examined to determine the dimension and volume of an AC developed between a major ocean and a continent. Discussion will focus on (i) the preservation and destruction of the primary accretion features in an AC edifice; (ii) absence of tectonic melange; (iii) growth rate of an AC; and (iv) the total volume of an AC and a possible material source.

JURASSIC TECTONIC SETTING OF EAST ASIA

One of the biggest plate reorganizations of Phan- erozoic tectonic history began in the Jurassic period, that involved the opening of the Atlantic and the resultant breakup of the Pangean supercontinent. On the other hand, in wesern Panthalassa, several continental blocks detached originally from an older supercontinent, Rodinia; that is, the Sino- Korean (North China) and Yangtze (South China) blocks merged into a larger continental mass (Asia) by the Middle Jurassic (Maruyama e t al. 1989). Figure 1 depicts a Late Jurassic paleogeographic reconstruction of Pangea after the Jurassic continental coalescence. After the several collision and amalgamation events in Asia, a simple plate tectonic setting appears that featured a continuous

Fig. 1 Late Jurassic (ca 150 Ma) paleogeography of the supercontinent Pangea and superocean Panthalassa (modified from Scotese & Langford 1995). Proto-Japan along the eastern margin of the amalgamated Yangtze and Sino-Korean blocks has represented a segment of the lengthy circum-Pangea subduction zone. The subduction of the Izanagi-Kula plate beneath East Asia was responsible for the development of the Jurassic arc-trench system around proto-Japan.

Jurassic accretion tectonics of Japan 27

side of the previously mentioned granite belt, an along-arc AC belt is recognized in East Asia from the western Philippines to the Koryak-Kamchatka region, northeast Russia. These observations con- firm the development of a well-matured arc-trench system along Jurassic East Asia induced by oceanic subduction from the Pacific side. Because the paleomagnetic data indicate no large latitudinal movement for the amalgamated China for this duration (Lin et al. 1985), an oceanic plate, probably the Izanagi-Kula plate that formed a part of the paleo-Pacific seafloor, actively subducted northwestward beneath the East Asian continental margin. This setting, that is, northwestward oceanic subduction beneath Eurasia from the Pacific side, persisted up to the present time (Maruyama et al. 1989), and the eastern continental margin of East Asia, including proto-Japan, has grown oceanward for -200 km in width since the Jurassic.

subduction zone between one major continent (Asia) and one major ocean (Pacific). The eastern continental margin of Asia, particularly that of China (composed of amalgamated North and South China blocks), faced the Jurassic Pacific Ocean directly. The analysis of relative plate motions in the Pacific domain was once attempted by Enge- bretson et al. (1985) on the basis of paleomagnetic data with respect to hot spot tracks, however, no practical constraint was given for the Jurassic period owing to the iscarcity of information.

The distribution of the Jurassic calc-alkaline granite belt in East Asia positively indicates that active subduction was taking place from the Pacific side; that is, the Middle-Late Jurassic (ca 180- 150 Ma) Daebo granite in the Korean Peninsula and the Early Yanshanian granitoids in South China (Takahashi 1983; Cheng 1986; Fig. 2). Run- ning almost parallel to but strictly on the ocean

n-related orogen in East Asia

Jurassic arc granite belt

Jurassic AC belt

Pucfic oceun

Jurassic Orogenic Units in Japan

500 km

Jurassic ( l a t e s t Triassic to earliest C r e t a c e o u s ) AC

Jurassic granite belt low-P/T metamorphosed Jurasslc AC

Fig. 2 Distribution of the Jurassic accretionary complex and granite in Japan (modified from lsozaki 1996). Note the two belts (Mino-Tanba belt and Chichibu belt) with the Jurassic accretionary complex in Southwest Japan. I-K.T.L., Ishigaki-Kuga Tectonic Line that bounds pre-Jurassic belts and the Mino-Tanbo belt; B.T.L., Butsuzo Tectonic Line that bounds Chichibu belt and the post-Jurassic belts; M.T.L., Median Tectonic Line; T.T.L., Tanakura Tectonic Line; I-S.T.L., Itoigawa- Shizuoka Tectonic Line. Inset shows an index map of Jurassic subduction-related orogenic elements (accretionary complex and granite) in East Asia (compiled from Takahashi 1983; Mizutani etal 1986; lsozaki eta/. 1987; lsozaki & Nishimura 1989; Kojima 1989; Vishnevskaya & Filatova 1994).

28 Y. Isoxaki

Marine to non-marine Jurassic sediments of shallow-water shelf facies occur sporadically in Asia, and they provide us with vital information on faunal-floral provinciality in Jurassic East Asia. From a paleogeographical viewpoint, faunal-floral distinction between the relatively cooler Boreal province mostly in Sikhote-Alin/Oloi, northeast Russia, and the warmer Tethys-related East Asian province in China is a key constraint in Jurassic East Asia and the western Pacific (Hayami 1990). The Japan segment of the Jurassic orogen belongs to the East Asian province, suggesting a close link to China. This is consistent with the pre-Japan Sea (pre-Miocene) paleogeographic reconstruction of Asia (Otofuji 1996).

JURASSIC OROGEN IN JAPAN

Nearly one-half of the supracrust of the Japanese Islands is occupied by the Jurassic orogenic complexes (Fig. 2). The Jurassic AC occurs widely throughout the Japanese Islands for more than 3000 km along-arc, from Ishigaki Island, the southern Ryukyus on the southwest, to west Hok- kaido on the northeast, and for up to 200 km wide across-arc. The other important component of the Jurassic orogen in Japan is the Early to Middle Jurassic (204-159 Ma) granitic rocks (I-type, tonalite-granodiorite) but their distribution is restricted exclusively to the Hida belt in central Japan.

The Jurassic AC is best exposed in Southwest Japan by virtue of rapid Quaternary uplifting. The distribution of the Jurassic AC in Southwest Japan is two-fold; that is, the AC belt on the Japan Sea side -70-100 km wide across-arc) is called the Mino-Tanba belt, and the AC belt on the Pacific side -10-30 km wide) is called the Chichibu belt (Fig. 2). In addition, a regionally metamorphosed Jurassic AC occurs in the domain called the Ryoke belt in Southwest Japan, just south of the Mino- Tanba belt. The Ryoke belt represents a regional low-PIT type metamorphic belt (forming the Cretaceous paired metamorphic belts with the high-PIT Sanbagawa belt) associated with Late Cretaceous arc granite. The non- to weakly metamorphosed Jurassic AC changes gradually into the Ryoke metamorphics via a contact aureole, and the Ryoke belt is regarded as a shallower part of the Late Cretaceous arc crust (Nakajima 1997).

In Northeast Japan across the Tanakura tectonic line (TTL), the Jurassic AC apparently occurs in a 200 km wide domain called the Northern Kitakami-

Oshima belt, and its low-P/T metamorphic (Ryoke) equivalents in the Gosaisho belt. However, the external geometry and internal structure of Juras- sic rocks in Northeast Japan are secondarily modified by the intense left-lateral strike-slip dislocation related to the Tertiary Japan Sea opening (Jolivert et al. 1994). The thick Quaternary volcano-sedimentary covers in Northeast Japan also prevent on-land geologists from pursuing detailed analyses.

The Jurassic granites occur strictly in the Hida belt in Japan, separated from their main distribution in mainland Asia (Fig. 2). The Hida belt is regarded as a continental fragment detached from mainland Asia, particularly from the suture zone between the Sino-Korean and Yangtze blocks characterized by a 250 Ma ultrahigh-pressure metamorphic belt (Isozaki 1997). Although the Hida belt with Jurassic granites and the Mino-Tanba belt with coeval AC are juxtaposed in modern central Japan, a minimum separation for 150-200 km is required between an arc and a coeval trench in a subduction zone. This suggests that a post-Jurassic across-arc shortening of more than 100 km occurred, in order to juxtapose the two belts in central Japan. The nappe-style occurrence of the Hida belt over the Mino-Tanba belt is consistent with these constraints, and relevant geological- geophysical lines of evidence indicate that the juxtaposition occurred probably in latest Jurassic (Komatsu 1990; Sohma & Kunugiza 1993). In addition, the restricted occurrence of the Boreal- type Jurassic shallow-water fuana and flora (Hayami 1990; Kimura & Ohana 1990) suggest a unique northerly position of the Hida belt with respect to the rest of Jurassic proto-Japan. Isozaki (1996, 1997) reviews the overall tectonic evolution of the Japanese Islands and the origin of the Hida belt.

JURASSIC ACCRETIONARY COMPLEX

Major aspects of the Jurassic AC in Japan are described here, demonstrating well-documented examples from several areas in the Mino-Tanba and Chichibu belts in Southwest Japan (Fig. 2). The description covers the lithologic characteristics of component rocks, their pre-accretion stratigraphy, internallexternal structures of the AC, and timing of subduction-related metamorphism.

The Jurassic AC can be regarded as a mixture of two distinct rock categories, that is volcanic- sedimentary rocks derived from oceanic plate, and


On the other hand, from the viewpoint of field occurrence in outcrop/map scale, units of the Jurassic AC can be classified into two types: coherent-type and chaotic-type. The coherent-type unit is composed of imbricated thrust sheets of sedimentary sequences that retain the primary stratigraphic coherency in most parts, while the chaotic-type unit consists of mixed rock assemblages in which numerous blocksAenses of various lithology and size are chaotically scattered in argillaceous matrix. The latter, with such a block- in-matrix fabric, has often been described as an olistostrome and/or a melange. In the following part of this section, a representative and classic example of the coherent-type unit in the Inuyama area and that of the chaotic-type unit in the Sanbosan area are described as the two end- members that limit the variety of the Jurassic AC units. The intermediate types between these two end-members are all regarded here as the chaotic- type units. The coherent-type units occur only exclusively in the Middle Jurassic part, while the chaotic-type units occur in all parts. On the basis of distribution on the surface, the latter occupies nearly 70% in volume of the whole Jurassic AC edifice in Southwest Japan.

External configuration and internal structures of the Jurassic AC in Japan are ubiquitously characterized by a regional-scale subhorizontal piled nappe structure (Fig. 3). The internal nappes show a clear tectonically downward-younging polarity, in accordance with the primary accretion polarity at the trench. The apparent total thickness of the whole Jurassic AC nappe is estimated a t more than 10 km in many areas, and in some cases up to nearly 20 km (Wakita 1988; Otsuka 1988; Nakae 1993). A typical example of a large-scale klippe- like occurrence of the Jurassic AC is described from the Omine area.

The Jurassic AC rocks usually suffered from a regional metamorphism of the prehnite-pumpellyite facies to lower greenschist facies, probably representing a low-grade part of the subduction- related high-PIT metamorphism. Chronometric dating on fine-grained meta-sediments of the Ju- rassic AC indicates that the metamorphic peak was reached immediately after the accretion a t the trench. Well-documented examples from the Mino- Tanba and Chichibu belts are given here.

In addition, the extent of the Jurassic AC in East Asia is briefly summarized in order to document the overall dimension and volume of the Jurassic AC. For further details of the Jurassic AC in Japan, particularly the results of regional mapping and

terrigenous clastic rocks derived from a continent or arc with a sialic basement. The former group consists of basaltic-gabbroic greenstones and fine- grained sediments, such as bedded radiolarian chert and pelagic limestone, that lack land-derived coarse-grained clastics. The latter group comprises coarse-grained terrigenous elastics, such as conglomerate, sandstone and mudstone, that contain abundant clasts of granites-gneiss and quartzo- feldspathic grains derived from sialic crust. Such a contrast in composition between the two groups indicates mutual isolation of their primary depositional sites, viz. land margin (of continent or arc) versus mid-ocean. An ancient oceanic trench is the best candidate for a mixing site of these contrasting rock types originally separated by a distance, and a subduction-related accretion origin of this mixed rock association is suggested. Layer-parallel shortening structures of the Jurassic AC indicate a great amount of contraction, and the resultant tectonic repetition of the sedimentary rocks appears identical to that observed in modern active AC.

By virtue of microfossil (conodont, radiolaria) dating, a primary stratigraphy can be reconstructed for the component rocks, called oceanic plate stratigraphy (OPS; refer to Isozaki 1996) because it records the sedimentary history upon an oceanic plate prior to accretion a t the trench. Accretion timing of an ancient AC can be determined by dating a horizon between oceanic sediments and terrigenous sediments in an OPS because this horizon approximates the arrival timing of an oceanic plate a t an ancient trench. Microfossil dating a t thousands of localities has clarified that the Jurassic AC in Southwest Japan has grown during a long interval from the latest Triassic to earliest Cretaceous, and predominantly in the Middle Jurassic. On the basis of accretion timing, the Jurassic AC in the Mino-Tanba belt has been subdivided into seven or more units (Wakita 1988; Nakae 1993; Fig. 3). The number of units discrim- inated by OPS, however, is totally dependent on the resolution of microfossil zoning (e.g. for the Jurassic, -7 million years for one radiolarian zone on average), and further detailed subdivision will be possible with enhanced finer zoning schemes in the future. In this article, from the chronological viewpoint of accretion timing, the Jurassic AC in Southwest Japan is roughly subdivided into the following three parts. (i) Latest Triassic to Early Jurassic (Early Jurassic part, for short); (ii) Middle Jurassic part; and (iii) Late Jurassic to earliest Cretaceous (Late Jurassic part, for short).

30 Y. Isoxaki

Fig. 3 Geologic map of the Mino-Tanba belt in central Japan (compiled from Wakita 1988; Otsuka 1988; Nakae 1993) . Inset shows a highly simplified geologic sketch map and profile of the same area (not to scale), demonstrating basic internal structure of the Jurassic accretionary complex (AC). Note the subhorizontal piled nappe structure with a tectonically downward younging polarity, and a nappe-oblique transverse fault cross-cutting it (fault with open and solid teeth). The south-neighboring Ryoke ( low-P/T metamorphic) belt contains metamorphosed equivalents of the Jurassic AC ot the Mino-Tanba belt; two belts are otten bounded by a secondary fault but they were primarily stitched by a regional contact aureole.

microfossil plus chronometric mapping in each belt, refer to the following literature; for the Mino- Tanba belt in Southwest Japan; Tamba Belt Re- search Group (1979), Wakita (1988), Otsuka (1988), Mizutani (1990), Fukudomi (1990), Nakae (1993), and Takami et al. (1993); for the Chichibu belt in southwest Japan: Yamato-Omine Reserach Group (1981, 1992), Yao (1984), Yamakita (1988), Hada and Kurimoto (1990), Matsuoka (1992), and Hisada et al. (1992); for Northern Kitakami- Oshima belt in Northeast Japan: Okami and Ehiro (1988), and Minoura (1990).

COHERENT TYPE: IMBRICATED LAYER-PARALLEL THRUST SHEETS

The best documented example of the coherent-type unit in the Jurassic AC, from the viewpoint of quality and quantity of data in microfossil age control, and petrographic, structural and paleo-

magnetic analyses, occurs in the Inuyama area in the central Mino-Tanba belt, Southwest Japan (Kondo & Adachi 1975; Yao e t al. 1980; Shibuya & Sasajima 1986; Matsuda & Isozaki 1991, Kimura & Hori 1993; Figs 3, 4). The Jurassic AC in this area is composed of two distinct lithologic units: bedded radiolarian chert and elastic rocks such as sandstone and mudstone. Four thick units of bedded chert (CH-1 to CH-4) are apparently interlay- ered with those of the elastic rocks. These rocks have no signature of melange-like tectonic mixing, and they appear to have a coherent stratigraphic succession characterized by the alternation of two lithologies. In fact, the entire apparent superposi- tion of the these rocks was long accepted by previous geologists as a primary coherent strati-

Microfossil dating, however, has demonstrated a striking age difference between the chert and clastic rocks; the bedded chert (CH-1 to CH-4) ranges

graphy.


Fig. 4 Geologic map and bloi:k diagram of the lnuyama area, Southwest Japan, showing the imbricated structure of the coherent- type unit of Jurassic accretioiiary complex (modified from Matsuda and I!;ozaki 1991) Note the tectonic repetition of the same stratigraphic sequences (chert and clastic rocks) four to five times within this area by layer-parallel faults

Middle- Late Jurassic clastic rocks

Mid Triassic- Early Jurassic bedded chert

/ fault

Tertiary upright folding axis , af te r hondo & A d a c h i (19751

in age from the Early Triassic to Early Jurassic (Figs 4, 5), while all the clastic rocks range from Middle to Late Jurassic, regardless of their structural horizons (Yao et al. 1980). In addition, the detailed microfossil zoning revealed that all chert units are facing right-side up. These age relations clearly refute the previous belief in total coherent stratigraphy and, instead, suggest a post- sedimentary tectonic repetition by layer-parallel faulting to form a pile of imbricated thrust sheets. The name coherent-type is given according to an appearance in contrast to the chaotic-type described later.

Constraints from rnicrofossil data are concordant with field observations, because most of the tops and soles of the major chert units are tectonically truncated by faults, nearly parallel to subparallel to bedding. For example, the base of the CH-2 (Mid- dle Triassic) is in contact with a structurally underlying elastic unit of the Middle Jurassic age by a fault with a 20 em wide black gouge. Unless a significant microfossil age gap is indicated, however, field recognition is quite difficult for a knife- sharp fault without a remarkable fault gouge, for example, that a t the sole of the CH-4 tectonic slice.

The repeated occurrence of the same microfossil sequence covering the Middle-Late Triassic interval further indicates that some apparently thick- looking chert units such as CH-2 and CH-4 are

actually composed of three or more layer-parallel subslices with the same primary stratigraphy and facing. Within the Inuyama area, -5 km across, the same bedded chert sequence is tectonically repeated a t least 10 times by layer-parallel shortening. The primary thickness of a bona fide stratigraphic sequence of the bedded chert is merely -100 m. By unfolding the secondary synform structure, a pile of imbricated thrust sheets with top-to-south vergence is reconstructed, and this appears almost identical to that of a modern accretionary prism formed a t the trench through offscraping (Matsuda & Isozaki 1991; Kimura & Hori 1993; Fig. 6).

The microfossil-guaranteed primary stratigraphic thickness and age coverage of the Middle Triassic to Early Jurassic bedded cherts give a very low average sedimentation rate, -0.5 g/cm2/1000 years (Matsuda & Isozaki 1991). Modern examples for such a slowly deposited siliceous sequence are only expected in open-ocean deep-sea floors. In addition, the purity of biogenic silica (> go%), absence of coarse-grained terrigenous elastics and carbonates, long-term successive deposition (> 50 million years), and extensive area of deposition (> 5000 km) all support the deep-sea pelagic origin for the bedded radiolarian chert. On the basis of these observations, Matsuda and Isozaki (1991) concluded that the bedded chert was prima-

32 Y. Isoxaki

I- - 7

281

Fig. 5 Stratigraphic distribution of microfossils (conodonts and radtolartans) from the middle one-third of the CH-2 chert unit in the lnuyama area (modified from Matsuda & lsozaki 1991) Recognition of 11 conodont zones in sequence within a 50 m thick chert suggests a low average sedimentation rate

rily accumulated in an open-ocean seafloor below the carbonate compensation depth (CCD), well prior to their encounter with the younger Jurassic coarser grained elastics derived from the continental margin. The stratigraphic base of the bedded chert sequence in the Inuyama area is abruptly truncated by a fault immediately beneath a char- acteristic siliceous claystone spanning the Permo- Triassic boundary (Isozaki 1994), however, MORB greenstones are expected as the most likely candidate for its basement. The paleomagnetism of the Middle Triassic part of the bedded chert indicates deposition in a low-latitude area (3"N or S; Shibuya & Sasajima 1986) that is concordant with the mid-oceanic origin, remote from the proto- Japan continental margin in the mid-latitude.

On the other hand, the Middle-Late Jurassic sandstone, mudstone and conglomerate, full of coarse quartzo-feldspathic grains, no doubt represent ancient near-continent sediments. In particular they contain typical continental derivatives such as cobble and/or boulders of 2000-1700 Ma, gneiss and/or granite (Shibata & Adachi 1974) and detrital monazite grains dated 1700-1400, 1250,

850 and 250 Ma (Suzuki et al. 1991). These detrital clastics suggest that the likely provenance of the terrigenous elastics of the Jurassic AC was the North and South China (Sino-Korean and Yangtze) blocks, including the 250 Ma suture zone between them (Isozaki & Maruyama 1991; Sohma & Kunugiza 1993; Isozaki 1997). Sedimentary structures suggest a turbidite origin for these terrigenous clastics (Kondo & Adachi 1975).

Microfossil dating and field observation clarified that a stratigraphic interval between the pelagic bedded chert and terrigenous elastics is occupied by a siliceous mud(si1t)stone that shows an intermediate lithology between chert and terrigenous mudstone both in grain size and composition. Figure 7 illustrates a gradual lithologic change from the underlying bedded chert to bedded siliceous mudstone within the same late Early Juras- sic radiolarian zone. The primary thickness of the siliceous mudstone reaches 40 m, and its age ranges up to the early Middle Jurassic. The siliceous mudstone is in turn covered by coarse- grained sandstone and/or mudstone of the late Middle Jurassic (Fig. 8). Its lithology and relatively


-OF-SEQUENCE THRUST ZONE IMBRICATE THRUST ZONE

7 SUBDUCTION DlnECTlON *, Z k r n TERRIGENOUS INTERVAL ~EMipE~AG,c iNTERvAL

.Ribbon ... h.e! .t............. INTERVAL s.i!ceous .c!a~s!o??. .............................................

Clastic rock

S.ili,c,eo.u.s mUdS!O,?e ............................................... ............................................

OCEANIC CRUST

~ accretionary complex > pohplslhomlpd.plc

Ir.n.lllon zone b I

Duplexlng-underplstlnp o f pelagic sediments h lerrlgenous cIas1Ics

- decollemont

(s.nd.1. + mud.1.) h.mlp.l.pls sed. (SII. m. )

Fig. 6 Schematic models for landward accretion of deep-sea pelagic sediments at an ancient trench proposed on the basis of the observations in the lnuyama area (a) Model showing the formation of imbricated thrust sheets by offscraping at the toe of a trench (modified from Kimura & Hori 1993), (b) model showing tectonic setting around a trench where deep-sea sediments are accreted and incorporated into duplex-like structures (modified from Matsuda & lsozaki 1991)

higher average sedimentation rate suggest a deep- sea hemipelagic origin; that is, deposition in a transitional environment between a pelagic realm and a land margin.

The horizon between the hemipelagic siliceous mudstone and overlying terrigenous elastics is quite important, because it marks the timing of the first encounter of oceanic (pelagic and/or hemipelagic) sediments with terrigenous elastics, probably of trench-fill turbidites. This horizon between the two lithologies can be a reliable marker for the accretion timing of an AC (Figs 6, 8). As far as a coherent-type unit with a reconstructed OPS is

concerned, precise dating of an accretion timing is therefore possible for an ancient AC. In the case of the Jurassic AC in the Inuyama area, it was accreted to proto-Japan in the late Middle Jurassic (ca 160 Ma). In addition, the age of the subducted oceanic plate responsible for accretion was dated to be more than 90 million years old, as indicated by the total duration of the pelagic and/or hemipelagic sedimentation on the oceanic plate (from 251 to 160 Ma; Matsuda & Isozaki 1991).

Shikoku Island, in the Togano area of the Chi- chibu belt (Fig. 2), is another well analyzed example of the Jurassic AC in which numerous imbri-

34 Y. Isoxaki

Bedded chert with thin siliceous mudstone layer

with thlck sillceous mudstone layer

Siliceous mudstone

brick red part

0 chocolate brown part

U Topmost of CH-2 radiolrrian

assemblage zone

5 2

p : Y . 0 . I

3 0

cated tectonic slices of the coherent-type unit occur in a 6 km-wide zone (Matsuoka 1992, Fig. 9a). Each slice is composed of a pelagic bedded chert and an overlying hemipelagic siliceous mudstone and terrigenous clastics, almost identical to the example in the Inuyama area. The detailed microfossil dating demonstrated that the slices can be clustered into four groups characterized by distinct OPS; OPS groups 1-4 as shown in Fig. 9(b). The accretion timing, indicated by the horizon of the siliceous mudstone/turbidite transition, is getting younger tectonically downward or oceanward. Such an imbricated structure of thrust sheets with

KISO RIVER -e=====

Fig. 7 Stratigraphic change from pelagic chert into hemipelagic siliceous mudstone, via a 2 m transitional interval at the topmost portion of the CH-2 chert unit in the lnuyama area (modified from Matsuda & lsozaki 1991) (a) Sketch of the outcrop, ( b ) columnar section of the outcrop with radiolarian occurrences

an oceanward/structurally downward growth polarity has a striking affinity to those observed in the modern AC in Barbados and the Nakai trough off Southwest Japan (Moore et al. 1988; Taira et al. 1992).

In particular, the well preserved asymmetrical deformation fabrics in the imbricated thrust sheets in the Inuyama area suggest subhorizontal layer- parallel shortening with a top-to-south sense (Kimura & Hori 1993; Fig. 6a) that is consistent with the Jurassic paleotectonic setting between Asia and the Pacific Ocean with a subduction zone along proto-Japan. The series of thrust sheets


;*

I I I I I I I I

Fig. 7 [

Fig. 5

bedded chert

l o o m

Sedimentation rate (AASR in m/m.y.)

70 20 30

7 /

/ /

terngsnour 1.oes

/

I i

* Paleolatltude of 0.723.4" or S

Fig. 8 Reconstructed stratigrdphy of the component rocks (oceanic plate stratigraphy) of the Jurassic accretionary complex in the lnuyama area on the basis of microfossil data and gradual lithologic changes recognized in field (modified from Matsuda & lsoiaki 1991) Note the upward increase in the average rate of sedimentation from chert to terrigenous clastics

composed of bedded chert and stratigraphically overlying siliceous mudstone plus terrigenous clastics forms a duplex-like structure. In analogy with modern examples, such an asymmetrical layer- parallel shortening structure was probably formed by an offscraping mechanism in the frontal part of an accretionary prism.

On the basis of the above observations, the coherent-type unit of the Jurassic AC is regarded as an initial product of the frontal accretion processes at the Jurassic subduction zone in East Asia that still retains the original accretion features.

CHAOTIC TYPE: BLOCK-IN-MATRIX MODE

The Jurassic AC in the Sanbosan area, Shikoku Island, represents one of the first and well- documented examples of the chaotic-type unit in Japan (Figs 2, 10a; Yamato-Omine Research Group 1981). I t is composed mostly of ill-sorted black argillaceous matrix that contains abundant lensoids and blocks of various rock types, such as

v r a yioup-3

2 km ..

Radiolarian Oceanic Plate Stratigraphy 'One

. group7 I group-;! I group-3 1 group-4

I trench-fill turbidlie

4

boundary siliceous claystc

' I ' Fig. 9 Geologic map of the Togano area, Chichibu belt in Shikoku, and the oceanic plate stratigraphy (OPS) of the Middle-Late Jurassic coherent-type accretionary complex (AC) (modified from Matsuoka 1992). The coherent- type units of the Middle-Late Jurassic accretionary complex are represented by striped patterns on the map. As shown in the inset map, all coherent-type units are composed of several imbricated sheets of chert and terrigenous clastics. The Late Jurassic slope sediments unconforrnably cover the AC units. Four distinct groups with unique OPS are recognized as shown below. Note open triangles in the OPS columns that indicate proxy ages of the primary accretion timing at the trench. A tectonically downward and oceanward younging polarity is clearly observed in the whole edifice.

greenstones, limestone, bedded chert, siliceous mudstqne, sandstone, and mudstone. With respect to the coherent-type unit, the most striking contrast in composition exists in the predominance of argillaceous rocks, mostly in the matrix, and of greenstones and limestone blocks. The size of these lensoids or blocks various from several kilometers in diameter to tiny pebbles, and they are scattered randomly and enveloped within argillaceous rocks

36 Y. Isoxaki

in a matrix-supported manner. Some large slabs composed of bedded chert and/or siliceous mudstone sequences possess primary stratigraphy within themselves, just like a thrust sheet of the coherent-type unit. Such chaotic features characterized by block-in-matrix characteristics is recognized not only in outcrop scale but also in map scale.

Detailed field observations and microfossil dating have documented the remarkable age difference between the relatively younger matrix and older blocks (Fig. 1 Ob; Yamato-Omine Research Group 1981). The predominant black mudstone matrix is of the Late Jurassic to earliest Cretaceous age, while the blocks or lensoids are older than Middle Jurassic; cherts range from the Late Carboniferous to Middle Jurassic, siliceous mudstone in Late Jurassic, and limestone from Late Carboniferous to Late Triassic, respectively. These age relations, in addition to their field occurrence, apparently indicate that the older blocks or lensoids were secondarily incorporated into the younger mudstone matrix. The documentation of the block-in-matrix nature also resolved an apparent discrepancy claimed for the Permian faunal provinciality in Japan (Ishii 1990) that was produced by an erro- neous assumption on the autochthoneity of limestone blocks embedded in the chaotic-type unit. Excepting these exotic fragments, the Jurassic AC per se represents an autochthonous orogenic unit formed in situ along East Asia, and has never formed an exotic or allochthonous terrane.

The chaotic-type units of the Jurassic AC have often been described as an olistostrome or a melange in previous literature because they lack clear bedding features or unique, coherent stratigraphy. Unlike the Franciscan-type tectonic melange, however, they usually lack strong sheared fabric in mudstone matrix, and serpentinite is totally absent in them. Some parts show remarkable scaly fabric in the matrix but they are not ubiquitous on a regional scale. Thus it is not correct to compare the chaotic-type units of the Jurassic AC with the Franciscan-type tectonic melange in which subduction-related tectonic mixing has been strongly emphasized (Hamilton 1969; Hsu 1971). Instead, a high-energy sedimentary mixing process with soft (plastic) matrix, such as a debris flow, mud flow or rock fall/avalanche, seems likely for the origin of the chaotic-type units. Some chaotic- type units with a strongly sheared fabric may represent a secondarily deformed example. In the present article, the term ‘melange’ is not used for the chaotic unit in order to avoid misconceptions

regarding tectonic mixing. Although a recommen- dation is made to use this term in a purely descrip- tive sense (Raymond 1984), it appears to be al- ready stigmatized by the connotation of predominant tectonic mixing related to subduction.

Excepting lensoids/blocks of limestone and greenstones, it is noteworthy that the rest of the component rocks of the chaotic-type units are essentially the same as those of the coherent-type units. In particular, almost identical rock types such as bedded chert, siliceous mudstone, and sandstone, strongly suggest a cognate origin for the two types, probably formed within the same trench. Us- ing microfossils, reconstruction of the primary stratigraphy of the component rocks is possible, and a quite similar OPS to those for the neighboring coherent-type units was reconstructed (Fig 1Oc). Judging from the block-in-matrix nature, the chaotic-type units are regarded as products of secondary mixing of AC materials that are supplied through the collapse of previously accreted AC. In fact, some chaotic-type units contain large coherent slabs (of greater than kilometric size) that internally retain part of the primary OPS, that is, a continuous stratigraphic sequence from bedded chert to overlying siliceous mudstone. The timing of the secondary chaotic mixing in the trench can also be estimated by dating the matrix, although .it is less clearly con- strained than the primary arrival timing a t the trench defined by the particular horizon in the OPS. In the case of the Sanbosan area, the age of the secondary mixing (and redeposition at trench plus successive re-accretion) is some time in earliest Cre- taceous or later.

Through comparison of the chaotic-type units in the Mino-Tanba belt and modern analogues, Oka- mura (1991) proposed a possible formation mechanism for the chaotic-type units, emphasizing the role of episodic collision-subduction of seamounts a t the trench (Fig. 11). According to this model, the incorporation of a large seamount mass into the trench may disturb the toe of the accretionary wedge and trigger huge landslides/avalanches to form chaotic deposits in the trench. Slope sediments covering the older AC may also be reworked and involved in the chaotic deposits as matrix- forming argillaceous materials. The chaotic deposits accumulated a t the trench are accreted (some are recycled) to the trench inner wall by successive subduction. The frequent occurrence of large blocks of alkali-basaltic (OIB-like) greenstones and reef limestone (Nakae 1993; Sano 1988) in the chaotic-type units is strong supporting evidence for this interpretation. The occurrences of Late Juras-


chaotic claslics

0 Late Jur Early Cret mudstone (M) 0 Late Jur sandstone (6) A Late Jur shelf limestone (L) A Mid LateTrias reef limestone (L) 0 Late Jur siliceous mudstone IA) a

L I ~ 0 0 + Early Tnas -Mid Jur bedded dhetl (C)

Reconstructed OPS for Chaotic-type Unit

chaotic-type unit

Fig. 10 Geologic map (a), fossil locality map (b), and a reconstructed column of the oceanic plate stratigraphy (c) of the chaotic-type unit of the Jurassic accretionary complex in the Sambosan area, Chichibu belt in Shikoku (modified from Yamato-Omine Research Group 1981). Note the block-in-matrix nature of the chaotic-type unit with numerous lensoids/blocks of limestone, greenstones and chert surrounded by argillaceous matrix (predominant mudstone plus lesser sandstone and acidic tuff). Abbreviations for fossil-bearing lithologies shown in fossil locality map are as follows: M, mudstone; L, limestone; C, chert; A, acidic tuff. In addition to the components of the coherent-type unit, materials from the greenstone-reef limestone complex (paleo-seamount) and slope carbonate/clastics are incorporated in the chaotic-type units.

sic shelf-type limestone blocks and bivalve-bearing shallow-water sandstone blocks within chaotic units (Yamato-Omine Research Group 1981; Yao 1984; Matsuoka 1992) also support the incorporation of coeval shelf/slope sediments into the chaotic mixing by gravity slide. The accretion process of the chaotic-type unit was probably similar to that for the coherent-type unit. However, any duplex- like layer-parallel-shortening structure has not

filling injection structures in the sandstone blocks observed in the Mino-Tanba belt (Wakita 1988) may suggest development of mud volcanoes on the Jurassic accretionary wedge, however, the predominance of the chaotic-type units throughout the 3000 km-long Jurassic AC in Japan and the extensive nappe occurrence of the chaotic-type units can hardly be explained solely by this mechanism.

been documented, probably Owing to the SUBHORlZONTAL NAPPE: INTERNAL AND EXTERNAL absence of key beds in the chaotic deposits.

Another possible mechanism proposed for chaotic (sedimentary) mixing of AC materials is mud diapirism in the lower trench slope induced by abnormal high pore-pressure within the accretionary wedge, analogous to modern examples in the Barbados AC (Westbrook & Smith 1983). Fracture-

GEOMETRY

The Jurassic AC consists of several nappe units and forms a subhorizontal piled nappe structure as a whole. The anatomy of the Jurassic AC and its external configuration with respect to the contacts with other AC nappes is described.

38 Y. Isoxaki

r I

fragment of seamount

__ I t’ 7 -’\ subducted seamount 1 chert 4-

i I: y

I (chaotic-type unit) 1 Fig. 11 Schematic model of formation of the chaotic-type unit of ancient accretionary complex by a seamount subduction (modified from Okamura 1991 ) The primary accretionary features preserved in the coherent-type units can be secondarily destroyed by the entering of a seamount into a trench collapsed materials from the older accretionary wedge are brought into trench floor and finally re-accreted to the trench inner wall

Internal nappe structure

The Jurassic AC in Southwest Japan is roughly subdivided into three parts on the basis of their distinct timing of the primary accretion suggested

by OPS: the Early Jurassic part, Middle Jurassic part, and Late Jurassic part, and these three all form subhorizontal nappes (Fig. 3). The oldest part of the three occurs in the northern and western areas of the Mino-Tanba belt, for example, occupy- ing the structurally highest horizon, while the youngest part is found mostly in the eastern area and in the structurally lowest horizon (Ishiga 1983; Wakita 1988; Otsuka 1988; Fukudomi 1990; Na- kae 1993). Similar relationships are observed also in the Chichibu belt (Yamato-Omine Research Group 1981, 1992; Yao 1984; Yamakita 1988; Matsuoka 1992; Hisada et al. 1992). On the basis of these observations, the fundamental internal structures of the Jurassic AC in the Mino-Tanba belt and Chichibu belt are shown in a schematic cross-section of Southwest Japan (Fig. 12a).

Each of the three aforementioned major nappes is composed of much thinner veneer-like subunits, viz. subnappes; four chaotic-type units and two coherent-type units are distinguished in terms of OPS in the eastern Mino-Tanba belt, and seven chaotic-type units in the western part (Wakita 1988; Nakae 1993). Some subnappes can be traced laterally for more than 200 km, sometimes discon- tinuously, while their thickness is mostly less than 3 km. Subnappe-bounding faults may correspond to out-of-sequence thrusts observed in modern AC.

N S Mino Tanhn belt

Pre Juiass c ACS Early Jurassic AC nappes \

Middle Jurassic AC mppes

~ ~~

a

b ~~ ~

t PT‘ wench -+

Chaotic-type unit

Fig. 12 Schematic cross-section of Southwest Japan, showing a large-scale subhorizontal piled nappe structure composed of nappes of the Paleozoic to Mesozoic accretionary complexes [(a) modified from lsozaki & ltaya 19911, and a simplified anatomy of the Jurassic accretionary complex in Southwest Japan after removing the Tertiary upright folding (b). (a) Note that the Jurassic accretionary complex occurs essentially as a large scale subhorizontal nappe tectonically intercalated between the overlying pre-Jurassic nappes and underlying post-Jurassic nappes. At present, the structural sole of Jurassic accretionary complex (AC) nappe in the Mino-Tanba belt is probably truncated by the Tertiary subhorizontal thrust (paleo-MTL) that dislocated the southern half of the Ryoke belt (Isozaki & Maruyama 1991; lsozaki 1996a). (b) Each nappe, both the coherent-type and chaotic-type units, is composed of numerous imbricated thrust sheets. Note the development of a nappe-oblique transverse fault of which sense of shear has not been well documented (see text).


of the accretionary wedge. Kinematic analyses of these faults are definitely needed.

1 Age I Oceanic Plate Stratigraphy of the Jurassic

A highly schematic diagram of the internal structure of the Jurassic AC is shown in Fig. 12(b).

A comparison of OPS among the subnappes distinguished in three areas of the Mino-Tanba belt is summarized in Fi,g. 13. A systematic younging polarity among these subunits is clearly demonstrated by the difference in the age of accretion for each subunit (primary accretion for the coherent- type and secondary accretion for the chaotic-type). I t is noteworthy that this tectonically downward- younging polarity is well maintained, not only within a coherent-type unit but also in the whole piled nappe edifice including the chaotic-type units. This suggests that accretion of the chaotic-type units has also followed the general growth trend of AC.

Another important observation is on the nappe- oblique transverse faults developed within the piled nappe edifice of the Jurassic AC; for example, a low-angle fault between the Middle Jurassic AC nappe and Late Jurassic AC nappe that cross-cuts some subnappe boundaries (Figs 3, 12). Such nappe-oblique transverse faults are regarded as a part of post-accretion large-scale deformations within the accretionary wedge. Analogy from modern examples suggests that they may correspond to a horizontal decollement or a back-thrusting (of a contraction sense), or normal faults (of an exten- sional sense) developed to maintain a critical taper

AC

Fig. 13 Comparison of oceanic plate stratigraphy of subunits of the Jurassic accretionary complex (AC) in the Mino-Tanba belt, Southwest Japan (compiled from Wakita 1988; Fukudomi 1990; Nakae 1993). Note the arrival timing of the oceanic plate at the trench for each AC nappe, including both coherent-type and chaotic-type units (a), and re-accretion timing for the chaotic-type unit (A). An oceanward and tectonically downward-younging polarity is observed throughout the piled nappe edifice of the Jurassic AC.

External nappe structure

The external configuration of the whole Jurassic AC in Japan has recently been revealed through the recognition of 3D geometry of the Jurassic AC in the Chichibu belt ( = Northern and Southern Chichibu belts in Isozaki 1996), Southwest Japan. The Chichibu belt represents an approximately 1000 km-long isolated distribution of the Jurassic AC separated by 50-80 km from the main distribution in the Mino-Tanba belt (Fig. 2). On the basis of the recent field research utilizing microfossil and radiometric mapping methods, a new tectonic interpretation was proposed that regards the whole Jurassic AC in the Chichibu belt in Southwest Japan as a large-scale klippe (Isozaki & Itaya 1991; Isozaki 1996) tectonically above younger AC nappes.

A typical example of a klippe-style field occurrence is observed in the Omine area in the Kii Peninsula, (Fig. 2) where the Jurassic AC nappes of the Chichibu belt, 20 km wide, develop tectonically above the younger Cretaceous AC nappes of the Shimanto belt (Yamato-Omine Research Group 1981, 1992; Fig. 14). The structural sole of the

I I M ~ , 1 - Kanoashi area I Tanba area

1'0-1 I I I I ~*Chaotic-type unit 4 re-amreton

mudstone/sandstone

siliceous mudstone Q primly amval at trench

H ' ' G L

...... , r ..._.. ....................... , I 4 , HHU ! I

Mino area

Coherent-type AC unit

H

I /

Sm Sk,Fn Km Nb

e Jurassic AC group

40 Y. Isoxaki

' -' Cretaceous AC (Shirnantc belt) ~~

Jurassic AC is bounded by a remarkable subhorizontal fault called the Butsuzo tectonic line (BTL). In the Omine area, the sinuous trajectory of this subhorizontal fault, almost running parallel to topographic contours, can be traced for more than 100 km. Although smaller in scale, similar modes of occurrence of the Jurassic AC, with respect to the underlying Cretaceous AC nappes of the Shi- manto and Sanbagawa belts, have been observed in other areas of Southwest Japan (Kurimoto 1982; Kawato et al. 1991; Suzuki & Itaya 1994). On the other hand, the structural top of the Jurassic AC klippe is, in turn, tectonically overlain by another klippe composed of older pre-Jurassic orogenic complexes of the Kurosegawa belt (Isozaki & Itaya 1991; Isozaki 1996, 1997).

Because the Jurassic AC in the Chichibu belt are characterized by identical OPS and age of subduction-related metamorphism (mentioned later) to that in the Mino-Tanba belt, their cognate origin in the same subduction zone in Jurassic Asia is suggested. An occurrence of a huge AC in ancient orogenic belts is generally regarded as a direct fingerprint of an ancient subduction by major oceanic plates. It seems quite unlikely to assume two

Y

Fig. 14 Large-scale klippe-like occurrence of the Jurassic accretionary complex in the Omine area, central Kii peninsula, Southwest Japan (modified from Yamato- Omine Research Group 1981, 1992). Note the internal piled nappe structure and a subhorizontal thrust bounding the sole of the complex (8utsuzo Tectonic Line; BTL) from the underlying Cretaceous accretionary complex. Compare the ages of the mudstone between the Jurassic and Creta- ceous accretionary complexes The precise surface trajectories of the nappe boundary faults have been determined for the first time by virtue of the microfossil mapping in such a thickly vegetated area An example of tHe mapping results is shown in the inset 0 Cretaceous mudstone; 0, Late Jurassic-Early Cretaceous mudstone; B, Permian-Jurassic limestone; 0, Triassic- Middle Jurassic chert.

or more parallel-running, coeval and similar- looking subduction zones within Jurassic East Asia. The absence of coeval parallel-running plural igneous belts of arc affinity in East Asia is another major difficulty in the previously proposed models that assumed parallel-running double or triple subduction zones.

Besides the rootless klippe occurrence of the Jurassic AC of the Chichibu belt, striking similari- ties in OPS and in structural horizon within the piled nappe edifice exist between the two Jurassic AC belts in Southwest Japan; for example, the Jurassic AC in the Mino-Tanba belt occurs immediately beneath the pre-Jurassic nappes (Hayasaka 1987) and that in the Chichibu belt occurs beneath the pre-Jurassic Kurosegawa klippe (Isozaki & Itaya 1991). All these observations strongly suggest that the Jurassic AC of the Chichibu belt belonged primarily to the same nappe as that of the Mino-Tanba belt which is tectonically sandwiched between the overlying pre-Jurassic nappes and underlying Cretaceous nappes (Isozaki & Itaya 1991; Fig. 12a). In other words, the Jurassic AC in the Chichibu belt corresponds most likely to a tectonic outlier of that in the Mino-Tanba belt.


was attempted (Shibata & Mizutani 1980), the age of this pre-Late Cretaceous regional metamorphism has long remained enigmatic.

Using a newly developed technique in fine- grained mineral separation, in particular for fine- grained metamorphic white (mostly phengitic) mi- cas smaller than 50 pm (Nishimura e t al. 1989), pelitic phyllites of the Jurassic AC were dated a t many localities (Takami et al. 1990, 1993; Isozaki et al. 1990a; Kawato e t al. 1991; Suzuki & Itaya 1994). Contamination of detrital mica was avoided by microscopic observation. The maximum metamorphic temperature is estimated close to 350°C but no higher than that, which corresponds to the closing (or blocking) temperature of the K-Ar system for phengite. Accordingly, the white mica K-Ar ages from these phyllite samples indicate a proxy age of the highest metamorphic temperature ( = age of peak metamorphism). The measured ages range from Middle Jurassic to early Early Cretaceous but they show tripartite clustering a t ca 130, ca 140 and ca 160 Ma.

Chronological constraints for the low-grade metamorphism are provided also by the age of the protolith AC and the age of post-metamorphic erosion; that is, the metamorphism should postdate the accretion timing and predate the surface erosion. The abovementioned three metamorphic age groups appear consistent with these constraints, in particular with the accretion timing; for example, an Early Jurassic AC unit accreted a t ca 175 Ma was metamorphosed a t 160 Ma, -15 million years after the accretion (Takami et al. 1990); a Middle Jurassic AC unit accreted a t ca 160 Ma was metamorphosed a t ca 140 Ma, -20 million years afterwards (Isozaki et al. 1990; Kawato et al. 1991); an earliest Cretaceous AC unit accreted a t ca 140 Ma was metamorphosed a t ca 130 Ma, -10 million years afterwards (Takami et al. 1993). For the Jurassic AC in Southwest Japan, the age gap between the accretion and the following low-grade metamorphism appears to be -10-20 million years. (Fig. 15). This supports the subduction- related origin of the low-grade metamorphism and continuous onset of metamorphism in a deeper level of the subduction zone after accretion. These results suggest that the age of this low-grade metamorphism may be used as an independent criterion, just like OPS, to distinguish neighboring AC in orogenic belts.

The metamorphic petrology of these low-grade rocks indicates a range of -2-8 kb for the low- grade metamorphic pressure, constraining the depth of burial to be shallower than 20 km. On the

Documentation of the Chichibu outlier or klippe of the Jurassic AC, together with that of the Kurosegawa klippe, completely renewed the general understanding of the 3D orogenic structure of Southwest Japan. In particular, the external geometry of the Jurassic AC in Southwest Japan prior to the upright folding displays a huge subhorizonal nappe -3000 km long and 200 km wide. This nappe appears thicker on the continent side (Mino- Tanba belt), up to -20 km, while thinner on the ocean side (Chichibu belt), less than 5 km (Fig. 12a). Thus an oceanward-tapered wedge geometry is suggested for the Jurassic AC as a whole. The 3D geometry of the structural base of the Jurassic AC in the Mino-Tanba belt is not well clarified, however, owing to the absence of surface exposure and deep seismic reflection data. The subhorizontal nature of the Tertiary paleo-MTL (Isozaki & Maruyama 1991; Yamakita et al. 1995; Isozaki 1996) suggests development of a subhorizontal intracrustal decollement separating the Ju- rassic AC from the underlying crustal rocks partly including Cretaceous AC nappes.

Also for the Jurassic AC in Northeast Japan, a nappe-favoring interpretation was proposed (Tazawa 1988); however, no detailed 3D configuration like that in Southwest Japan has yet been demonstrated, owing to the thick Cenozoic covers and severe overprint of the Tertiary deformation.

SU BDUCTION-RELATED L OW-GRADE METAMORPHISM AND ITS PEAK AGE

Components of the Jurassic AC, both of the coherent-type and chaotic-type, suffered from a low-grade, regional metamorphism of the prehnite-pumpellyite facies to the pumpellyite- actinolite facies (Hashimoto & Saito 1970; Takami et al. 1990; Banno & Sakai 1989; Kawato e t al. 1991). This regional metamorphism clearly pre- dates another younger regional metamorphism associated with Late Cretaceous granitic intrusions (called the Ryoke low-P/T metamorphism; see Na- kajima 1997) because the metamorphic isograds of the Late Cretaceous Ryoke metamorphism cross- cut obliquely the structures of the weakly metamorphosed Jurassic AC in the Mino-Tanba belt. Judging from these geologic relations and newly obtained metamorphic ages mentioned later, the pumpellyite-bearing low-grade regional metamorphism was probably related to the Jurassic subduction processes, representing lower grade parts of the high-PIT metamorphic facies series. Although preliminary whole rock K-Ar and Rb-Sr dating

42 Y. Isoxaki

Age I OPS of the Jurassic AC and Age of Metamorphism I

.. .. . . .. .. . . . . . . .. .. . . . . . . . . . . . . . . . .. . . . . .. . . . . 20 m y . gap

B

. . . . . . . .. . .

g$$j Chaotic-type unit

Mudstond

Siliceous rnudstone

Beddedcheri

P-T Boundary unit

sandstone

Fig. 15 A chronological summary of the Jurassic accretionary complex (AC) in Southwest Japan, showing a relation between the timing of primary accretion and following low-grade regional metamorphism in oceanic plate stratigraphy (compiled from Takami eta / . 1990, 1993; lsozaki e ta / . 1990) Note that the low-grade regional metamorphism most likely occurred ca 10-20 million years later than the final accretion at trench, that is, the onset of burial in the subduction zone. Also noteworthy is a tectonically downward- younging polarity recognized also in the timing of the low-grade metamorphism. *, peak metamorphism; A, final accretion; a, primary arrival at trench

basis of the aforementioned 10-20 million year interval, an average burial rate for the Jurassic AC in the ancient subduction zone is estimated a t -0.5-1 km/million years. A realistic rate for most of the Jurassic AC is probably much smaller because an oceanic plate can subduct from a trench to a 20 km-deep level within 5 million years with a subduction rate (orthogonal component to trench) of 6 mm/year ( = km/million years). There may be a differential movement between the oceanic plate per se and the overriding AC, probably reflecting the difference between the dragging solid plate and dragged soft AC material.

Concerning post-metamorphic erosion-related constraints, on the other hand, no discrepancy has

been recognized. The oldest unconformably overlying sediments are Late Cretaceous volcaniclas- tics, called Nohi rhyolites, in central Japan. The evidence for much older erosion of the Jurassic AC is given by some Early Cretaceous shallow-water conglomerates in Southwest Japan and the southeast Korean Peninsula that contain pebbles of Early Jurassic chert and mid-Jurassic siliceous mudstone, probably derived from the Jurassic AC (Saida 1987; Chang et al. 1990; Umeda et al. 1995). However, there is no information for metamorphic grade and age obtained from these fossil- bearing siliceous rock clasts, thus no age comparison is possible between the metamorphism and erosion.

EXTENT OF JURASSIC AC IN EAST ASIA

In addition to the 3000 km along-arc and 200 km across-arc distribution in Japan from the Ryukyus to Hokkaido, the Jurassic AC has a much greater extent in East Asia, from the western Philippines to Far East Russia (Fig. 2). The total length of the Jurassic AC belt hitherto known in East Asia is more than 6000 km. Except for Southwest Japan, detailed analysis on the Jurassic AC using microfossil and chronometric mapping methods is now in progress in East Asia; quality and quantity of data vary considerably among areas. Nevertheless a rough estimate is definitely needed for the spatial dimension of the Jurassic AC in East Asia because it may suggest characteristics of the plate interac- tions and of paleogeography of Jurassic Asia and the western Pacific. The distributions of the Juras- sic AC in the western Philippines and Northeast China-Far East Russia are briefly reviewed.

West Philippines

From the Calamian Islands in the western Philip- pines (Fig. 2), the occurrence of Jurassic AC was first reported by Isozaki et al. (1987). Additional lines of microfossil evidence were provided by Faure and Ishida (1989), Cheng (1989), and Tu- manda et al. (1990). The Jurassic AC in this area contains Permian, Triassic and Jurassic bedded chert embedded within Jurassic clastic rocks, and is probably correlated to the chaotic-type unit of the Jurassic AC in Southwest Japan. Detailed analysis of internal structures, however, has not been done yet. Surrounded by several Cretaceous and younger back-arc basins, these islands with the Jurassic AC currently form a small continental block called the North Palawan. Prior to the South


of oceanic subduction/accretion-related orogen (Isozaki 1996a; Maruyama 1997). As these general aspects, related to long-term (more than 100 million years long) orogenic processes involving older and younger AC in Japan, are summarized in other articles in this issue of the journal, the discussion here focuses on other interesting facets of the Jurassic AC in Japan: (i) predominance of the chaotic-type units with respect to seamount subduction; (ii) absence of the Franciscan-type melange; (iii) growth rate in comparison with modern analogues; and (iv) total volume and a material source along a 250 Ma collision suture in central China.

China Sea opening in the Cretaceous-Tertiary, the North Palawan block was attached to mainland Asia (somewhere close to the present Hainan island, China) as a southerly segment of the Jurassic AC belt in East Asia (Isozaki et al. 1987). To date, the Jurassic AC in the western Philippines represents the southwestern extremity of the Jurassic AC in East Asia.

Northeast China and Far East Russia

From the Pacific northwest, occurrences of the Jurassic AC have lately been reported in several areas in northeast China and Far East Russia (Fig. 2); for example, the Nadanhada and Khaba- lovsk areas (along the China-Russian border) in Sikhote-Alin (Mizutani et al. 1986; Kojima 1989; Natal’in 1993). Lithologic assemblage and microfossil ages are completely identical to those of the Jurassic AC in the Mino-Tanba belt, Southwest Japan. In addition, a recent report on the Jurassic radiolarians from cherts in the Koryak-Kamchatka region, northeast Russia (Vishnevskaya & Filatova 1994), suggests further extension of the Jurassic AC belt in northeast Asia. The precise distribution and internal structures of these Jurassic AC in Russia, however, have not yet been demonstrated. The northern extension of the Jurassic AC in Japan dives into the Japan Sea off western Hokkaido. The paleogeographic reconstruction concerning the Ter- tiary opening of the Japan Sea (Otofuji 1996), however, suggests that the Jurassic AC found in northeast China and Far East Russia correspond to the missing northern extension. The occurrence of the Jurassic granite belt in the Sikhote-Alin region also supports the northerly continuity of the subduction-related orogen.

DISCUSSION

The previously described characteristics of the Ju- rassic AC in Japan provide new insights and practical clues to analyze products of ancient oceanic subduction and accretion. In particular, the utility of the OPS analysis using microfossils, sometimes combined with chronometric mapping, has been well proven. These methods will be standard pro- cedures in future studies of ancient AC in any orogenic belt related to oceanic subduction, regardless of its age. The subhorizontal nature and structurally downward-younging polarity demonstrated in the Jurassic AC in Japan also suggest a general model of growth pattern and resultant structure

PRESERVATION OF COHERENT-TYPE UNIT

The coherent-type units retaining the primary accretionary structures occupy less than one-third in area of the whole Jurassic AC, as typically observed in the eastern Mino-Tanba belt, South- west Japan (Wakita 1988; Otsuka 1988). Some parts of the Jurassic AC, for example, in the western Mino-Tanba belt (Fukudomi 1990; Nakae 1993) are occupied mostly by the chaotic-type units, with minor coherent slabs contained as isolated lensoids. Such a predominance of the chaotic- type units suggests that the primary accretionary features were rarely preserved, and instead were destroyed and/or modified by later deformation. As the components of the chaotic-type units are mostly derived from older coherent-type units previously accreted, the degree of preservation (or destruction) of the coherent-type units within the accretionary wedge was probably related to successive formation of the chaotic-type units. Judging from the area ratio on the surface, an apparent preservation ratio of the primary accretionary structures is around 30% for the Jurassic AC in Southwest Japan.

As mentioned before, episodic collision-subduction of seamounts a t the trench may have destroyed the primary accretionary structures such as imbricated thrust sheets, and formed the chaotic-type units (Okamura 199 1). The frequent occurrence of blocks or lenses of OIB greenstones and reef limestone in the chaotic-type units (and their absence in the coherent-type units) strongly supports this interpretation. The sporadic occurrence of the coherent-type unit sandwiched between nappes of the chaotic-type unit (Figs 3, 12b) may indicate that the destruction of the coherent- type unit may have occurred not continuously but episodically. The predominant occurrence of the

44 Y. Isoxaki

chaotic-type units in the Jurassic AC in Southwest Japan may indicate that an ancient oceanic plate responsible for the Jurassic AC formation, probably the Izanagi-Kula plate, had plenty of seamounts on it, and that these seamounts collided one after another, intermittently, with the Jurassic trench off proto-Japan and subducted beneath the Asian continental margin. Thus, the preservation ratio of the coherent-type unit within a whole AC edifice can be a monitor for superficial smoothness of an ancient subducted oceanic plate (Fig. 16), as pointed out by Okamura (1991).

The collision and subduction of a seamount a t the trench, however, is generally regarded as the cause of tectonic erosion rather than accretion, on the basis of observations of modern examples off the coast of Japan (von Huene & Lallemand 1990; von Huene & Scholle 1991). In the case of the Jurassic trench, a large amount of material may have been removed from the accretionary wedge in deeper levels a t the same time, even though accretion of chaotic-type units occurred in the frontal wedge. When such a subduction erosion process is taken into account, the net preservation ratio of

the coherent-type unit will be much smaller than the apparent ratio of 30%. Furthermore, the predominance of mud-sized materials in the chaotic- type unit may suggest that a huge amount of slope mud from the the trench inner wall (primarily accumulated on AC edifice) was incorporated into the chaotic-type units in addition to the recycled mud-sized materials from the collapsed coherent- type units. If this is so, the net preservation ratio will be much smaller. Thus I conclude that the primary accretionary features observed in modern examples, such as offscraped imbricated thrust sheet, are rarely preserved because major oceanic plates generally have various topographic highs like seamounts, rises, plateaus and arcs. Alterna- tively, the chaotic-type units in the ancient AC may represent a general consequence of long-term subduction processes comprising primary accretion and following modification by seamount collisions. Likewise, a predominant occurrence of the coherent-type unit in an ancient orogenic belt may imply a subduction-accretion by an exceptionally smooth-surfaced oceanic plate that was probably formed a t a rapidly spreading mid-oceanic ridge.

horse of chaotic-type AC

J. horse of coherent-tvue AC

seamount chain

/

Fig. 16 Schematic model of a subduction zone showing the secondary effect by seamount(s) subduction at trench The coherent-type unit of an accretionary complex represents an initial product of offscraping processes at the trench, while the chaotic-type unit is regarded as a product of seamount subduction composed of collapsed materials of the older accretionary wedge A subduction zone on a srnooth- surface oceanic plate may build the coherent-type dominant accretionary complex, while a subduction zone with a rugged- surfaced oceanic plate with seamounts and other topographic highs may build the chaotic-type dominant accretionary complex and/or suffer from tectonic erosion Insets show resultant internal structures for the two types formed under the two contrasting subduction modes


Cretaceous part of the AC belt) is -3000 km long from Washington State to Baja California, one of the closest in magnitude among ancient examples, but is no more than half the length of the Jurassic AC belt in East Asia. Because the Jurassic AC belt in East Asia represents a consequence of an inter- action between a major continent (Asia) and a major ocean (Pacific), its total volume and growth rate can provide important references in consider- ing continental growth in the Phanerozoic. In this section, I attempt to estimate an average growth rate and resulting total volume of the Jurassic AC in East Asia, and also to examine a possible source of AC building materials from the viewpoints of the paleogeographic and paleotectonic setting of Juras- sic East Asia.

As frequently discussed by many others (Cowan 1994), an apparent length of an ancient AC belt may have been modified by strike-slip dislocation in the fore-arc domain after the primary AC formation. Concerning the Jurassic AC in East Asia, the length of -6000 km appears essential because the coeval granite belt runs almost entirely parallel to the AC belt (Fig. 2). Some geologists in Japan also emphasize the effect of the post-Jurassic left- lateral strike-slip system in East Asia in order to explain the origin of doubled (or tripled) Jurassic AC belts (Mino-Tanba and Chichibu belts) in Southwest Japan, although no practical evidence has been presented for the location of putative strike-slip faults and/or their amount of dislocation indicated by offset points. Because there is no modern example of a fore-arc sliver documented to have been displaced for more than 1000 km along- arc, a modification in length of the Jurassic AC belt over 1000 km appears unrealistic. Thus, in this article, the length, across-arc width, and thickness of the Jurassic AC are measured based fundamen- tally on the present configuration of the Jurassic AC belt in East Asia.

The maximum across-arc width of the Jurassic AC belt is -200 km in central Southwest Japan (Fig. 2) and the thickness of the Jurassic AC nappes is a t least 10 km, and possibly reaches 20 km in the eastern Mino-Tanba belt (estimated from descriptions by Wakita 1988 and Otsuka 1988). The width and thickness of the Jurassic AC in Southwest Japan are compatible with those of the modern Barbados AC. Taking the maximum estimates for width and thickness and assuming a simple wedge shape, the volume of the Jurassic AC for a unit length is roughly calculated: 200 km x 20 km x 1/2 = -2000 km3 per km of trench. As materials of this amount were accreted

ABSENCE OF THE FRANCISCAN-TYPE MELANGE

One more important point to be emphasized here is the absence of the so-called ‘Franciscan-type tectonic melange’ (Hamilton 1969; Hsu 1971) in the Jurassic AC in Japan. Occurrence of strongly sheared chaotic-type units with scaly argillaceous matrix is known in the Jurassic AC (Wakita 1988; Hada & Kurimoto 1990; Okamura 1991); however, their distribution is highly limited. Furthermore, the matrices of such highly deformed parts do not show a powder-ground nature like the typical Franciscan melange in California, and ‘melange with serpentinite matrix and clasts of blueschists and eclogite’ is completely absent in the Jurassic AC in Japan. Thus, in short, the Jurassic AC in Japan is composed of the coherent-type units which retain the primary accretion features and the chaotic-type units derived from the former, mostly through sedimentary mixing, without any significant contribution from putative ‘Franciscan-type tectonic melange’.

Concerning the problematic origin of the ‘Fran- ciscan-type melange’, a recent study of the Mari- ana fore-arc recognized serpentinite seamounts with blueschists clasts, which were probably formed by the release of high pore-fluid pressure within the accretionary wedge (Fryer et al. 1985; Maekawa et al. 1993) like mud diapirism. This may provide a possible modern analogue for a ‘non-tectonic’ serpentinite melange of the Fran- ciscan complex. Froim the Franciscan complex in Alta and Baja California, occurrences of the coherent-type AC units have recently been demonstrated in several areas (Murchey 1984; Sedlock & Isozaki 1990; Isozaki & Blake 1994; Kimura et al. 1996) and these suggest a potential share of the coherent-type units also within the Franciscan complex. In addition, the development of a subhorizontal piled nappe structure within the Franciscan complex has been proposed in northern California (Wakabayashi 1992; Isozaki & Maruyama 1992; Maruyama et al. 1!392). Future researches may clarify similar characteristics and anatomy in a mirror image between the Franciscan complex and the Jurassic AC in Japan.

GROWTH RATE OF JURASSIC AC

The Jurassic AC belt in East Asia, -6000 km long, is one of the largest examples among modern and ancient AC belts; that is, its length is compatible to that of the modern Chile trench with AC. The Franciscan complex in western North America (the

46 Y. Isoxaki

mostly during the interval of the Middle-Late Jurassic, -50 million years, the rate of accretionary growth is -40 km3/million years per km of trench. According to the recent summary on AC volume by von Huene and Scholle (1991), small- to medium-sized AC around the Pacific show similar growth rates; for example, -400 km3/km during 6 million years in the central Aleutian trench, and -150 km3/km during 6.5 million years in the Peru trench a t 9 " s ( = -67-23 km3/million years per km of trench). The abovementioned rough estimate for the growth rate of the Jurassic AC is compa- rable to those for modern examples in the same order of magnitude, therefore, it appears consistent.

VOLUME OF JURASSIC AC AND MATERIAL SOURCE

Besides a reasonable growth rate, development of a huge AC edifice like the Jurassic AC in Asia should be guaranteed by a longlasting and abundant supply of materials to the trench. In general, an abundant material supply into the trench is regarded as one of the major factors in building huge AC (von Huene & Scholle 1991). Concerning the Jurassic AC in Japan, the average percentage of oceanic plate-derived materials is less than 15% in volume, virtually -10% or less (Isozaki et al. 1990b). This means that the total volume of the Jurassic AC depends mostly on that of terrigenous clastics supplied from land areas, in this case definitely from Jurassic Asia.

By extrapolating the abovementioned estimate of AC volume per unit length of trench to the full length of the Japan segment (-3000 km), the total volume of the Jurassic AC in Japan is estimated to -6 x l o 6 km3 (Fig. 17a). Even though restricted to a half length of the AC belt in East Asia, this estimated volume is quite enormous, almost corresponding to the volume of all detritus made by complete erosion of the upper 3 km crust of the whole Yangtze craton (-2000 x 1500 km) (Fig. 17b). This comparison implies that an unusu- ally fertile source area, together with extensive distributory drainage, existed to supply abundant materials to the ancient trench along Jurassic East Asia.

Sandstone petrography combined with detrital mineral dating (Shibata & Adachi 1974; Suzuki e t al. 1991) suggests that the main provenance of the terrigenous elastics in the Jurassic AC in South- west Japan was the Sino-Korean (North China) and Yangtze (South China) blocks with Precam- brian basements. In addition, the predominant oc-

a - 6 x lo6 km3

3000 km I

- 6 x lo6 Ikm' b

71 1500

- 3 x loe km' C I

Fig. 17 Estimate of the total volume of the Jurassic accretionary complex (AC) in Japan (a) and a comparison with a possible material source. Based on the apparent length, width and thickness in Japan, the total amount of the Jurassic AC in Japan is estimated at - 6 x l o 6 km3 (a), which roughly corresponds to the total volume of the upper 3 km of continental crust of the entire Yangtze craton (b). As suggested by sandstone petrography and detrital zircon ages of the Jurassic accretionary complex, the 250 Ma ultrahigh- pressure metamorphic belt in the Qinling-Dabie suture zone with diamond/ coesite-bearing eclogite is the best candidate for the material source of the Jurassic AC in East Asia. With respect to the quick exhumation of the ultrahigh-pressure metamorphic rocks to the surface, a minimum estimate on the eroded amount of crustal materials along the suture was estimated at 3 x 106 km3 (c), which appears consistent in order of magnitude with the above estimate o n the Jurassic accretionary complex.

currence of 250 Ma detrital grains (Suzuki et al. 1991) strongly suggests a large contribution from the Qinling-Dabie collisional suture zone between the Sino-Korean and Yangtze blocks, which is characterized by a 250 Ma ultrahigh-pressure (UHP) metamorphic belt and its lateral equivalents (Isozaki & Maruyama 1991; Sohma & Kunugiza 1993; Isozaki 1997). The preservation of the UHP rocks from the diamond depth (more than 100 km deep) requires a rapid exhumation after peak metamorphism and associated erosion of the overlying units (quick removal of abundant crustal materials


-200 km wide, and -20 km thick) forms a segment of an -6000 km-long AC belt along East Asia.

(2) The Jurassic AC represents subhorizontal piled nappes that are composed of two types of rock assemblage; that is, a coherent-type unit characterized by imbricated thrust sheets and chaotic-type units characterized by (melange-like) block-in-matrix fabric.

(3) The whole piled nappe edifice of the Jurassic AC shows a tectonically downward-younging polarity, regardless of the difference between the coherent-type and chaotic-type units. (4) The predominance of the chaotic-type units

in the Jurassic AC suggests frequent secondary reorganization of the AC edifice, probably by seamount collision and subduction a t the trench.

( 5 ) The component materials of the Jurassic AC were mostly derived from the 250 Ma suture zone in central China.

above them). The occurrence of the UHP rocks including diamond/coesite-bearing eclogites is known from an approximately 1200 km-long and -60 km-wide zone along the suture from the Dabie mountains to Shandon peninsula (Fig. 17c). In order to bring -100 km-deep rocks to the surface for that length, remloval of -3 x l o 6 km3 crustal materials above them is required (as a minimum amount that corresponds to nearly half of the Jurassic AC volume). Furthermore, the lateral ex- tensions of the suture with a medium-pressure- type metamorphic belt likewise have removed additional amounts of 'crustal materials. The rough estimate shown above thus suggests that the 250 Ma suture zone in central China is the most likely supplier of terrigenous clastics to the Juras- sic trench, with no other competitive candidate known in Jurassic East Asia, as pointed out by Isozaki and Maruyama (1991).

The relatively minor contribution from the adjacent coeval arc (Adachi & Suzuki 1994) suggests that the major distributory system of terrigenous clastics was along-trench rather than across-arc, just like the modern Nankai Trough. According to the paleogeographic reconstruction for Jurassic Asia (Maruyama et al. 1997), the eastern margin of the Qinling-Dabie suture was connected to the trench facing the Pacific. Thus voluminous terrigenous clastics, eroded out from the collision suture and adjacent two cratons on both sides, were transported along the suture to the trench, via a huge delta developed a t the eastern suture margin (Isozaki & Maruyama 1991; Isozaki 1997). Con- cerning the northern segment of the Jurassic AC in northeast Russia, northerly continental blocks amalgamated into Asia, that is, the Bureya (Amuria), Siberia, and Kolyma-Omolon blocks, and their mutual suture zones may have worked as major provenances for trench-fill clastics.

CONCLUSION

Recent studies of the Jurassic accretionary complex in Japan and East Asia clarified the following new aspects of the subduction-related orogenic belt between Asia and the Pacific Ocean. These new observations suggest some general and significant insights to on-land exposed ancient accretionary complexes; for example, spatial dimension, externalhnternal 3D structures, preservation of primary accretionary features, and controlling factors of voluminous accretion.

(1) The Jurassic AC in Japan (-3000 km long,

ACKNOWLEDGEMENTS

I would like to thank Tetsuo Matsuda and Shi- genori Maruyama for their valuable suggestions regarding various aspects of the Jurassic AC in Japan. Koji Wakita, Akira Yao, and Goran Ekstrom have reviewed the manuscript and provided con- structive comments.

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