3. Tephras of the Izu-Bonin Forearc (Sites 787, 792, and 793)

Taylor, B., Fujioka, K., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 126

3. TEPHRAS OF THE IZU-BONIN FOREARC (SITES 787,792, AND 793)1

Kantaro Fujioka,2 Yoshiko Matsuo,3 Akira Nishimura,4 Masato Koyama,3 and Kelvin S. Rodolfo5

ABSTRACT

Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information aboutthe timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-BoninArc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes andchemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra ofthe high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few wereproduced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources,such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, mayreflect thickening of the island-arc crust.

INTRODUCTION

Studies of marine tephra of the Izu-Bonin Arc (Figs. 1 and 2) havelagged behind those of other regions because virtually the entire area issubmerged, and the islands are so small that it is difficult to correlatemarine tephras to their land sources (Machida and Arai, 1983, 1988;Furuta et al., 1986). Before the marine surveys began, geologic workalong the volcanic front of the Izu-Bonin Arc emphasized detailedmapping of the younger volcanic islands of the Izu-Bonin Arc (Oshima,Toshima, Miyakejima, Hachijojima, and Myojinsho; Nakamura, 1964;Isshiki, 1959, 1960, 1978, 1980, 1984; Niino et al., 1953) and olderbasaltic rocks of the Bonin Islands, as well as the rhyolites on Kozushima,Shikinejima, and Niijima islands, which obliquely cross the Izu-Boninvolcanic front (Shiraki et al., 1978; Isshiki, 1982,1987).

The bathymetry of the Izu-Bonin region has been mapped duringsurveys by the Geological Survey of Japan on the cruise ship Hakurei-Maru since 1974. Karig and Moore (1975a, 1975b) summarized thegeology and geophysics of the Mariana and Izu-Bonin arcs from theresults of Deep Sea Drilling Project (DSDP) Leg 31. A geologic mapof the Izu-Bonin Arc at a scale of 1:1,000,000 was published by theGeological Survey of Japan (Honza et al., 1982). More recently,Honza and Tamaki (1985) and Yuasa and Murakami (1985) compiledthe geology and geophysics of the Izu-Bonin Arc using all the dataavailable at that time.

Nakamura (1964) performed excellent tephrochronological workon Oshima volcano; however, marine tephrochronological works ofthe Izu-Bonin Arc are scarce because of the lack of good piston cores,and because all previous DSDP/IPOD (International Phase of OceanDrilling) sites along the arc were limited to the transect across theMariana Arc during IPOD Legs 59 and 60. From these studies,volcanism in the Philippine Sea area has been placed in the contextof the opening of the Shikoku and Parece Vela basins and marginalseas (Kobayashi and Nakada, 1978; Seno and Maruyama, 1984).However, drilling in the Izu-Bonin Arc did not begin until OceanDrilling Program (ODP) Leg 126 in 1989.

1 Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station,TX (Ocean Drilling Program).

2 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo164, Japan (present address: Japan Marine Science and Technology Center, 2-14 Nat-sushima, Yokosuka, Kanagowa 238, Japan).

3 Institute of Geosciences, Shizuoka University, 836 Oya, Shizuoka 422, Japan.4 Marine Geology Department, Geological Survey of Japan, 1-1-3 Higashi, Tsukuba,

Ibaraki 305, Japan.5 Department of Geological Sciences, M/C 186, University of Illinois at Chicago, P.O.

Box 4348, Chicago, EL 60680, U.S.A.

After Karig (1971a, 1971b) initially demonstrated that the Philip-pine Sea had formed by backarc spreading, an intense debate aroseabout the temporal relationship between arc volcanism and backarcspreading. The volcanic activity of the Mariana Arc was documentedduring DSDP/IPOD Legs 31 (Karig, Ingle, et al., 1975), 58 (Kleinand Kobayashi, 1980), 59 (Scott and Kroenke, 1980; Rodolfo, 1980),and 60 (Hussong and Uyeda, 1981). Karig, Ingle, et al. (1975)concluded from the results of Leg 31 that backarc spreading occurredduring maxima in arc volcanism; and Karig (1983) restated anddefended this position. In contrast, Scott and Kroenke (1980), Ro-dolfo (1980), Crawford et al. (1981), Scott et al. (1980), and Sharaskinet al. (1981) interpreted the successions drilled during Leg 59 asevidence that backarc opening behind the Mariana Arc occurredduring periods of relative quiescence in arc volcanism. Discussing theresults of Leg 60, Hussong and Uyeda (1981) suggested that backarcopening had no temporal relationship with arc volcanism. The resultsof Leg 126 bear importantly on this question.

During Leg 126, three sites were drilled in the forearc basin of theIzu-Bonin arc-trench system (Figs. 1 and 2): Sites 787 in the AogashimaCanyon, Site 792 at a fork of this canyon, and Site 793 in the middle ofthe upper slope basin. Oligocene to Holocene sediments and rocks ofsimilar types (gravity-flow deposits, hemipelagites, and volcaniclasticrocks in stratigraphic ascending order) were recovered from the three sites(Fig. 3; Taylor, Fujioka, et al., 1990). The sedimentation rates for the samerock types at each of the three sites, based on biostratigraphic andmagnetostratigraphic age controls (Leg 126 Shipboard Scientific Party,1989; Taylor, Fujioka, et al., 1990), are also similar to each other: fast inthe gravity-flow deposits, very slow for hemipelagic rocks, and interme-diate for volcaniclastic rocks. More than 300 tephra layers were recov-ered, from APC cores in the shallower sediment sections of these threesites and from rotary cores (RCB) taken from deeper, sedimentary rockportions. During ODP Leg 125, five other sites were also drilled in theIzu-Bonin forearc (Leg 125 Shipboard Scientific Party, 1989). Two ofthese sites (782 and 786) reached the Eocene to Oligocene volcanicbasement of the outer-arc high, and 116 tephra layers were recovered thatoffer important information about the history of both explosive andeffusive volcanism that took place on the outer-arc high during theTertiary (Leg 125 Shipboard Scientific Party, 1989).

We present here shipboard descriptions and microscopic charac-teristics of the tephra layers obtained during Leg 126, and the resultsof laboratory data on grain-size and chemical analyses conducted ontephras. We discuss the nature and origin of the marine tephras aroundthe Izu-Bonin forearc as inferred from this information, which wasdetermined at the Ocean Research Institute of the University ofTokyo. Site 792 continuously cored Oligocene sedimentary succes-sions with good recovery from Holes 792A, 792B, and 792E; thus,

47

K. FUJIOKA ET AL.

34°N

32C

Figure 1. Bathymetric map of the Izu-Bonin arc-trench system between 30° and 34°N and drilling sites of Legs 125 and 126. Contour interval500 m (numbers on contour lines indicate depth in kilometers). Locations of the drill sites are indicated by filled circles.

Site 792 serves as our reference site for the Izu-Bonin forearc (Tables1 and 2).

METHODS

Shipboard descriptions of the tephra layers (Taylor, Fujioka, et al.,1990) are used here (Tables 1 and 2). The mineral assemblages,morphologies of the glass shards, and characters of the lithic frag-ments in each tephra layer were determined from smear slides bothat sea and onshore at the Ocean Research Institute, University ofTokyo, using standard ODP techniques (Taylor, Fujioka, et al, 1990).

Grain-size distributions for 64 tephra layers were determined byusing standard sieving methods for grains >500 µm, and a ShimadzuCo, Ltd., Model SALD-1100 laserbeam particle-size analyzer forgrains 44-500 µm in diameter. These results are presented in Appen-dix A (in back-pocket microfiche). The finer fractions <44 µm werefirst removed by suspension in distilled water; the samples were thenagitated in an ultrasonic cleaner and decanted. The accuracy ofgrain-size distributions using this instrument is >95%.

Refractive indices of glass shards from the tephra layers were meas-ured with the Refractive Index Measurement System, Model RIMS-86of Kyoto Fission Track Co., Ltd., using the experimental procedures ofYokoyama et al. (1986). This refractometer is useful for the charac-terization of glass shards in tephras because the refractive indices of manyglass shards can be determined with ease even if they are present in smallamounts and are mixed with mineral and pumice grains. The accuracy ofthe refractive index for a single shard is >99%. Table 3 summarizes the

rock types, thicknesses, grain sizes, glass/crystal/lithic ratios, con-stituent minerals, glass shard morphologies, and refractive indices for116 of the tephra layers, those that we think represent primary ashfalls. Reworked tephra layers are not included.

Concentrations of 12 major elements were determined by electronmicroprobe analyses for glass shards and mineral grains using meth-ods similar to those described by Smith and Westgate (1969). Tephrasamples were suspended in distilled water and then agitated in anultrasonic cleaner. The clay fractions (<2-µm fraction) were removedby decanting the supernatant suspension. After drying, hand-pickedshards were impregnated with epoxy resin in 5-mm holes punched inacrylic plates. After polishing, these mounts were analyzed with aJEOL Model JCXA-733 electron probe microanalyzer, with a 10-µmdiameter defocused beam at 15 kV and 1.2 × I0'7 Å, using 10-scounting times for every oxide. The results of these analyses are listedin Appendix B (in back-pocket microfiche). Albite, corundum, quartz,periclase, adularia, hematite, Ni-olivine, wollastonite, ilmenite, Mn-olivine V2O3, and spinel were used as standards for each oxide. Theaccuracy of the values for the different oxides range from 90% to98%. Other experimental procedures and conditions used in thevolcanic glass analyses are discussed in detail by Fujioka et al. (1980)and Furuta and Arai (1980a, 1980b).

VISUAL ASPECTS OF TEPHRA LAYERS

Colors of the tephras are white, gray, green, and black, dependingon the chemistry and the presence of lithic fragments and minerals

48

TEPHRAS OF THE IZU-BONIN FOREARC

Arc

Outerarc high

787 7853000m

790/1Back a re rift

6000m

9000m 100 km

VE=14

Figure 2. Schematic cross section of the Izu-Bonin arc-trench system, showing the locations of the drilling sites of Legs 125 and 126. VE = vertical exaggeration.

(Tables 1 and 2). Glass shards are typically silt size, whereas scoriaand pumice grains are sand and granule size. The tephra layers rangein thickness from several millimeters to several decimeters (maxi-mum = 146 cm), reflecting on their distance from source areas anddegrees of postdepositional bioturbation (Walker, 1971). Most of thetephra layers (Fig. 4) were <IO cm thick. Beds greater than 60 cmthick commonly contain scoria and pumice, possibly resulting frommixing and resedimentation by turbidites. Some tephra layers showeddistinct size grading, some have mixed assemblages of rhyolitic andbasaltic compositions, and some display resedimented structures.Pronounced structures caused by bioturbation of bottom-dwellingorganisms such as zoophycos, chondrites, and planolites were rare(Taylor, Fujioka, et al., 1990).

The tephras occur in definite layers (L; Fig. 4), small pods orpockets (P), or dispersed in the muddy matrix (D). These occurrenceshad previously been observed in cores taken during IPOD Leg 57 inthe Japan Trench forearc area (Fujioka et al., 1980). Each layer maybe either a single bed (Fig. 4A) or a set of tephra beds with sharpcontacts and no intervening mud (Figs. 4B, 4C, and 4D), therebydocumenting explosive eruptions that followed each other closely intime. Both simple and multiple tephras may be a single color ormottled black and white beds. Two types of dark colored tephra layerswere common; one consisted of black, coarse scoria, and the other ofblack, fine, well-sorted ash. Two types of pale-colored tephra layersare also common: white pumiceous debris and fine white ash.

Smear Slide and Binocular Observations

In smear slides, the volcanic glass shards are translucent-colorless,black to brown, and mixed (see "Comments" column, Table 3). Layersdominated by the translucent shards typically are composed of more than85% glass shards with few lithic fragments. The beds with black to brownshards also have minor amounts of lithic fragments and \5%-A0% crystalcontent in addition to black and brown glass shards. The mixed typecontains black and translucent volcanic glass shards. Shard morphologiesare given in the "Glass Shape" column of Table 3. The colorless fragmentsare typically bubble-wall (bw) fragments, tubular (t), or pumiceous(pum). Brown bubble-wall shards are denoted as "brbw"; black ashesnormally are crystal-rich (cry). Representative morphologies are shownin the stereographic photomicrographs of Figure 5 and in the smear-slidephotomicrographs of Figure 6. Plagioclase, clinopyroxenes, and opaque

minerals are common in all the types of tephra layers; orthopyroxeneand biotite are rare. Rare hornblendes occur in the white tephra layersand crystal-rich black ashes.

FREQUENCY OF THE TEPHRA IAYERS

The frequency of volcanic tephra layer occurrences was deter-mined by using biostratigraphic-controlled ages from Taylor, Fujioka,et al. (1990). Site 792, with continuous recovery through the Oligo-cene, is our reference site for the Izu-Bonin forearc. In Figure 7,frequencies of the Oligocene tephra beds are not included becausethese deposits were predominantly thick, resedimented, volcaniclas-tic deposits, and discrete tephra layers were difficult to identify(Taylor, Fujioka, et al., 1990); however, the scientific party of Leg125 counted the ash frequencies at Sites 782 and 786 (Tables 4 and5), which were drilled on the outer-arc basement high southeast ofSite 792 (Fig. 1). The ash-layer frequencies display a maximum inlate Eocene to early Oligocene time (Leg 125 Shipboard ScientificParty, 1989).

Several lacunas occur in the sedimentary succession recoveredfrom Site 792 at 1-2.2, 3.5-6, 8-9, and 13-19 Ma (Taylor, Fujioka,et al., 1990), which must be considered in analyzing the frequency.Figure 7 illustrates the frequency of tephra-layer occurrences in thesuccessions of uppermost Oligocene to Pleistocene rocks and sedi-ments at Site 792, our reference site. There appear to be minor peaksin tephra-layer frequencies in the middle and upper Miocene (10-11and 6.7-6.8 Ma, respectively), and in the lower part of the upperPliocene succession (4.5-2 Ma). In Figure 8, a similar frequencydiagram for the tephra layers of Quaternary age shows two peaks: onefrom 1.0 to 0.6 Ma and the other from 0.4 to the Holocene. In thisQuaternary succession, the only significantly higher frequency ofblack tephra layers occurred at 400 Ka. Figure 9 indicates the fre-quency and thickness of the Quaternary tephras at Site 793. A similarpattern of tephra frequency has also been seen in the volcaniclasticmaterials of the Miocene Shirahama formation of the south BosoPeninsula in central Japan (Kotake, 1988). Along the northern Izu-Bonin Arc, rhyolitic volcanism on Niijima, Kozushima, and Shikine-jima islands characteristically produced white tephra layers duringthe Pleistocene, whereas the pyroclastic materials of Oshima, Mi-yakejima, and Hachijojima islands along the volcanic front of the arcwere generally basaltic.

49

K. FUJIOKA ET AL.

o -

5 0 ^

100^

15Chj

2 0 0 ^

250^

3 0 0 ^

350^

4 0 0 -

Q.Φ

Q 450H

5 0 0 -

550-^

6 0 0 ^

650-

700-

750-

800-

850-

900-

Site 792

T~j ?~i n*<•

α. -i. J. j . xJ. J- -1. X JL X J. JL XJ. -1- J- J- -

• • • •• • • • •

• • • •

>~.o~.o~.o~.o~'o'o'o'o'

SΛΛΛΛ

Δ. A A Λ A

Ù. Λ Δ. i. Ú

» T T t 1Λ Λ Λ Λ /

1 T T T 1

Nannofossils

Silt/siltstone and clay/clay stone

Vrtric and pumiceous sand/sandstone

Pumiceous gravel

Conglomerate

Volcanic-dast breccia

Lapilli tuff

Mafic volcanic rocks

Site 793 Site 787

50-^

10CH

200^

25CH

350-^

400^

450^

500^

550

600^

650^

700^

750^

800^

850^

90CH

950^

1000^

1050^

1150^

1200^

Figure 3. Lithologic columns of forearc Sites 787, 792, and 793, Izu-Bonin Arc.

50


35

40

A

65

70

75

80 L-B

25

30

D

Figure 4. A. Simple, white tephra layer with distinct normal grading of pumice grains; Sample 126-793A-7H-3, 31-38 cm. B. Multiple tephralayers consisting of medium-grained white, gray, and black tephra layers; Sample 126-792A-3H-5, 65-80 cm. C. Multiple layer consisting oftwo black tephra layers, the lower one distinctly laminated; Sample 126-793A-3H-2,50-70 cm. D. Multiple layer of thin, coarse black and finewhite ashes; Sample 126-792B-10X-CC, 22-30 cm.

GRAIN SIZE

Source regions and settling mechanisms of the volcanic ashes maybe evaluated from grain-size data. Three types of grain-size distribu-tion patterns are apparent (Figs. 10, 11 A, and HB):

1. White, coarse tephra layers typically have median diameters ofabout 3Φ, are well sorted, and are fairly unskewed.

2. Layers of fine white ash show both well-sorted and Unsortedcumulative curves with median diameters of 4((x Well-sorted parts are

generally unskewed (Fig. 11 A), whereas poorly sorted parts may bestrongly skewed toward fine sizes (Fig. 1 IB). Well-sorted, fine tephralayers may represent ash falls, whereas those skewed toward the fineend may be the products of subaqueous volcanism (Walker, 1971;Sparks et al., 1981).

3. The grains in black tephra layers are well sorted, have mediandiameters of 3^Φ, and are unskewed or only slightly skewed, al-though most layers are leptokurtic. Examples are the layers in Sam-ples 126-792A-3H-5, 78-80 cm, and 126-793A-3H-2, 50-52 cm.This distribution pattern indicates proximal Izu-Bonin Arc sources,

51

K. FUJIOKA ET AL.

Table 1. Description of marine tephras at Hole 792A in the Izu-Bonin forearc.

No Core Sec Interval Th.(cm) (cm)

Description and remarks

12

3456789

101112

131415161718192021222324252627282930

31323334

35

36

37

38394041

42434445

4647484950515253545556575859

11

1111111

111

111111111111111111

122222

2

22

222223333

33333333333333

11

1111122

222

333345555555666666

711122

2

23

3334

CC1111

11222223334444

0-1934-42

57-5863-6467-6980-82

124-13030-3451-79

120-124125-132139-140

4-524-2573-80

126-145144-146

15-1628-3232-3847-5260-6280-8398-9944-55

81-110118-122122-124131-132144-147

37-39

193/5

111264

9/12/7

471

11

4/31921

2/24/23/22

2/111129421

1/2

2+0-90+60/14/16

100-105140-150*

*0-3064-74

82-94

133-150**0-8

40-4659-6490-98

27-113**0-12+27-3950-5269-7080-94

108-113122-127

3-625-2781-84

97-117147-14855-5765-73

102-1152-6

17-2277-78

99-104

518/22

8/2

7/3/2

12/13

658

98+

1221

10/4

54/11/22320178

12/1451

4/1

fine scoria(dispersed in silty clay)light brownish gray(5YR6/1)/very light gray(N8);silt-cl ay/sandc.sand-granule scoriac.sand-granule scoriac.sand-granule scoriablack ;f.sandbrownish black(5YR2/1);scoria lapilli(max.dia=4 mm)medium light gray(N6);c.sand(pumice & lithic)light gray(N7)/dark gray(N3);silt-clay/f.sand/c.sand-granuledark gray(N3);f.sandwhite;fine ash(dispersed)brownish black(5YR2/1);scoria v.c.sand(ma×.dia=1mm)scoria c.sandolive gray(5Y4/1);m.sandbrownish gray(5YR4/1);silt/c.sand(pumice & scoria)light gray(N7)(purple);clay-f.sand(grading)olive black(5Y2/1);siltmedium light gray(N6);siltblack/black;m.sand/scoria(max.dia=2 mm)medium light gray(N6)/white;clay/silt-clayblack/black;f.sand/scoria(max.dia=2 mm)dark gray(N3);scoria(max.dia=1 mm)medium light gray(N6)/gray;silt/c.sanddark gray(N3);scoria c.sand(dispersed)dark gray(N3);f.-c.sand(grading)darkgray(N3);f.-c.sand with granules(grading)medium light gray(N6);f.sandblack ;f.sandblack;f.-m.sandgrayish green(10GY5/2)/medium light gray(N6);silt-claywhite;fine ash(dispersed)medium dark gray(N4)/grayish black(N2);silt/f.sandgrayish black(N2);f.sandm.d.gray(N4)/light olive gray(5Y6/1);pumiceous v.csand(pum.max.dia=2 cm)/m.sandlight olive gray(5Y6/1)/grayish black(N2);coarse tuff(pumice & lithic, granules)/scoria c.sandlight gray(5YR6/1)/olive gray(5Y4/1 )/brownish gray(5YR4/1 )(purple);clay/sand/siltlight olive gray(5Y6/1);pumiceous lapilli(pum.max.dia=2 cm,lithic max.dia=5 mm)/pumiceous sandylapilli(pum.max.dia=1 cm)dark gray(N3);silt-m.sand(grading)light gray;claydark gray(N3);silt-f.sand(grading)f.sand-pumiceous gravel(pum.max.dia=1 cm, twokinds of pumice,white porous and gray dense)gray;pumice lapilli tuff(pum.max.dia=2 cm)medium gray(N5);lithic tuff,silt-v.f.sand(grading)lithic tuff.c.sandmedium light gray(N6)/medium gray(N5); silt-v.f.sand/pumice c.sand, crystal (Plagioclase) rich (dia=1 mm)medium gray(N5); tuff m.-v.f.sandmedium light gary(N6)/medium gray(N5);silt/v.f.sandgrayish green (5G5/2) / light gray (N7); siltgrayish black(N2);siltmedium light gray(N6);siltolive gray(5Y4/1);clayey siltwhite;fine tuffm.d.gray(N4); silt -v.f.sandgray;pumice lapilli(pum max.dia=2 cm)white;silt / pumiceous c.sand(lithic tuff)grayish black(N2);f.sanddark gray(N3);silt.black;v.f.sand(dispersed)dark gray (N3); v.f.sand/scoria v.c.sand(max.dia=2 mm)

52


Table 1 (continued).

6061

626364656667686970717273747576777879

8081828384858687888990919293949596979899

100101102103104105106107108109110111112113114115116

117

118119120121122123

124125

33

333333333333444444

4444444444444444444444444444444444553

5

555555

55

45

55556666677

cc112222

222333333444444455555556666666667

cc111

1

112222

22

141-14715-19

54-5767-7678-79

110-11429-32

4758-6082-88

100-11516-17

42-45**0-1410-16

115-1208-1519-2333-4650-53

73-80101

123-13110-1530-3350-52

79100-104128-129

6-731-32

3539

48-5090-91

94-10310-1728-3038-4349-64

81102-105108-110

0-212-1535-3648-5068-6986-87

107-108126-128146-150

12-1312-17+0-318-2038-43

69-72

118-119140-14220-2231-3340-4451-55

65-6880-89

62/2

2/19143<12

3/315117

65

4/33/1133

7<18532<14111

<1<121

8/1725

13/2<13223121112415+32

4/<1

3

122244

34/5

medium dark gray(N4);silt-m.sand(grading)olive gray(5Y4/1)/dark gray(N3);v.f.sand/v.c.sand(lithic tuff, lithic max.dia=i mm)dark gray(N3);silt/scoria c.sand(max.dia=1 mm)grayish black (N2);v.c.siltmedium light gray(N6);siltwhite;fine tuff pocketlight olive gray(5Y6/1); siltpumice tuff(max.dia=1 mm)grayish black(N2); scoria c.sand(max.dia=2 mm)light gray(N7)/very light gray(N8);siltmedium dark gray(N4);silt-m.sandolive gray(5Y4/1);siltlight gray(N7);silt

dark gray(N3);silt/v.f.sanddark gray(N3);siltolive gray(5Y4/1)/grayish black(N2);silt / f.sandgrayish black(N2);silt/c.sandlight olive gary(5Y6/1);silty/c.-m.sandgrayish black(N2);v.c.sand with granule(max.dia=4 mm)dark gray(N3);silt - v.f.sand.(grading)dark gray(N3);lithic tuff(5 mm thick)dark gray(N3);siltblack; c.sand (burrowed)olive gray(5Y4/1);pumnice pebble(max.dia=5 mm)dark gray(N3);siltdark gray(N3);siltdark gray(N3);silt-f.sand(grading)grayish black(N2); m.sandlight olive gray(5Y6/1); f.sandgray;fine tuffwhite ;fine tuff (dispersed)dark gray(N3);fine tuff(5 mm thick)light olive gray(5Y6/1);f.sanddark gray(N3);f.sandlight olive gray(5Y6/1);silt/m.sandgrayish black(N2);scoria c.sandgrayish black(N2);scoria c.sandgrayish black(N2); scoria granuledark gray(N3); silt / m.sandgrayish black(N2);scoria c.sand(5 mm thick)grayish black(N2);scoria c.sand(max.dia=2 mm)grayish black(N2);scoriagrayish black(N2);scoria c.sandlight olive gray(5Y6/1);siltgrayish black(N2);f.sand -siltdark.gray(N3);silt with scoria(max.dia=1 mm)dark. gray(N3);m. sanddark gray(N3);siltdark gray(N3);siltdark.gray(N3);siltdark gray(N3);siltlight olive gray(5Y6/1)(reddish);siltdark.gray(N3);siltlight olive gray(5Y6/1);siltolive gray(5Y4/1);siltlight olive gray(5Y6/1)/olive gray(5Y4/1);silt/ v.f.sand (5 mm thick)black(5Y2/1 J/light olive gray(5Y4/1) ;silt/m.sandpumice (5 mm thick)very dark gray(5Y3/1);v.f.sandolive gray(5Y4/1);silt/ v.f.sand(5 mm thick)olive gray(5Y4/1);pumice v.c.sandolive gray(5Y4/1);siltlight olive gray(5Y6/1 );siltvery dark gray(5Y3/1);m.sand with scoria(max.dia=1 cm)olive gray(5Y4/1);siltolive gray(5Y4/1);silt/pumice f.sand(max.dia=3 mm)

53

K. FUJIOKA ET AL.


126127128129130

131132133

134135136137

138139

140141142143

144

145146147

148149150

151

152153154155

156157158159160161162163164165

55555

55556666

66

6666

6

666

6666667777

7777777777

22333

33341112

22

2333

3

444

44455

cc1111

1222233333

129-130140-143

8-1930

54-61

75-76103-108114-150*

*0-5414-70

96124-125

8-26

27-2838-45

92-1014-1034-4658-68

114-136

3-1021-2434-85

93-'104114-120140-150*

*0-7

13

10/1<1

3/1/3

15

11/79

56<11

7/6/5

15/2

6/36

10/210

5/17

1/63

4/47

10/1617

14-135113/5/17+*0-14+

+0-112-1321-2940-57 :

103-13119-2141-52

112-114119-121

12-1427-3238-3946-4857-67

14+ 118

3/6/3/5

28211222512

3/5/2

166 7 81-86 3/2

167

168

169

170

171172173174

777

7

77778888

344

4

456

cc1111

98-114**0-5687-92

101-106

143-145*6*0-150****0-16******0-14+

+0-633-4248-5575-77

72

1/1/

3/2

/14(

+6972

olive gray(5Y4/2);siltgrayish black(N2); siltolive gray(5Y4/1);silt/v.f.sandblack;silt(5 mm thick.dispersed)light.olive gray(5Y6/1 )/olive gray(5Y4/1 )/light olivegray(5Y6/1 );silt/silt/fsilt with a 5 mm thick v.f.sandbasal partlight olive gray(5Y6/1); fine tuffblack;fine tuff (dispersed)light gray(5Y4/1)/light(5Y4/1);silt/clayey silt

olive gray(5Y4/1);pumiceous pebbly sandblack;silt (dispersed)grayish black(N2);siltlight olive gray(5Y6/1)/light gray(N7)/olive gray(5Y4/1);silt/silt/ pumiceous v.f.sandolive gray(5Y4/2);v.f.sandlight olive gray(5Y6/1)/dark olive gray(5Y3/2); silt/m.sandolive gray(5Y4/1);silt/v.f. sand (laminated)light olive gray(5Y6/1);siltgrayish black(N2);f.sand/v.f.sand (laminated)brownish black(5YR2/1);silt with scoria scatterd basalpart(max.dia=5 mm)dark olive gray(5Y3/2); grayish black(N2) silt / silt-f.sand (grading)olive gray(5YR4/1 )/olive gray(5Y4/2);silt/clayey siltlight olive gray(5Y6/1);silt pocketdark olive gray(5Y3/2); grayish black(N2); silt / m.-f.sandlight olive gray(5Y6/1)/olive gray(5Y4/1);silt/v.f.sandlight olive gray(5Y6/1);siltdark olive gray(5Y3/2)/olive black(5Y2/1); clayey silt /siltgrayish black(N2);f.sand with pumice m-c.sand/silt/m.sandolive black(5Y2/1);v.f.sandolive gray(5Y4/1);v.f.sandlight olive gray(5Y6/1);siltgreenish gray(5GY6/1)/olive black(5Y2/1 )/light olivegray(5Y6/1) / olive gray(5Y4/1 );silt/v.f.sand/silt/sandysiltgrayish black(N2); alternation of f.sand and v.f.sandlight olive gray(5Y6/1);silt pocketlight olive gray(5Y6/1);siltgrayish black (N2); v.f.sand pocketgrayish black(N2);v.f.sandbrownish black(5YR2/1);v.f.sandolive gray(5YR4/1); v.f.sandlight olive gray(5Y6/1);siltgrayish black(N2); scoria sand pocketdark reddish brown(5YR2/1 )/dark reddish brown(5YR2/1);v.f.sand/sandy mud with scattered pumice(max.dia=δ mm)/v.f.sanddark reddish brown(5YR2/1)/grayish black(N2); silt/f.sand(laminated)light olive gray(5Y6/1 );silt

brownish black(5YR2/1);silt /f.sand(pumice & foramsincluded)/v.f.sand with thin(2 mm) scoria basebrownish black(5YR2/1)/grayish black(N2);silt/v.f.sand

143-145*6/146/30+dark reddish brown(5YR3/2)/grayish black(N2);clayeysilt/f.sand with gravel(max.dia=10 mm)/scoria pebble

grayish black(N2);scoria m.sandgrayish black(N2); v.f.sandgrayish black(N2);scoria c.sandgrayish black(N2);m.sand

54



175176

177

178

179

180181182183184185186187188189190191192

193194

195196197198

199200201

202203204205206207208209210211

212

8888

8

8

888888899999999999

101010

1010101010101010101010101010

10

1122

2

2

233333

cc1112223334

CC111

12233333333344

45

cc

85-86131-137*

*0-4766-83

93-135

135-148

148-15010-1219-21

2530

39-43*0*-16++0-50

6060-7829-6584-99

113-150**0-3

37-5454-87**4-64

7+0-716-18

36-100

132-1486-8

29-31*4*-818-3439-44

789395

118-124129-132144-145

0-524-84

101-150**0-40****0-20+

16/47

2/12/3

4/38

13

222

<1<1

9/11 +

+50<1183615

3/37

1793

1+7264

162

2/4

12/45<1<1<16315

16/44

109+

grayish black(N2);m.sanddark olive gray(5Y3/2)/olive black(5Y2/1);clayey silt /v.f.sand(m.sand pumice included)dark reddish brown(5YR2/1)/olive black(5Y2/1)/grayish black(N2);silt/v.f.sand/sandy siltbrownish black(5YR2/1 )/olive black(5Y2/1); silt/f.-m.sand (grading)(pumice and forams included)scoria pebble (max.dia=7 mm) scattered in dark olivegray sandy mud)grayish black(N2);m.sandpumice granulepumice granulepumice granule, seampumice granule, seamlight olive gray(5Y6/1 )/olive black(5Y2/1);sandy silt/m.sandolive black(5Y2/1);scoria pebble-granule gravelm.sand (pumice) seamolive gray(5Y4/2);siltolive black(5Y2/1);f.sand - v.f.sand (grading)olive black(5Y2/1);v.f.sandolive black(5Y2/1);siltolive black(5Y2/1);m.sand (laminated)light olive gray(5Y6/1);siltolive gray(5Y4/1);v.f.sand

light olive gray(5Y6/1);silt pocketgrayish black(N2);scoria granulegrayish black(N2);scoria granuleolive gray(5Y4/1);pumiceous granule-c.sand (pumicepebble concentrated at 65-68, 72-73,and 85-87cm)olive gray(5Y5/2);pumiceous m.sand(laminated)olive gray(5Y4/1);siltgrayish black(N2);scoria m.sand/scoria granule

light olive gray(5Y6/1);silt/f.sandlight olive gray(5Y6/1);siltmedium dark gray(N4);v.f.sandmedium gray(N5);silty claygrayish black(N2);siltolive black(5YR2/1);scoria granulesolive black(5YR2/1);f.sandmedium dark gray(N4);siltgrayish black(N2);v.f.sandolive gray(5Y4/2)/olive black(5Y2/1); pumiceousm.sand/ pumiceous m.sand (scoria concentrated)olive black(5Y2/1);pumiceous m.sand(pum.max.dia=6 mm, laminated in the upper part)

Notes: Data after Taylor, Fujioka, et al., 1990. Abbreviations are as follows: v.f. sand = very fine sand; f. sand = finesand; f.-m. sand = fine to medium sand; f.-c. sand = fine to coarse sand; m.-v.f. sand = medium to very fine sand;m. sand = medium sand; c.-m. sand = coarse to medium sand; c. sand = coarse sand; v.c. sand = very coarse sand;v.c. silt = very coarse silt; pum. = pumice; and max. dia. = maximum diameter.

55

K. FUJIOKA ET AL.

Table 2. Description of marine tephras at Hole 792B in the Izu-Bonin forearc.

No Core Sec Interval Th Description and remarks of tephra(cm) (cm)

dark gray(N3);granule-pebbledark gray(N3);silt/f.sanddark gray(N3)/olive gray(5Y4/1);silt/m.sand(grading)light gray(N7);siltdark gray(N3)/grayish black(N2);m.sand/m.sand

light gray(N7);siltlight gray(N7);siltdark gray(N3);c.sandgrayish black(N2)/grayish black(N2);f.sand-granule(grading)/m.-f.sandsiltm.-c.sand (grading)dark gray(N3)/dark gray(N3);silt/f.-m.sand(grading)olive black(5Y2/1);siltolive black(5Y2/1);siltdark gray(N3);siltdark gray(N3);siltolive black(5Y2/1);f.-m.sanddark gray(N3);m.-c.sand

grayish black(N2);scoria pebble(max.dia=1 cm)dark gray(N3);silt-v.f.sand(grading)scoria (v.c.sand size)olive black(5Y2/1)/grayish black(N2);silt/m.-v.c.sand(grading)dark gray(N3);sand with granuleblack(N1 )/black(N1 );silt/f.sandolive black(5Y2/1)/grayish black(N2);silt/f.sandolive black(5Y2/1)/grayish black(N2);silt/f.sand

grayish black(N2)/very light gray(N8)/olive black(5Y2/1);f.sand(grading)/silt/silt-v.f.sand(grading)v.c.sandolive black(5Y2/1 )/grayish black(N2);silt/sand

scoria granule(ma×.dia=4 mm)grayish green(10GY5/2);siltolive black(5Y2/1 )/olive black(5Y2/1)/grayish black(N2);silt/v.f.sand/v.f.sandolive black(5Y2/1 )/grayish black(N2);silt/sand

olive black(5Y2/1 )/grayish black(N2);silt/sand withgranule at basescoria v.c.sandolive gray(5Y4/1);f.sandbrownish gray(5YR4/1)/olive gray(5Y4/1 )/brownishgray(5YR4/1 )(pinkish);f.sandlight gray(N7);siltmedium dark gray(N4);f.sandmedium gray(N5);siltvery dark gray(N3);f.sandmedium gray(N5);siltdark gray(N3);siltolive black(5Y2/1);c.sandolive black(5Y2/1);c.sanddark gray(N3)/olive black(5Y2/1 )/dark gray(N3);silt/clayey silt/siltdark gray(N3);siltdark gray(N3);c.sand(5 mm thick)dark gray(N3)/(N3)/(N3);clay/sand scoria c.sand(max.dia=4 mm)dark gray(N3)/dark gray(N3)/pale olive(10Y6/2);c.sand/scoria f.sand/scoria (max.dia=4 mm)dark gray(N3)/dark gray(N3);silt/c.sand(grading)dark gray(N3);siltgreenish gray(5Y6/1)/light gray(N7);silt/siltdark gray(N3);m.sandolive gray(5Y4/1);siltlight gray(N7);siltgreenish gray(5GY6/1);scoria granule in mud (max.dia=5 mm)

12345

6789

101 112131415161718

19202122

23242526

27

2829

303132

33

34

353637

383940414243444546

474849

50

51525354555657

2222222222222222222223333

333333

333333

333

333

444444444

444

4

4444445

11111222223334555556

cc1112

222233

334444

455

556

111111112

222

2

23333

cccc

+0-2835-5055-90

120-122140-150*

*0-1017-3040-4572-85

89-150**0-102107-112116-15082-13033-4060-6270-7174-7686-97

+5-150**0-15+40-5567-88

120-1217-47

54-6476-92

100-125137-150*

*0-4170-95

117-118145-150*

*0-526-2959-61

70-115 1

147-150**0-3050-95

111-112127-15015-50 1

11-2046-4954-5555-5860-6288-90

113-115119-12026-42

63-6673

84-90

101-109

122-1371-3

22-3060-6185-87

28-30++0-15

+2815352

10/10

13513

81/82

534

8/40721211

+ 160+

15211

10/30

104/127/187/41

7/8/10

15/5

32

5/10/30

3/30

10/35

123

15/8/12

93132221

6/6/4

3<1

1/2/3

2/5/1

10/52

4/4122+

+ 15

56



5859

60616263646566676869707172737475767778798081

8283

8485

868788899091

92

9394

95

969798

99100

101102

103104

105106

56

788888888888888889999999

10

1010

101010101010

10101010

10

101010

1111

1111

1111

1111

ccCC

cc111222223333444411122

cccc

1

11

112222

2333

3

33

cc

11

12

22

2cc

15-29+0-13+

8-126-7

19-2661-6331-3254-5572-7886-91

115-12318-2636-4160-6682-8540-4680-86

110-115123-12932-33

99-100124-13027-3036-37*

*0-19-1032-36

72-7985-93

107-109135-13620-2146-5181-84

101-103

147-150**0-6

15-1933-36

47-51

61-62109-11426-33

13-1547-49

65-660-13

4248-57

83-8820-24

14+13+

4172116588563665611632

12/2

3/44/3/1

21153

1/1

7/2

41/2

2/1/2

15

2/1/4

21/1

11/4/2/6

<13/6

52/3/1

olive gray(5Y4/1);f.sanddark gray(N3);scoria granule(scoria ma×.dia=5 mm,pum.max.dia=4 mm)dark greenish gray(5GY4/1);m.sandolive black(5Y2/1);silty clayolive black(5Y2/1);silty clayolive black(5Y2/1);silty clayvery light gray(N7);fine ashlight olive gray(5Y6/1);siltgrayish black(N2);claygrayish black(N2);claygrayish black(N2);clay with Chondrites type burrowdark gray(N3);silt(grading)grayish black(N2);m.-c.sand(grading, max.dia=δ mm)grayish black(N2);m.sandolive gray(5Y4/1);siltlight gray(5Y6/1);siltmedium dark gray(N4);silty claylight gray(N7);silt with m.sand baselight gray(N7);siltlight gray(N7);siltlight gray(N7);siltlight gray(N7); siltlight gray(N7);sandy siltlight gray(N7);silt

light gray(N7);v.f.sandvery dark gray(5Y3/1)/olive gray(5Y4/1);silt/pumicem.sand(laminated)light olive gray(5Y6/2)/light gray(5Y7/1);silt/siltolive gray(5Y4/2)/olive gray(5Y4/2)/very dark gray(5Y3/1);pumice granule(max.dia=3 mm)/silt/v.f.sand(laminated)very dark gray(5Y3/1);c.sandvery dark gray(5Y3/1);siltolive gray(5Y5/2);siltlolive gray(5Y4/1)pumice m.sand(max.dia=4 mm)olive gray(5Y5/2);siltolive gray(5Y5/2)/very dark gray(5Y3/1);silt/pumicem.sandolive gray(5Y4/2)/gray(5Y5/1 );silt/silt

gray(5Y5/1);silt with f.sand at base(5 mm thick)gray(5Y5/1)/olive gray(5Y4/2);silt/pumice v.c.sand(max.dia=δ mm)gray(5Y5/1 )/very dark gray(5Y3/1 )/gray(5Y5/1 );silt/silt/siltgray(5Y5/1);siltlight olive gray(5Y6/1);siltlight olive gray(5Y6/1)/grayish black(N2)/brownishblack(5YR2/1 );silt/f.sand/silty claygray(5Y5/1);siltgray(5Y5/1)/(5Y5/1);f.sand/silt with scoria c.sand atbase(5 mm thick, max.dia=2 mm)light olive gray(5Y6/1);siltlight olive gray/olive gray(5Y4/2)/(5Y4/2)/grayishblack(N2);silt/silt with scoria/m.sand scattered in siltyclay/silt with scoria(max.dia=3 mm)black;f.sand seam(5 mm thick)gray(5Y5/1)/light olive gray(5Y6/1);silt with pumicem.sand/siltolive gray(5Y5/1);siltgrayish black(N2)/olive gray(5Y4/1 )/dark olive gray(5Y3/2);silt/f.sand/f.sand

107 11 cc 27-31 4 grayish black(N2);siltVOLCANIC ASH LAYERS AT SITE 792E

1 2 1 17 <1 reddish gray(10R5/1);silt, pocket2 3 1 +0-7 +3/4 olive gray(5Y5/2)/light gray(N7);silt/silt with pumice at

base(max.dia=5 mm)3 3 1 15 <1 pumice m.sand seam(5 mm thick)4 3 1 21-23 2 brownish black(5YR2/1);v.f.sand5 3 1 41-46 1/2/2 olive gray(5Y4/2)/grayish black(N2)/light olive gray

(5Y6/1);scoria pebble to granule/ditto with silt(max.dia=δ mm)

57

K. FUJIOKA ET AL.


6789

10

1112

13

1415161718192021

22

232425

26272829303132

3334353637383940414243444546474849505152536455565758

33444

5558

99999999

9

91012

16161717171717

1717171717171717181818181818181818181818181818181818

1111

2

11

cc1

11111233

3

312

2211111222222234111122333333444444

5559

21-2280-15010-75

25-3270-75*

*0-625-34

27-2976-8394-95

115-116144-14694-96

0-716-20

96-103

110-117144-14683-90

6-1192-9850-5172-7385-95

100-107148-150*

*0-1014-1516-2238-4051-5866-71

119-12353-5519-4148-5079-82

107-111114-117103-110142-14720-2337-4449-5262-6385-87

112-1130-9

16-2043-47

112-114116-117120-121

111

7015/50

75/4/2

4/5

22/51122

3/1

7

727

5611

10712

162754222234375373121944211

black;f.sand(diiseminated)black;f.sand(disseminated)olive black(5Y2/1);silt with scoria sand at basescoria & pumice granule scattered in clayolive black(5Y2/1)/grayish black(N2);silty clay/scoriasilty v.c.sand(max.dia=4 mm)olive gray(5Y4/1);siltolive(5Y5/3)/olive gray(5Y4/2)/gray(5Y5/1 );silt/si;t/silt

greenish black(5GY2/1)/olive gray(5Y4/1);silty clay/clayey siltolive black(5Y2/1);siltolive gray(5Y4/1)/olive gray(5Y4/1);silt/v.f.sandolive gray(5Y4/1);siltgrayish black(N2);siltolive black(5Y2/1);siltdark olive gray(5Y3/2);silt(laminated)greenish black(5GY2/1);clayey silt(laminated)greenish black(5GY2/1)/grayish black(N2);clayey silt/silt(laminated)dark greenish gray(5G4/1);pumice granule-pebble(max.dia=IO mm)greenish black(5GY2/1);clayey silt(laminated)grayish black(N2);siltolive gray(5Y4/1);scoria pebble(max.dia=7 mm)scatterred in muddy sandstoneolive black(5Y2/1);clayey silt(laminated)olive black(5Y2/1);clayey silt(laminated)grayish black(N2); ***grayish black(N2); ***grayish black(N2);silt-v.c.sand(grading)grayish black(N2);silt-v.c.sand(grading)grayish black(N2)/dark olive gray(5Y3/2);silt(laminatin in lower part)pale red(5R6/2); ***dark olive gray(5Y3/2);silt(grading)dark olive gray(5Y3/2);scoria granule-fine pebblegreenish black(5GY2/1);silt/m,sand(grading)dark olive gray(5Y3/2);siltdark olive gray(5Y3/2);silty v.f.sandlight gray(N7);silt*** clayey siltolive black(5Y2/1 );silt-f.sand(grading)olive black(5Y2/1);siltolive black(5Y2/1);siltolive black(5Y2/1);f.sandgrayish black(N2);silt-f.sand(grading)brownish gray(5YR4/1);silt-silty sand(grading)dark greenish gray(5GY4/1);v.f.sandolive black(5Y2/1 );silt-v.f.sand(grading)greenish gray(5GY6/1);f.sandgrayish black(N2);v.f.sand(laminated)scoria granuleolive gray(5Y4/1);c.sandolive black(5Y2/1);siltolive black(5Y2/1);siltolive black(5Y2/1 );silt-v.f.sand(grading)olive black(5Y2/1);siltolive black(5Y2/1);c.sandolive black(5Y2/1);v.f.sand

Notes: Data after Taylor, Fujioka, et al., 1990. Abbreviations are as in Table 1.

58


Figure 5. Stereographic photomicrographs of tephra layers. A. Coarser tephra with both translucent bubble-walland pumice-type glass shards; Sample 126-792A-1H-5, 81-83 cm. B. Example of tephra composed of veryfine, white, bubble-wall glass shards; Sample 126-792A-1H-1, 38^0 cm.

but more stenokurtic black tephra layers may have resulted fromsecondary sources, remobilized as turbidity currents and grain flows.

CHEMICAL COMPOSITIONSOF THE TEPHRA LAYERS

We analyzed a total of 1240 shards from 145 layers (2-35 analysesper layer) by electron probe (Microfiche). Our results document thechemical history of volcanism in the Izu-Bonin arc-trench systemsince the Oligocene. Variations in major oxides with stratigraphicposition are shown in Figure 12. Separate curves are drawn for black

glass shards, which normally contain <65 wt% silica, and white ones,which are rhyolitic and have silica contents >65 wt%. Notably, thechemical compositions of the two glass types do not overlap, stronglysuggesting that the black and white ashes were separate basalt andrhyolite eruptions. Most of the glasses from the black tephra layersfall into the basalt and/or basaltic andesite clans, but some are an-desite. TiO2 contents are invariably <1.5%, and some are <1%. TheK2O contents of most of the glass is <0.4%, values lower than thoseof volcanic rocks typical of island arcs (Kurosawa, 1959; Miyashiro,1974; Kurosawa and Michino, 1976; Yuasa, 1985). The K2O contentsof the colorless volcanic glass shards are especially notable. Most

59

K. FUJIOKA ET AL.

Figure 6. Photomicrographs of tephra layers. A. Stereographic photomicrograph of fine, black volcanic tephra layer having considerableamounts of lithic and crystals; Sample 126-793A-8H-5, 28-30 cm. B, C. Photomicrographs of the smear slides of the tephra layers; (B)black tephra with brown glass and clear tabular glass shards, Sample 126-792A-6H-3,130-132 cm; (C) bubble-wall-type translucent glasses,Sample 126-792A-1H-5, 81-83 cm.

values are low, generally <1%; however, several beds have shardswith high K2O in the section younger than 3 Ma.

Silica values of the Miocene and Pliocene ashes are clearly bimo-dal (Fig. 13), as is the case in the middle Miocene northeast Japan Arc(Konda, 1974) and in the Cascade region of the northwestern UnitedStates (Christiansen and Lipman, 1972). SiO2 in Quaternary tephralayers display similar spreads; however, their basaltic, and especiallytheir rhyolitic peaks, are dominant compared with the andesitic ones,as in the Cascade region of North America (Suga and Fujioka, 1990;Eaton, 1982, 1984).

Siθ2-Alkali Relationship

Figures 14A and 14B present silica-alkali plots of all the data. TheSiO2 contents of the shards range from 48 to 79 wt%, but arebimodally clustered into two sets, with values centered on 54 and 72wt%, respectively, reflecting the definite bimodal color pattern ob-served visually in the cores on board the JOIDES Resolution. As notedearlier, the 65% silica value separates the white and black ashes. Mostof the tephra layers belong to the low-alkali tholeiitic series of Kuno(1960, 1966), and some belong to the high-alkali and alkali-olivine-

60


100-1

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I

300

2 0 0 -

po sibl ashin unr cov r d part

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0)o><

cα> (D

i5 o

Graphic lithology

~-:-:-:-:-: fi

Grain sizec s fs cs g

I I I I

Figure 7. Frequency diagram of the tephra layers for forearc Site 792. Columns from left to right show core number, recovery rate, ash frequency, age, graphic

lithology, grain size, and lithologic unit, respectively.

61

K. FUJIOKA ET AL.

ß

0.0 T

Ash frequency (no./m.y.)

100 200 300 400 500

i 0.5-

1.0-

0.0

Light colored ash

Indeterminate and undermined ash

Dark colored ash

Possible ash in unrecovered part 0.5-

Iα>O)

1.0-

Ash accumulation rate (m/m.y.)

20 40 60 80

Figure 8. Accumulation rate of the Quaternary volcaniclastic materials of the forearc Site 792. A. Ash frequency indicated by ash number per million years,

calculating by the constant sedimentation rates of muddy hemipelagites. B. Ash sedimentation rate obtained by the same method as in Figure 8A.

basalt series of that classification scheme. Yuasa (1985) compiled allthe published chemical compositions of the Izu-Bonin volcanic rocksand classified them into two types: a primitive, low-alkali tholeiiteseries in the southern part of the arc, and a high-alkali or calc-alkalicseries in the northern part. Yuasa (1985), Notsu et al. (1983), and Ikedaand Yuasa (1989) also state that the volcanic rocks are more primitivein the south compared with those in the north. The boundary betweenthe two major types is the Sofugan Tectonic line near the Sofugandistrict (Yuasa, 1985; Yuasa and Murakami, 1985). Alkali olivinebasalt also exists near the Iwojima Islands near the backarc side ofthe arc (Tsuya, 1937).

SiO2-TiO2 Relationship

An inverse relationship exists between SiO2 and TiO2 (Figs. 15Aand 15B), as has been noted in other island-arc volcanic suites,especially in calc-alkaline andesites (Gill, 1981), reflecting earlycrystallization of iron-titanium oxides. Taylor and Nesbitt (1990)showed that the TiO2 contents of volcanic rocks are minimal neartrench axes and increase toward the backarc.

SiO2-K2O Relationship

Based on K2O contents, Gill (1981) classified andesitic rocks intolow-K, medium-K, and high-K series. Miyashiro (1973, 1974) pro-posed another classification for the andesitic rocks, based on theirSiO2 and FeO/MgO ratios. No definite classification has been pro-posed for more silicic rocks. Figure 16 shows that most of the tephralayers fall into GilFs low-K series, a few into his medium-K series,and very rare ones into his high-K series. Thus, mostly the volcanicrocks of the Izu-Bonin Arc belong to Gill's low-K series and Mi-yashiro^ tholeiite and calc-alkalic rock series. The relative proportionof calc-alkalic to tholeiitic tephra is low compared with values in otherarcs and active continental margins. However, this may be an artifact

arising from the occurrence of older data published for the rocks ofthe northern part of the Izu-Bonin Arc. Recently, Hamuro et al. (1980,1983), Ikeda and Yuasa (1989), and Hochstaedter and Gill (1990)published new data for submarine volcanic rocks that indicate thetephra layers of the low-K series were brought to the depositional sitefrom the volcanic front by subaerial and subaqueous processes.

DISCUSSION

Frequency of Tephra Occurrence

The frequency pattern of the tephra layers in the forearc sitesdocument two definite episodes of increased volcanism: in the lateEocene to early Oligocene (Leg 125 Shipboard Scientific Party, 1989)and in the Pliocene-Pleistocene. In addition, a small peak occurs inthe late middle Miocene (ca. 10 Ma). These peaks correspond to thefollowing regional tectonic and geologic events: before the openingof the Parece Vela Basin, after the cessation of Shikoku Basin spread-ing (Seno and Maruyama, 1984; Kobayashi and Nakada, 1978), andbefore the rifting of the Izu-Bonin backarc regions (Fujioka, 1988).The frequency of tephra layers means that the episodes of backarcspreading started just after the cessation of the intense arc volcanism.A major debate about the relationship between backarc spreading andarc volcanism has been discussed. Our results corroborate the conten-tion (e.g., cf. Scott and Kroenke, 1980; Rodolfo, 1980) that backarcspreading started during periods of quiescence in arc volcanism. Ourresults also provide additional details concerning the petrology ofmagmas erupted during arc rifting.

The younger tephras from the forearc sites are colored and chemi-cally bimodal; that is, they are black ash and white ash, or basalticand rhyolitic, respectively. This bimodality increases from Plioceneto Holocene time. Similar bimodal volcanism was noted in the Basinand Range province and the Cascade region of the North America,and in volcanic rocks produced from Eocene to Holocene time, whenregional extensional stress fields were predominant (Christiansen and

62


ß

I

1P 4

0.275

623

O.β3OMa

MO 100 80 60 40 20

Ivv>>V 410

!705

335

196

20 40 60 80 100

Black White

Granule Size

Sand Size

Sat Size

(number) Black White (cm)

Figure 9. Frequency (A) and thickness (B) of the tephra layers at Site 793, Izu-Bonin forearc. The thickness and frequency of the black and white tephra layersare plotted vs. the core number.

Lipman, 1972; Eaton, 1982,1984). Kennett et al. (1979) documentedand discussed a pulse of circum-Pacific volcanism during the Quater-nary period and named it the Cascadian pulse. The Izu-Bonin explo-sive volcanism was similar in time to the Cascadian event.

Cadet and Fujioka (1980) and Fujioka (1983a) documented thatmaxima of explosive volcanism in the northeast Japan (Tohoku) arcoccurred during the middle Miocene and Pliocene-Pleistocene. Theyounger of these two maxima (ca. 3 Ma) is contemporaneous withthat of the Izu-Bonin Arc. In the Tohoku arc, Pliocene-Pleistoceneexplosive volcanism was not bimodal, although the middle Miocenevolcanism was (Konda, 1974; Fujioka, 1983b). We speculate that thedifference of the Pliocene-Pleistocene volcanic products in the Izu-Bonin and Tohoku arcs may be ascribed to the difference of the crustalthickness and nature in crust of the two arcs.

Origin of the Black Tephras

The chemical composition and grain-size distribution of the blacktephras at Site 792 are comparable to those produced by several basalticeruptions in the Aogashima Islands (Takada et al., in press; Tsuya,1937). The chemical composition of the Aogashima basalt, especiallythe Kurosaki volcanics (the lowermost exposure of the island, ofunknown age), displays low Ti and K2O and high CaO and FeO. Site792 black tephras are chemically comparable to the Kurosaki volcanics(Takada et al., in press). The location of Site 792 is at the fork of theAogashima Canyon, about 60 km east of Aogashima Island. The headof one branch of the canyon, South Canyon, is located immediatelyeast of Aogashima. Some of the black tephra layers may possibly be

correlated by chemistry with the Kurosaki volcanics of AogashimaVolcano. If they are correlated, basaltic tephra at Site 792 were eithertransported in the air as air fall or were remobilized along the canyonafter air-fall deposition.

K2O Contents of Volcanic Glass

Dickinson and Hatherton (1967), Dickinson (1968), and Nielsonand Stoiber (1973) demonstrated that the K2O contents of volcanicrocks bear a close relationship to the depth of the magma from whichthey were generated. Consequently, the K2O contents of the tephrasshould also reflect the depth of the magma from which the tephralayers were derived. High-K2O tephra layers increased in abundancesince 3 Ma along the Izu-Bonin Arc. If they were derived from theIzu-Bonin Arc, they might indicate gradual thickening of the arc crustsince the Pliocene, consistent with Miyashiro's (1974) observationthat the proportion of calc-alkalic series rocks increases as the crustthickens. This interpretation is invalid if the high-K2O ash came fromanother source.

Possible Correlation of High-K Tephraswith Widespread Regional Deposits

The fine-grained, well-sorted, white tephra layers consisting mainlyof bubble-wall glass shards may belong to other tephras than those of theIzu-Bonin Arc, widespread throughout the region, such as those describedby Machida and Arai (1988) and Furuta et al. (1986). Fujioka et al. (thisvolume) correlated the tephras at Hole 442A in the Shikoku Basin with

63

Table 3. Petrographic characteristics of tephra layers from the forearc sites.

Core, section,

Interval

126-792A-

1H-1,1H-2,

1H-3,

1H-5,

1H-6,

1H-6,

2H-1,

2H-2,

2H-3,

2H-4,

2H-4,

3H-1

3H-1,

3H-1,

3H-2,

3H-5,

3H-CC

4H-1,

4H-2,

4H-3,

4H-3,

4H -4,

4H-5

4H-5,

4H-6,4H-7,

5H-1,

5H-2,

5H-2,

5H-3,

5H-3,6H-2,

6H-2,

6H-3,

6H-4,

7H-2,

7H-2,

7H-3,

7H-3,

7H-4,

8H-2,

8H-3,

9H-4,

10H-2,

126-792B-

(cm)

38-40

64-66

138-140

81-83

119-121

146-148

117-119

87-8960-62

18-20

23-2550-52

86-88

122-124

83-85

78-80

(5-7)

117-119

53-5520-22

101-103

96-98

19-21

57-59

48-50

11-13

37-39

45-47

141-143

57-59

131-133

17-19

44-46

130-132

97-99

20-21

48-50

81-83

110-112

103-104

64-66

40-42

6-8

6-8

Lithology

ML

ML

SL

ML

SL

D

D

MLSL

D

DSLMLMLgS

ssDDQ

MLQ

ML

SL

SL

ML

SL

SL

ML

ML

ML

ML

ML

ML

P

SL

ML

SL

ML

D

ML

SL

SL

Thickness(cm)

8

28

19

3

4

12

5

D

D2

1 4

5

1

14

5DD

9Q

1 5

21

5

4

3

7

90

18

7

221 1

2

1 1

5725

70932

Grain size

v.f.sand-v.f.sand-v.f.sand-v.f.sand-

v.f.sand-v.f.sand•v.f.sand•v.f.sand-v f Sand-ys.sand-

f ç a n ( 4

v.f.sand-

v.f.sand-v.f.sand-

v.f.sand-f.sand -v.f.sand-v.f.sand-

v.f.sand-

v.f.sand-f.sand -f.sandf.sandv.f.sand-

v.f.sand-v.f.sand-v.f.sand-f.sandv.f.sand-v.f.sand-v.f.sand-v.f.sand-

v.f.sandv.f.sandm.sandc.sandc.sandc.sand

si l tm.sandc.sandf.sandf.sandc sandf.sand

v.f.sandf canH

m.sandv.f.sandf.sandm.sandv.f.sandv f sandv.f.sand

j

c.sand

m.sand

f.sand

f.sand

v.f.sand

c.sand

v.f.sand

f.sand

m.sand

c.sand

m.sand

f.sand

s i l t

f.sand

f.sand

f.sand

c.sand

m.sand

f.sand

m.sand

m.sand

Max gl

(mm)

0.150.20.4

0.55

0.55

0.2

0.1

0.35

0.90.2

0.35

0 9

0.25

0.15

0 25

0.35

0.15

0.30.4

0.15

0 2

0.15

0 40.4

0.4

0.4

0.3

0.15

>0.1

0.08

0.2

0.4

0.15

0.7

0.15

0.15

0.2

0.2

0.2

0.4

0.3

0.2

0.5

0.3

Max li

(mm)

0.15

0.15

0.15

>0.9

0.1

>0.6

0.15

0.150 15

0.1

0 10.4

0.1

>0.5

0 2

0 4

0.1

0.3

0.15

>0.1

>0.6>0.7

0.08

0.1

0.30.1

0.7

0.1

0.05

m t

m t

m t

m t

m t

m t

m tm t

m t

Mode

90/1 0/090/1 0/090/ 8/293/ 5/290/ 8/2

50/1 5/3595/ 5/096/12/2

50/20/3080/10/1091/ 8/1

95/ 5/095/ 4/19 4/1 5/1

30/30/4095/ 5/065/15/2045/1 5/4090/1 0/0

45/1 5/4095/ 5/0

50/20/3065/15/2020/20/6080/1 0/1095/ 5/095/ 5/060/10/3095/ 5/095/ 5/095/ 5/0

20/50/3040/40/2095/ 5/095/ 5/095/ 5/085/ 8/790/ 8/260/15/2090/ 9/191/ 8/187/10/391/ 8/1

Constituent mineralspi

+ +

++

++

++

++++

++

+++++++

++

+ +

+ +

+++++

++++

++

+ +

+ +

+++++

++++

++

++

++

++

+++

+++++

++++

++

++

++

++++++

+ +

cpx

+

+++

++++

++++

++

++

+

+

++

++

+ +

+

++

+

+

++

++

++

++

++

++++

+ +

opx

+++

+ +

+

++

++

++

hb

++

+

++

+

+

+

+

+

+

++++++

+ +

bi op

++

++

++

++++

++

++

++

++

+ +

++++

++

++

+ +

+ +

++

++

+

++

++++

++

++

++++++

Glass shape

b w

bw

bw

bw

tub,pumbw

bw

bw(fine)>pum(coarse)br.cry.bw

br.cry>pumbw

pum, tub.brbwbwh w

cry.tub,bwb w

bw.br,pumbr,tub,pum

bw

br.cry.bwbw

hw hrpum.bwcry,se

bw>pum.brbw

bw

cry.br>tub.bwbwbw

bw

cry

br,cry,pum,tubbw

bwbw

br.bw.crybw>brbr.cry

bw

bw

bw>tub, pum, cry.brbw.pum>br

Refractive Index(mean)

1.50521.51071.51151.50961.51221.5051

1.51961.5204

1.5179

1.50671.50161 5044

1.50841 .5095

1.516

1.5309

1.5031.507

1.5105

1.51451.50131.5046

1.5078

1.51 181.50821.509

1.50311.49961.52091.5206

(min)

1.50191.50871.50971.509

1.51071.5046

1.50821.516

1.5156

1.50581.50121 501 9

1.50691.5087

1.5148

1.5195

1.50211.50481.5087

1.51221.49981.5042

1.5067

1.51061.50651.5078

1.50261.49891.50561.5197

(max)

1.50711.513

1.51321.51081.50311.5057

1.52111.5285

1.5195

1.5081.502

1 5061

1.50911.5103

1.5168

1.533

1.5041.50981.5112

1.51561.50241.5053

1.5084

1.5131.50931.5099

1.50381.50031.53471.5218

Comment

br glass(f.sand) mixedbw(m.sand;Max 0.4mm)&bw(v.f.sand)

br glass(m.sand) mixed

br glass nixed

br glass & clear glass mixed

br glass&bw glass mixed

pum, cry(m.sand)&bw(v.f.sand)mixed

tub(c.sand) mixedbw,pum&br glass mixed

Max crystal 0.7mm

very finebr (0.5mm)mixed

br glass (0.2mm) mixed

br glass(0.2mm) mixedtub,pum(Max 0.5mm),br&bw mixed

br glass mixed

muddymuddy

muddy

muddymuddymuddy

muddy

muddymuddymuddy

muddy

muddy

muddymuddy

muddymuddy


Core, section,Interval

1H-1,1H-1,1H 21H-2,1H-21H-3,8X-1,8X-2,8X-38X-3,8X-4,9X-1,9X-2,

10X-1,10X-1,10X-3,10X-CC

126-792C-

1X-CC,

126-792D-

1X-1,

126-792E-

1R-1,3R-1,3R-1,4R-1,5R-1,5R-1,8R-1,9R-1,9R-3,

10R-2,11X-1,12R-1,15R-1,16R-2,17R-1,17R-2,17R-2,17R-4,18R-1,

(cm)

50-52139-141

36-3769-71

1 37-1 3863-6520-2274-7624-2660-6280-82

125-12736-38

5-7

88-9048-50

(24-26)

(16-18)

46-48

64-664-6

43-4521-2329-3159-6131-3378-80

115-117146-148104-106

87-89132-134

78-80101-103

13-15120-122

20-22108-1 10

Lithology

SL

SL

SLMLSLD

SLSLSLSLSLSLSLD

MLMLML

SL

ML

DMLMLSLSLD

MLMLSLDDDDD

SLSLSLSLD

Thickness(cm)

7

103

20•j

7

6Qß

66

2

8

57

4

30

7

5

1

7

9

7

7

7

1

4

22

Grain size

s i l tf.sand - granuleu f canri m canH

v.f.sand- m.sand

v.i.saπu- csdπov.f.sand- f.sandv.f.sand- f.sandv.f.sand- c.sand

V.T.SαflU" L.bdllU

v f sand- c sand

v.f.sand- f.sand

v.f.sand• f.sand

v.f.sand- f.sand

v.f.sand• m.sand

silt - f.sand

silt - f.sand

v.f.sand- f.sand

v.f.sand

v.f.sand- m.sand

v.f.sand- f.sand

v.f.sand- f.sand

v.f.sand- f.sand

v.f.sand- c.sand

v.f.sand- f.sand

v.f.sand• f.sand

v.f.sand- f.sand

v.f.sand- m.sand

f.sand

f.sand - c.sand

v.f.sand- c.sand

v.f.sand- m.sand

v.f.sand- m.sand

f.sand - m.sand

f.sand

v.f.sand- f.sand

v.f.sand• granule

v.f.sand- f.sand

v.f.sand- f.sand

Max gl

(mm)

0.05

>0.5

0 4

0.3

>O 7

0.2

0.1

0.2

0 25

0 15

0.2

0.2

0.2

0.25

0.15

0.15

0.25

0.15

0.4

0.2

0.2

0.15

0.3

0.250.20.20.40.20.30.30.5

0.5

0.3

0.2

0.4

0.4

0.2

0.15

Max li

(mm)

>0.5

0.1

>O 8

0.1

0.1

>0.15

>O 7

>O 7

0.15

0.35

0.1

0.07

0.1

0.15

>1.0

0.2

0.1

0.1

0.3

0.1

>0.8

>1.5

0.15

0.2

0.2

0.1

0.2

>1.5

0.5

0.1

m t

m t

m t

m tm t

m t

m t

m t

m t

m t

m t

m t

m t

Mode(gl%/cry%/li%)

95/ 5/020/40/4092/ 8/091/ 8/1

Δ,C\ i O(\ 1 A{\

7 5 / 1 5 / 0

4 0 / 4 0 / 2 0

5 0 / 2 5 / 2 5

4UMU'JU

20/40/4080/1 2/892/ 8/095/ 5/0

75/1 0/1 595/ 5/092/ 8/092/ 8/0

90/10/0

94/ 5/1

90/ 9/190/ 9/192/ 7/1

50/20/3090/1 0/085/10/588/1 0/289/10/1

72/ 8/2080/10/1080/10/1085/10/582/10/8

80/10/1085/ 8/7

80/10/1070/15/1580/10/1080/10/10

Constituent minerals

p l

+++

+ +

+ +

+++

+++

+ +

++++

++

++

++

+ +

+ +

++++

+++

++

++

++

++++

++

++++

++

++

++

++

++

++

++

cpx

++

+ +

++

++

+ +

++

++

+ +

++++

+

++

+

++

++

++

++

++

++

++

++

++

++

op×

++

+

hb

++

+

+

++

+

+

+

+

++

+

+

+

bi

+

op

++

++

+ +

++

++

++++

++

+ +

+ +

++

++

++++

+++++

++

Glass shape

bwtub,pum

h w

bwbr.crypum.br

br.cry>bwcry.bw

h w

cry.bwbw>br.cry

bwbw>pum

bw.br,pum,crybw

bw

bw>pum

bw

bw>cry

bw>br.crybw

bwcry,pumbw>pum

bw>br.crybw>pumbw>pumbr.crybr.cry

br.cry,pumcry.br

tub,pum,cry.bwpum,crybr.cry

bw.br,crybw.br

bw.br,sebr.cry>bw


1.5065

1 51.514

1.5281

1.503

1.50251.5141

1.51041.5059

1.5038

1.5014

1.5315

1.503

1.5133

(min)

1.5056

1 49881.5098

1.5218

1.5015

1.49921.5099

1.50821.505

1.502

1.5002

1.5072

1.5024

1.5127

(max)

1.5073

1 501 21.5184

1.5367

1.5106

1.50531.5207

1.51281.5069

1.505

1.5034

1.5366

1.5035

1.5137

Comment

very fine

br glass mixed

br glass mixed

very fine

pum glass (0.2mm) mixed

sc.br glass mixed

br glass mixed

br.cry glass mixed

br.cry glass & bw mixed

muddy

muddymuddymuddymuddymuddy

muddy

muddy

muddy

muddy

muddy

muddy

muddymuddymuddymuddymuddymuddy

muddymuddymuddy

85


Core, section,Interval

18R-2,18R-4,

126-793A-

1H-1,1H-1,1H-2,2H-2,3H-1,3H-2,3H-2,3H-3,3H-4,4H-2,4H-3,5H-1,5H-3,5H-4,5H-5,5H-6,6H-3,6H-5,7H-1,7H-2,7H-2,7H-3,8H-4,8H-4,8H-5,8H-5,8H-5,9H-3,

10H-2,10H-CC,11H-1,11H-2,11H-2,11H-3,

(cm)

144-1461 18-120

10-1254-56

1 18-120130-132147-149

50-5269-71

113-115116-118

96-98108-1 10

65-673-5

50-5215-1797-99

131-13390-9299-10132-34

1 18-12035-3718-20

137-13928-3073-75

115-117129-131129-131

(3-5)40-4275-77

115-1179-11

Lithology

SL

D

Thickpumiceous

layer

SL

ML

SL

SL

D

D

D

D

SL

DDD

MLSLMLSLSLSLSLSLSLSLSLSLMLSLMLDD

SL

Thickness(cm)

5

12051

4

7.5

2

2

5

7 3

21

3 0

1 04

5 3 0

1 3

1 31 0

4 0

1 118

8

7

3 0

Grain size

v.f.saπd-

f.sandv.f.sand-v.f.sand-m.sand -m.sand -t.saπdf.sand

f.sandv.f.sand-v.f.sand-v.f.sand-v.f.sand-v.f.sand-v.f.sand-v.f.sand-f.sandv.f.sand-v.f.sandf.sand

v.f.sand-f.saπdv.f.sand-v.(.Sand-ys.sand-v.f.sand-f.sandv.f.sand-v.f.Sand-ys.sand-f.sandv.f.sand-v.f.sand-

f.sandf.sand

granulec.sandm.sandgranulegranulem.sandm.sandf.sandm.sandf.sandf.sandm.sandf.sandf.sandf.sandf.sandm.sandc.sandm.sandm.sandf.sandf.sandm.sandf.sandf.sandf.sandm.sandm.sandm.sandf.sandf.sandm.sandm.sandm.sand

Max gl(mm)

0.2

0.25

1.5

0.4

>0.7>1.5>0.50.4

0.350.2

0.5

0.150.1

0.2

0.150.2

0.2

0.150.4

0.8

0.3

0.4

0.2

0.3

0.3

0.2

0.2

0.250.4

0.3

0.4

0.3

0.250.4

0.4

0.3

Max li(mm)

0.3

0.15

0.3

0.35>0.6

>0.80.3

0.251.5

0.2

0.1

0.150.150.1

0.150.1

0.15

0.150.5

0.2

0.4

0.15

0.150.2

0.150.1

0.150.2

0.15

m t

m t

m t

m t

m t

m t

m t

m t

m t

m t

m t

Mode

75/10/1 582/ 8/10

40/40/2077/ 8/ 1580/10/1 0

40/40/2030/30/4030/40/3080/10/1 085/1 0/594/ 5/1

70/1 0/2085/1 0/590/ 8/2

70/20/1095/ 5/0

80/1 0/1 075/1 5/1 090/ 8/291/ 8/185/10/580/1 5/590/ 8/291/ 8/185/1 0/580/1 5/590/ 8/295/ 5/075/10/582/15/285/1 0/592/ 8/0

Constituent minerals

p i

+ +

+ +

+++

++

++

++

+++

+++

+++

++

++

++

++

++

++

++

++

++

+++

++

++

++

++++

++

++

++

++

++

++

++

++

++

++

+++ +

cpx

+ +

+ +

+ +

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

++

+ +

opx

+

+

++

hb

+

+

++

++

++

++

+

++

++

++

++

++

++

++

++

++

+ +

bi op

++

++

++

++

++

++

++

+ +

Glass shape

cry,secry,se,cry

pum,br.crypum,tubbw.pum

pum

pum.brcry.br,pum

cry,pumpum.bw

pum.bw.brb w

bw.crybr.cry,pum

br.crybw.br

br.cry.bwpum.bwbr.cry

bw.pumpum.br.bw

br.bwpum,tub

b w

brbwbrbwbrbwbrbwbrbw

bw.brbwbrbw.bw

b w

cry,pumpum,tub

pum

b w


1.5186

1.5067

1 .5122

1.50841.5027

1.51261.512

1.52281.51071.5117

(min)

1.5132

1.504

1.5079

1.50111.5016

1.50911.509

1.51041.50761.5099

(max)

1.5258

1.5247

1.5105

1.51231.5039

1.51421.5149

1.52691.51391.5138

Comment

pum.scpl(f.sand) mixed

muddy

muddymuddy

muddy

muddymuddymuddy

muddymuddy

muddy

muddy

muddymuddy

muddy

muddy

Notes: v.f. sand = very fine sand, f. sand = fine sand, m. sand = medium sand, c. sand = coarse sand, gl = glass, liop. = opaque mineral, pum. = pumice, br = brown glass, tub = tubular, and bw = bubble wall.

= lithic fragment, cry = crystal, pi = Plagioclase, cpx = clinopyroxene, opx = orthopyroxene, hb = hornblende, bi = biotite,


Table 4. Catalog of the ash intervals in Hole 782A(tephra layers), Leg 125, in the Izu-Bonin forearchigh.


Core, section,interval (cm)

125-782A1H-1, 901H-3, 241H-3, 1302H-4, 862H-4, 902H-4, 103-1042H-4, 1142H-4, 1312H-5, 107-1152H-5, 127-1502H-6, 0-82H-6, 85-865H-6, 18-196H-3, 21-276H-4, 84-916H-6 55-576H-6, 87-907H-3, 67-697H-3, 106-1107H-4, 79-847H-7, 59-618H-4, 94-958H-4, 122-12311X-1, 54-5711X-1, 110-11711X-2, 37-3811X-3, 0-111X-3, 9.5-1011X-3, 51-5412X-1, 2012X-1, 28-2912X-2, 9-1012X-2, 16-1712X-CC, 22-2313X-2, 103-10413X-3, 51.5-52.514X-2, 10-1114X-2, 82-8314X-2, 112-11614X-2, 140-14414X-3, 41-4514X-5, 15-1815X-2,105.5-106.515X-2, 11515X-2, 11715X-3, 8-1615X-3, 101-10515X-3, 113-11515X-4, 13.5-17.016X-3, 96-9717X-1,54-5517X-2. 3-517X-2, 102-10317X-2, 124-12517X-3, 12117X-4, 0-217X-4, 73-74

Depth(mbsQ

0.93.244.3

15.1615.215.3315.4415.6116.87-16.9517.318.118.1645.9851.01-51.0753.14-53.2155.85-55.8756.17-56.2060.97-60.9861.36-61.4062.53-62.6466.89-66.9172.2472.5296.24-96.2796.80-96.8797.5798.798.899.21-99.24

105.5105.58106.8106.96110.27117.53118.52126.2126.52117.22-127.26127.50-127.54128.01-128.05130.0-130.65136.86

136.95136.97137.38-137.54138.31-138.35138.41-138.43138.93-138.97147.86154.14155.13-155.15156.12156.34157.81158.08-158.12158.83

Volcaniclayer

123456789

1011121314151617IS19202122232425262728293031323334353637383940414243

4445464748495051525354555657


17X-5, 139-14118X-1, 10-1619X-1, 147-14819X-2, 23-2719X-3, 104-10519X-CC, 13-1420X-CC, 1021X-1, 51-5321X-1. 64-6621X-2. 4S-5121X-2, 82-86.521X-2, 96-9721X-2, 127-12821X-3, 0-121X-3, 7221X-5, 42-4523X-2, 15-1623X-3, 23-2623X-3, 93-9423X-4, 107-10923X-4, 132-13323X-5, 40-4523X-5, 83-8623X-6, 67-6823X-6, 134-13723X-6, 141-14424X-1, 44-4524X-2, 2-325X-4, 32-3625X-4, 11426X-1, 13826X-1, 14026X-2, 83-8528X-1, 98-9928X-3, 1729X-3, 9-1029X-3, 43-4629X-3, 9029X-4, 85-9029X-5,12-1529X-6, 19-2129X-6,30-4129X-7, 35-373OX-2, 9-1030X-2, 30-3131X-1, 1731X-1.22-2432X-2, 41-4632X-4, 12432X-6, 32-3533X-3, 82-8533X-5, 87-8833X-7, 27-29 534X-CC, 20-26.537X-5, 85-8739X-2, 939X-2, 44-45.539X-2, 70-7141X-4, 23-24

Depth(mbsf)

160.99-161.01163.36-163.38174.37174.63-174.67176.94178.08183.26192.61-192.62192.74-192.76194.08-194.10194.42-194.46194.56194.87195.1195.82198.52-198.55213.15214.73-214.76215.33217.07-217.09217.32217.90-217.9521S.33-218.36219.67220.34-220.37220.41-220.44221.44222.7235.52-235.56236.34241.68241.7242.63-242.65260.58262.77273.1272.63-272.66273.1274.55-274.67275.32-272.35276.89-276.91277.00-277.11278.55-278.57280.39280.6288.67288.72-288.74295.91-295.%299.74301.82-301.85307.82-307.35310.37312.77-312.80314.79-314.85348.45-348.47362.49362.88363.1384.93

Volcaniclayer

58596061626364656667686970717273747576777879808182838485S687888990919293949596979899

100101102103104105106107108109110111112113114115116

Note: Data from Fryer, Pearce, Stokking, et al. (1990).

67

K. FUJIOKA ET AL.

Table 5. Catalog of the ash intervals in Hole 786A (volcaniclasticlayers), Leg 125, in the Izu-Bonin forearc high.

A . 100


125-786A-1H-1, 62-771H-2, 3-51H-2, 5-181H-4, 12-161H-4, 23.5-251H-5, 62-1111H-6, 77-781H-7, 5-82H-1, 10-142H-1, 29-302H-1.802H-2, 99-1132H-2, 143-1462H-3, 145-1462H-4, 36-402H-4, 84-852H-4, 93-962H-4, 141-1452H-5, 28-302H-5, 122-1252H-6, 14-173H-1, 41-443H-1, 54-583H-1, 87-903H-2, 100-1033H-3, 116-1203H-3, 116-1203H-5, 79-80.53H-6, 102-1123H-CC, 10-173H-CC, 214X-1, 60-615X-1, 39-415X-1, 74-765X-1, 74-765X-1, 78-795X-1, 106-1075X-2, 14-156X-1,21-256X-1,93-946X-2, 37-466X-2, 102-1036X-3, 126.5-1286X-5, 8-96X-5, 22-276X-5, 36-407X-1, 138-1407X-2, 125-1277X-3, 3-47X-3, 15.5-167X-5, 9-107X-5, 14-157X-5, 18-247X-5, 64-667X-6, 27-289X-3, 43-449X-3, 85-899X-3, 128-1319X-4, 23-269X-5, 56-66

Depth oftop ofsection(mbsf)

0

1.51.54.54.567.599.79.79.7

11.211.212.714.214.214.214.215.715.717.219.219.219.220.722.222.225.226.728.8928.8928.738.238.238.238.238.239.747.647.649.149.150.653.653.653.657.158.660.160.163.163.163.163.164.2979.879.879.881.382.8

True depth(mbsf)

0.62-0.771.53-1.551.55-1.684.62-4.66

4.735-4.756.62-7.118.27-8.289.05-9.089.80-9.949.99-10.0

10.512.19-12.3312.63-12.6614.10-14.1414.56-14.6015.04-15.0515.13-15.1615.60-15.6515.98-16.0016.92-16.9517.34-17.3719.61-19.6419.74-19.7820.07-20.1021.70-21.7323.36-23.4023.36-23.4025.99-26.00527.72-27.8228.99-29.06

29.129.30-29.3138.59-38.6138.94-38.9638.94-38.9638.98-38.9939.26-39.2739.84-39.8547.81-47.8548.53-48.5449.91-49.9250.12-50.13

51.865-51.8853.68-53.6953.82-53.8753.98-54.058.48-58.5059.85-59.8760.13-60.14

60.255-60.2663.19-63.2063.24-63.2563.28-63.3463.74-63.7664.56-64.5780.23-80.2480.65-80.6981.08-81.1181.53-81.5683.36-83.46

Volcaniclasticlayer

123456789

101112131415161718192021222324252626272829303132333334353637383940414243444546474849505152535455565758

Note: Data from Fryer, Pearce, Stokking, et al. (1990).

those in the Izu-Bonin forearc. Of these, tephras having high-K contentsmay have come from sources far to the west (e.g., from the Ryukyu Arc),carried by the prevailing westerly winds around Japan (Horn et al., 1969;Kennett, 1981). The volcanic history of the Ryukyu Arc is not wellknown, however, as its stratigraphy has been studied by only a fewworkers (Konishi, 1965; Furukawa and Isezaki, in press; Fujioka et al.,

3 4 5Grain size(0)

1 2 3 4Grain size($)

— α — White fine (poor sorting)

— Coarse White

*» Black white

°— Fine (well sorting)

Figure 10. Grain-size characteristics of four types of tephras. A. Cumulative

frequency grain-size curve for all the tephras at Site 792, Izu-Bonin forearc.

B. Frequency grain-size distribution curve for tephra layers at Site 792. The

scale of the grain size is represented by Φ (phi).

this volume). The Pliocene Shimajiri Group of the Ryukyu Arccontains much tephra material. Backarc rifting of the Ryukyu Troughduring the Quaternary, characterized by notably bimodal chemistry,has been documented by Letouzey and Kimura (1985).

CONCLUSIONS

In the Izu-Bonin forearc region, 393 white and black tephras weredeposited from the Oligocene to the present. The tephra frequencyincreased in the late Eocene to early Oligocene and the Pliocene-Pleis-tocene, with a small increase in the middle Miocene (about 10 Ma).Quaternary tephras, with variable ratios of basaltic and rhyoliticcompositions, increase in frequency at around 0.6 and 0.3 Ma.

68


A A) Sample 126-792A-1H-3,138-140 cm

C) Sample 126-792A-5H-1,37-39 cm

D) Sample 126-792A-8H-3,40-42 cm

F) Sample 126-792A-5H-2,141-143 cm

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.41

B) Sample 126-792A-3H-1,86-88 cm

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.66

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.69

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.66

E) Sample 126-792E-3R-1, G) Sample 126-792E-18R-1,4-6 cm 108-110 cm

100., . 100,.

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.65

Low K2O

B A> Sample 126-792A-1H-1,38-40 cm

Medium K2O

B) Sample 126-792A-4H-4,96-98 cm

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.69

High K2O

C) Sample 126-792A-5H-2,45-47 cm

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.68

Black

D) Sample 126-792A-3H-2,83-85 cm

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.94

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.90

2 2.5 3 3.5 4 4.5 5

MdΦ = 3.98

J

Low K2O

2 2.5 3 3.5 4 4.5 5

MdΦ - 3.86

Medium K2O

Figure 11. Cumulative curve for grain size of the tephras of the forearc sites of the Izu-Bonin Arc. A. Examples of cumulative curves showing nearly symmetrical

grain-size distribution. B. Examples of fine, skewed grain-size distribution.

The Eocene through Quaternary tephras have three principalcombinations of color and grain-size distribution: (1) white, coarse,well-sorted, and unskewed; (2) fine, white, well to poorly sorted,generally unskewed but rarely skewed toward the fine end; and (3)black, well-sorted, unskewed, and generally leptokurtic. The first andthird combinations may reflect proximal sources in the Izu-Bonin Arc,but some tephra layers of the second type may have been derived fromdistal sources far to the west (e.g., the Ryukyu Arc).

The chemistry of the tephras is notably bimodal (basaltic andrhyolitic) since the late Pliocene, which suggests that the predominantstress field over that period has been tensional, as in the Cascaderegion of western North America. Most of the tephras belong to thelow-alkali tholeiitic series of Kuno (1960,1966) and the low-K seriesof Gill (1981). High-K series tephras occurred from 3 Ma to theHolocene, some of which may have been derived from the RyukyuArc and other sources far to the west.

ACKNOWLEDGMENTS

During Leg 126 cruise, Captain Ribbon and the crew of JOIDESResolution were very helpful to us, sometimes far beyond theirofficial duties, and we are very thankful. We greatly appreciate themany valuable discussions and suggestions of B. Taylor, J. Gill, R.Hiscott, and K. Marsaglia. Thanks are also owed to Drs. F. McCoy,M. Yuasa, S. Nagaoka, N. Kotake, Y. Hirata, H. Matsuoka, and R. U.Solidum for their critical comments. Mrs. Kanehara, Miss K. Ikari,and Ms. Minnie Jones helped us prepare the manuscript.

REFERENCES

Cadet, J. P., and Fujioka, K., 1980. Neogene volcanic ashes and explosivevolcanism: Japan Trench transect, Leg 57, Deep Sea Drilling Project. Invon Heune, R., Nasu, N., et al., Initial Repts. DSDP, 56, 57, Pt. 2:Washington (U.S. Govt. Printing Office), 1027-1041.

69

K. FUJIOKA ET AL.

300

35050 55 60 65 70 75 0 0.5

SIO,10 11 12 13 14 0 2 4 6 8 10 12 0 1 2 3 4 5 6

AI2O3 (%) FeO (%) MgO (%)

0 2 4 6 8 10 1 2 3 4 1 2 3 4 5 2 3 4 5 6 7 8

CaO (%) Na2O (%) K2O (%) Total Alkali (%)

Figure 12. Oxide variation diagram (Harker diagram) of glass shards with time at Site 792 Izu-Bonin forearc. Open circles = black tephras, and open square withdot = white tephras. Horizontal scales are represented by weight percent of oxides.

Christiansen, R. L., and Lipman, P. W., 1972. Cenozoic volcanism and plate-tectonic evolution of the western United States. II. Late Cenozoic. Philos.Trans. R. Soc. London, Ser. A, 271:249-284.

Crawford, A. J., Beccaluva, L., and Serri, G., 1981. Tectono-magmatic evolu-tion of the West Philippine—Mariana region and the origin of boninites.Earth Planet. Sci. Lett., 54:346-356.

Dickinson, W. R., 1968. Circum-Pacific andesite types. J. Geophys. Res.,73:2261-2269.

Dickinson, W. R., and Hatherton, T., 1967. Andesitic volcanism and seismicityaround the Pacific. Science, 157:801-803.

Eaton, G. P., 1982. The Basin and Range Province: origin and tectonicsignificance. Annu. Rev. Earth Planet. Sci., 10:409^140.

, 1984. The Miocene Great Basin of western North America as anextending back-arc region. Tectonophysics, 102:275-295.

Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init. Repts.,125: College Station TX (Ocean Drilling Program).

Fujioka, K., 1983a. History of the explosive volcanism of the Tohoku Arc fromthe core sediment samples of the Japan Trench. /. Volcanol. Soc. Jpn.,28:41-58.

, 1983b. Where were the "Kuroko deposits" formed—lookingfor the present day analogy. Min. Geol. (Tokyo) Spec. Vol., 11:55-68.

-, 1988. Apossible nascent rift in northern Izu-Ogasawara arc. Tectonics

In Klein, G. déV., Kobayashi, K. et al., Init. Repts. DSDP, 58: Washington(U.S. Govt. Printing Office), 617-627.

1980b. Petrographic properties of tephras, Leg 56, Deep Sea

of pastern Asia and Western Pacific Continental Margin. 1988 DELP TokyoInt. Symp, and Sixth Japan-U.S.S.R. Geotectonic Symp., 25. (Abstract)

Fujioka, K., Furuta, T, and Arai, F., 1980. Petrography and geochemistry ofvolcanic glass: Leg 57, Deep Sea Drilling Project. In von Huene, R., Nasu,N., et al., Init. Repts. DSDP, 56,57, Pt. 2: Washington (U.S. Govt. PrintingOffice), 1049-1066.

Furukawa, M., and Isezaki, N., in press. Tectonics of the Okinawa Trough. J. Geol.Furuta, T., and Arai, F., 1980a. Petrographic and geochemical properties of

tephras in Deep Sea Drilling Project cores from the north Philippine Sea.

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Furuta, T, Fujioka, K., and Arai, E, 1986. Widespread submarine tephra aroundJapan, petrographic and chemical property. Mar. Geol., 72:125-146.

Gill, J. B., 1981. Orogenic Andesites and Plate Tectonics: New York (Sprin-ger-Verlag).

Hamuro, K., Aramaki, S., Fujioka, K., Ishii, T, Tanaka, T, and Uto, K., 1983.The Higashi-Izu-oki submarine volcanoes, Part 2, and the submarinevolcanoes near the Izu Shoto Islands. Tokyo Daigaku Jishin Kenkyusho Iho,58:527-557.

Hamuro, K., Aramaki, S., Kagami, H., and Fujioka, K., 1980. The Higashi-Izu-oki Submarine Volcanoes, Part 1. Tokyo Daigaku Jishin KenkyushoIho, 55:259-297.

Hochstaedter, A. G., and Gill, J. B., 1990. Petrology and geochemistry of lavasfrom the Sumisu Rift: an incipient back-arc basin. Earth Planet. Sci. Lett.,100:195-209.

Honza, E., and Tamaki, K., 1985. The Bonin Arc. In Nairn, A.E.M., Stehli, F.G., and Uyeda, S. (Eds.), The Ocean Basins and Margins (Vol. 7): ThePacific Ocean: New York (Plenum Press), 459-502.

Honza, E., Tamaki, K., Yuasa, M., Tanahashi, M., and Nishimura, A., 1982.Geological map of the Northern Ogasawara Arc. Geol. Surv. Jpn., Mar.Geol. Map Ser., 17 (Scale 1:1,000,000).

Horn, D. R., Delach, M. N., andHorn, B. N., 1969. Distribution of tephra layersand turbidities in the north Pacific. Geol. Soc. Am. Bull, 80:1715-1724.

Hussong, D. M., and Uyeda, S., 1982. Tectonic processes and the history ofthe Mariana Arc: a synthesis of the results of Deep Sea Drilling ProjectLeg 60. In Hussong, D. M., Uyeda, S., et al , Init. Repts. DSDP, 60:Washington (U.S. Govt. Printing Office), 909-929.

70


oü

>JO3

3O

20

15

10 -

Quaternary

!:

!

m

in

I50 55 60 65 70 75 80

SiO2 wt. %

20

i 15OOg> 10

JS

I 5ü

0

I

PlioceneI

π ggggggj

•

I

50 55 60 65 70 75 80SiO2 wt. %

20

c38

ive

*-»CO3

3ü

15

10

5

Miocenei i

MI i

_Bftff8888a

_

50 55 60 65 70 75 80

Siθ2wt. %

Figure 13. Frequency diagram of SiO2 contents of the tephras examined fromthe Quaternary, Pliocene, and Miocene.

Ikeda, Y., and Yuasa, M., 1989. Volcanism in nascent back-arc basins behindthe Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, northwestPacific: evidence for mixing between E-type MORB and island arc mag-mas at the initiation of back-arc rifting. Contrib. Mineral. Petrol,101:377-393.

Isshiki, N., 1959. Hachijo-jima. Geol. Surv. Jpn., Quadrangle. Ser. (Scale1:50,000).

, 1960. Miyake-jima. Geol. Surv. Jpn., Geol. Map Jpn. (Scale1:50,000).

-, 1978. Geology of the Toshima district. Geol. Surv. Jpn., Quadran-gle Ser. (Scale 1:50,000).

-, 1980. Geology of the Mikurajima, Inambajima and Zenisu district.Geol. Surv. Jpn., Quadrangle Ser. (Scale 1:50,000).

-, 1982. Geology of the Kozushima district. Geol. Surv. Jpn., Quad-rangle Ser. (Scale 1:50,000).

-, 1984. Geology of the O Shima district. Geol. Surv. Jpn, Quadran-gle Ser. (Scale 1:50,000).

-, 1987. Geology of the Nii Jima district. With geological sheet mapat 1:50,000. Geol. Surv. Jpn.

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o 792A&792B (-7X)+ 793A

792B(8X-)792E(-5R)

792E(6R-)

50 55 60 65 70 75 80

Pliocene10

^ 8tq , 6

o "O"""

)°o

o

o

50 55 60 65 70 75 80

10

^ 8%

6 4

Miocene

J ) Oi

o

5 ° O

o

SiO2 wt.

2

50 55 60 65 70 75 80SiO2 wt.%

Figure 14. Silica vs. total alkali diagram of the tephras examined at Sites 792 and 793 from the Quaternary, Pliocene, and Miocene. A. Plotof all the data. Open circles = Holes 792A and 792B, and plus signs = Hole 793A. B. Plot of averaged data for selected volcanic tephra layer.

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72


MO 50 60 70

SiO2 wt.

o 792A&792B (-7X)+ 793A

80

40 50 60 70

792B(8X-)792E(-5R)

792E(6R-)

B

Quaternary1.5

0.5 -

Oo

o Co

0°oa>0 o

°o 8

° %oc

o....,,.o3

50 55 60 65 70 75 80

Pliocene

1.5

o

1 -

0.5-

0

" • " o 1

°O

!j

oo

ö"

Q U

°OQ....öqoc

Q -

O

50 55 60 65 70 75 80

1.5

= 0.5

0

Miocene

j>oo

I.. .0

50 55 60 65 70 75 80SiO2 wt.%

Figure 15. SiO2 vs. TiO2 diagram for tephras of the forearc Sites 792 and 793, Izu-Bonin Arc, from the Quaternary, Pliocene, andMiocene. A. Plot of all the data. Open circles = Holes 792A and 792B, and plus signs = Hole 793A. B. Plot of averaged data forselected volcanic tephra layer.

Islands. Fifth Circum-Pacific Energy and Mineral Resources Conferenceand Exhibition, Honolulu, Hawaii. (Abstract)

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Date of initial receipt: 18 March 1991Date of acceptance: 2 August 1991Ms 126B-117

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K. FUJIOKA ET AL.

o

Quaternary

α o

°

×o j

00*o

40

o

u40

u40

50 60 70

Pliocene

50 60 70

792A&792B (-7X)+ 793A

80

792B(8X-)792E(-5R)

80

50 60 70

SiO9 wt.%80

B

Quaternary

-o βrf×

,

High-K

Mediui

Low-K

o

µ^"o

i-K,

c

sëocPo

>

I

50 55 60 65 70 75 80

60 65 70Siθ2wt.%

Figure 16. SiO2 vs. K2O diagram for tephras of the forearc Sites 792 and 793, Izu-Bonin Arc, from the Quaternary, Pliocene, andMiocene. Categories low-k to high-K are the same as in Gill (1981). A. Plot of all the data. Open circle = Holes 792A and 792B,and cross = Hole 793A. B. Plot of averaged data for selected volcanic tephra layer.

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3. Tephras of the Izu-Bonin Forearc (Sites 787, 792, and 793)

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