episodic exhumation of accretionary complexes: fission-track thermochronologic evidence from the...

The Island Arc (1995) 4, 209-230

Research Article Episodic exhumation of accretionary complexes: Fission-track

thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan

TAKAHIRO TAGAMI, NORIKO HASEBE AND CHICA SHIMADA Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606, Japan

Abstract Apatite and zircon fission-track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (Tr) generally indicate -10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C T, exhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion-burial-exhumation process. Zircon data suggest two cooling episodes: -70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and -15 Ma cooling in the central Kii Peninsula. The former is consistent with 40Ar/39Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano-plutonic complexes contemporaneously placed inland. Available geo- and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphism concepts.

Key words: cordilleran orogeny, episodic exhumation, subduction zone, thermal history,

INTRODUCTION

Accretionary complexes are remarkable geological features formed a t a forearc margin of overriding lithosphere in subduction zones. Study of the evolution of the complexes would give a clue to the geologic processes that control the growth of con- tinent and orogeny. The structure and development of accretionary complexes were better understood in the 1980s due to extensive seismological and drilling investigations of modern subduction zones (Moore & Silver 1987). The framework of material

Accepted 17 July 1995

migration was first established a t the toe of modern accretionary prisms [e.g. Barbados (Moore et al. 1982, 1988) and Nankai (Kagami 1986; Ashi & Taira 1992)l . In addition, mechanical characteristics of accretionary complexes were better constrained by wedge models, in which various physical analogies, such a s Coulomb failure, were applied to describe behaviors of massive sediments accreted during plate subduction (Davis et al. 1983; Dahlen & Barr 1989). Despite these progresses, relatively little is known of accretionary processes and material migration at deeper

210 T. Tayarrii et al.

interiors of accretionary complexes (Isozaki e t al. 1990). Multidisciplinary studies on the ancient accretionary complexes currently outcropped on land could shed further light on the evolution of the entire accretionary history.

The Shimanto Belt, which lies along the Pacific coastal range of southwest Japan subparallel to the modern Nankai Trough (Fig. l ) , is a well studied onland accretionary complex and preserves an accretionary history from Cretaceous to Miocene time (Taira et al. 1988; Underwood e t al. 1993). The area consists of unmetamorphosed to low-grade metamorphosed coherent turbidites and melanges, with estimated maximum burial depths of about 10 to 15 km and maximum temperatures of about 200 to 300°C (Toriumi & Teruya 1988). England and Thompson (1984), among others, suggested that analysis of pressure-temperature-time paths pro- vides critical information on material migration and tectonics of geological bodies in the lithosphere. In the case of low-grade metamorphosed terrains, such as the Shimanto Belt, thermal his-

tory (i.e. temperature-time path) analysis would be more informative for reconstructing their histories. Fission-track (FT) thermochronology is particularly effective for tackling the problem due to greater sensitivities to elevated temperatures a t about 100 to 300°C (Table 1). We compile a series of apatite and zircon FT data from the Shimanto Belt and its vicinities (Tagami et al. 1988; Hasebe e t al. 1993a, b; Shinjoe & Tagami 1994; Tagami & Shibata 1994, and unpubl. data, 1995). Material migration and exhumation a t the deep interiors of accretionary complexes will be deduced from reconstructed thermal histories of rocks, and their implications will be discussed in the context of accretionary complex evolution.

GEOLOGIC OUTLINE

Southwest Japan is divided by the Median Tectonic Line (MTL) into the Inner Zone to the north and the Outer Zone to the south (Fig. 1). The Outer

Fig. 1 Geological map of studied areas. 1, Kyushu region; 2, Ashizuri region; 3, Muroto region, including areas around Tosa Bay; 4, central to western Kii region. 5, Kumano Acidic Rocks; 6, Oboke unit, 7, eastern Ki i region; 8, Cyubu region; 9, Nagano region. MTL , Median Tectonic Line; BTL, Butsuzo Tectonic Line; ATL, Aki Tectonic Line; ISTL, ltoigawa-Shizuoka Tectonic Line; PP, Pacific Plate; PSP, Philippine Sea Plate; EP, Eurasia Plate; SB, Shikoku Basin; JS, Sea of Japan; NT, Nankai Trough; RT, Ryukyu Trench; IBT, Izu-Bonin Trench; JT. Japan Trench

Episodic exhumat ion of accretionary complexes 21 1

Oligocene to Early Miocene age, separated by the Shiina-Narashi fault. (The subdivision is generally applicable to other regions. Those three units will be hereafter referred to as Cretaceous, Eocene and Miocene Shimanto Belts, respectively.) Toriumi and Teruya (1 988) studied petrologically the low-grade metamorphism of the Cretaceous Shimanto Belt, and suggested the maximum temperature-pressure condition of about 200 to 300°C and 3 to 5 kb. Detailed vitrinite reflectance studies on the Muroto Peninsula (DiTullio e t al. 1993; Hibbard et al. 1993; Laughland & Underwood 1993; Mori & Taguchi 1988) revealed the maximum paleotemperatures of 140-320°C according to the correlation of Barker (1988).

The Shimanto Belt was intruded by the great mass of the Miocene granitic rocks (Shibata 1978; Fig. 1). This intrusion has been attributed to the rapid subduction of the newly formed hot Shikoku Basin (Takahashi 1986). On the Muroto Peninsula, however, only gabbroic intrusion having a Rb-Sr whole-rock biotite isochron age of 14.4 k 0.4 Ma (Hamamoto & Sakai 1987) has been found a t the toe, producing hornfels aureole of approximately 40 m in width (Yoshizawa 1953). Vitrinite reflectance data from the Eocene Shimanto Belt are consistent with the structural features related to accretionary processes (DiTullio e t al. 1993; Laughland & Underwood 1993). Hence, the Mio- cene thermal event would not have substantially affected the pre-Miocene thermal structure of the Cretaceous-Eocene Shimanto Belt currently outcropped in the Muroto region, and was localized to the Miocene Shimanto Belt.

Samples used for fission-track analysis were collected from sandstones outcropped on land, either as sandstone beds within coherent turbiditic units or as sandstone blocks surrounded by shaly matrix within melange units. In both cases, the depositional age of each unit was estimated from fossils, mainly radiolaria, included in adjacent shale beds (or shaly matrix).

Table 1 Closure temperatures of radiometric dating mcthods at geologic timescale (e.g. Harrison et al. 1980; Hurford 1986).

Method Mineral Closure Cooling rate temperature ("C/Ma)

("C) FT K-Ar K-Ar FT FT K-Ar Rb-Sr K-Ar Rb-Sr K-Ar U-Pb U-Pb Rb-Sr

Apatite Microcline Plagioclase Zircon Sphene Biotite Biotite Muscovite Muscovite Hornblende Monazite Zircon Whole-rock

100 i 10 150 i 30 200-250 260 k 50 290 i 40

320 i 40 -350 500 f 50

-530

Crystallization

280-345

480-580

-650-750

1-10

-10 -10 -10

1-100 -10

-

- 5-1000 -

-

Zone consists primarily of the Sambagawa Meta- morphic Belt, the Chichibu Belt, and the Shimanto Belt from north to south. The Sambagawa Belt is a high-pressure regional metamorphic belt and is formed within a Mesozoic accretionary complex a t depth (Takasu et al. 1994). The Chichibu Belt is a Jurassic accretionary complex (Isozaki e t al. 1990). The Shimanto Belt is a Cretaceous to Mio- cene accretionary complex (Taira et al. 1988) and has tectonic contact with the Chichibu Belt along the Butsuzo Tectonic Line (BTL). The Miocene accretionary complex continues southwards to the modern, submarine Nankai accretionary prism, which is being formed a t Nankai Trough as a consequence of subduction of the Philippine Sea plate beneath Eurasia (Ashi & Taira 1992). The Ryoke Belt is a Cretaceous low-pressure regional metamorphic belt that has tectonic contact with the Sambagawa Belt at the MTL, showing a pair of contrasting metamorphic conditions (Miyashiro 1961). The Belt is characterized by a large number of associated granitic rocks, as part of Cretaceous granitic province that probably extends further southwest and northeast along the Eurasian continental margin (Nakajima 1994).

The Shimanto Belt is composed mainly of coherent turbidites, tectonic melanges and slope basin deposits (Taira et al. 1988). In the Muroto region, it is divided by the Aki Tectonic Line (ATL; the Nobeoka thrust is its equivalent in eastern Kyushu) into the Northern Shimanto Belt of Cretaceous age and the Southern Shimanto Belt of Tertiary age (Fig. 1). The latter is further subdivided into the Murotohanto sub-belt of Eocene to Early Oligocene age and the Nabae sub-belt of predominantly Late

THERMOCHRONOLOGY OF ACCRETED SEDIMENTS

Thermal history analysis using radiometric dating methods, called thermochronology, showed rapid growth in 1980s and have been applied to various geological problems in which either temperature elevation or cooling are critical constraints (MeDougall & Harrison 1988; Wagner & Van den haute 1992). The concept was successful in particular with FT and 40Ar/39Ar methods, exhibiting a

212 T. Taganzi et al.

wide range of applications to orogenic belts, lifted continental margins, sedimentary basins and geothermal areas. The principle of thermochronology is that daughter elements (or fission tracks) formed by radiometric decay processes show significant variations in effective thermal diffusion (or annealing) between minerals and techniques. Hence, individual ages from a rock are supposed to record individual periods of time that have elapsed since the rock’s paleotemperature decreased through closure (or blocking) temperatures, below which chro- nometers begin to accumulate daughter elements effectively (Dodson 1973). Hence temperature- time paths of rocks can be reconstructed by plot- ting their ages against closure temperatures (Wag- ner e t al. 1977). Estimated closure temperatures vary from approximately that of crystallization temperature (whole rock Rb-Sr system) to around 100°C (apatite FT system; Table I), covering various geologic processes. The FT method is particularly useful for studying accretionary complexes because their estimated paleotemperatures coincide approximately with FT closure temperatures of -100 to 300°C.

In order to reconstruct thermal histories with good confidence, the closure temperature and partial annealing zone need to be known for geologic periods of time. The basic approach to quantify the thermal stability of fission tracks in a mineral is to observe their behavior during laboratory heating experiments. The confined track length measurement has played a major role in quantifying pre- cisely the degree of track annealing due to its advantage in greater resolution over the track density measurement (Gleadow et al. 1986). By heating samples a t various temperature and time conditions, kinetic parameters to describe track annealing were determined for apatite (Laslett et al. 1987) and zircon (Yamada et al. 1995a; Fig. 2) . The apatite annealing models were then subjected to geological tests using oil exploration boreholes in sedimentary basins (Green et al. 1989a,b; Fig. 3), which led to the quantitative thermal history reconstruction using modeling techniques (Green et al. 198913). In contrast, the natural long-term annealing characteristics of zircon FT system have been ambiguous because of the diffi- culty to observe its annealing a t higher temperatures of -200-300°C (Table 1) under well- controlled geothermal regimes. The first systematic characterization has been achieved recently a t a contact thermal aureole around the Takatsukiyama granite on the Hata Peninsula, Shikoku Island, which intruded the Cretaceous Shimanto Belt a t

Temperature 800600 400 300 200 (“C)

Annealing Zone for Geologic Timescale

h

0 Q, v) W

28

24

-

-

h 20

- 16

12

- + W

c -

-

8 -

E i=

1@h 1@h 10h l h 101 h

4 ‘ J J ’ J / h I I I I I 0.8 1.2 1.6 2.0 2.4

1 OOO/T( K-1) Fig. 2 Arrhenius Plot for thermal annealing of fission tracks in Lircon (after Yamada e t a / 1995a) Degree of track annealing is measured by reduction in confined track lengths, as represented by various symbols and corresponding six contours fitted by five parameter linear fan-shape model Also shown is a plausible partial annealing zone for geologic timescale estimated by extrapolating short-term laboratory heating data followed by various geologic con trols

-15 Ma (Tagami & Shimada, in press; Fig. 1). The obtained age and length data set will answer how to extract thermal history information from zircon FT data of sediments that were heated a t depth after accretion.

Figure 4 illustrates a mean zircon FT age profile of Cretaceous coherent units along an eastward traverse from the intrusion contact. Approaching the contact, original ages of around 100 Ma show a continuous reduction to approximately 15 Ma at -3 km distance. This demonstrates clearly the existence of fossil partial annealing zone (FPAZ) under geological heating. Mean track lengths also show a characteristic variation against ages within the FPAZ (Fig. 5), which correlates with the case of apatite (Green 1986). In addition, distributions of both single-grain ages and track lengths show a remarkable variation. The distribution of single- grain ages changes from a multi-modal, broad pattern reflecting provenance ages to a unimodal pattern characterizing the secondary 15 Ma heating by granite intrusion (Figs 5, 6). The P(xz) parameter to test statistically the concordance of

Episodic exhumation of accretionary complexes 213

Fig. 3 Long-term natural annealing of fis- - sion tracks in apatite observed in borehole :loo samples from Otway Basin Australia (after y

Green eta / 1989a) Because the evolution of 2 80 this basin is well controlled geologically the ; data set place valuable constraints on the 60 -

partial annealing zone of apatite fission-track ; system for geologlc timescale Both fission- a, 40 -

track age (left) and length (r ight) are reduced 2 progressively down to zero in the tempera- 9 20 - ture range of about 60-120°C due to geothermal temperatures increasing with the

at 2 0 Temperature ("C)

I

- 0 0 20 40 60 80 100 120 140

Temperature ( "C) depth of the boreholes Error bars are shown 0 20 40 60 80 100 120 140

single-grain ages (Green 1981) also shows a systematic variation in the FPAZ from < 0.1 (e.g. SMTZ54 in Fig. 5) to > 5 (SMTZ03), which indicates a change from multiple populations to a single population. However, distributions of lengths can change from the initial unimodal pattern of provenance (SMTZ54) to a bimodal pattern characterizing mixed ages (SMTZ23), and finally reverting to a unimodal pattern in a totally reset sample (SMTZ03; Fig. 6). These are the result of mixing both pre-existent tracks shortened to various degrees by the Miocene heating and unannealed tracks with original, long lengths formed and accumulated thereafter (such a mixed track length pattern typically is observed for SMTZ23 in Fig. 5; see also SMTZ19 and 07 in Fig. 6).

The kinetic parameters predict the partial annealing zone (PAZ) of zircon as 210 to 320°C ( k 6 O " C a t k20), 190 to 350°C ( k5OoC), or 170 to 390°C (k 50°C) for a heating duration of 10 Ma, depending on the model fit used (Yamada et al. 1995a). The authors defined the lower temperature limit as being that which caused 5% age reduction, whereas the upper limit is essentially total resetting to zero age. Recent age and length data from the Vienna Basin ultradeep borehole (Tagami et al. unpubl. data, 1995) show no significant annealing a t -200°C subnormal geotherm, which has probably been kept for -5 Ma according to the basin formation history since Miocene reconstructed by numerous borehole data (Wessely 1987). In addition, thermal modeling using the data in Fig. 4 suggests a relatively narrow temperature interval of about 100°C for the PAZ of zircon (Tagami & Shimada, in press). We accordingly

*0° 1 t

n 2 150 1 W

t 100

i Distance from contact (m) i I I I I

- -Accretionary - -Prism (97-73Ma)- - - - - - - - - - - + + + +

Fig. 4 Zircon fission-track age profile of Cretaceous Shimanto Belt along an eastward traverse from the intrusion contact of -15 M a Takatsukiyama Gran- ite in the Ashizuri region (Fig 1) Approaching the contact mean fission-track ages show a systematic reduction at -3 km distance from -100 M a detrital ages predominantly older than deposition to -15 M a total reset ages concordant with the emplacement age of granite The age profile is reasonably explained by long-term heating of Shimanto rocks due to contact metamorphism of granite intrusion These results first demonstrate a transient zone of track retention (I e the fossil partial annealing zone), of zircon fission-track systems for geologic periods of t ime Error bars are shown at 20 See Figure 5 for detailed analysis

214 T. Tugami et al.

SMTZ03 SMTZ23 SMTZ54

(Ma)

Deposition

+?. 0

- 2 - 40

Heating 15

..* * \

15

I5Ma heating Mean Zircon FT Age (Ma)

Fig. 5 Mean fission-track age vs length relationship lor Cretaceous Shimanto zircons from the fossil partial annealing zone lound in the contact thermal aureole of -15 M a Takatsukiyama Granite in the Ashizuri region Data points tend to fall in a trend convex downward called the boomerang shape with a characteristic change in distributions of single-grain ages [using the radial plot (Galbraith 1990) see Fig 8 for detailed explanation of the plot] and confined track lengths Unannealed zircon samples first predate deposition with long mean lengths of -10 p m and are characterized by single-grain ages consistently coeval or older than deposition and unimodal length distributions (e g SMTZ54) As annealing proceeded due to paleotemperature increase samples tend to postdate deposition and show mixed ages between original detrital ages ( I e -150 to 100 Ma) and secondary heating age (-15 Ma) Track lengths of those samples show a progressive decline from -10 to -7 p m substantially shorter than unannealed ones Single grain ages show a great scatter between the original and heating ages with bimodal (or mult imodal) broader distributions of track lengths (e g SMTZ23) Eventually mean ages of samples became concordant with the -15 M a secondary heating age and mean track lengths increased to -1 1 p m (e g SMTZO3) Here single-grain ages show a good agreement with each other at -15 M a with passing the f ( x 2 ) statistical test The track length distribution also shows a unimodal tight distribution 0 1 0 1 member of Oguwa Formation F2 F2 member of Furushiroyama Formation (Teraoka e t a / 1986) Error bars are shown at 2 0 See Figure 4 for comparison

adopted the PAZ of -230 to 310°C for zircon FT system for a heating duration of 10 Ma. (Note that the lower temperature limit here represents 10% age reduction.) These recent results settled the controversy on the zircon closure temperature (Tr); 170°C (Harrison et al. 1979) or 240°C (Hurford 1986), with support for the latter.

Therefore, plausible estimates on the PAZ are presented for apatite and zircon FT systems

(Fig. 6). I t is noteworthy that the depositional age estimated from fossils also places important constraints on the partial resetting of thermochronologic system. As shown in Figures 5 and 6, single- grain ages are consistently older than the depositional age if the maximum paleotemperature was lower than the PAZ (e.g. < 230°C for zircon FT system). In contrast, a significant number of grains are younger than the depositional age if samples

Episodic exlzzcmatio?c of accretionary complexes 2 15

.i ____,_____ u;i / Sample Fig. 6 Summary of thermal stability of fis-

sion tracks in apatite and zircon TSZ total stability zone PAZ partial annealing zone TAZ total annealing zone Also shown are variations in distributions of single-grain ages using conventional age spectra (Hur- ford et a/ 1984) and confined track lengths which allow the maximum paleotemperature of an unknown samde to be examined

Temp.( "C)

un 0 100 200 300 0 5 10 15

C

Track length (prn)

are heated up to the PAZ (230-310°C). For samples heated above the PAZ (> 31OoC), all grain ages are consistently younger than the depositional age. Therefore, the following information, if available, should be taken into account to interpret appropriately FT thermochronologic data:

(i) the distribution of single-grain ages in each sample [e.g. radial plot (Galbraith 1990) and/or age spectra (Hurford e t al. 1984); the former is more informative and preferred along with a statistical test (e.g. P(x2) (Green 1981) and/or age dispersion (Galbraith & Laslett 1994)];

(ii) the distribution of confined track lengths in each sample; and

(iii) a comparison of single-grain ages with the depositional age for each sample;

(iv) the correlation between mean age and length for a series of samples.

THERMAL HISTORIES OF THE SHIMANTO BELT AND ITS VICINITIES

Figure 1 compiles the localities of FT analyses in and around the Shimanto Belt. Prior to regional comparisons, we need to understand how fundamental accretionary processes are reflected in FT

data. Hence the Muroto region will be examined first. The results will then be compared and contrasted with other regions in order to unravel the along-arc as well as across-arc variation.

MUROTO REGION

Apatite FT data are concordant at - 10 Ma regardless of the depositional ages that range from -200 to -20 Ma (Fig. 7), and thus reflect post-depositional thermal histories. The P(xz) test indicates that single-grain ages are concordant for each sample. I t suggests that the samples were heated, after deposition, to the higher temperature limit of apatite partial annealing zone (APAZ), -120 C, and then cooled through APAZ a t -10 Ma. It is stressed that the ages are concordant regardless of structural facies of rocks (e.g. coherent vs melange units), as well as the timing of deposition and distance from the ancient trench. Therefore, the 10 Ma cooling should have been regional and should cover the entire Muroto region.

In contrast, zircon FT data are significantly different between samples and reflect various maximum paleotemperatures (TmaX) a t around the zircon partial annealing zone (ZPAZ; Fig. 8a-c). The

216 T. Tagami et al.

L v Emplacement 0) & cooling ages

Y 80 0

Chichibu Shinanto coherent

a

Shimanto melange

u- Shinanto Ryoke granite

Muroto Kyushu Ashizuri

obtained P(x2) values are consistently less than 5, and this suggests that grains in each zircon sample are not homogeneous in age but consist of multiple age populations. Hence the samples were probably not heated to the higher temperature limit of ZPAZ (i.e., -310°C). In such a case, the sample's mean age does not necessarily represent any specific thermal event and thus the distribution of single- grain ages need to be examined.

I t is generally observed in the Cretaceous Shi- manto Belt (CSB) that single-grain ages tend to distribute around the depositional age, with some grains being younger than it (Fig. 8a). Track length distributions show that a significant number of tracks are shorter than 9 pm in most samples. In rapidly cooled volcanics, the fraction of such tracks are -2% on the average (Hasebe et al. 1994). These lines of evidence suggest that zircon samples were more or less thermally affected after their deposition. The existence of secondary heating is also supported by the mean age vs mean length plot (Fig. 9). The six samples plotted in the diagram were collected from Zone 2 melange around the Tosa Bay (Taira et al. 1988), from which the Campanian deposition of both argilla- ceous melange matrix and related sandy flysch was well established by the microfossil dating. Data points tend to fall in a trend convex downward as seen in Fig. 5, with three characteristic age-length groups. The group A tends to predate deposition with mean lengths of -9.5 pm. The group B is characterized by mean lengths of -7-9 pm, which is substantially shorter than the other two and tends to slightly predate deposition. The group C postdates deposition with a mean length of

Kii

Fig. 7 Mean apatite fission-track ages from four regions of the Shimanto Belt and its vicinities (Fig 1) Also shown are depositional ages of individual samples (Taira ei a/ 1980) Emplace- ment and cooling ages are shown for samples from Ryoke granitic rocks All apatite ages from the Shimanto Belt postdate deposition and generally pass the P ( x 2 ) test suggesting -15- 10 M a regional cooling Error bars are shown at 2 0

-9.5 pm. This trend is most obviously observed for three data from the Tei melange. In addition, the groups also have characteristic track-length distributions: unimodal with tails of short tracks (group A and C; SHMICOG and SH32, respectively) and bimodal (or multimodal) with lengths ranging from -12 pm down to 2-3 pm (group B; SHMIC08). The timing of heating is estimated as 70-60 Ma by extrapolating the left (younger) wing of the trend to the 10.5-11 pm [i.e., mean track lengths in unannealed samples (Hasebe et a/. 1994; see Fig. 5)]. Quantitative analysis of these data is undertaken currently using the thermal modeling technique to estimate time of heating more objec- tively. In contrast with the Zone 2 melange, samples from the Zones 3a and 3b melanges to the south (e.g. SHKGll in Fig. 8a) do not show any evidence of reheating to the ZPAZ, as seen clearly by the unimodal length distribution with a mean of 10.1 pm as well as by grain ages consistently older than deposition. This suggests a significant local variation in paleotemperature within the CSB and may reflect a general paleotemperature increase towards the north.

In the Eocene Shimanto Belt (ESB), distributions of single-grain ages are consistently multimodal as suggested by extremely low P(x') values (Fig. 8b). Hence, the T,,, should have been lower than the top of the ZPAZ (-310°C). In addition, no samples yielded grain ages younger than the depositional age, with no signs of significant reheating to ZPAZ. Distributions of track lengths are consistently unimodal and tight with a few tracks shorter than 9 pm. Therefore, the secondary heating, if it existed, was probably not effective to anneal tracks

LIZ

0

10

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SHMICI 3 20

15

10

5

' 0 50 100 150 200

105 f 10 Ma y 2 .

20

15

10

- SHMIC17

- P(x2): 0.0% 75+7Ma

AD: 29% - 8 O L "& , -2L

' 0 50 100 150 200

40

70

40

SH18 Depositional 101 +9 Ma age

ln 15 P(x2): 0.0% \ e

70 E m & 10

z 5 4a 0

25 ' 0 50 100 150 200 Relative standard error

1.9 Frn (34) >60": 10.1 + 0.3 urn.

I

1.5 Frn (22) I

n 0 5 10 15

- m 5 a w m Y 0 ? c c 0 In In

LL

.-

.- 0 5 10 15

Fission track age (Ma) 5p" 2p" ITA [ 15 Track length (pm)

0 4 8 12 16 Precision (reciprocal relative standard error)

Fig. 8b Distributions of single-grain ages and confined track lengths for representative zircon samples from the Eocene Shimanto Belt in the Muroto region

to a great degree but limited to around the lower temperature limit of the ZPAZ (-230°C). Overall, therefore, these data suggest that the T,,, of ESB were around or below -230°C.

Distributions of single-grain ages and track lengths do not exhibit signs of effective secondary heating for zircons from the Miocene Shimanto Belt (MSB, Fig. 8c). Hence the T,,, was around or below -230°C. However, a sandstone sample -30 m from the contact with the Gabbroic intrusion a t the toe of Cape Muroto (SH23 in Fig. 10a) yielded a partial resetting pattern of zircon FT system, suggesting that the thermal effect was localized to the tip of the Muroto Peninsula (see Hasebe et al. 1993b for more detailed discussion).

KYUSHU REGION

Apatite FT ages are concordant at - 10 Ma regardless of depositional ages with high P(xz) values. Hence they are interpreted to represent cooling through APAZ (-120°C) a t -10 Ma covering both CSB and ESB, similar to data in the Muroto region. Zircon ages are also similar to those in the Muroto region. The P(x2) values are consistently less than 5, suggesting that samples were not heated to the higher temperature limit of ZPAZ (-310°C). The distribution of single-grain ages show contrasting patterns between units (Fig. 10a). In the southern area of the CSB (e.g. SHQOS), grain ages are predominantly younger

Episodic exhumation of acc?>etionary complexes 219

SHMIC05 101 f 13 Ma

20[ 15 P(y2): 0.0% 70

40

25 0 5 10 15

< 15

Fission track age (Ma) Track length (pm) Relative standard error

50% 20% 10% I I I ~ I ~ I ~ I

0 4 8 12 16 Precision (reciprocal relative standard error)

Fig. 8c Distributions of single-grain ages and confined track lengths for representative zircon samples from the Miocene Shimanto Belt in the Muroto region

than its deposition, and this suggests reheating well into the ZPAZ (230-310°C). In the northern area of CSB, however, grain ages are mostly older than deposition with a few grains younger [-go- 70 Ma (SHQ07)], showing smaller magnitudes of reheating to the ZPAZ. In the ESB, grain ages are consistently older than deposition with no signs of reheating to ZPAZ (SHQ22). Samples from southern area of CSB are characterized by concentric patterns of grain age distributions a t -70-60 Ma, with rather high P(xz) values (e.g. SHQO2) or with a few outliers of grain ages. They are similar to the pattern of the sample SH32 (Figs 8a, 9) and thus best interpreted by nearly complete resetting of detrital ages. The peak ages in age spectra (74-

59 Ma) should represent approximately the onset of cooling from ZPAZ.

In the studied region, the Chichibu Belt, CSB and ESB from northwest to southeast show tectonic contacts along north-dipping thrusts (Imai et al. 1971; Sakai & Kanmera 1981; Toriumi & Teruya 1988; Fig. 1) . The CSB is subdivided into two units: the northern Hinokage unit mainly composed of sandy flysch, and the southern Makimine unit which contains greenstone and chert in phyl- lite, showing melange structures. The Makimine unit shows intense deformation and metamorphosed to greenschist facies, whereas the Hino- kage unit is less deformed with a lower grade of metamorphism to prehnite-pumpellyite facies (Imai

220 T. Tugami et al.

SH32 SHMICOS SHMIC06

12 h

E E v

60 80 100 120

Mean Zircon FT Age (Ma) Fig. 9 Mean fission track age vs length relationship for zircon samples from Cretaceous Zone 2 melange around Tosa Bay, Shikoku Island Data points tend to fall in a trend convex downward and can be divided into three groups (a b and c in diagram), each of which shows characteristic distributions of both single-grain ages and confined track lengths Error bars are shown at 20

et al. 1971; Sakai & Kanmera 1981; Toriumi & Teruya 1988). Hence the T,,, variation discussed above is reasonably concordant with the metamorphic zonation and thus reflects the thermal histories a t deeper interiors of the Shimanto accretionary complex.

KII REGION

Apatite FT ages represent cooling through APAZ (-120°C) a t 35-15 Ma. The older 35 Ma apatite age from the western part of CSB near the Sam- bagawa Belt (Fig. l), allows two alternative inter-


(a) ( b )

20 SHNWOG 20

15 15 74+9Ma

5

0

10 10

5

0 0 50 100 150 200 0 50 100 150 200

20 SHQ07 117 f12Ma P(x'): 0.0% AD: 30%

lo[ 5 ,&:F 0 0 50 100 150 200

.. 3. :* \ 7 0

i- 40

/- 25

20 SHNW21 77+9Ma P(x2): 7% AD: 7%

10

5

n "0 50 100 150 200

f! 15 1 1 5

20 SHNW25

5

10

0

145 f 14 Ma

AD 26% 15 . .* . . ..

0 50 100 150 200

Lo c 70

0 z 0

rn

20 SHNW32 Depositional

20

15 15 E m 10

2 5

m cn 10

0 ' 0 50 100 150 200 25 ; Fission track age (Ma) Fission track age (Ma)

Relative standard error Relative standard error 50% 20% 10% 50% 20% 10%

0 4 8 12 16 0 4 8 12 16 Precison (reciprocal relative standard error) Precision (reciprocal relative standard error)

Fig. 10 (a) Distributions of single grain ages for representative zircon samples from the Kyushu region, with one from the toe of Muroto Peninsula (SH23) (b) Distributions of single-grain ages for representative zircon samples from the Kii region

pretations: 35 Ma cooling through APAZ, or 15 Ma cooling from within APAZ resulting in a mixed age. Track-length analysis with quantitative modeling is currently undertaken and will decide the interpretation. It is remarkable that some areas of the Shimanto Belt in the Kii region were cooler than the higher temperature limit of APAZ (-120°C) at 15 Ma, as also implied by the thermal history of Kumano Acidic Rocks occurring in the southeastern Kii area (Hasebe et aL. 1993a). The apatites -15 Ma are from the northernmost area of the CSB, -2 km from the Ryoke Belt across the

Median Tectonic Line (MTL; Fig. 1). A similar range of apatite cooling ages were found on granitic rocks in the Ryoke Belt (Fig. 7): -15 Ma in the central Kii area and -30 Ma to the east (Tagami et al. 1988; Tagami & Shibata 1994). The 30 Ma apatite samples have unimodal track-length distribution characterizing simple cooling (Tagami et al. 1988), and may therefore represent a regional cooling episode as well as the 15 Ma cooling in the central Kii area.

Zircon ages from CSB yielded a wide range of mean ages from -180 to -20 Ma. By compar-

222 T. Tcrgawi et al.

ing single-grain data with the depositional age, the following three groups were recognized (Fig. lob):

(i) late Cretaceous cooling ages, characterized by clustered grain ages younger than deposition (e.g. SHNWO6);

(ii) unreset or slightly reset ages, with single- grain ages predominantly older than deposition (SHNW25) and occasionally a few grains younger than the deposition age; and

(iii) Miocene cooling ages with grain ages clus- tering at -15 Ma (SHNW32).

The groups (i) and (ii) may be transitional, as seen i n the case of Zone 2 melange in the Muroto region (Fig. 9). In those samples, youngest grain ages.roughly agree at -70 Ma, concordant with a 74 Ma total reset age of sample SHNWO6 (Fig. lob). Therefore these data suggest regional cooling from various parts of ZPAZ a t -70 Ma. Three samples belonging to grqup (iii) appear to retain no detrital age components as even their oldest grains are much younger than the depositional age (Fig. lob). Hence they indicate the onset of cooling from ZPAZ a t -15 Ma, after heating above the higher temperature limit of ZPAZ (-310°C). The three samples are from the northernmost area of the CSB, about 5 km away from the MTL, where the metamorphic grade belongs to greenschist facies (Seki et al. 1971; Toriumi & Teruya 1988). In particular, the samples were collected from outcrops that exhibit block-in- matrix structures with characteristic clear scaly cleavages. Because -15 Ma apatite ages were obtained from localities nearby, the 15 Ma rapid cooling from -310°C to below 60°C was likely localized around the area and related to the exhumation of the highly deformed metamorphic rocks.

Zircon samples from granitic rocks in the Ryoke Belt yielded -68--55 Ma cooling ages in the central Kii region, which is in accordance with previous results from the eastern Kii area (Tagami e t nl. 1988; Fig. 1). The ages are interpreted by post-emplacement cooling of granitic rocks (Tag- ami ef al. 1988; Fig. 7). The Sambagawa Belt in the western Kii region has resulted in zircon ages of 94-76 Ma, with single-grain ages ranging from 100 to 70 Ma. In particular, the SHNW21 sample with a relatively high P(x') value shows grain ages clustered around 75-70 Ma, with two grains slightly older (Fig. lob). This is similar to the pattern of the sample SH32 (Figs 8a, 9) and best interpreted by the nearly complete resetting of detrital ages due to secondary heating to the ZPAZ. Youngest grain-age populations in the sam-

ples roughly agree at -70 Ma and are concordant with a 72 Ma peak age of the sample SHNW21. Therefore, the Sambagawa Belt in this region should have cooled from ZPAZ a t -75-70 Ma.

ASHlZURl REGION

Apatite FT data represent cooling of the CSB through APAZ (-120°C) at -10 Ma. Note that the six data are from the traverse described in Fig. 4 and thus possibly suffered the 15 Ma heating from the granite intrusion. The right two age lengths in Fig. 7, however, are > 10 km from the contact. The zircon age reduction a t -3 km (Fig. 4) and the paleotemperature profile estimated by thermal modeling suggest that the two samples were not substantially reheated by granite but reflect a regional paleogeothermal profile. Zircon ages > 5 km distant from the contact also reflect thermal histories of the CSB associated with the accretionary process. Mean ages tend to be older than deposition with occasionally younger grains, similar to the CSB in the Muroto region.

SUMMARY OF RECONSTRUCTED THERMAL HISTORIES

Figure 11 illustrates the thermal histories of representative localities. Three notable cooling ages are found therein. First, -70 Ma regional cooling in the CSB and the Sambagawa Belt (the western Kii area discussed above and the Oboke unit in central Shikoku (Shinjoe & Tagami 1994; see Fig. 1). Note that the former is probably correlated to the Besshi unit according to the similarities in its protolith, with zircon FT cooling ages of 75-70 Ma also being consistent with slightly older muscovite 40Ar/ ,3!3Ar cooling ages of -89-76 Ma having a higher closure temperature (Table 1) from the Besshi unit (Takasu & Dallmeyer 1990). Also note that the timing of cessation of metamorphism and subsequent onset of cooling is constrained by their whole-rock 10Ar/3gAr ages as -70 and -85 Ma for Oboke and Besshi units, respectively. Second, -10 Ma regional cooling that covers the entire Shimanto Belt and most likely its vicinities may not cover the Kii region. Third, -15 Ma rapid cooling localized in the central Kii area. In addition, there may be -35 Ma cooling in the Kii region although it is open for further studies. I t is also remarked here that systematic differences in T,,,,, are presented in the diagram, with -300°C for CSB (e.g. SH32 and SHQO2) and -150°C for both ESB (SH28 and SHQ22) and MSB (SHMIC05).

Episod ic e x h u m a t i o n of accretionary complexes 223

Time (Ma) 120 100 80 60 40 20 0

L MUROTO

SH32 SH28 SHMIC05

KYUSHU 0 SHQ02 0 SHQ22

Kll SHNWOG SHNW32

SAMBAGA WA +6- 0843 + SHNW21

IMPLICATIONS FOR ACCRETIONARY COMPLEX EVOLUTION

MAXIMUM PALEOTEMPERATURES AND MATERIAL MIGRATION PATH WAYS

The reconstructed thermal histories in the Muroto region (Fig. 11) show a significant across-arc variation in the T,,,; higher values in the CSB and lower values in the ESB and MSB, with some variations within individual units. No evidence is known, however, to support a higher paleogeother- ma1 gradient in Cretaceous time than in Eocene and Miocene; to the contrary, it was in the Miocene time that an anomalous, high gradient was likely to have existed (Hibbard e t al . 1993). The overall trenchward decrease in T,,, therefore suggests that the maximum burial tends to increase to the north within the Shimanto Belt. It implies that older accretionary units currently outcropped have experienced deeper pathways of material migration than younger accretionary units. This is in agreement with the material migration trajectories predicted by the critical taper Coulomb wedge model of accreted sediments (Dahlen & Barr 1989). When taking into account vitrinite reflectance data that suggest a southward increase in each structural unit (Mori & Taguchi 1988), the overall northward increase in the maximum burial is considered to be stepwise, as a result of imbrication of accretionary units bounded by north-dipping thrusts.

An overall northward increase in T,,, trend was

50"

100" 0

150" - ? 200"

250"

300"

350"

400"

3

W

0, I-

Fig. 11 Reconstructed thermal histories in and around the Shimanto Belt Three samples from the Muroto region represent the Cretaceous (SH32) Eocene (SH28) and Miocene (SHMIC05) units, whereas two from the Kyushu represent the Cretaceous (SHQ02) and Eocene (SHQ22) Two in the Kii region represent areas that yielded contrasting thermal histories despite indistinguishable depositional ages Samples from [he Oboke unit ( 0 8 4 3 ) and the Sambagawa Belt in the western Kii area (SHNW21) are also shown Error bars are shown at 20

also found between the CSB and ESB in the Kyushu region (Fig. 11). In addition, a southward increase in the metamorphic grade is reported within the CSB (Imai et al. 1971; Toriumi & Teruya 1988). This is also suggested by the T,,,,, variations within the CSB estimated in this study, being higher in the south and lower in the north. Hence, the entire CSB may represent a rather large imbricate thrust package. Both the regional metamorphic grade and the T,,,,i, of the CSB are higher in this region than in the Muroto. This implies greater exhumation in order to expose deeper interiors of the Shimanto accretionary complex, that probably resulted from the enhanced motions of the Nobeoka thrust.

Figure 12 presents a schematic diagram on plausible material migration pathways of the three units of the Shimanto Belt: CSB, ESB and MSB. In order to reconstruct such pathways on the basis of thermal histories of accreted sediments, the following three constraints need to be known or reasonably assumed: (i) the shape of the accretionary wedge, (ii) the thermal structure of the wedge, and (iii) the fundamental pattern of material migration in the wedge. The shape of an accretionary wedge can vary through time in response to various factors, such as the subduction rate and angle, rheology of decollement, material input or output, thermal structure, etc. However, it tends to be kept more or less constant as long as the subducting oceanic plates are of similar characteristics (Davis et al. 1983; Platt 1986). The ancient slabs subducting beneath southwest Japan had similar ages

224 T. Tagnmi et al.

10-

50 -

100 -

I50 -

250 -

300 -

350 ~

400 -

Fig. 12 Plausible material migration pathways estimated for three units of the Shimanto Belt CSB Cretaceous Shimanto Belt, ESB, Eocene Shimanto Belt, MSB, Miocene Shi- manto Belt APAZ apatite partial annealing zone ZPAZ zircon partial annealing zone Note the vertical exaggeration

0 Trench-fill turbtdite

10

Plate subduction

20

in the order of 10 Ma from Late Cretaceous to the present, in addition to rather constant convergence rates of -5-10 cm/yr (Engebretson et al. 1985), except during the subduction of the newly opened Shikoku Basin that formed a t several stages from -26 to -13 Ma (Chamot-Rooke et al. 1987). The present shape of the southwest Japan forearc system is thus adopted here as a first-order apgroximation of the Shimanto accretionary wedge.

With respect to thermal structure, modeling based on reasonably assumed subduction parameters predicts subnormal geothermal gradients within a wedge (Dumitru 1991). Accordingly, a gradient of 20°C per km is assumed in this study for the Cretaceous to Eocene Shimanto wedge, based on the pressure-temperature path of the Shimanto metamorphism (Toriumi & Teruya 1988). This value is also consistent with the modeling by Dumitru (1991), assuming a slab age of -10 Ma and convergence rate of -5 cm/yr. I t is remarked, however, that the value can not be applied to the thermal structure in middle Miocene time, when a hot Shikoku Basin rapidly subducted beneath the southwest Japan forearc as a probable consequence of the opening of the Sea of Japan and a clockwise rotation of southwest Japan (Otofuji et al. 1985). Vitrinite reflectance and illite crystanity data from various rock units within the MSB suggest anom- alously high geothermal gradient, presumably as high as -60-90°C per km (Hibbard et al. 1993). Nevertheless, the vitrinite reflectance data from the ESB are consistent with the structural features related to accretionary processes (DiTullio et al. 1993; Laughland & Underwood 1993), indicating that the high gradient was localized to the MSB. The material migration pathway for the MSB was thus probably shallower than that for the ESB

despite their similar T,,, values, as expressed qualitatively in Fig. 12.

Material migration a t shallow levels of accretionary wedges has been better understood in the past two decades by drilling of modern accretionary prisms and their seismic reflection profiles (Moore & Silver 1987). The trench-fill turbidites accrete as offscraped imbricate packages, which subsequently thicken by thrusting in addition to forearc slope sedimentation and move away from the trench due to succeeding frontal accretion (Moore 1989). This growth pattern of imbricate thrust slices might be maintained a t deeper levels as well. Meanwhile, the pelagic and hemipelagic sediments and oceanic basalts accrete mainly a t depth, probably due to a step in the decollement associated with the formation of imbricate duplex structures (Moore 1989). Those underplated materials may migrate progressively away from the subducting plate and be incorporated in the overlying wedge as the result of successive underplating. In addition to these smaller-scale patterns of material migration near the subducting slab, their overall trajectories within an entire accretionary wedge were better constrained by the physical modeling on the as- sumption of Coulomb failure for accreted sediments (Dahlen & Barr 1989). Here we adopted their steady-state critical taper wedge model that incorporates uniform erosion and moderate 25% sediment influx due to underplating.

The material migration pathways estimated on these guidelines (Fig. 12) show the following three notable features: (i) no crossover between three material migration pathways, (ii) the depth of accelerated underplating, and (iii) episodic material migration when age data are incorporated. The absence of crossover confirms that the overall, general trend of T,,, estimated for the three units

Episodic exh imution oj' ncc?*etionary complexes 225

subsequent, shallower stage of exhumation. The Oboke unit that underlies the Besshi unit (Takasu & Dallmeyer 1990), however, experienced younger exhumation a t -70-60 Ma, closer to the -75- 60 Ma exhumation of the CSB. These lines of evidence strongly suggest that the Sambagawa Belt and the northern area of the Shimanto Belt were exhumed during a series of episodic events, which extended regionally in both along-arc and across-arc directions. These data also support the speculation that the Oboke unit is the high pressure equivalent of the Shimanto Belt (Shinjoe & Tagami 1994).

When considering the geologic and tectonic implications of these episodic exhumations for the evolution of the southwest Japan forearc, we eval- uated tectonic models proposed previously in order to explain the uplift and exhumation of high- pressure metamorphic belts in the subduction zones. Since the birth of plate tectonics, this type of uplift has been discussed in the context of the cordilleran-type orogeny (Dewey & Bird 1970), which is characterized by paired metamorphic belts (Miyashiro 1961) and divergent thrusting and synorogenic sediment transport from the high- temperature volcanic axis. As a typical example, the Franciscan high-pressure metamorphism and approximately coeval Sierra Nevada magmatism were interpreted as a result of such orogeny (Hamilton 1969; Dewey & Bird 1970; Ernst 1971). Ernst (1 975) proposed successive underthrusting and imbrication of down-going younger materials according to plate subduction in order to explain the systematic trenchward decrease in both age and metamorphic intensity of high-pressure metamorphic belts. Many geological and geophysical investigations of subduction zones in the past two decades attributed the growth and uplift of an accretionary complex to various material transfer processes [i.e., offscraping, underplating and out- of-sequence thrusting (Moore e t al. 1982, 1988; see also Morley 1988)l. Based on theoretical and experimental studies (Cowan & Silling 1978; Cloos 1982; Davis et al. 1983; Dahlen & Barr 1989) the evolution and uplift of a high-pressure metamorphic belt in the subduction zones have been recognized as the manifestation of material migration in, or deformation of, an accretionary complex having wedge-shape geometry (Platt 1986; Dahlen & Suppe 1988; Barr et al. 1991).

The regional exhumation episodes of the southwest Japan forearc are characterized in the context of accretionary wedge evolution by episodic up-

are qualitatively consistent with the material migration trajectories predicted from the steady- state model. For a smaller scale within individual units, however, pathways probably show cross- overs due to increased tilting of imbricate thrust sheets during successive accretion. The depth of underplating is probably highly variable, as suggested by different T,,,, for several melange units in the Shimanto Belt. In general, however, melange units in the CSB tend to have experienced higher T,,, well into the ZPAZ (Fig. 12) along with their more voluminous occurrences therein (e.g. the Makimine unit in the Kyushu region and four melange zones in the Muroto region) than in the ESB. The volume of underplated oceanic materials appears to increase a t further depths of an accretionary wedge [i.e., the blueschist metamorphism zone such as the Sambagawa Belt (Isozaki et nl. 1990)l. Therefore it seems that the underplating is accelerated with increasing depths a t around 15 km (Fig. 12). The third feature possesses most profound implications for the evolution of accretionary complexes and thus will be discussed below in detail.

EPISODIC ACCRETION AND EXHUMATION

On the basis of the paleogeothermal structure of the Shimanto accretionary wedge, we assume a constant geothermal gradient of 20' C per km for the CSB and ESB to convert the thermal histories (temperature-time relationship; Fig. 11) into exhumation histories (depth-time relationship). Note that this value is approximately applicable to cooling histories of the Sambagawa Belt, as suggested by the pressure-temperature path for the retro- grade stage of metamorphism (Takasu e t al. 1994). Accordingly, three notable exhumation episodes were found for the Shimanto Belt and vicinities: (i) -70 Ma regional exhumation in the CSB and Sambagawa Belt from various depths of -20- 10 km to -8 km, (ii) -10 Ma regional exhumation in the CSB and ESB from -6 to < 2 km, and (5) -15 Ma exhumation localized to the central Kii area from -15 to < 2 km (Figs 11, 12). The first episode is approximately synchronous with the initial stage of exhumation of the highest grade Besshi unit of the Sambagawa Belt in central Shikoku, estimated as -85-75 Ma from -30 to -15 km depth by muscovite, hornblende and whole-rock 4"Ar/3gAr ages (Takasu & Dallmeyer 1990). In particular, the Sambagawa rocks from the western Kii area, in agreement with the Besshi unit, yielded consistent -75-70 Ma ages for the

226 T. Tugnrni et al.

ward motions of structural units that were accreted and metamorphosed in different conditions. In addition, the areal distributions of those units extend along-arc as well as across-arc. Exhumation was syn-subduction because continuous subduction is suggested by the reconstruction of past plate motion; Pacific plate (or Kula plate) subduction a t -70 Ma (Engebretson et al. 1985) and Philippine Sea plate a t -10 Ma (Seno & Maruyama 1984). With the absence of paleo-island arc remnants or other large buoyant crustal fragments, it is not likely to be attributable to any collisional event. Hence the upward motions should be a consequence of overall material migration associated with deformation of the accretionary wedge. By taking into account the general material migration pattern within an accretionary wedge (Dahlen & Barr 1989), the regional exhumation episodes should be best explained by the accelerated accretion at about 70 and 10 Ma a t the front as well as base of the southwest Japan forearc wedge, accom- panied by surface erosion and/or extension (Platt 1986; Harms et al. 1992; Ring & Brandon 1994).

In order to test this hypothesis, estimated ages of accretion and exhumation were compared and contrasted for southwest Japan (Fig. 13). Three major stages were recognized for accretionary his-

AGE(M=) MHUMAllON _

ACCRmoN MAGMATlSM ........................................................................................

............................. .............................

............................

............................

............................

Fig. 13 Comparison of estimated ages among accretion exhumation and magmatism Dark shaded zones including those for the Oboke, Besshi and Kii represent major periods of each episode, whereas light shaded zones correspond to outliers in geochronologic data The extensive magmatism appears to have triggered the accretion-exhumation twice in the past 100 Ma Note that the accretion age is estimated here as the age of rock deposition in and around the ancient trench Therefore we need to correct for the travel time of accreted materials from trench to the accretionary position for rigorous discussion CSB Cretaceous Shimanto Belt, ESB, Eocene Shimanto Belt, MSB, Miocene Shimanto Belt, SB Shimanto Belt (denoting the -10 M a widespread cooling except for Kii region)

tory therein (Taira et al. 1988): Late Cretaceous, predominantly Campanian; Middle Eocene to Early Oligocene, predominantly Late Eocene; and Late Oligocene to Early Miocene, predominantly Early Miocene, although only a small portion of accreted units appears to outcrop on land. With respect to regional exhumation, two episodes were found in the present study and both relate closely in age to the massive accretion, with being coeval or by slightly postdating them. These correlations likely support exhumation caused by accretion. I t should be remembered that the timing of accretion is represented by the depositional age of shale formed in and around the trench and thus inherently tends to predate the real accretion, particularly for deeply buried units. This is caused by the travel time of sediments from trench to deep interiors of the accretionary prism. The duration would be controlled by the subduction rate, depth of effective accretion to cause exhumation, and material migration pathway thereto [i.e. either by subduction with the slab followed by underplating or by downward flow within the prism after accretion (Dahlen & Barr 1989)l. If we assume a subduction rate (exactly that of its downward component) of 3 cm/yr, a depth of 15 km and underplating after subduction, the duration is given as about 5 Ma (Fig. 12). When this effect is taken into account, the temporal correlation between accretion and exhumation seems to be improved for both late Cretaceous and Middle Miocene episodes. One ambiguous factor is the Late Eocene to Early Oli- gocene exhumation, which is predicted by the hypothesis but does not seem widespread, except for a sample from the CSB in the western Kii area. Further thermochronologic mapping into the Chichibu Belt and other units to the north is strongly needed to constrain the areal extent of the exhumation. With respect to the -15 Ma local exhumation in the central Kii area its tectonic cause is not clear and allows alternative interpre- tations: such as the magmatic doming related to the Middle Miocene igneous activities of the Omine and Kumano Acidic Rocks to the south (Fig. 1); the rapid, local accretion of volcano-plutonic materials supplied from the large Kumano Acidic Rocks and associated bodies during the rapid clockwise rotation of southwest Japan, etc. Nevertheless, it is implied that the disturbance of zonal structures in the central Kii area (Fig. 1) was formed by this rapid Middle Miocene exhumation.

What promoted the accretion exhumation? It is considered that several tectonic factors in a subduction zone control the development (or erosion)


units intercalated with 15 Ma mudstones (Taira et ai. 1991). This indicates a voluminous supply of igneous rock fragments to the ancient trench in Middle Miocene, presumably derived from felsic volcano-plutonic terranes contemporaneously ac- tive in land. In contrast to these cases, the sandstone composition of the flysch units in the ESB is more quartzose and less feldspartic than those in the CSB (Teraoka 1979; Kumon 1983). It suggests that the Eocene flysch had greater contribution from the exhumed older accretionary prisms than the Cretaceous one (Taira e l al. 1988), and this favors the accretion caused by the change of geometry discussed in the last paragraph.

These lines of geochronologic and geologic evidence, in conjunction with mechanics of accretionary prisms, imply that the extensive magmatic activities caused the exhumation of southwest Japan forearc twice during the past 100 Ma. A similar correlation in age among magmatism, accretion and regional exhumation was recognized recently in the Mesozoic Cordilleran system of California (Tagami & Dumitru 1995). The present scenario is thus a modern trans- lation of the cordilleran-type orogeny as well as paired metamorphism, and predicts that the extensive magmatism triggers the regional exhumation of accretionary complexes including high-pressure metamorphic belts. What caused such large igneous provinces a t continental margins? The question prompts discussion and will be answered by high- resolution geo- and thermochronology, but is beyond the scope of this study.

of an accretionary wedge: subduction velocity, slab dip-angle and incoming sediment flux (Hilde 1983; Shreve & Cloos 1986). The reconstructed past plate motion suggests that the subduction parameters were relatively constant during two of the three accretionary stages: Late Cretaceous rapid subduction of Pacific (or Kula) plate toward northwest (Engebretson et al. 1985) and moderate subduction of Philippine Sea plate toward northwest (Seno & Maruyama 1984). I t is expected in these cases, therefore, that the incoming sediment flux dominantly controlled the tectonics of the accretionary wedge. In Middle Eocene, however, the rotation vector of the Pacific plate changed substantially, from northward to westward in terms of the convergence to Eurasia around Japa- nese islands. Accordingly, this resulted in the change of subduction kinematics from a highly oblique left-lateral motion to a nearly head-on motion against southwest Japan a t its paleoposi- tion (Engebretson e t al. 1985; note this configura- tion is before the Middle Miocene -50" clockwise rotation of southwest Japan relative to Eurasia). This may have influenced the geometric configura- tion of the Shimanto accretionary wedge and promoted the accretion.

The southwest Japan forearc was probably subjected to a large amount of sediment transport from volcano-plutonic provinces twice in the past 100 Ma (Fig. 13); the Cretaceous felsic magmatism widespread in the Inner Zone and other Eurasian margins, the duration of which was -100-75 Ma with a predominant period of 90-80 Ma, according to Rb-Sr whole rock ages complied by Nakajima (1994); and Middle Miocene felsic magmatism predominantly in the Outer Zone near the ancient trench, having an age of -15-12 Ma with a predominant period of 15-13 Ma, estimated by extensive K-Ar ages on volcanics and granitic rocks with fine-grained porphyritic textures (Shibata 1978; Tatsumi & Ishizaka 1982). Both agreed with individual accretionary (depositional) ages (Fig. 13). The resultant supply of massive sediments to the ancient trench was in fact recorded in the contem- poraneous sedimentary sequence. The upper Creta- ceous flysch units in the CSB exhibit the paleocur- rent dominated by southward (trenchward) as well as east-west (along-arc) directions (Kumon 1983), with abundant rock fragments derived from acidic and intermediate volcano-plutonic terranes (Ter- aoka 1979; Kumon 1983). With respect to Miocene magmatism, the lithologic data from the ODP drill hole a t Site 808 showed thick acidic tuff layers, situated below Middle Miocene and above basaltic

ACKNOWLEDGEMENTS

Asahiko Taira, Gaku Kimura and Shigenori Maruyama encouraged us to consider the evolution of accretionary complexes. We also thank Susumu Nishimura, Masayuki Torii, Yoshitsugu Furukawa, Hironao Shinjoe, Rod Brown and Tony Hurford for their helpful suggestions on various parts of the study and Tim Byrne and Simon Wallis for review- ing the manuscript. This work has been supported by a Grant-in-Aid No. 04854085 from the Ministry of Education, Science and Culture and by a visiting research program of the Kyoto University research reactor.

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