Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks

Tectonophysics 393 (2004) 241 – 261 www.elsevier.com/locate/tecto

Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks Costas Xenophontosa, Soichi Osozawab,* a

Geol. Survey Department, Natural Resources and Environment, 2064 Nicosia, Cyprus Graduate School of Science, Institute of Geology and Paleontology, Tohoku University, Sendai, 980-8578, Japan

b

Accepted 21 April 2004 Available online 15 September 2004

Abstract Accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts, ocean ridges or subduction-related ophiolites. Seamount type basalts are associated with shallow water sediments— mostly reefoidal limestones. Ocean ridge type basalts are generally overlain by pelagic cherts. Subduction-related ophiolitic eruptives, often underlain by gabbroic and ultramafic rocks, are associated with hemipelagic mudstones. The age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin. This relationship has been established for each type of accretionary complex, examples being taken mostly from Japan and the western Pacific rim in order to represent evolutionary processes at a typical active plate margin. In general, the seamount types are older, ridge types are of intermediate age, and the ophiolitic types are by far the youngest, usually close to zero age. Seamount type basalts are accreted by shallower scraping of the seamount’s sediment apron together with fragments of seamount basalt, ridge type, by peeling due to permeability contrast, and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient. Accordingly, it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction. D 2004 Elsevier B.V. All rights reserved. Keywords: Travel time; Seamount; Ocean ridge; Ophiolite; Accretionary complex; Ridge subduction

1. Introduction Basalt is a minor but significant component of ancient accretionary complexes representing the upper part of the oceanic crust, and may be accreted to continental margins during subduction (Isozaki et al., * Corresponding author. E-mail address: [email protected] (S. Osozawa). 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.037

1990). Factors controlling the accretion of basalt and the detailed mechanisms controlling such processes are, however uncertain. It has been proposed that accreted basaltic slabs are very thin, up to several hundred meters only (Kimura and Ludden, 1995). According to Kimura and Ludden (1995), the delamination of thin crustal slivers is related to the sealing and hardening of the uppermost part of the oceanic crust by alteration, in contrast to the porous

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and relatively weak unaltered crust below. Considering actual examples of accreted basalt, however, the basaltic age appears to be an essential factor. In this sense, the age does not refer to the date of eruption as a geologic age, but to the time interval between the eruption and its incorporation into the accretionary complex, similar to the travel time invoked by Isozaki et al. (1990). Accreted basaltic assemblages are not only those formed by sea floor spreading at mid-ocean ridges, as implied by the Kimura and Ludden (1995), but include those formed by intra-plate volcanism that give rise to seamounts (Sano and Kanmera, 1988), whose volcanic edifices are built on pre-existing basaltic ocean floor. In accretionary complexes; however, the basalts derived from such edifices are relatively abundant and their ages tend to be greater than those of ocean floor basalts, suggesting that the mechanism of accretion of seamounts differs from that proposed by Kimura and Ludden (1995). A third type of accreted igneous assemblages is recognized, differing from those produced at mid-ocean ridges plus seamounts, and associated with convergent plate margins. These basalts are extremely young and appear to have been accreted during or shortly after their formation. Accretionary complexes containing this type of basalts are not rare. Basaltic and some rhyolitic sequences are accompanied by gabbro and ultramafic rocks and are generally considered to represent ophiolite assemblages (e.g., Osozawa et al., 1990). The detachment surface is in this case much thicker than those where basaltic sheets are detached from the upper crust formed at mid-ocean ridges, and typically incorporates deeper mantle and gabbroic sequences. In this paper, three-fold division of igneous assemblages is corroborated on the basis of associated sedimentary rocks, prior to considering their age relationships. On this basis it is shown that the age, hence temperature, correlates with basaltic type. The age estimates are then related to the relative depth of detachment, which is shallower for seamounts and deeper for the ophiolitic bodies. An important conclusion is that the ophiolitic assemblages can be accreted when the oceanic lithosphere is still very young, for example, when a spreading center is approaching a subduction zone. The implications of sub-ophiolitic metamorphic soles, high P/T meta-

morphism characterizing subduction zones and anomalous magma chemistry observed in these assemblages are discussed.

2. Three types of accreted igneous assemblages Igneous assemblages, mostly basaltic assemblages, concerned in this paper are from accretionary complexes formed mostly at circum Pacific active margins (Fig. 1). The 3-fold distinction of igneous assemblages in ancient accretionary complexes (Table 1) is clear and simple, being fundamentally matched by the type of accompanying sediment. It is not dependent on other criteria such as the geochemical signature of basaltic rocks. Shallow water limestones lacking terrigenous detritus are typically associated with seamount type basalts, pelagic cherts cover ridge type basalts, and hemipelagic mudstones and terrigenous sandstones are intercalated with the ophiolitic lithologies. Oceanic cherts lack terrigenous fragments such as quartz larger than silt size, in contrast with hemipelagic mudstones. The seamount and ridge type sediments are already well recognized or distinguished by most workers studying accretionary complexes (e.g., Isozaki et al., 1990). However, those associated with the ophiolites are less well understood and need to be considered further. 2.1. Seamount type Basalts of seamount type are closely associated with shallow-water, fossiliferous limestones, devoid of terrigenous material, which are considered to have formed as fringing reefs (e.g., Sano and Kanmera, 1988). Such basalts and limestone associations are found as fragments in debris flow deposits (Sano, 1991), basalt fragments being mostly vesicular, indicating eruption under relatively shallow water. Such debris flow deposits are typically intercalated in both pelagic and hemipelagic sediments (Osozawa, 1986; Matsuda and Ogawa, 1993), suggesting that sedimentation in both types of environment occurred at variable times prior to their final accretion. In general, these deep-water debris flow deposits accumulated at some distance from the seamounts.

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243

Fig. 1. Index of places of data obtained from the western Pacific region. Abbreviations: see Table 1. The other data are only from western North America and New Zealand.

They sometimes include fragments of slump folded cherts, scraped from the slope during emplacement of the debris flow. Although such deposits sometimes occupy the lowermost stratigraphic position of a thrust sheet, they would have originally covered pelagic chert. In other cases, such deposits might have buried the basaltic lavas, representing the flanks of seamounts (Osozawa, 1986; Osozawa

et al., 1990; Sano and Kanmera, 1988). These lavas, including lava clasts in debris flow, are distinct from ocean floor basalts, and coexistence of these types in accretionary complexes is not reported. Mass wasting, also subduction related when seamounts approach and collide with a trench, is characterized by a small amount of matrix. The matrix

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Table 1 Age data of accreted basalt Zones

Basalt type

Basalt chemistry

References for basalt chemistry

References for ages

I—inner zone I1: Akiyoshi I2: Maizuru

seamount ophiolite

intra-plate MORB? and intra-plate and arc

Tazaki et al. (1994) Ishiwatari et al. (1990)

I3: Ultra-Tamba

seamount

I4: Tamba-II (Minamiyama) I5: Tamba-II (Natasyo) I6: Tamba-I (Tsurugaoka) I7: Tamba-I (Yugawara) Ashio-1 Ashio-2

seamount

Kanmera et al. (1990) Hayasaka (1990), Herzig et al. (1997) Ishiga (1990a), Musashino et al. (1987) Goto (1986)

seamount seamount seamount seamount seamount

MORB? and intra-plate intra-plate

Nakae (1992), Sano (1999) Nakae (1992)

Nakae (1992) Nakae (1990) Nakae (1991) Kamata (1997) Kamata (1997)

N—northern outer zone N1: Shingai N2: Yusukawa-1 N3: Yusukawa-2 N4: Tenjinmaru N5: Mikabu

seamount seamount seamount seamount ophiolite

arc

Tazaki et al. (1994)

MORB? and intra-plate

Ozawa et al. (1999)

Isozaki (1986) Nagata (1993 MS) Nagata (1993 MS) Yamakita (1988) Faure et al. (1991)

S—southern outer zone including komatiite S1: Nomura-1 seamount S2: Nomura-2 seamount S3: Nomura-3 seamount S4: Yasuodani seamount S5: Sambosan seamount S6: Yuwan S7: Tsukimiyama S8: Yokonami Miyama

ophiolite ridge ridge ridge

arc and intra-plate

Osozawa and Yoshida (1997)

arc and intra-plate

Asaki (unpublished data)

S9: Mugi

ophiolite

arc

Osozawa and Yoshida (1997)

S10: Sakihama

ridge

arc and intra-plate

Asaki (unpublished data)

S11: Muroto

ophiolite

arc

Asaki (unpublished data)

Setogawa-B1, C2, D1 Setogawa-B2 Setogawa-B3 Setogawa-C1 Setogawa-D2 Setogawa-D3, D4 Setogawa-D5, D6 Setogawa-E1, E2

ridge ophiolite ophiolite ridge seamount ridge seamount ridge

arc and intra-plate arc and intra-plate

Osozawa and Yoshida (1997) Osozawa and Yoshida (1997)

arc arc arc

Osozawa and Yoshida (1997) Osozawa and Yoshida (1997) Osozawa and Yoshida (1997)

W—western Hokkaido Taro W1: Oshima-1

seamount seamount

intra-plate

Tsuchiya et al. (1999)

Nagata (1993 MS) Nagata (1993 MS) Nagata (1993 MS) Nagata (1993 MS) Matsuoka (1984), Aita (1987) Osozawa (1984) Okamura (1980, 1992) Okamura (1980, 1992) Kishu Shimanto Research Group (1991) Osozawa, (unpublished data) Osozawa (unpublished data) Hamamoto and Sakai (1987) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990) Osozawa et al. (1990)

Matsuoka and Oji (1990) Ishiga and Ishiyama (1987)

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245

Table 1 (continued) Zones W—western Hokkaido W2: Oshima-2 W3: Sorachi W4: Hidaka Western Greenstone W5: Hidaka-1 (Toma) W6: Hidaka-2 (Idonnappu) W7: Hidaka-3 (Kamiokoppe) W8: Hidaka-4 (Kamimaru)

Basalt type

Basalt chemistry

References for basalt chemistry

References for ages

seamount ophiolite seamount

intra-plate

Tsuchiya et al. (1999) Takashima (2000)

Tajika et al. (1984) Kiminami et al. (1992) Kiminami et al. (1985)

seamount seamount ophiolite

MORB?

Miyashita and Yoshida (1994)

Kato et al. (1986) Kiyokawa (1992) Tajika and Iwata (1990)

seamount

Iwata and Tajika (1989)

E—eastern Hokkaido E1: Tokoro

seamount

E2: Nakanogawa-Yubetsu

seamount

E3: Hidaka main

ophiolite

Okada et al. (1989), Kanamatsu et al. (1992) Nanayama (1992), Okamura (unpublished data) Owada et al. (1991)

Western Shikhote-Alin Khabarovsk

seamount

Nadanhara

seamount

Eastern Shikhote-Alin Chernaya

seamount

Tetyukhe-Dalnegorsk

seamount

Sakhalin Aniva

seamount

Kimura et al. (1992), Rikhter and Bragin (1985)

Okinawa Iheya

seamount

Ie Nakijin

seamount ophiolite

Motobu

seamount

Ujiie and Oba (1991a,b), Ishibashi (1968) Ujiie and Oba (1991a,b) Ishibashi (1970), Kobayashi and Ishibashi (1970) Fujita (1989), Hanzawa (1935)

Ishigaki Fusaki

seamount

Isozaki and Nishimura (1989)

Taiwan Tailuko

seamount

Jahn (1988), Yen et al. (1951)

MORB?

Maeda and Kagami (1996)

Kojima et al. (1991), Nalivkin (1973) Kojima (1989)

Kemkin et al. (1992), Nalivkin (1973) Kojima (1989), Krasilov and Parnyakov (1984)

(continued on next page)

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Table 1 (continued) Zones

Basalt type

Philippines Palawan

seamount

Faure and Ishida (1990), Hashimoto (1981)

Indonesia Luk-Ulo

ridge

Wakita et al. (1994)

New Zealand Marble Bay Kawakawa Bay

seamount ridge

Aita and Sporli (1992) Aita and Sporli (1992)

Franciscan Yolla Bolly Baja California

ridge ridge

Marin Headlands

ridge

Zones

I—inner zone I1: Akiyoshi I2: Maizuru I3: Ultra-Tamba I4: Tamba-II (Minamiyama) I5: Tamba-II (Natasyo) I6: Tamba-I (Tsurugaoka) I7: Tamba-I (Yugawara)

Kazanian Tatarian Tatarian Norian–Rhaetian

Follicucullus monacanthus Follicucullus scholasticus Neoalbaillella ornithoformis Canuptum triassicum

Ishiga (1990) Ishiga (1990) Ishiga (1990) Yao (1990)

Bathonian–Callovian

Trilocapsa tetragona– Guexella nudata Stlocapsa (?) spiralis– Foremanella hipposidericus Pseudodictyomitra cosmoconica–Alivium helenae Stlocapsa (?) spiralis– Foremanella hipposidericus Gongylothorax sakawaensis– Foremanella hipposidericus

Oxfordian Berriasian–Valanginian

Ashio-2

Oxfordian

N—northern outer zone N1: Shingai Tatarian N2: Yusukawa-1 Rhaetian N3: Yusukawa-2 Hettangian–Sinemurian N4: Tenjinmaru Bathonian

Callovian–Oxfordian

S—southern outer zone S1: Nomura-1 Tatarian Sinemurian

Neoalbaillella ornithoformis Canuptum triassicum lower Parahsuum simplum upper Eucyrtidiellum unumaense–Tricolocapsa tetragona Amphipyndax trunoensis– Foromanalla hipposidericus

lower Neoalbailella ornithoformis lower Parahsuum simplum

References for ages

Isozaki and Blake (1994) Sedlock and Isozaki (1990) Murchey (1984)

Sedlock and Isozaki (1990)

References for fossil zone

Oxfordian

S2: Nomura-2

MORB?

References for basalt chemistry

t (time): age of youngest hemiperalic rock (=age of terrigenous rock) Stages Fossil zones

Ashio-1

N5: Mikabu

Basalt chemistry

Min. (a)

Max. (b)

Means (a+b)/2

Deviations |a b|/2

265 259 253 219

259 256 250 210

262 257.5 251.5 214.5

3 1.5 1.5 4.5

Aita (1987)

159

155

157

2

Aita (1987)

151

146

148.5

2.5

Schaaf (1985)

134

127

130.5

3.5

Aita (1987)

151

146

148.5

2.5

Aita (1987)

149

146

147.5

1.5

Ishiga (1990) Yao (1990) Yao (1990) Aita (1987)

253 215 210 165

250 213 194 157

251.5 214 202 161

1.5 1 8 4

Aita (1987)

153

146

149.5

3.5

Ishiga (1990)

253

252

252.5

0.5

Yao (1990)

201

194

197.5

3.5

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247

Table 1 (continued) Zones

t (time): age of youngest hemiperalic rock (=age of terrigenous rock) Stages Fossil zones

S—southern outer zone S3: Nomura-3 Bathonian

S4: Yasuodani

Oxfordian

S5: Sambosan

Oxfordian

S6: Yuwan S7: Tsukimiyama S8: Yokonami

Barremian Turonian–Coniacian Turonian–Santonian

Miyama S9: Mugi

Coniasian Companian

upper Eucyrtidiellum unumaense–Tricolocapsa tetragona Stlocapsa (?) spiralis– Gongylothorax sakawaensis Stlocapsa (?) spiralis– Foremanella hipposidericus lower Crolanium pythiae Alievium superbum Alievium superbum–lower Amphipyndax pseudoconulus Theocampe urna lower Amphipyndax tylotus

S10: Sakihama

Priabonian

Thysocyrtis bromia

S11: Muroto Setogawa-B1, C2, D1 Setogawa-B2

Serravallian Aquitanian

(Rb–Sr) Stichocorys delmontensis

Aquitanian

Stichocorys delmontensis

Setogawa-B3

Aquitanian

Stichocorys delmontensis

Setogawa-C1

Aquitanian

Stichocorys delmontensis

Setogawa-D2

Aquitanian

Stichocorys delmontensis

Setogawa-D3, D4

Aquitanian

Stichocorys delmontensis

Setogawa-D5, D6

Aquitanian

Stichocorys delmontensis

Setogawa-E1, E2

Burdigalian–Serravallian

Calocycletta costata

W—western Hokkaido Taro Oxfordian

Campanian

Amphipyndax tsunoensis– Stylocapsa (?) spiralis Stylocapsa (?) spiralis– Gongylothorax sakawaensis Sethocapsa cetia– Sethocapsa trachyostraca upper Cecrops septemporatus Cecrops septemporatus– Dibolachras tythopora upper Cecrops septemporatus–Crolanium pythiae lower Amphipyndax tylotus

Danian

Stichomitra granulata

W1: Oshima-1

Oxfordian

W2: Oshima-2

Tithonian–Valanginian

W3: Sorachi Hauterivian W4: Hidaka Western Hauterivian Greenstone W5: Hidaka-1 (Toma) Hauterivian–Barremian

W6: Hidaka-2 (Idonnappu) W7: Hidaka-3 (Kamiokoppe)

References for fossil zone

Min. (a)

Max. (b)

Means (a+b)/2

Deviations |a b|/2

Aita (1987)

165

157

161

4

Aita (1987)

151

148

149.5

1.5

Aita (1987)

151

146

148.5

2.5

Schaaf (1985) Schaaf (1985) Schaaf (1985)

117 92 92

115 88.5 84

116 90.25 88

1 1.75 4

Schaaf (1985) Sanfilippo and Riedel (1985) Sanfilippo and Riedel (1985)

89 78

88 74

88.5 76

0.5 2

39.5

36

37.75

1.75

et al.

14 21.5

14.8 20

14.4 20.75

0.4 0.75

et al.

21.5

20

20.75

0.75

et al.

21.5

20

20.75

0.75

et al.

21.5

20

20.75

0.75

et al.

21.5

20

20.75

0.75

et al.

21.5

20

20.75

0.75

et al.

21.5

20

20.75

0.75

et al.

17

14.5

15.75

1.25

Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985) Sanfilippo (1985)

Aita (1987)

151

149

150

1

Aita (1987)

151

148

149.5

1.5

Aita (1987), Schaaf (1985) Schaaf (1985) Schaaf (1985)

140

122

131

9

121 122

120 117

120.5 119.25

0.5 2.75

Schaaf (1985)

121

113

117

4

Sanfilippo and Riedel (1985) Hollis (1993)

78

74

76

2

64.5

63

63.75

0.75

(continued on next page)

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Table 1 (continued) Zones

t (time): age of youngest hemiperalic rock (=age of terrigenous rock) Stages Fossil zones

W—western Hokkaido W8: Hidaka-4 Danian (Kamimaru) E—eastern Hokkaido E1:Tokoro E2: NakanogawaYubetsu E3: Hidaka main

Campanian–Maastrichtian ? Danian Amphisphaera kina–Buryella foremanae Danian–Thanetian (Rb–Sr)

Western Shikhote-Alin Khabarovsk Bejocian Nadanhara

Buryella foremanae

Callovian

Eastern Shikhote-Alin Chernaya Hauterivian

lower Eucyrtidiellum unumaense Guexella nudata

upper Cecrops septemporatus–Dibolachras tythopora Pseudoctictyomitra cosmoconica–Cecrops septemporatus

References for fossil zone

Min. (a)

Max. (b)

Means (a+b)/2

Deviations |a b|/2

Hollis (1993)

63

61

62

1

Hollis (1993)

78 66

66.5 61

72.25 63.5

5.75 2.5

49.9

62.1

56

6.1

Aita (1987)

171

165

168

3

Aita (1987)

157

155

156

1

Schaaf (1985)

121

119

120

1

Schaaf (1985)

134

120

127

7

Tetyukhe-Dalnegorsk

Berriasian–Hauterivian

Sakhalin Aniva

Valanginian–Hauterivian

Alievium helenae– Dibolachras tythopora

Schaaf (1985)

128

117

122.25

5.75

Okinawa Iheya

Callovian–Oxfordian

155

146

150.5

4.5

Ie Nakijin Motobu

Valanginian–Hauterivian Carnian Valanginian–Hauterivian

Archaeodictyomitra (?) Aita (1987) mirabilis–Foremanella hipposidericus upper Cecrops septemporatus Schaaf (1985) (Ammonite) upper Cecrops Schaaf (1985) septemporatus

121 231 121

120 223 120

120.5 227 120.5

0.5 4 0.5

210

186

198

12

199

133

166

33

153

150

151.25

1.25

89

78

83.5

5.5

Ishigaki Fusaki

Hettangian–Pliensbachian Pseudoctictyomitra cosmoconica

Schaaf (1985)

Taiwan Tailuko

Sinemurian–Berriasian

(Pb–Pb)

Philippines Palawan

Callovian–Oxfordian

Amphipyndax tsunoensis–Stylocapsa (?) spiralis

Aita (1987)

Indonesia Luk-Ulo

Coniasian–Campanian

Theocampe urna–lower Amphipyndax pseudoconulus

Schaaf (1985)

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Table 1 (continued) Zones

t (time): age of youngest hemiperalic rock (=age of terrigenous rock) Stages Fossil zones

References for fossil zone

Min. (a)

Max. (b)

Means (a+b)/2

Deviations |a b|/2

New Zealand Marble Bay Kawakawa Bay

Norian Berriasian–Valanginian

lower Canoptum triassicum Acanthocircus dicranacanthos–Cecrops septemporatus

Yao (1990) Schaaf (1985)

219 135

215 121

217 128

2 7

Franciscan Yolla Bolly

Kimmeridgian–Tithonian

Aita (1987)

143

134

138.5

4.5

Baja California

Berriasian–Barremian

Schaaf (1985)

134

113

123.5

10.5

Marin Headlands

Albian–Cenomanian

Zhamoidellum mikamense– Sethocapsa cetia Pseudodictyomitra cosmoconica–Cecrops septemporatus Thanarta veneta– Obesacapsula somphedia

Schaaf (1985)

89

94

96

2

Data used in Figs. 2, 3 and 4 are means of At, but also the deviations in Fig. 2. Basalt chemistry is also compiled. Zones are after Osozawa (1994, 1998). Informal fossil zones of fusulina and others, and radiometric age determinations are in parentheses. Correlation of absolute age and biostratigraphy is after Haq et al. (1987) and his related studies. References related to Table 1 are in Appendix A.

is mostly hemipelagic mudstone but partly sandstone. Fragments and blocks comprise limestone and small amount of basalt, almost free of chert. The limestone, whose block dimensions range up to several hundred meters (Sano, 1991), was thought to have overlain the basalt, although original stratigraphic relations are poorly preserved (Osozawa, 1986). No example of a complete and unblocked seamount, together with the expected basal thrust, have been not reported from any accretionary complex. Another type of near-trench mass wasting is indicated by large amounts of terrigenous muddy matrix supporting blocks of deep-water debris flows and cherts. Because such deposits are usually pervasively sheared, they have been interpreted as me´langes. However, an origin by mass wasting is preferred (e.g., Osozawa, 1984; 1993).

pelagic sequence of the Setogawa zone (Osozawa et al., 1990), although shallow water limestones are completely absent from sequences of ridge type basalts. Some basalts and pelagic rocks are, on the other hand, found as blocks. Conformable relation of basalt and chert is sometimes observed within some blocks, again lacking, shallow-water limestone, but accompanied by inter-pillow pelagic limestones (Okamura et al., 1980). The original sequence, reconstructed from the ages and lithologies of the blocks, is the same as sequences observed in thrust sheets (e.g., Taira et al., 1989). Sometimes interpreted as tectonic me´langes (e.g., Hashimoto and Kimura, 1999), we attribute these to a sedimentary origin (e.g., Osozawa, 1993). 2.3. Ophiolite type

2.2. Ridge type Mid-ocean ridge type basalts typically underlie pelagic, hemipelagic and terrigenous sequences in accretionary complexes and thrust sheets. These basalts themselves are less common than might be expected (Table 1), and represented only a part of the Setogawa zone, Japan (Osozawa et al., 1990) and the Marin Headlands area, California (Murchey, 1984). Deep-water limestones are present only in the lower

Ophiolitic assemblages are considerably more diverse, and associated with hemipelagic mudstone and partly terrigenous sandstone rather than pelagic chert in most cases. Basalts of seamount and ridge type also occur as blocks in a muddy matrix, but basalts of present type are extrusions and intrusions in mudstone, the latter identified from their chilled margin with surrounding mudstones (e.g., Miyashita and Katsushima, 1986; Osozawa et al., 1990; Asaki

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et al., 1999). Minor mudstone dikes are observed to intrude into the glasses of such chilled margins (Osozawa et al., 1990; Osozawa, 1993), indicating that mudstones were unconsolidated and intrusions were syn-depositional, probably feeding the overlying pillow lavas. This activity is thought to have occurred in a hemipelagic environment. Because some of the pillows are lavas underlain by gabbroic and ultramafic rocks, these associations are regarded as ophiolitic, even when lacking a sheeted dike complex. The sheeted dike complex in Japan is only reported from the Hayachine ophiolite (Ehiro and Kanisawa, 1999). Ophiolites have been subdivided into Tethyan and Cordilleran types (Moores, 1982; Nicolas, 1989), former associated with active margins, latter with passive margins. The ophiolites considered in this paper, in Japan may be characteristic as Tethyan in type. Distinctions between the two groups in terms of their origin and emplacement mechanisms are discussed later. Ophiolites such as the Zambales (Schweller et al., 1984; Faure et al., 1989), New Guinea and New Caledonian, Dun Mountain, Trinity and Josephin (e.g., Wright and Wyld, 1994), Coast Range (Dickinson et al., 1996) are typical of the circum-Pacific region, and may be distinguished from Cordilleran ophiolites (Nicolas, 1989). Ophiolitic assemblages occur as thrust sheets in accretionary complexes. In the latter, thick mudstones and then sandstones cover the igneous basement. The Yakuno (Ishiwatari, 1985), Horokanai (Ishizuka, 1985), Shimokawa (Miyashita and Yoshida, 1994) and Poroshiri (e.g., Miyashita, 1983) ophiolites shows a typical ophiolite sequences. However, in the Maizure, Sorachi, and Hidaka zones that include these ophiolites, the bases of these thrust sheets do not always consist of ultramafic rocks, some comprising gabbros and others, basalts with mudstone intercalations. Eruptive sequences in part of the Setogawa zone are accompanied by gabbroic and ultramafic lithologies, which occur as xenoliths, and fragments in secondary debris-flow deposits (Osozawa et al., 1990; Osozawa and Yoshida, 1997), originally derived from the ophiolite. The Mineoka ophiolite has a similar origin (e.g., Osozawa, 1992), although gabbros and basalts occur as intrusions in terrige-

nous sediments of the Nabae complex (Osozawa, 1993). Several accretionary complexes along the seaward margin of the northern Shimanto zone consist mostly of eruptives (Osozawa, 1992), although some ultramafics and gabbros are reported (Nakagawa, 1967; Ishikawa and Shibano, 1974; Yoshida and Kashima, 1976; Suzuki and Hada, 1983). Extrusives and intrusives in mudstone have undergone intense accretion-related pressure-solution and shearing. Some of these have floated like blocks in matrix (Onishi and Kimura, 1995; Asaki et al., 1999), although these blocks do not constitute tectonic me´langes.

3. Travel time and its dependence on igneous assemblages Defined as the difference in geologic ages between youngest hemipelagic rocks and the oldest sediments associated with igneous components of these accretionary complexes (Osozawa, 1992, 1994), travel times of the latter are compared. The date of the youngest hemipelagic rocks is the arrival time of an igneous assemblage at trench, reflecting its travel time from the place of origin, whether hot spot, mid-ocean ridge, or arc-trench system (Isozaki et al., 1990). The oldest or stratigraphically lowest sedimentary rocks for the seamount, ridge, ophiolitic types are, respec-

Fig. 2. Relation of travel time and three types of accreted igneous assemblages.

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tively, shallow water limestones, pelagic rocks and hemipelagic rocks. These ages, mostly based on fossil data, were established for ancient accretionary complexes mostly from Japan and the western Pacific region (Fig. 1 and Table 1). Each of these complexes has available date set for compilation. Some ophiolite complexes, e.g., the Klamath Mountains and Alaska, are considered not to be simple accretionary complexes as assumed for those in Japan (e.g., Isozaki et al., 1990; Hacker et al., 1995; Plafker and Berg, 1994). Accretionary complexes in collisional belts are not yet enough for their identification as well as the age data (e.g., Collins and Robertson, 1997). Ages of seamount type basalts range from 13 to 175 myr with a mean of 99 myr, those of the ridge, from 2 to 94 myr with a mean of 39 myr, while ophiolite type igneous rocks mostly show to be 0 myr travel times. The latter two examples are however, up to 30 myr (Fig. 2). While there is a significant age overlap, it is clear that travel times of the seamount type are greatest, ridge types are intermediate and ophiolite types the shortest (Figs. 3 and 4). Thus, seamount and ridge type basalt may be erupted relatively large distance from a trench, while ophiolites appear to be exclusively formed close to the trench. An important implication is that the magmatic components of ophiolites could be generated at midocean ridges that are approaching a trench.

Fig. 3. Frequency of travel time for three magmatic types.

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Fig. 4. Cumulative frequency of travel time for three magmatic types.

4. Emplacement mechanism The accretion of igneous complexes is expected to consist of two processes, off-scraping at shallower levels and underplating at deeper levels. Off-scraping is expressed by imbricated thrust fans and underplating, by duplexing. Unfortunately, processes of roof thrusting are usually not exposed, while reports of duplexing are scarce. Tectonic me´langes have recently been considered to be components of duplex formations (Hashimoto and Kimura, 1999), contrasting to the axial-planer pressure-solution cleavages typically in the strata of imbricated thrust fans (e.g., Ujiie, 1997). Severe shearing is easily recognized in association with the thrusts and decollement, but the presence of basalt fragments in a muddy matrix is not necessarily explained as evidence of a tectonic me´lange. Me´lange fabrics mostly appear to result from layer-parallel extension (e.g., Onishi and Kimura, 1995), and are not able to readily incorporate underlying basalts into an unconsolidated muddy matrix (Osozawa, 1984). Some me´langes such as in the Asio and Motobu zones (e.g., Schoonover and Osozawa, this volume) are characterized only by axial-planer pressure-solution cleavages printed over block-inmatrix fabric, which is then a fabric of debris-flow. Contrastingly, the Kunigami zone has asymmetric shear fabric, but exotic blocks are not included (Schoonover and Osozawa, this volume). Accordingly, Fig. 5 does not distinguish the duplex and shows only the thrust fan. In either case eruptive sequences are

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detached along thrusts or decollement. It is therefore likely that accretionary complexes showing highergrade metamorphism contain a relatively large eruptive components (Isozaki et al., 1990). In the following, eruptive type rather than volumetric proportion is considered. The types of accreted magmatic product reflect their respective travel times, and, in turn, show a close relation to the thermal state of the crust, whether generated at a hot spot or mid-ocean ridge. Larger travel times correspond to cooler, denser oceanic crust while shorter times correspond to warmer, more

buoyant oceanic crust. Cool ocean floor subjected to hot spot volcanism would become reheated, likely to be accreted, in contrast to older ocean floor without hot spot. In addition to the difference in lithology, the depths of detachment thrusts and decollement zones differ for each of the three types. For older oceanic plates, the detachment depth is shallower than the top of the ocean-floor basalt, debris-flow deposits in the sedimentary sequence including limestones and seamount basalts. If the emplaced complex shows intermediate travel time, the detachment penetrates to greater depth

Fig. 5. Three modes of accretion in accord with three magmatic. Not to scale.

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in the uppermost oceanic crust, while if the oceanic plate is extremely young, the detachment plane may sometimes reach the upper mantle, extending the oceanic crust, and contributing to ophiolite formation (Fig. 5). It can be said that the depth and the consequent thickness of accreted igneous materials is a function of both travel time and temperature of the oceanic plate. The approach to and collision of seamount topographic highs with the trench would be expected to concentrate stress, leading to collapse of the seamount rather than truncating its summit, as assumed by Von Huene and Scholl (1991). Only collapsed material could be accreted, thereby relinquishing major part of the seamount basalts and underlying ocean floor. Decollement could not penetrate this far into anisotropic sediment layers, because of the weaker stress caused by high angle subduction of an older, denser oceanic plate (Uyeda, 1983). When an oceanic plate is of intermediate age and shows a correspondingly intermediate stress field, decollement may reach more resistant basaltic basement penetrating the sediment apron. The detachment mechanism allows for concentration of pore water pressure in the porous part of the basalt layer just below the altered carapace where pores are filled by secondary minerals (Kimura and Ludden, 1995). When crustal age is extremely young, an ophiolitic assemblage is indicated. The relatively complete preservation of ophiolite sequences indicates that the accretion mechanism consists essentially of offscraping. In fact, our detailed structural studies show that the Yakuno ophiolite reflects off-scraping of the dominant accretionary complex as characterized by coaxial deformation shown by the axial-planer pressure-solution cleavages (Osozawa et al., this volume). Ophiolites are thus interpreted to represent off-scraped remnant of very young lithosphere structurally from above the decollement. According to rheological studies (Hoffman and Ranalli, 1988; Van den Beukel, 1990), the thermal state of a young, buoyant slab is sufficient to reduce its strength to allow delamination or break-off. Because coupling between the overriding and subducting plates is strong, the shear stress is probably large enough to become detached to form decollement within the mantle (Uyeda, 1983).

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The mechanism and processes of ophiolite emplacement at an active margin is very similar to that proposed for Tethyan ophiolite by Nicolas (1989). Tethyan ophiolites are emplaced following the oceanic detachment and rapidly formed giant thrusting, by obduction onto the continental crust of the passive margin. It is clear from the data presented here that the genesis of ophiolites and accompanied thrusting are closely associated in space and time. Rapid thrusting is further expressed by the negligible time gap between the thrust ophiolite and sediments deposited in overlying fore-arc basins, also exemplified by the Yakuno ophiolite (Osozawa et al., this volume) among others (e.g., Osozawa et al., 1990). Metamorphic soles also characterize Tethyan ophiolites show the hot condition and relative youth of the detached mantle, although unfortunately, there is no evidence of this in the Japanese ophiolites. The secondary deformation, evidenced by brittle faults with veins has completely modified the basal contact of the Yakuno ophiolite. The obduction process may therefore not apply in the case of Japanese ophiolites, although the active margin model proposed by Nicolas (1989), invoking a young subduction zone is well reconciled to the present considerations.

5. Discussion On the basis of sediment and age relations of accreted magmatic assemblages, we show that their genetic and emplacement processes, especially of ophiolites, are highly characteristic. We now discuss these in relation to the constraints offered by metamorphism, geochemistry and sedimentary processes. 5.1. Low to medium P/T metamorphism Low P/T and the high P/T metamorphic suites commonly underlie Tethyan ophiolites, and the ophiolites themselves have low P/T trajectories (Nicolas, 1989). The Japanese ophiolites also show lower P/T metamorphic records in common with Tethyan ophiolites, but lack high P/T trajectories expected for Cordilleran type ophiolites (Nicolas, 1989) and active margin assemblages. Only the Mikabu ophiolite (e.g., Faure et al., 1991) shows high

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P/T character, presumably an effect of overprinting of the high P/T Sambagawa metamorphism, not touched in this paper by lacking available age data. The Yakuno and Horokanai ophiolites reflect a history of medium P/T metamorphism (Ishiwatari, 1985; Osozawa et al., this volume and unpublished data), while the latter was explained to be low P/T based on metamorphic mineral assemblages (Ishizuka, 1985). In general, metamorphism extends down to the mantle sequence and lower the ophiolite stratigraphic position corresponds to higher the metamorphic grade. Prograde metamorphism shown by the Yakuno ophiolite was only once, coeval with the deformations like pressure-solution cleavages at the time of thrusting (Osozawa et al., this volume). High thermal gradients at subduction zones could be accomplished if a young, hot oceanic plate or midocean ridge was approaching and then subducted beneath the Japan arc and involved in thrusting. The ophiolite type igneous assemblages in the Shimanto zone lava have been recently interpreted to reflect medium P/T conditions in the higher metamorphic grade part (Nagae and Miyashita, 1999; Schoonover and Osozawa, this volume). Abnormally high thermal gradients of the Shimanto zone are indicated by studies of illite and vitrinite reflectance of mudstones (e.g., Sakaguchi, 1996). 5.2. Magma geochemistry Magmatic chemical compositions have been used to discriminate geotectonic settings of ophiolites (e.g., Isozaki et al., 1990), and it is hopefully expected to fit their subdivision by associated sediments in this paper. For two reasons, however, this paper does not rely on geochemistry as a primitive tool for establishing the ophiolite origins and as a criterion for the subdivision of basalts. Firstly, basalts can not be distinguished on the basis of immobile element abundance alone or by using diagrams based on such elements (e.g., Nicolas, 1989). Moreover, the distinction of mid-ocean ridge from island-arc basalts (Pearce, 1983) relies on large ion lithophile elements (LILE), whose mobility in hydrous solution limits their use in regard to metamorphosed basalts. Secondly, accreted magmatic assemblages commonly include island-arc basalts, as concluded from our recent studies (e.g., Osozawa and Yoshida, 1997),

although these have also reinterpreted as MORBs by Asaki (2000). In either case, magmas of mid-ocean ridge and ophiolite types vary considerably such that the simple applications of geochemical fingerprints (e.g., Pearce et al., 1984) are somewhat limited. All three magmatic types, allow to include alkali basalts (e.g., Osozawa and Yoshida, 1997). A raising question is whether island-arc basalts or MORBs are included. LILE enriched types may be less susceptible chloritization and albitization, processes that effectively result in depleted LILE patterns (Asaki and Yoshida, 1999). The enrichment is then considered to be close to original and Table 1 is expressed by this consideration that the magmas would be island-arc basalts. On the other hand, illite is crystallized in basalts at the time of thrusting and pressure-solution cleavaging, prograde metamorphism ignored by Asaki and Yoshida (1999). This process might effect enriching LILE (Asaki, 2000). In either case, evidence of negative Nb anomalies in most samples other than ocean ridge and intra-plate type basalts (e.g., Osozawa and Yoshida, 1997) invariably suggests island arc affinity of magmas, mostly generated by melting of very depleted mantle sources (Bach et al., 1996). Geochemical studies of accreted assemblages indicate that at least some mantle heterogeneity exists, supported by the modern analogy of the Chile ridge (Kleln and Karsten, 1995). Whereas these basalts are said to have major element compositions within MORB, LILE are enriched for some basalts of the Chile ridge. Some basalts of seamount type are not intra-plate type and explained to be island arc type (e.g., Tazaki et al., 1994). Ophiolite type rock assemblages sometimes contain intermediate and silicic rocks, which can be interpreted to form by fractional crystallization and assimilation (e.g., Maeda and Kagami, 1996; Osozawa and Yoshida, 1997). However, Th contents in basalts are small as MORBs (Asaki, 2000), indicating a little contribution of sediments (Hawkesworth et al., 1997). 5.3. Sediment Basaltic magmas have been classified on the basis of their associated sediments (Table 1). This simple classification is sometimes not applied and such studies mislead the tectonic setting of igneous assemblages, as briefly considered below.

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Reviewing the accretion mechanisms associated with ridge type basalts, Kimura and Ludden (1995) mistakenly identified seamount type basalts accompanying limestone in the Tamba II zone (e.g., Nakae, 1991). Likewise, basalts of the Sorachi zone were interpreted to be products of a hypothetical midPacific super plume (Kimura et al., 1994; Sakakibara et al., 1999), rather than ophiolitic types. However, sediments overlying the late Jurassic basalts were not cherts but hemipelagic mudstones and silicic tuffs, unrepresentative of the mid-Pacific, where pelagic cherts would have been expected. In contrast, Sakakibara et al. (1999) considered that Jurassic super-plume basalts rest on the Triassic chert, which in turn, was expected to overlie typical ocean-floor basalt. Considering the lack of evidence, however, this stratigraphy is not supported. While Jurassic basalts occupying the top part of the Horokanai ophiolite (Ishizuka, 1985) in the Sorachi zone, Triassic cherts and basalt occupy a discrete accretionary component of the Hidaka zone. One problem in the Sorachi zone is that it includes limestones of shallow-water origin (e.g., Kimura et al., 1994), which in general, characterize seamount rather than ophiolite as proposed for the Sorachi zone. However, because the age of fragmental fossils is probably Jurassic to Cretaceous, the fragments need not be interpreted as older accreted seamounts, but may be interpreted as accreted topographic highs within, and contemporary with, the Sorachi sedimentary basin. These highs may be explained in terms of off-ridge volcanism near to a trench, consistent with ophiolite rather than seamount type. Lithologically similar limestones are reported from the Maizuru and Setogawa zones associated the Yakuno and Setogawa ophiolites (Osozawa et al., this volume; 1990). Magmatic assemblages of the Mikabu zone, also in Jurassic age, have also been ascribed to a super plume (Ozawa et al., 1999) or hot spots (Isozaki et al., 1990) origin. These include rocks of komatiite affinity (Yoshida et al., 1984; Ozawa et al., 1999). Lavas sometimes accompany chert, as xenoliths (Sakakibara et al., 1993) or in thin layers, but in most case, are alternated with or directly covered by, hemipelagic mudstone (Faure et al., 1991). Lavas from the Mikabu zone are thus also of ophiolite type, and accompany gabbroic and ultramafic sequences.

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The Maizuru Group is a sedimentary section of the Yakuno ophiolite. The mudstone and sandstone are not thought to represent a member of accretionary complex (e.g., Takemura et al., 1996), and the ophiolite with silicic association is thought to represent an immature island arc (Herzig et al., 1997). These, however, constitute an imbricated thrust sheet of accretionary complex (Osozawa et al., this volume). Igneous and metamorphic rocks of the Hidaka Main zone are considered to represent deep arc crust (e.g., Komatsu et al., 1989), but treated as an ophiolite, following Maeda and Kagami (1996). The basalts again associate mudstone and sandstone. Some accretionary complexes with the ophiolite type basalts contain exotic rocks older than the basalts. One reason misled Onishi and Kimura (1995) to identify the Mugi complex as a tectonic melange is that it contains a small amount of exotic cherts and basalts. The Yuwan complex (Osozawa, 1986; Osozawa and Yoshida, 1997) is another typical example. This complex contains Cretaceous basalts of ophiolite type, but also contains seamount type basalts of Carboniferous to Triassic, shallow water limestones of Carboniferous and Permian, and cherts of Permian to Jurassic. The latter seamount type basalts are not examined for age relation, so that these are derived and recycled from some older accretionary complexes.

6. Summary and conclusions The accreted igneous assemblages in orogenic belts maybe subdivided according to whether they derive from seamounts, ocean ridges, or subductionrelated ophiolites. Seamount basalts are of intraplate type, associated with shallow water sediments—mostly reefoidal limestones. Ocean ridge basalts are invariably of MORB-type and overlain by pelagic cherts. Subduction-related ophiolite type eruptives, sometimes accompanied by gabbroic and ultramafic rocks, are usually associated with hemipelagic mudstones. The age of such eruptive lithologic assemblages reflects the time taken for them to have traveled from the point of generation to the locus of accretion. This has been established for each of these types of accretionary complex in Japan and the western Pacific active margin settings. In general, seamount

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types are older, ridge types are of intermediate age, and the ophiolite types are by far the youngest, usually close to zero age. Seamount type basalts are accreted by shallower scraping of the seamount’s sediment apron together with fragments of seamount basalt, ridge type, by peeling due to permeability contrast and the ophiolite type by deeper scraping as a consequence of increasing temperature. The ophiolitic rocks concerned are generated close to the trench and may be accreted as a result of ridge subduction. The associated limestone, chert and mudstone can divide igneous assemblages of seamount, ridge and ophiolite types. These types indicate the places where these assemblages were formed, far from the trench or not, but additionally they have close relation to the ages and temperatures of oceanic plates of their origin. Basalts of the seamount type are old and cold, the ridge type intermediate, and the igneous assemblages of ophiolite type young and hot. Genesis of the ophiolites at active margin is probably by the ridge subduction and deep-seated thrusting within very young oceanic lithosphere. Me´lange process has no direct relation to accretion process.

Acknowledgments M.F.J. Flower, A. Ishiwatari and an anonymous reviewer improved the manuscript. A. Nicolas, G. Kimura and T. Byrne read early versions of the manuscript, while T. Asaki discussed various aspects of basalt geochemistry.

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