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Contrasting styles and rates of tectonic uplift of coral reef terraces in the Ryukyu and Daito Islands, southwestern Japan
QuaternaEv International, Vol. 15/16, pp. 17-29, 1992. Printed in Great Britain. All rights reserved.
1040-6182/92 $15.00 © 1992 INQUA/Pergamon Press Ltd
C O N T R A S T I N G STYLES AND RATES O F T E C T O N I C U P L I F T O F C O R A L R E E F
TERRACES IN THE RYUKYU AND DAITO ISLANDS, SOUTHWESTERN JAPAN Yoko Ota* and Akio O m u r a ?
*Department of Geography, Yokohama National University, Yokohama, 240, Japan ~Department of Earth Sciences, Kanazawa University, Kanazawa, 920, Japan
Coral reef terraces on Kikai, Hateruma and Yonaguni in the Ryukyu Islands on the Eurasian lithospheric Plate and those of Kita-Daito and Minami-Daito Islands on the Philippine Sea Plate indicate three distinct patterns of coral reef terrace formation and preservation, primarily reflecting different uplift rates. The Kikai-type, which includes many terraces from Oxygen Isotope Stages 5e to 1, is a result of high uplift rate of ca. 1.8 m/kyr on the fore-arc rise of a Chilean-type subduction zone. Arcward (westward) tilting and faulting are also characteristic of the Kikai-type. The Hateruma-type is characterized by two coral reef terraces younger than the Stage 5e terrace and they are arranged in a concentric annular pattern. Slow uplift rate of ca. 0.3 m/kyr, adjacent to a Mariana-type subduction margin is typical of the Hateruma-type. The emergence of Kikai and Hateruma Islands is recent, younger than Isotope Stage 5e and 7, respectively, reflecting the active deformations of the leading edge at the overriding Eurasian Plate. In the Daito-type, atolls surrounded by fringing reefs have been uplifted very slowly (less than 0.05 m/kyr), but over a time period of at least several million years. Slow uplift and long history of Daito-type islands is the result of up-bulging of the Philippine Sea Plate as it migrates toward the Ryukyu Trench.
INTRODUCTION The Ryukyu Arc-Okinawa Trough system is one of the active arc-backarc systems along the margin of the western Pacific. The Ryukyu Trench marks the plate boundary between the overriding Eurasian Plate to the west and the subducting Philippine Sea Plate to the east (Fig. 1). The Wadachi-Benioff Zone at this boundary dips more gently (ca. 25°-35 °) in the northern part of the plate boundary, than the southern part, where the dip is ca. 55°-65 ° (Konishi, 1980; Carr et al., 1973). No historical large earthquakes accompanied by coastal uplift are known in this active subduction zone, however. Okinawa Trough is interpreted as an actively rifting backarc basin (Furukawa, 1991). The islands in this subduction zone setting, located between ca. latitude 24°N. and 29°N., are divided into the Ryukyu Islands which lie on the Eurasian Plate, and the Daito Islands which lie on the Philippine Sea Plate (Fig. 1). Well developed coral reef terraces, which record the former sea-level position, are found on these islands. But the islands on opposite sides of the trench have experienced different tectonic histories, which are reflected in different uplift histories recorded by coral reef terraces fringing the islands. Differential movements are also recorded on some of the Ryukyu Islands, depending on the variation in the detailed tectonic setting of each island. Many geological and geomorphological studies have been carried out on the coral reef terraces on the Ryukyu and Daito Islands, since the pioneer work by Hanzawa (1935) who used the name Riukiu Limestone for the reef limestone composing the coral reef terraces. However, many different ideas and chronologies have been proposed for the coral reef terraces, because of the difficulties in radiometric dating of the 17
coral limestone due to recrystallization, and problems of terrace correlation due to localized tectonic deformation. Middle to Late Quaternary terrace chronology based on radiometric dating has been established for only a few islands of the Ryukyu and Daito Islands (e.g. Kikai, Hateruma and Yonaguni islands on the Eurasian Plate and Kita- and Minami-Daito islands on the Philippine Sea Plate). Interpretation on the different uplift on some of the above-mentioned islands was proposed by Konishi (1980). However, the number of dated samples was limited and the relation between the dated samples and morpho-stratigraphy of the terrace sequences were not fully examined at that time. In this paper, we describe geologic and geomorphic features of these islands, based on detailed morphostratigraphic studies of coral reef terraces and the alpha spectrometric 23°Th/234U dating (ages obtained by this method will be simply described as 23°Th/234U age in the following sentences) of coral limestones. We also compare the different tectonic histories deduced from coral reef studies on different islands, and from that, we propose three different patterns of coral reef terraces to explain their variable evolution in different tectonic settings. CORAL REEF TERRACES ON THE EURASIAN PLATE Most islands on the Eurasian Plate are fringed by a flight of coral reef terraces, suggesting continued uplift of the islands. The highest parts of some islands, such as Kikai and Hateruma, are overlain by Pleistocene reef sediments, that rest on Neogene sedimentary rocks and form a flat, table-like surface. The table reefs suggest that uplift of these islands started only recently. However, on other islands underlain by older rocks,
18
Y. Ota and A. Omura
FIG. 1. Study area and tectonic setting of the Ryukyu Island Arc region. Inset shows the plate boundaries and ocean-floor ridges (coarse dotted pattern). Solid line with triangles denotes active subduction zones. Large numbered arrow shows orientation and rate (mm/yr) of relative plate motion. Broken line with open arrows indicate a back-arc spreading centre (Moore, 1982). RT, Ryukyu Trench; KPR, Kyushu-Palao Ridge; AP, Amami Plateau; DR, Daito Ridge; ODR, Oki-Daito Ridge. Isobaths (500 m interval) are from the Chart No. 6302, published by the Maritime Safety Agency of Japan. Labelled islands are solid: K, Kikai Is.; T, Toku Is.; OE, Okierabu Is.; OK, Okinawa Is.; H. Hateruma Is.; Y, Yonaguni Is.; KD, Kita-Daito Is.; MD, MinamiDaito Is.; OD, Oki-Daito Is.: OT, Okinawa Trough. such as Paleozoic or Tertiary volcanic or clastic rocks (e.g. Toku, Okierabu, Okinawa and Yonaguni), fringing reefs surround the higher parts of the islands. K i k a i Island
Kikai Island is the closest of the Ryukyu Islands to the Ryukyu Trench (about 80 km, Fig. 1) and is the only place throughout Japan, where all the high stillstands of sea-level younger than the last interglacial m a x i m u m are preserved. Konishi et al. (1974) reported the first U-series ages (23°Th/234U and 234U/238Uages) of fossil corals from Kikai: ages of ca. 120-130 ka, 80100 ka, 55-65 ka and 35-45 ka correspond roughly to Oxygen-Isotope Stages 5e, 5c, 5a and 3 (Shackleton and Opdyke, 1973), respectively (Fig. 2). The highest point on the island at 224 m elevation is underlain by the reef limestone of Isotope Stage 5e (weighted average 23°Th/234U age is 129+2 ka). Coral limestones of Middle Pleistocene age are reported from the southern part of Kikai by O m u r a et al. (1985), that are ca. 200 ka by the 23°Th/234U method and 400-500 ka by the Electron Spin Resonance (ESR) method. Recently, older ages of ca. 570-840 ka are also reported by the ESR method (Ikeda et al., 1991). However, these older reef complexes are not exposed on the Pleistocene terrace surfaces, and are almost entirely overlain by upper Pleistocene units (Fig. 3). On Kikai, the uplifted reef complexes are not distributed in a concentric annular pattern, because
they have been complicately deformed by faulting associated with uplift of the island. This deformation makes it difficult to identify and correlate the terraces, and to reconstruct their tectonic history. Even on this small island it is difficult to m a p the exact distribution of all the terraces. We interpret the relationship between terrace surfaces and coral reef complexes as shown in a schematic diagram (Fig. 3). This interpretation is based on the radiometric age determinations, observation of terrace morphology and coral limestone facies. Late Pleistocene corals are usually thin and overlie an eroded surface in thicker Middle Pleistocene coral limestone. Coral limestone of Stage 3 rests unconformably on an eroded surface cut into coral limestone of Stage 5a. A flat terrace-like surface at about 70-80 m in elevation is an erosion surface cut into the Middle Pleistocene units. Kikai is characterized by very active tectonic movement. Boundaries of terraces of different ages usually coincide with active faults, some of which also displace a single terrace. Slips on these faults have apparently occurred several times on Kikai after the emergence of the ca. 120 ka coral reefs. Coral reefs younger than Stage 5e have developed on downthrown blocks (Fig. 3). Two faults, Tobiyozaki Fault to the northeast and Araki Fault to the southwest of the island, displace Holocene terraces as well as the ca. 60 ka and the ca. 40 ka terraces. The altitude of coral reef terrace surfaces
Tectonic Uplift of Coral Reef Terraces 129"55'
19
130"E
E
TOBIYO-ZAKI
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FIG. 2. Map of Kikai Island showing Holocene and Pleistocene coral reef terraces and the locations and approximate ages (seven types of symbols) of representative coral samples from terraces (compiledmainlyfrom Konishiet al., 1974;Omura, 1988; Ota et al., 1978). A-B shows location of profile in Fig. 3.
indicates overall a westerly tilt of the island, away from the trench axis (Ota et al., 1978). Kikai is the only island in the Ryukyu Islands that is fringed by Holocene terraces. These emerged Holocene coral reef terraces reach 13 m in altitude. Four distinctive terraces suggest repeated episodic, probably coseismic, uplifts since the culmination of the postglacial transgression (Figs 2 and 3, Ota et al., 1978 and Nakata et al., 1978). Radiocarbon ages of these terraces from highest to lowest are ca. 6.8 to 6.0, 5.2 to 3.5, 3.5 to 3.0 and 2.5 to 1.5 ka BP. Logs of drillholes through the Holocene coral reefs show that the Holocene reef began forming by ca. 8 ka BP (Konishi et al., 1983). H a t e r u m a Island
Hateruma Island is located at the southwestern end of Ryukyu Islands, about 100 km north of the Ryukyu Trench (Fig. 1). A concentric pattern of coral reef terraces has developed on this island. A Middle and Late Quaternary chronology based on a morphostratigraphic study of the six terraces by Ota and Hori (1980) was confirmed with 23°Th/234Udating of terrace coral limestone by Ota et al. (1982) and Omura (1984).
The coral reef terraces I, III, IV and V correspond to Oxygen-Isotope Stages 7, 5e, 5c and 5a, respectively (Figs 4 and 5). The uppermost terrace 1 has a maximum elevation of 59.5 m, and the most extensive terrace III has a maximum elevation of 41 m. Both terraces are constructional and are underlain by thick transgressive coral reef limestone, that rests unconformably on an irregular, upper Pliocene, bedrock surface. These terraces, I and III, were formed during the penultimate and last interglacials, respectively. Terraces IV and V (Isotope Stages 5c and 5a) are underlain by only very thin coral reefs on erosion surfaces cut into older reef limestones. There is a small but progressive westward tilt of the coral reef terraces as indicated by the altitude of the terraces at their inner margins (Fig. 4). Terraces of ca. 60 ka, 40 ka, and of Holocene age like those on Kikai Island, are not present on Hateruma. Y o n a g u n i Island
Yonaguni Island is located at the western end of the Ryukyu Islands Arc, and is close to the southern margin of Okinawa Trough. Thus Yonaguni Island has a somewhat different tectonic setting from Kikai and
20
Y. Ota and A. Omura
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,,
OCCURRENCEOF DATEDCORALS ISOTOPE STAGE
AVERAGE AGE (ka)
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Distance
'
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FIG. 3. Geologic section and topographic profile of southern part of Kikai Island along line A-B in Fig. 2. Locations of dated coral samples are shown. Data sources are as in Fig. 2, Horizontal boxes with light shading show the extent of coral limestone, corresponding to each isotope stage. Vertical lines with arrows are faults that displace coral reef terraces. Dip of fault planes are unknown.
g T-O >200 /i 120 100 ~_ 80
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I II III
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IV V VI
• 12 Altitude of inner margin of terrace (m) Fault . . . . I'N -
~'03'-
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FIG. 4. Map of coral reef terraces of Hateruma Island. Altitude of inner margin of each terrace (m), faults, and localities and ages (four types of symbols) of dated corals are shown (compiled from Ota and Hori, 1980; Omura, 1984; revised from Ota and Machida, 1987). A-B shows location of profile in Fig. 5.
Hateruma islands (Fig. 1). The island has been complexly deformed through subduction of Philippine Sea Plate and backarc spreading in the adjacent Okinawa Trough. The tilted blocks of coral reef terraces (Fig. 6) are the outstanding morphologic features of this island (e.g. Ota and Hori, 1980; Koba et al., 1987; Kuramoto and Konishi, 1989). Four distinct coral reef terraces fringe the island, and rest on a bedrock of Tertiary age (Fig. 6). Based on
ages, Terrace IV (ca. 30 m) is correlated with the last interglacial maximum of Isotope Stage 5e (Kodama et al., 1990). Terrace III is correlated with the penultimate interglacial, and occurs on the northwestern part of the island at a maximum elevation of 54 m. Two younger radiometric ages, correlative with Stage 5c, occur at one site on Yonaguni (Kodama and Omura, 1991). No radiometric ages are available from the higher terraces I and II. 23°Th/234U
21
Tectonic Uplift of Coral Reef Terraces A
OCCURRENCE OF DATED CORALS
B
ISOTOPE AVERAGE STAGE AGE (ka)
2
0
4
6 km
FIG. 5. Geologic section and topographic profile of coral reef terraces of Hatcruma Island along line A-B in Fig. 4 and locations of dated corals. Boxes with light shading show the extent of coral limestone corresponding to each isotope stage. Data sources are as in Fig. 4.
i
i
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23°Th/'~U age (ka)
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Prominent sea cliff FIG. 6. Map showing coral reef terraces, representative locations and ages (three types of symbols) of dated samples and active faults on Yonaguni Island. Coral ages are from Kodama et al. (1990) and Kodama and Omura (1991).
CORAL REEFS OF THE DAITO ISLANDS ON THE PHILIPPINE SEA PLATE
Kita-Daito and Minami-Daito on the Daito Ridge (Fig. 1) are two of only a few uplifted atolls (Figs 7 and 8) in the world, and have attracted attention because of their unique tectonic histories. Vertical tectonic movement of these islands has changed from subsidence, during their formation as atolls, to uplift, which resulted in emergence of the atolls, followed by the formation of fringing reef terraces around the atolls. A 431 m deep drillhole in the lagoon of the Kita-Daito raised atoll did not reach basement (Sugiyama, 1934, 1936), indicating subsidence since at least the early Miocene (Hanzawa, 1939). The first radiometric ages of Pleistocene corals from the Daito Islands were obtained from Minami-Daito by Konishi et al. 41978) and Konishi (1980), who reported
the last interglacial corals of ca. 98-123 ka by the Z26Ra/ 23SU method for corals at less than 11 m elevation. Koba et al. (1987b) also reported the last interglacial ages for corals on Kita-Daito Island using the ESR method. Recently, detailed geomorphic and geologic observation of small fringing reefs that exist sporadically along the coast of both islands, and radiometric age determinations of many coral specimens by the 23°Th/Z34u, ESR, and non-destructive 226Ra/238U methods have been carried out (Ota et al., 1991; Omura et al., 1981; Koba et al., 1991; Kawana et al., 1991). We use only the 23°Th/234U ages in the following discussion because all samples have been dated by this method, and there is good agreement (at the one standard deviation level) between the results by this method and those obtained by other methods. Scattered autochthonous fossil corals occur on coastal benches truncating the older calcareous sedi-
22
Y. Ota and A. Omura 131J17'E
131"18'E
131'~19'E
131120'E
-25"57'N
A -
-
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FIG. 7. Contour map of Kita-baito Island, an example of a raised atoll. Contours (interval, 10 m with supplementary 5 m
contours shown in broken line) are from 1/25,000 topographic map 'Kita-Daito-jima' published by the Geographical Institute of Japan. Solid circle is the borehole site described by Sugiyama (1934, 1936). A-B and C-D (N-l) show location of profiles in Fig. 8.
A
KITA-DAITO ISLAND
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FIG. 8. Geologic sections and topographic profiles of Daito Islands. Upper; Kita-Daito Island along line A-B in Fig. 7. The profile is drawn based on the 1/5,000 scale map published by Kita-Daito Village. Lower; Profile showing the occurrence of autochthonous corals (fine dot pattern) and their 23°Th/234U ages (ka) (sample locations shown in solid circles). Left; N-1 on Kita-Daito Island along line C-D in Fig. 7. Right; profile at S-3, in north-western part of Minami-Daito Island (after Ota et al., 1991). Note that autochthonous corals only sporadically occur on Daito Islands and that all dated corals correlate with Isotope Stage 5e.
m e n t s c o m p o s i n g atolls a n d l a g o o n s ( D a i t o D o l o s t o n e ) at s e v e r a l localities on K i t a - D a i t o a n d M i n a m i - D a i t o , at the e l e v a t i o n s u p to ca. 8 m a n d 11 m, r e s p e c t i v e l y (Fig. 8). C o r a l ages c l u s t e r in t h e r a n g e ca. 115-130 ka ( w e i g h t e d a v e r a g e age is 123_+ 1 k a ) a n d a r e c o r r e l a t e d with I s o t o p e S t a g e 5e. T h e e l e v a t i o n o f t h e last i n t e r g l a c i a l s h o r e l i n e o n t h e s e islands is e s t i m a t e d to b e ca. 10-12 m a b o v e p r e s e n t s e a level, j u d g e d by the
a l t i t u d e o f n o t c h e s a n d b e n c h e s a s s o c i a t e d with the d a t e d corals. N o e m e r g e d fringing reefs y o u n g e r t h a n I s o t o p e S t a g e 5e a n d no H o l o c e n e coral reefs are f o u n d a b o v e sea level. N o 23°Th/234U a g e s a r e o b t a i n e d f r o m the atoll l i m e s t o n e o r o l d e r fringing reefs b e c a u s e the c a l c a r e o u s sediments (Daito Dolostone) are entirely dolomitized. T h e r e f o r e , w e can n o t d e t e r m i n e the history o f e m e r -
Tectonic Uplift of Coral Reef Terraces
23
gence after the initial period of subsidence on Kita- and Minami-Daito. Recently, three coral samples from the crest of atoll (45 m in elevation) on Kita-Daito were dated at ca. 3.6-5.3 Ma by the ESR method (Koba et al., 1991), indicating that the emergence of the atolls began sometime before the early Pliocene, because ESR ages give only the minimum, representing the age of recrystallization of coral limestone. Thus, emergence of the Daito Islands began much earlier than the emergence of the other islands on the Eurasian Plate.
characterized by the low average uplift rate less than 0.2 m/kyr. The average uplift rate is also low (0,3 m/ kyr) in Hateruma, however, which is close (ca. 100 kin) to the Ryukyu Trench. A marked contrast of the uplift rate between Kikai and Hateruma suggests that there is a difference in subduction style at the northern and southern part of the Ryukyu Trench.
LATE QUATERNARY UPLIFT RATES AND CORAL REEF T E R R A C E SEQUENCES
On Kikai and Hateruma Islands sufficient data exists to examine uplift histories and whether the uplift rate has been uniform through time. Both Kikai and Hateruma have a short geological history having been uplifted above sea level after the Isotope Stage 5e and 7, respectively (Table 3, Fig. 9). This is much more recently than not only on Daito Islands but also on other Ryukyu Islands (e.g. Yonaguni and Okierabu Islands), which have been emergent since the Early Pleistocene or Pliocene. This suggests the islands located on the leading edge of overriding plate have a shorter but more intense tectonic history than islands located far from the trench and on the margin of backarc basin or on the outer swell of subducting plate. The expected elevations of terraces on Kikai and Hateruma (Table 2) can be calculated from the average long term uplift rate since Stage 5e and the eustatic correction as defined by Chappell and Shackleton (1986). There are marked discrepancies between the expected elevations and those observed, and imply uplift rates have varied considerably with time, if we accept that eustatic sea level curves of Chappell and Shackleton (1986) are applicable to the Ryukyu Islands. For example, discrepancies at ca. 100 ka and 80 ka on Kikai Island (+24 m and +60 m, respectively) are particularly marked. This suggests that, in addition to long-term variation in tectonic activity, differential movements with major faulting took place in association with the emergence of these terraces, resulting in variable uplift rates with time. In contrast, the differences between calculated and observed elevations are not so large on Hateruma (Table 2), where only minor faulting has occurred during the Late Quaternary. There is an increase in the average uplift rate toward the present both on Kikai and Hateruma, although the increase is not consistent on Kikai (Table 2). This may suggest a change in convergence rate between the Eurasian and Philippine Sea plates, as in the case of increased uplift rates deduced from coral reef terraces on Santo and Malekula Islands, New Hebrides Island Arc (Jouannic et al., 1980).
The formation of a flight of marine terraces is considered to be the combined result of eustatic sealevel fluctuation and tectonic uplift (e.g. Yoshikawa et al., 1964; Bloom, 1980; Lajoie, 1986 for full discussion). Rapid uplift is essential for the formation of many separate terraces, formed during successive high sea level stillstands, corresponding to interglacial or interstadial episodes. The last interglacial coral reef terrace, correlated with Isotope Stage 5e, has been identified on each of the five islands described in this paper (Table 1, Fig. 9). Assuming a constant rate of uplift since the last interglacial maximum, and that sea level at that time was ca. 6 m higher than present (e.g. Chappell and Shackleton, 1986), then the average uplift rate during the last ca. 120 ka is estimated to be 1.8 m/kyr on Kikai, 0.3 m/kyr on Hateruma, 0.2 m/kyr on Yonaguni, 0.03 m/kyr on Kita-Daito and 0.05 m/kyr on Minami-Daito (Table 1). Many terraces are preserved on Kikai Island and are related to Isotope Stages 5e to 3, as well as multiple Holocene terraces of Stage 1. These data provide a good example of where rapid uplift results in multiple terrace formation. In contrast, slow uplift results in a limited number of terraces, for example, on Hateruma Island, where only two terraces younger than Stage 5e are observed. Similarly on Yonaguni, only two samples have been found to correlate corals correlated with Stage 5c. The last interglacial terrace is the youngest terrace on Okierabu, away from the trench (Fig. 1) where the average uplift rate is 0.1 m/kyr (Omura, unpub.). No terrace younger than 5e has been found on the Daito Islands, which have very low average uplift rates. Thus, considerable differences exist in the average uplift rate between the Ryukyu Islands on the overriding and uplifting Eurasian Plate and the Daito Islands on the subducting Philippine Sea Plate. Within the Ryukyu Islands, Kikai, which is the closest to the Ryukyu Trench, has the highest uplift rate and may be regarded as a typical island of a fore-arc rise tectonic setting. This rate (1.8 m/kyr), is the highest rate measured during the Late Quaternary in the coastal areas of Japanese Islands. In contrast, the islands away from the trench (Okierabu) or close to the backarc spreading in the Okinawa Trough (Yonaguni) are
DIFFERENCES IN UPLIFT HISTORIES IN THE KIKAI AND H A T E R U M A ISLANDS
CORAL REEF T E R R A C E D E V E L O P M E N T IN DIFFERENT TECTONIC SETTINGS
Three Patterns o f Coral Reef Terrace Evolution As noted above, different island groups have different
24
Y. Ota and A. Omura
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Tectonic Uplift of Coral Reef Terraces
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FIG. 9. S u m m a r y of the radiometric ages of fossil corals from the Ryukyu and Daito Islands. Time scale is logarithmic. Numerals in the boxes show n u m b e r of dated Pleistocene coral samples. Radiometric data are from Konishi et al. (1974), O m u r a (1982, 1984, 1988), O m u r a et al. (1985, 1991), Ota et aL (1978, 1991) and K o d a m a et al. (1990). Older ESR ages by Ikeda et al. (1991) are not shown in this figure. Width of age bars shows range between the oldest and youngest ages. A g e bar for one sample of Stage 7 on Kikai represents one standard deviation in the age. O p e n arrows and vertical dashed lines show the correlation with isotope stages.
T A B L E 2. Shoreline altitudes and a m o u n t of uplift on Kikai and H a t e r u m a Islands since the last interglacial m a x i m u m
A p p r o x i m a t e age of high sea-level stand (ka) 120-13(1 100 80 60 40
Present highest altitude of former shoreline* (m) Kikai Hateruma > 224 195 185 70 50
41 30 23 ---
Paleo sea-level heightt (m) +6 -9 -19 -30 -41
Apparent amount and rate of uplifts (m) Kikai Hateruma 218 204 204 100 91
(1.7) (2.0) (2.6) (1.7) (2.3)
35 (0.3) 39 (0.4) 42 (0.5) ---
Expected shoreline altitude assuming constant uplift since 120 ka (m) Kikai Hateruma . 171 125 78 31
.
. 19 4 ---
Difference between expected altitude and present altitude (m) Kikai Hateruma . +24 +60 -8 + 19
+l 1 +19 ---
*Height errors are within _+ 2 m. t H e i g h t from Chappell and Shackleton (1986). ~:Numbers in parentheses denote apparent rate of uplift (ndkyr).
patterns of coral reef terraces which appear to be related to their tectonic setting. We propose three distinct patterns for the development of sequences of coral reef terraces, based mainly on variation in uplift rates (Fig. 10, Table 3). The Kikai-type is characterized by rapid emergence and formation of terraces corresponding to each of the high stands of sea-level during and since Stage 5e, and the formation of multiple Holocene terraces. The highest terrace, Stage 5e, comprises a table reef, but other younger terraces are a succession of fringing reefs. The terrace distribution pattern does not show a concentric annular pattern, however, because of faulting, which has caused substantial dislocation of the
terrace sequences during the Late Quaternary. Progressive westward tilting away from the trench axis is also characteristic in this island. The Hateruma-type has an annular arrangement of coral reef terraces on a slowly uplifting island. A series of emergent fringing reefs surrounds the central uplifted table reef. Due to slight, but progressive westward tilting, coral reef terraces are elliptical in plan view with long axes oriented E-W. Faulting is not significant here and the number of terraces younger than Stage 5e is limited. The Daito-type is a unique type of emergent atoll, being surrounded by a flight of fringing reefs. It has a long history of uplift, but the rate has been very low,
26
Y. Ota and A. Omura
MAP VIEW
CROSS-SECTION
TR
KIKAI-Type
J
m
5c~
~00
5a
~~.
. .
;~ 13,/
9
Holocene terraces
/"
!
00
erosional terrac a
~0
5
~
HATERUMA-Type ~ ~ m 50 FR .,,,--.-,:'~,;:;:; :',:1:: :;:;:,'5e 5cFR 0 DAITO-Type 5e
5e
;km
5e__~ ........_L L 6km
i
R:Table reef
'
........
2
5e
6 °
6
stage 1
~
Tertiary siltstone
R:Fringing reef
Q) ~ tO ~ ~
stage 3,5a.5c
~
Fault
r:Atoll
"-- ~
stage 5e
~
Boundary of coral reef terraces
:Lagoon
~ ~ stage 7 and older O 0 i::::::::7: Daito Dolostone
~) E
FIG. 10. Schematic diagrams showing three patterns of coral reef terraces that reflect differing styles and rates of vertical deformation. See Table 3 for details.
T A B L E 3. Comparison of coral reef terraces on islands of the Eurasian and Philippine Sea Plates
Plate
Island Kikai Hateruma Distance from the trench (km) 80 100 Age of the oldest reef (ka) 570-840* 210 Age of the oldest reef terrace (ka) 120 210 Altitude of terrace related to the last interglacial maximum (m) > 224t 41~ Average uplift rate since the last interglacial maximum (m/kyr) 1.8 0.3 Altitude of Hoiocene terraces (m) 13§ -Estimated age of emergence of island (ka) < 120 < 210 Evolutional history of reef terraces table reef--*fringing reef table reef---~fringing reef Style of uplift
Subducting Philippine Sea Plate
Overriding Eurasian Plate
rapid uplift with faulting slow uplift with small and westerly tilting amount of westerly tilting
*Ikeda et al. (1991). tHighest altitude of the table reef terrace. :~Highest altitude of the inner margin of fringing reef terrace. §Subdivided into four steps. [IKoba et al. (1991).
Yonaguni 130 > 350 > 350
Kita- and Minami-Daito 150 > 4,600tl unknown
30~:
10-12
0.2 --
0.03-0.05 --
> > 350
> 4,6001]
fringing reef
atoll---~fringing reef
slow uplift with block movement
very slow uplift
27
Tectonic Uplift of Coral Reef Terraces
only less than 0.05 m/kyr since the last interglacial maximum, and no tilting or faulting has occurred. The youngest of the emergent fringing reefs belongs to Isotope Stage 5e.
Chilean-type subduction zone o
I
~
.c
Discussion
We relate the tectonic setting of these three types to the models of subduction proposed by Uyeda and Kanamori (1979) and Uyeda (1982); that are the Chilean-type and Mariana-type subductions. Konishi (1980) interpreted the different uplift rates of Kikai, Hateruma and Daito islands in terms of these models. Generally we agree with this interpretation but we refine and extend it, based on newly obtained data on morpho-stratigraphy and uplift rates. Data from Yonaguni and Okierabu Islands which were not included in Konishi's interpretation, are also considered in our discussion. The upper diagram of Fig. 11 is of Chilean-type subduction, which is characterized by the presence of a shallow dipping Wadachi-Benioff Zone and associated strong compression as well as great earthquakes. This type of subduction is considered representative of the northern part of the Ryukyu Trench (Carr et al., 1973; Konishi, 1980). The Kikai-type is interpreted to result from recent and intense tectonic activity on the fore-arc rise of the overriding plate along the Chilean-type subduction. The change from island submergence to uplift of the Daito-type may reflect the slow trenchward migration of the Daito Islands over a swell in the subducting Philippine Sea Plate during much of the Quaternary. We can assume that the Daito Islands were at least about 220 km, probably more than 300 km, away from the present location when the atolls emerged, judged by the velocity of the plate motion (60 mm/kyr) and the minimum age for Daito Dolostone (ca. 3.6-5.3 Ma). Mariana-type subduction is inferred for the southern part of the Ryukyu Trench (Konishi, 1980; lower diagram of Fig. 11), where a steeply-dipping WadachiBenioff Zone without great earthquakes exists. This type of subduction results in an extension behind the subduction zone and only small amount of uplift. The formation of Hateruma-type sequence of coral reefs may, therefore, be the result of a slow uplift on the overriding plate with a Mariana-type subduction setting. In addition, relative plate motion might be more oblique in the southern part of the Ryukyu Trench, judged by a change in direction of the trench (Fig. 1). This could explain the low uplift rate on Hateruma also. Other islands such as Yonaguni and Okierabu Islands may also belong to a Hateruma-type setting, if we only consider the uplift rate and number of preserved terraces younger than Stage 5e as the criteria. However, it must be recognized that evolution of coral reef terraces on these islands is quite different from Hateruma Island, judged by the presence of many tilted blocks bounded by normal faults on Yonaguni,
bu/ge
Pacific Ocean
Mariana-type subduction zone
w
~ 0
~C ~
W ~1~ rr
~. slow up#tt - r
b~ck-arc I ) a ~ r ¢ ~
__
no bulge P a c i f i c O c e a n
FIG. 11. Schematic diagrams showing the location of the four islands in the two types of subduction zone tectonic settings. Tectonic models are from Ota and Kaizuka (1991), after Uyeda (1982). Solid circles show approximate location of four islands discussed in the text.
and presence of fringing reefs around the central former islands on Yonaguni and Okierabu. In other parts of the Pacific, similar patterns of terraces to those on the Ryukyu and Daito Islands exist. Coral reef terraces on Santo and Malekula of New Hebrides (Jouannic et al., 1980) and those on the Huon Peninsula, Papua New Guinea (Chappell, 1974), are similar to Kikai Island, judged by their high uplift rate, presence of terraces corresponding to each high sea-level stand, multiple Holocene terraces, and the presence of many active faults. However, in contrast to Kikai Island, all the coral reef terraces on the Huon Peninsula are emergent fringing reefs. Therefore, the detailed evolutionary history must be different from Kikai Island. Coral reef terraces on islands of the southern Tonga group illustrate a very small amount of uplift (Taylor and Bloom, 1977), suggesting the similarities with the Daito-type terrace sequence. However, there are no emergent atolls in the Tonga Islands and their tectonic setting, located on the overriding plate of
28
Y. Ota and A. Omura
a Mariana-type subduction setting, is certainly different from Daito Islands setting. These comparisons suggest that many other factors, in addition to uplift rate, control the evolution of coral reef terrace sequences.
CONCLUSION The chronology of Late Quaternary coral reef terraces on Kikai, Hateruma and Yonaguni on the overriding Eurasian Plate and Kita- and Minami-Daito Islands on the subducting Philippine Sea Plate have been established, based on the morpho-stratigraphical study on coral reef terraces and 23°Th/234U ages of corals comprising terraces. Uplift rates since the last interglacial (Isotope Stage 5el are very different from island to island, reflecting different tectonic settings of these islands. Kikai Island (uplift rate 1.8 m/kyr) on the fore-arc rise along a Chilean-type subduction zone compares with 0.3 m/kyr on Hateruma Island, located on the fore-arc rise along a Mariana-type subduction zone. On Kita-Daito and Minami-Daito Islands uplift rates 0.03-0.05 m/kyr occur, and these islands are located on a subduction-related swell in the subducting plate. Uplift rate is the chief controlling factor in the number of terraces preserved, particularly the presence of terraces formed during high sea level stillstands younger than Isotope Stage 5e. Three distinct patterns of coral reef terrace formation and morphology are apparent. The Kikai-type is characterized by many terraces, westward tilting, and active faults disrupting the terrace surfaces. The Hateruma-type has a concentric distribution of a limited number of terraces. Emergence of both types have started in the Late Quaternary as the emergent table reefs of Stage 5e and 7, respectively, and indicate recent active tectonics on the overriding plate. The Daito-type is characterized by very slow uplift but a long geologic record of coral reef development. Non-uniform uplift rate through time is obtained for Kikai and Hateruma Islands. This requires us the further reconsideration of an assumption of uniform rate of uplift during the Late Quaternary which has been used as a fundamental concept for the estimation of paleo sea level height. Judged by non-uniform uplift rate on these two islands, it seems likely that the tectonic processes responsible for uplift, formation and preservation of coral reef terraces has changed through time.
ACKNOWLEDGEMENTS This work was financially supported by Grant-in-aid-for scientific research, provided by the Ministry of Education, Science and Culture, Japan at various stages. This paper is improved by the critical reading by A.L. Bloom, P. Pirazzoli, K.R. Berryman and A.R. Nelson.
REFERENCES Bloom, A.L. (1980). Late Quaternary sea level change on South Pacific coasts: A study in tectonic diversity. In: Morner, N.A. (ed.), Earth Rheology, lsostasy and Eustasy, pp. 505-516. John Wiley and Sons. Cart, M.J., Stoiber, R.E. and Drake, C.L. (1973). Discontinuities in the deep seismic zones and the Japanese arcs. Geological Society of America Bulletin, 84, 2917-2930. Chappell, J. (1974). Geology of coral terraces, Huon Peninsula, New Guinea: a study of Quaternary tectonic movements and sea-level changes. Geological Society of America Bulletin, 85, 553--570. ChappeU, J. and Shackleton, N.J. (1986). Oxygen isotopes and sea level. Nature, 324, 137-140. Furukawa, H. (1991). Formation age of the Ryukyu Arc Okinawa Trough system. Journal of Geography (Chigaku-zasshi), 100, 552564. * Hanzawa, S. (1935). Topography and geology of the Riukiu Islands. Science Report of Tohoku University, 2nd Ser. (Geology), 17, 1-61. Hanzawa, S. (1939). Studies on foraminifera fauna found in the bore cores from the deep well in Kita-Daito-Zima (North Borodino Island). Proceedings of Imperial Academy, Japan, 14, 384-390. Ikeda, S., Kasuya, M. and Ikeya, M. (1991). ESR ages of middle Pleistocene corals from the Ryukyu Islands. Quaternary Research, 36, 61-71. Jouannic, C., Taylor, F.W., Bloom, A . L and Bernat, M. (1980). Late Quaternary uplift history from emerged reef terraces on Santo and Malekula Islands, central New Hebrides island arc. UN ESCAP CCOP/SOPFC Technical Bulletin, 3, 91-108. Kawana, T., Taira, H., Tanahara, A., Aoki, H., Ota, Y., Omura, A. and Koba, M. (1991). 22ORa/238U dates by non-destructive y-ray spectrometry of late Pleistocene corals in Minami- and Kita-Daito Islands, Okinawa, Japan. Journal of Geography (Chigaku-zasshi), 100, 367-377.* Koba, M., Kaigara, T., Ikeya, M., Miki, M., Pirazzoli, P.A., Nakashima, Y., Kan, H. and Tamura, M. (1987a). Marine terraces and their ages of Yonaguni Islands, western extremity of the Ryukyu Arc, Japan. The Earth (Chikyu), 9, 168-176.t Koba, M., Tamura, M., Ikeya, M., Kaigara, T., Nakashima, H. and Kan, H. (1987b). Quaternary shorelines and crustal movements on Minamidaito-jima, northwestern Pacific. In: Qin, Y. and Zhao, S. (eds), Late Quaternary Sea-level Changes, pp. 188-198. China Ocean Press. Koba, M., Tamura, M., Kaigara, T., Ikeda, S., Ota, Y., Omura, A. and Kawana, T. (1991). Electron spin resonance coral ages obtained from a raised atoll, Kita-Daito Island, in the North Philippine Sea. Journal of Geography (Chigaku-zasshi), 100, 351366. * Kodama, K. and Omura, A. (1991). 23°Xh/234U age of the Riukiu Limestone on Yonaguni Island, southwest Ryukyu, Japan. Pro-
gramme and Abstracts of Annual Meeting qf Japan Association for Quaternary Research, pp. 52-53. Kodama, K.. Omura, A. and Suzuki, A. (1990). 23°yhf234U age of the Riukiu Limestone on Yonaguni Island, Okinawa Prefecture.
Abstracts of the 97th Annual Meeting of the Geological Society of Japan, 245 pp.t Konishi, K. (1980). Diverse plate convergence as deduced from raised coral reefs since the last interglacial. The Quaternary Research (Daiyonki-kenkyu), 18, 241-250.* Kuramoto, S. and Konishi, K. (1989). The Southwest Ryukyu Arc is migrating microplate (forearc sliver). Tectonophysics, 163, 7,5--91. Konishi, K., Omura, A. and Nakamichi, O. (1974). Radiometric coral age and sea level records from the late Quaternary reef complexes of the Ryukyu Islands. Proceedings of the 2nd International Coral Reef Symposium, 2, 595-613. Konishi, K., Komura, K. and Motoya, Y. (1978). An early Wisconsin reef on the Daito Ridge, northern Philippine Sea-isotope evidence. Proceedings of the Japanese Academy, 54B, 516-521. Konishi, K., Tsuji, Y., Goto, J., Tanaka, T. and Futakuchi, K. (1983). Multihole shallow drilling of coral reef. Marine Science (Kaiyo-kagaku), 15, 154-164.t Lajoie, K.L. 11986). Coastal tectonics. In: Wallace, R. (ed.), Active Tectonics, pp. 95-124. National Academy Press. Moore, G.M. (1982). Plate tectonic map of the Circum-Pacific region with explanatory notes. American Association of Petroleum Geo-
logy. • Japanese with English abstract. t l n Japanese.
Tectonic Uplift of Coral Reef Terraces Nakata, T., Takahashi, T. and Koba, M. (19781. Holocene emerged coral reefs and sea-level changes in the Ryukyu Islands. Geographical Review of Japan, 51, 87-108.* Omura, A. (1984). Uranium-series age of the Riukiu Limestone on Hateruma Island, southwestern Ryukyus. Transactions and Proceedings of the Palaeontological Society of Japan, New Series, 135, 415-426. Omura, A. (1988). Geologic history of the Kikai Island, central Ryukyus, Japan: summary of uranium-series dating of fossil corals from the Riukiu Limestone. The Memoirs of the Geological Society of Japan, 29, 253-268.* Omura, A., Tsuji, Y., Ohmura, K. and Sakuramoto, Y. (1985). New data on uranium-series ages of hermatypic corals from the Pleistocene limestone on Kikai, Ryukyu Islands. Transactions and
the Proceedings of the Palaeontological Society of Japan, New Series, 139, 196-205. Omura, A., Iwata, H., Ota, Y., Koba, M. and Kawana, T. (1991). z~Th/23aU dates of late Pleistocene corals from Kita- and MinamiDaito Islands, Okinawa, Japan. Journal of Geography (Chigakuzasshi), 100, 337-350.* Omura, K. (1982). Uranium-series age of the "'Kametsu Formation", Riukiu Limestone on the Tokuno-shima, Ryukyu Islands. Trans-
actions and Proceedings of the Palaeontological Society of Japan. New Series, 126, 327-333. Ota, Y. and Hori, N. (198(I). Late Quaternary tectonic movement of the Ryukyu Islands, Japan. The Quaternary Research (Daiyonkikenkyu), 18, 221-24(I.* Ota, Y. and Kaizuka, S. (19911. Tectonic geomorphology at active plate boundaries - - examples from the Pacific rim. Zeitschrift fiir Geomorphologie, N.F., Suppl., 82, 119-146. Ota, Y. and Machida, H. (1987). Quaternary sea-level changes in Japan. In: Tooley, M.J. and Shennan, I. (eds), Sea-level Changes. Institute of British Geography, Special Publication Series, 20, 182224.
29
Ota, Y., Machida, H., Hori, N., Konishi, K. and Omura, A. (1978). Holocene raised coral reefs of Kikai-jima (Ryukyu Islands) - - an approach to Holocene sea level study. Geographical Review of Japan, 51, 109-130.* Ota, Y., Hori, N. and Omura, A. (1982). Age and deformation of marine terraces of Hateruma Island, southwestern Japan. Abstracts of XI INQUA Congress, Moscow, 2, 232. Ota, Y., Omura, A., Koba, M., Kawana, T. and Miyauchi, T. (1991). Late Quaternary tectonic movement as deduced from raised coral reefs of Daito Islands on the northwestern part of Philippine Sea Plate. Journal of Geography (Chigaku-zasshi), 100, 317-336.* Shackleton, N.J. and Opdyke, N.D. (1973). Oxygen isotope and palaeomagnetic stratigraphy of equatorial Pacific core V28-238: oxygen isotope temperature on a 105 and 1(1~' years time scale. Quaternary Research, 3, 39-55. Sugiyama, T. (19341. On the drilling in Kita-Daito Island. Contribu-
tion of Institute of Geology and Paleontology. Tohoku University, 11, 44.t Sugiyama, T. (19361. On the second drilling in Kita-Daito Island.
Contribution of Institute of Geology and Paleontology. Tohoku University, 25, 35.+ Taylor, F.W. and Bloom, A.L. (19771. Coral reefs on tectonic blocks, Tonga Island Arc. Proceedings, Third International Coral Reef Symposium, pp. 275-28l. Uyeda, S. (1982). Subduction zones: an introduction to comparative subductology. Tectonophysics, 81, 133-159. Uyeda, S. and Kanamori, H. (1979). Back-arc opening and the mode of subduction. Journal of Geophysical Research, 84, 1049-1061. Yoshikawa, T., Kaizuka, S. and Ota, Y. (19641. Mode of crustal movement in the late Quaternary on the southeast coast of Shikoku, southwestern Japan. Geographical Review of Japan, 37, 1-22. *
1040-6182/92 $15.00 © 1992 INQUA/Pergamon Press Ltd
C O N T R A S T I N G STYLES AND RATES O F T E C T O N I C U P L I F T O F C O R A L R E E F
TERRACES IN THE RYUKYU AND DAITO ISLANDS, SOUTHWESTERN JAPAN Yoko Ota* and Akio O m u r a ?
*Department of Geography, Yokohama National University, Yokohama, 240, Japan ~Department of Earth Sciences, Kanazawa University, Kanazawa, 920, Japan
Coral reef terraces on Kikai, Hateruma and Yonaguni in the Ryukyu Islands on the Eurasian lithospheric Plate and those of Kita-Daito and Minami-Daito Islands on the Philippine Sea Plate indicate three distinct patterns of coral reef terrace formation and preservation, primarily reflecting different uplift rates. The Kikai-type, which includes many terraces from Oxygen Isotope Stages 5e to 1, is a result of high uplift rate of ca. 1.8 m/kyr on the fore-arc rise of a Chilean-type subduction zone. Arcward (westward) tilting and faulting are also characteristic of the Kikai-type. The Hateruma-type is characterized by two coral reef terraces younger than the Stage 5e terrace and they are arranged in a concentric annular pattern. Slow uplift rate of ca. 0.3 m/kyr, adjacent to a Mariana-type subduction margin is typical of the Hateruma-type. The emergence of Kikai and Hateruma Islands is recent, younger than Isotope Stage 5e and 7, respectively, reflecting the active deformations of the leading edge at the overriding Eurasian Plate. In the Daito-type, atolls surrounded by fringing reefs have been uplifted very slowly (less than 0.05 m/kyr), but over a time period of at least several million years. Slow uplift and long history of Daito-type islands is the result of up-bulging of the Philippine Sea Plate as it migrates toward the Ryukyu Trench.
INTRODUCTION The Ryukyu Arc-Okinawa Trough system is one of the active arc-backarc systems along the margin of the western Pacific. The Ryukyu Trench marks the plate boundary between the overriding Eurasian Plate to the west and the subducting Philippine Sea Plate to the east (Fig. 1). The Wadachi-Benioff Zone at this boundary dips more gently (ca. 25°-35 °) in the northern part of the plate boundary, than the southern part, where the dip is ca. 55°-65 ° (Konishi, 1980; Carr et al., 1973). No historical large earthquakes accompanied by coastal uplift are known in this active subduction zone, however. Okinawa Trough is interpreted as an actively rifting backarc basin (Furukawa, 1991). The islands in this subduction zone setting, located between ca. latitude 24°N. and 29°N., are divided into the Ryukyu Islands which lie on the Eurasian Plate, and the Daito Islands which lie on the Philippine Sea Plate (Fig. 1). Well developed coral reef terraces, which record the former sea-level position, are found on these islands. But the islands on opposite sides of the trench have experienced different tectonic histories, which are reflected in different uplift histories recorded by coral reef terraces fringing the islands. Differential movements are also recorded on some of the Ryukyu Islands, depending on the variation in the detailed tectonic setting of each island. Many geological and geomorphological studies have been carried out on the coral reef terraces on the Ryukyu and Daito Islands, since the pioneer work by Hanzawa (1935) who used the name Riukiu Limestone for the reef limestone composing the coral reef terraces. However, many different ideas and chronologies have been proposed for the coral reef terraces, because of the difficulties in radiometric dating of the 17
coral limestone due to recrystallization, and problems of terrace correlation due to localized tectonic deformation. Middle to Late Quaternary terrace chronology based on radiometric dating has been established for only a few islands of the Ryukyu and Daito Islands (e.g. Kikai, Hateruma and Yonaguni islands on the Eurasian Plate and Kita- and Minami-Daito islands on the Philippine Sea Plate). Interpretation on the different uplift on some of the above-mentioned islands was proposed by Konishi (1980). However, the number of dated samples was limited and the relation between the dated samples and morpho-stratigraphy of the terrace sequences were not fully examined at that time. In this paper, we describe geologic and geomorphic features of these islands, based on detailed morphostratigraphic studies of coral reef terraces and the alpha spectrometric 23°Th/234U dating (ages obtained by this method will be simply described as 23°Th/234U age in the following sentences) of coral limestones. We also compare the different tectonic histories deduced from coral reef studies on different islands, and from that, we propose three different patterns of coral reef terraces to explain their variable evolution in different tectonic settings. CORAL REEF TERRACES ON THE EURASIAN PLATE Most islands on the Eurasian Plate are fringed by a flight of coral reef terraces, suggesting continued uplift of the islands. The highest parts of some islands, such as Kikai and Hateruma, are overlain by Pleistocene reef sediments, that rest on Neogene sedimentary rocks and form a flat, table-like surface. The table reefs suggest that uplift of these islands started only recently. However, on other islands underlain by older rocks,
18
Y. Ota and A. Omura
FIG. 1. Study area and tectonic setting of the Ryukyu Island Arc region. Inset shows the plate boundaries and ocean-floor ridges (coarse dotted pattern). Solid line with triangles denotes active subduction zones. Large numbered arrow shows orientation and rate (mm/yr) of relative plate motion. Broken line with open arrows indicate a back-arc spreading centre (Moore, 1982). RT, Ryukyu Trench; KPR, Kyushu-Palao Ridge; AP, Amami Plateau; DR, Daito Ridge; ODR, Oki-Daito Ridge. Isobaths (500 m interval) are from the Chart No. 6302, published by the Maritime Safety Agency of Japan. Labelled islands are solid: K, Kikai Is.; T, Toku Is.; OE, Okierabu Is.; OK, Okinawa Is.; H. Hateruma Is.; Y, Yonaguni Is.; KD, Kita-Daito Is.; MD, MinamiDaito Is.; OD, Oki-Daito Is.: OT, Okinawa Trough. such as Paleozoic or Tertiary volcanic or clastic rocks (e.g. Toku, Okierabu, Okinawa and Yonaguni), fringing reefs surround the higher parts of the islands. K i k a i Island
Kikai Island is the closest of the Ryukyu Islands to the Ryukyu Trench (about 80 km, Fig. 1) and is the only place throughout Japan, where all the high stillstands of sea-level younger than the last interglacial m a x i m u m are preserved. Konishi et al. (1974) reported the first U-series ages (23°Th/234U and 234U/238Uages) of fossil corals from Kikai: ages of ca. 120-130 ka, 80100 ka, 55-65 ka and 35-45 ka correspond roughly to Oxygen-Isotope Stages 5e, 5c, 5a and 3 (Shackleton and Opdyke, 1973), respectively (Fig. 2). The highest point on the island at 224 m elevation is underlain by the reef limestone of Isotope Stage 5e (weighted average 23°Th/234U age is 129+2 ka). Coral limestones of Middle Pleistocene age are reported from the southern part of Kikai by O m u r a et al. (1985), that are ca. 200 ka by the 23°Th/234U method and 400-500 ka by the Electron Spin Resonance (ESR) method. Recently, older ages of ca. 570-840 ka are also reported by the ESR method (Ikeda et al., 1991). However, these older reef complexes are not exposed on the Pleistocene terrace surfaces, and are almost entirely overlain by upper Pleistocene units (Fig. 3). On Kikai, the uplifted reef complexes are not distributed in a concentric annular pattern, because
they have been complicately deformed by faulting associated with uplift of the island. This deformation makes it difficult to identify and correlate the terraces, and to reconstruct their tectonic history. Even on this small island it is difficult to m a p the exact distribution of all the terraces. We interpret the relationship between terrace surfaces and coral reef complexes as shown in a schematic diagram (Fig. 3). This interpretation is based on the radiometric age determinations, observation of terrace morphology and coral limestone facies. Late Pleistocene corals are usually thin and overlie an eroded surface in thicker Middle Pleistocene coral limestone. Coral limestone of Stage 3 rests unconformably on an eroded surface cut into coral limestone of Stage 5a. A flat terrace-like surface at about 70-80 m in elevation is an erosion surface cut into the Middle Pleistocene units. Kikai is characterized by very active tectonic movement. Boundaries of terraces of different ages usually coincide with active faults, some of which also displace a single terrace. Slips on these faults have apparently occurred several times on Kikai after the emergence of the ca. 120 ka coral reefs. Coral reefs younger than Stage 5e have developed on downthrown blocks (Fig. 3). Two faults, Tobiyozaki Fault to the northeast and Araki Fault to the southwest of the island, displace Holocene terraces as well as the ca. 60 ka and the ca. 40 ka terraces. The altitude of coral reef terrace surfaces
Tectonic Uplift of Coral Reef Terraces 129"55'
19
130"E
E
TOBIYO-ZAKI
'~°ThP=U ag e (ka) o
400-550
o 200 • 120 •
100
•
80 60
[]
28"20'N
40
- -
i~i !~,i iiiiiii B
0
1
I
i
2km ....
,
~ ~Reef --
,
!28o15,N
~ ~*" ~
Pleistocene terrace - 7
_
;
60-68ka
II 3.5-5.2 III 3.0-3.5
facies of coral limestone
-I- ~ j Terrace riser between Pleistocene ~' and Holocene coral terrace [ ~
IV 1.5-2.5 Fault
FIG. 2. Map of Kikai Island showing Holocene and Pleistocene coral reef terraces and the locations and approximate ages (seven types of symbols) of representative coral samples from terraces (compiledmainlyfrom Konishiet al., 1974;Omura, 1988; Ota et al., 1978). A-B shows location of profile in Fig. 3.
indicates overall a westerly tilt of the island, away from the trench axis (Ota et al., 1978). Kikai is the only island in the Ryukyu Islands that is fringed by Holocene terraces. These emerged Holocene coral reef terraces reach 13 m in altitude. Four distinctive terraces suggest repeated episodic, probably coseismic, uplifts since the culmination of the postglacial transgression (Figs 2 and 3, Ota et al., 1978 and Nakata et al., 1978). Radiocarbon ages of these terraces from highest to lowest are ca. 6.8 to 6.0, 5.2 to 3.5, 3.5 to 3.0 and 2.5 to 1.5 ka BP. Logs of drillholes through the Holocene coral reefs show that the Holocene reef began forming by ca. 8 ka BP (Konishi et al., 1983). H a t e r u m a Island
Hateruma Island is located at the southwestern end of Ryukyu Islands, about 100 km north of the Ryukyu Trench (Fig. 1). A concentric pattern of coral reef terraces has developed on this island. A Middle and Late Quaternary chronology based on a morphostratigraphic study of the six terraces by Ota and Hori (1980) was confirmed with 23°Th/234Udating of terrace coral limestone by Ota et al. (1982) and Omura (1984).
The coral reef terraces I, III, IV and V correspond to Oxygen-Isotope Stages 7, 5e, 5c and 5a, respectively (Figs 4 and 5). The uppermost terrace 1 has a maximum elevation of 59.5 m, and the most extensive terrace III has a maximum elevation of 41 m. Both terraces are constructional and are underlain by thick transgressive coral reef limestone, that rests unconformably on an irregular, upper Pliocene, bedrock surface. These terraces, I and III, were formed during the penultimate and last interglacials, respectively. Terraces IV and V (Isotope Stages 5c and 5a) are underlain by only very thin coral reefs on erosion surfaces cut into older reef limestones. There is a small but progressive westward tilt of the coral reef terraces as indicated by the altitude of the terraces at their inner margins (Fig. 4). Terraces of ca. 60 ka, 40 ka, and of Holocene age like those on Kikai Island, are not present on Hateruma. Y o n a g u n i Island
Yonaguni Island is located at the western end of the Ryukyu Islands Arc, and is close to the southern margin of Okinawa Trough. Thus Yonaguni Island has a somewhat different tectonic setting from Kikai and
20
Y. Ota and A. Omura
[
,,
OCCURRENCEOF DATEDCORALS ISOTOPE STAGE
AVERAGE AGE (ka)
, , ;
~
E- : 2:]Z:
,,|,~
2::~:2
2Z~2-3~"
:
.l
2 : . - :. .: ~ I R ~ ] ZZ ~
ISOTOPE STAGE
5c 10~jd~ - - -~. - ~
~ -
Coral limestone of Stage 5e
A 2O0
>¢~
B
5
Olderthanstage7 ~
f
~100
Stage3 Sanddune Stage3
6
'
Distance
'
~k~
FIG. 3. Geologic section and topographic profile of southern part of Kikai Island along line A-B in Fig. 2. Locations of dated coral samples are shown. Data sources are as in Fig. 2, Horizontal boxes with light shading show the extent of coral limestone, corresponding to each isotope stage. Vertical lines with arrows are faults that displace coral reef terraces. Dip of fault planes are unknown.
g T-O >200 /i 120 100 ~_ 80
0~ ~ ~ ~ ~ ~
I II III
~ ~ ~
IV V VI
• 12 Altitude of inner margin of terrace (m) Fault . . . . I'N -
~'03'-
L
123'46'
123"48'E
FIG. 4. Map of coral reef terraces of Hateruma Island. Altitude of inner margin of each terrace (m), faults, and localities and ages (four types of symbols) of dated corals are shown (compiled from Ota and Hori, 1980; Omura, 1984; revised from Ota and Machida, 1987). A-B shows location of profile in Fig. 5.
Hateruma islands (Fig. 1). The island has been complexly deformed through subduction of Philippine Sea Plate and backarc spreading in the adjacent Okinawa Trough. The tilted blocks of coral reef terraces (Fig. 6) are the outstanding morphologic features of this island (e.g. Ota and Hori, 1980; Koba et al., 1987; Kuramoto and Konishi, 1989). Four distinct coral reef terraces fringe the island, and rest on a bedrock of Tertiary age (Fig. 6). Based on
ages, Terrace IV (ca. 30 m) is correlated with the last interglacial maximum of Isotope Stage 5e (Kodama et al., 1990). Terrace III is correlated with the penultimate interglacial, and occurs on the northwestern part of the island at a maximum elevation of 54 m. Two younger radiometric ages, correlative with Stage 5c, occur at one site on Yonaguni (Kodama and Omura, 1991). No radiometric ages are available from the higher terraces I and II. 23°Th/234U
21
Tectonic Uplift of Coral Reef Terraces A
OCCURRENCE OF DATED CORALS
B
ISOTOPE AVERAGE STAGE AGE (ka)
2
0
4
6 km
FIG. 5. Geologic section and topographic profile of coral reef terraces of Hatcruma Island along line A-B in Fig. 4 and locations of dated corals. Boxes with light shading show the extent of coral limestone corresponding to each isotope stage. Data sources are as in Fig. 4.
i
i
i
i
122 °58'
123°02'E
-24°2
24°2_
[ ~
I
~
Holocene lowland
II
[ ~
Hills, slope and spot height (m)
o
200
•
120
[]
100
o I"~ ~
III
~
Former shoreline
IV
~
Fault scarp
23°Th/'~U age (ka)
0 L
2km i
Prominent sea cliff FIG. 6. Map showing coral reef terraces, representative locations and ages (three types of symbols) of dated samples and active faults on Yonaguni Island. Coral ages are from Kodama et al. (1990) and Kodama and Omura (1991).
CORAL REEFS OF THE DAITO ISLANDS ON THE PHILIPPINE SEA PLATE
Kita-Daito and Minami-Daito on the Daito Ridge (Fig. 1) are two of only a few uplifted atolls (Figs 7 and 8) in the world, and have attracted attention because of their unique tectonic histories. Vertical tectonic movement of these islands has changed from subsidence, during their formation as atolls, to uplift, which resulted in emergence of the atolls, followed by the formation of fringing reef terraces around the atolls. A 431 m deep drillhole in the lagoon of the Kita-Daito raised atoll did not reach basement (Sugiyama, 1934, 1936), indicating subsidence since at least the early Miocene (Hanzawa, 1939). The first radiometric ages of Pleistocene corals from the Daito Islands were obtained from Minami-Daito by Konishi et al. 41978) and Konishi (1980), who reported
the last interglacial corals of ca. 98-123 ka by the Z26Ra/ 23SU method for corals at less than 11 m elevation. Koba et al. (1987b) also reported the last interglacial ages for corals on Kita-Daito Island using the ESR method. Recently, detailed geomorphic and geologic observation of small fringing reefs that exist sporadically along the coast of both islands, and radiometric age determinations of many coral specimens by the 23°Th/Z34u, ESR, and non-destructive 226Ra/238U methods have been carried out (Ota et al., 1991; Omura et al., 1981; Koba et al., 1991; Kawana et al., 1991). We use only the 23°Th/234U ages in the following discussion because all samples have been dated by this method, and there is good agreement (at the one standard deviation level) between the results by this method and those obtained by other methods. Scattered autochthonous fossil corals occur on coastal benches truncating the older calcareous sedi-
22
Y. Ota and A. Omura 131J17'E
131"18'E
131'~19'E
131120'E
-25"57'N
A -
-
1 - - B
25 56 N
0 I
I
,
] kr
f
I
FIG. 7. Contour map of Kita-baito Island, an example of a raised atoll. Contours (interval, 10 m with supplementary 5 m
contours shown in broken line) are from 1/25,000 topographic map 'Kita-Daito-jima' published by the Geographical Institute of Japan. Solid circle is the borehole site described by Sugiyama (1934, 1936). A-B and C-D (N-l) show location of profiles in Fig. 8.
A
KITA-DAITO ISLAND
5 r ~ t ~
0
m 7
B
~
Borehole
2
4
C N-1
D
_ _
~
6
NW
8km
S-3
SE /
m
10
115+6ka
133:~6 ka
6
~"~
5
KITA-DAITO 4
,
0
-. . . .
L'4~ ',' ,'' ',' ',' : ' ',' ',' ', -''~" i.:i:i:.i:i:
10m
, ~, , ~ ' 30m
Daito Dolostone , MINAM!-DAITO
0
0
FIG. 8. Geologic sections and topographic profiles of Daito Islands. Upper; Kita-Daito Island along line A-B in Fig. 7. The profile is drawn based on the 1/5,000 scale map published by Kita-Daito Village. Lower; Profile showing the occurrence of autochthonous corals (fine dot pattern) and their 23°Th/234U ages (ka) (sample locations shown in solid circles). Left; N-1 on Kita-Daito Island along line C-D in Fig. 7. Right; profile at S-3, in north-western part of Minami-Daito Island (after Ota et al., 1991). Note that autochthonous corals only sporadically occur on Daito Islands and that all dated corals correlate with Isotope Stage 5e.
m e n t s c o m p o s i n g atolls a n d l a g o o n s ( D a i t o D o l o s t o n e ) at s e v e r a l localities on K i t a - D a i t o a n d M i n a m i - D a i t o , at the e l e v a t i o n s u p to ca. 8 m a n d 11 m, r e s p e c t i v e l y (Fig. 8). C o r a l ages c l u s t e r in t h e r a n g e ca. 115-130 ka ( w e i g h t e d a v e r a g e age is 123_+ 1 k a ) a n d a r e c o r r e l a t e d with I s o t o p e S t a g e 5e. T h e e l e v a t i o n o f t h e last i n t e r g l a c i a l s h o r e l i n e o n t h e s e islands is e s t i m a t e d to b e ca. 10-12 m a b o v e p r e s e n t s e a level, j u d g e d by the
a l t i t u d e o f n o t c h e s a n d b e n c h e s a s s o c i a t e d with the d a t e d corals. N o e m e r g e d fringing reefs y o u n g e r t h a n I s o t o p e S t a g e 5e a n d no H o l o c e n e coral reefs are f o u n d a b o v e sea level. N o 23°Th/234U a g e s a r e o b t a i n e d f r o m the atoll l i m e s t o n e o r o l d e r fringing reefs b e c a u s e the c a l c a r e o u s sediments (Daito Dolostone) are entirely dolomitized. T h e r e f o r e , w e can n o t d e t e r m i n e the history o f e m e r -
Tectonic Uplift of Coral Reef Terraces
23
gence after the initial period of subsidence on Kita- and Minami-Daito. Recently, three coral samples from the crest of atoll (45 m in elevation) on Kita-Daito were dated at ca. 3.6-5.3 Ma by the ESR method (Koba et al., 1991), indicating that the emergence of the atolls began sometime before the early Pliocene, because ESR ages give only the minimum, representing the age of recrystallization of coral limestone. Thus, emergence of the Daito Islands began much earlier than the emergence of the other islands on the Eurasian Plate.
characterized by the low average uplift rate less than 0.2 m/kyr. The average uplift rate is also low (0,3 m/ kyr) in Hateruma, however, which is close (ca. 100 kin) to the Ryukyu Trench. A marked contrast of the uplift rate between Kikai and Hateruma suggests that there is a difference in subduction style at the northern and southern part of the Ryukyu Trench.
LATE QUATERNARY UPLIFT RATES AND CORAL REEF T E R R A C E SEQUENCES
On Kikai and Hateruma Islands sufficient data exists to examine uplift histories and whether the uplift rate has been uniform through time. Both Kikai and Hateruma have a short geological history having been uplifted above sea level after the Isotope Stage 5e and 7, respectively (Table 3, Fig. 9). This is much more recently than not only on Daito Islands but also on other Ryukyu Islands (e.g. Yonaguni and Okierabu Islands), which have been emergent since the Early Pleistocene or Pliocene. This suggests the islands located on the leading edge of overriding plate have a shorter but more intense tectonic history than islands located far from the trench and on the margin of backarc basin or on the outer swell of subducting plate. The expected elevations of terraces on Kikai and Hateruma (Table 2) can be calculated from the average long term uplift rate since Stage 5e and the eustatic correction as defined by Chappell and Shackleton (1986). There are marked discrepancies between the expected elevations and those observed, and imply uplift rates have varied considerably with time, if we accept that eustatic sea level curves of Chappell and Shackleton (1986) are applicable to the Ryukyu Islands. For example, discrepancies at ca. 100 ka and 80 ka on Kikai Island (+24 m and +60 m, respectively) are particularly marked. This suggests that, in addition to long-term variation in tectonic activity, differential movements with major faulting took place in association with the emergence of these terraces, resulting in variable uplift rates with time. In contrast, the differences between calculated and observed elevations are not so large on Hateruma (Table 2), where only minor faulting has occurred during the Late Quaternary. There is an increase in the average uplift rate toward the present both on Kikai and Hateruma, although the increase is not consistent on Kikai (Table 2). This may suggest a change in convergence rate between the Eurasian and Philippine Sea plates, as in the case of increased uplift rates deduced from coral reef terraces on Santo and Malekula Islands, New Hebrides Island Arc (Jouannic et al., 1980).
The formation of a flight of marine terraces is considered to be the combined result of eustatic sealevel fluctuation and tectonic uplift (e.g. Yoshikawa et al., 1964; Bloom, 1980; Lajoie, 1986 for full discussion). Rapid uplift is essential for the formation of many separate terraces, formed during successive high sea level stillstands, corresponding to interglacial or interstadial episodes. The last interglacial coral reef terrace, correlated with Isotope Stage 5e, has been identified on each of the five islands described in this paper (Table 1, Fig. 9). Assuming a constant rate of uplift since the last interglacial maximum, and that sea level at that time was ca. 6 m higher than present (e.g. Chappell and Shackleton, 1986), then the average uplift rate during the last ca. 120 ka is estimated to be 1.8 m/kyr on Kikai, 0.3 m/kyr on Hateruma, 0.2 m/kyr on Yonaguni, 0.03 m/kyr on Kita-Daito and 0.05 m/kyr on Minami-Daito (Table 1). Many terraces are preserved on Kikai Island and are related to Isotope Stages 5e to 3, as well as multiple Holocene terraces of Stage 1. These data provide a good example of where rapid uplift results in multiple terrace formation. In contrast, slow uplift results in a limited number of terraces, for example, on Hateruma Island, where only two terraces younger than Stage 5e are observed. Similarly on Yonaguni, only two samples have been found to correlate corals correlated with Stage 5c. The last interglacial terrace is the youngest terrace on Okierabu, away from the trench (Fig. 1) where the average uplift rate is 0.1 m/kyr (Omura, unpub.). No terrace younger than 5e has been found on the Daito Islands, which have very low average uplift rates. Thus, considerable differences exist in the average uplift rate between the Ryukyu Islands on the overriding and uplifting Eurasian Plate and the Daito Islands on the subducting Philippine Sea Plate. Within the Ryukyu Islands, Kikai, which is the closest to the Ryukyu Trench, has the highest uplift rate and may be regarded as a typical island of a fore-arc rise tectonic setting. This rate (1.8 m/kyr), is the highest rate measured during the Late Quaternary in the coastal areas of Japanese Islands. In contrast, the islands away from the trench (Okierabu) or close to the backarc spreading in the Okinawa Trough (Yonaguni) are
DIFFERENCES IN UPLIFT HISTORIES IN THE KIKAI AND H A T E R U M A ISLANDS
CORAL REEF T E R R A C E D E V E L O P M E N T IN DIFFERENT TECTONIC SETTINGS
Three Patterns o f Coral Reef Terrace Evolution As noted above, different island groups have different
24
Y. Ota and A. Omura
r.~
",~0
000
0
0
O
E ¢-q
+1
0
O
¢J
o +1
+1
+1
+1 +1
.=.
O
E +1 .=.
,2o
O D.. ,.1
¢m
.<
.<
O
Tectonic Uplift of Coral Reef Terraces
1
5
~--
~
p
Isotope stage
r
A g e (ka) 50
10
I r llll
I
r
i
100
I I lllr
1
3
25
........
1-
5 a ¢__e
7
I
500 r F r iq-r-
\
Island i
i
111
Kikai
r- % - - ]
Toku
'l
Okierabu /
Hateruma
Method of dating /
14C ages
Yonaguni
23°Th/234Uages 234U/238U ages
Kita-Daito
ESR ages
Minami-Daito J_
k
I
i IliJJl
I
i
J J IJllJ
I
J
J_ J I Jii
FIG. 9. S u m m a r y of the radiometric ages of fossil corals from the Ryukyu and Daito Islands. Time scale is logarithmic. Numerals in the boxes show n u m b e r of dated Pleistocene coral samples. Radiometric data are from Konishi et al. (1974), O m u r a (1982, 1984, 1988), O m u r a et al. (1985, 1991), Ota et aL (1978, 1991) and K o d a m a et al. (1990). Older ESR ages by Ikeda et al. (1991) are not shown in this figure. Width of age bars shows range between the oldest and youngest ages. A g e bar for one sample of Stage 7 on Kikai represents one standard deviation in the age. O p e n arrows and vertical dashed lines show the correlation with isotope stages.
T A B L E 2. Shoreline altitudes and a m o u n t of uplift on Kikai and H a t e r u m a Islands since the last interglacial m a x i m u m
A p p r o x i m a t e age of high sea-level stand (ka) 120-13(1 100 80 60 40
Present highest altitude of former shoreline* (m) Kikai Hateruma > 224 195 185 70 50
41 30 23 ---
Paleo sea-level heightt (m) +6 -9 -19 -30 -41
Apparent amount and rate of uplifts (m) Kikai Hateruma 218 204 204 100 91
(1.7) (2.0) (2.6) (1.7) (2.3)
35 (0.3) 39 (0.4) 42 (0.5) ---
Expected shoreline altitude assuming constant uplift since 120 ka (m) Kikai Hateruma . 171 125 78 31
.
. 19 4 ---
Difference between expected altitude and present altitude (m) Kikai Hateruma . +24 +60 -8 + 19
+l 1 +19 ---
*Height errors are within _+ 2 m. t H e i g h t from Chappell and Shackleton (1986). ~:Numbers in parentheses denote apparent rate of uplift (ndkyr).
patterns of coral reef terraces which appear to be related to their tectonic setting. We propose three distinct patterns for the development of sequences of coral reef terraces, based mainly on variation in uplift rates (Fig. 10, Table 3). The Kikai-type is characterized by rapid emergence and formation of terraces corresponding to each of the high stands of sea-level during and since Stage 5e, and the formation of multiple Holocene terraces. The highest terrace, Stage 5e, comprises a table reef, but other younger terraces are a succession of fringing reefs. The terrace distribution pattern does not show a concentric annular pattern, however, because of faulting, which has caused substantial dislocation of the
terrace sequences during the Late Quaternary. Progressive westward tilting away from the trench axis is also characteristic in this island. The Hateruma-type has an annular arrangement of coral reef terraces on a slowly uplifting island. A series of emergent fringing reefs surrounds the central uplifted table reef. Due to slight, but progressive westward tilting, coral reef terraces are elliptical in plan view with long axes oriented E-W. Faulting is not significant here and the number of terraces younger than Stage 5e is limited. The Daito-type is a unique type of emergent atoll, being surrounded by a flight of fringing reefs. It has a long history of uplift, but the rate has been very low,
26
Y. Ota and A. Omura
MAP VIEW
CROSS-SECTION
TR
KIKAI-Type
J
m
5c~
~00
5a
~~.
. .
;~ 13,/
9
Holocene terraces
/"
!
00
erosional terrac a
~0
5
~
HATERUMA-Type ~ ~ m 50 FR .,,,--.-,:'~,;:;:; :',:1:: :;:;:,'5e 5cFR 0 DAITO-Type 5e
5e
;km
5e__~ ........_L L 6km
i
R:Table reef
'
........
2
5e
6 °
6
stage 1
~
Tertiary siltstone
R:Fringing reef
Q) ~ tO ~ ~
stage 3,5a.5c
~
Fault
r:Atoll
"-- ~
stage 5e
~
Boundary of coral reef terraces
:Lagoon
~ ~ stage 7 and older O 0 i::::::::7: Daito Dolostone
~) E
FIG. 10. Schematic diagrams showing three patterns of coral reef terraces that reflect differing styles and rates of vertical deformation. See Table 3 for details.
T A B L E 3. Comparison of coral reef terraces on islands of the Eurasian and Philippine Sea Plates
Plate
Island Kikai Hateruma Distance from the trench (km) 80 100 Age of the oldest reef (ka) 570-840* 210 Age of the oldest reef terrace (ka) 120 210 Altitude of terrace related to the last interglacial maximum (m) > 224t 41~ Average uplift rate since the last interglacial maximum (m/kyr) 1.8 0.3 Altitude of Hoiocene terraces (m) 13§ -Estimated age of emergence of island (ka) < 120 < 210 Evolutional history of reef terraces table reef--*fringing reef table reef---~fringing reef Style of uplift
Subducting Philippine Sea Plate
Overriding Eurasian Plate
rapid uplift with faulting slow uplift with small and westerly tilting amount of westerly tilting
*Ikeda et al. (1991). tHighest altitude of the table reef terrace. :~Highest altitude of the inner margin of fringing reef terrace. §Subdivided into four steps. [IKoba et al. (1991).
Yonaguni 130 > 350 > 350
Kita- and Minami-Daito 150 > 4,600tl unknown
30~:
10-12
0.2 --
0.03-0.05 --
> > 350
> 4,6001]
fringing reef
atoll---~fringing reef
slow uplift with block movement
very slow uplift
27
Tectonic Uplift of Coral Reef Terraces
only less than 0.05 m/kyr since the last interglacial maximum, and no tilting or faulting has occurred. The youngest of the emergent fringing reefs belongs to Isotope Stage 5e.
Chilean-type subduction zone o
I
~
.c
Discussion
We relate the tectonic setting of these three types to the models of subduction proposed by Uyeda and Kanamori (1979) and Uyeda (1982); that are the Chilean-type and Mariana-type subductions. Konishi (1980) interpreted the different uplift rates of Kikai, Hateruma and Daito islands in terms of these models. Generally we agree with this interpretation but we refine and extend it, based on newly obtained data on morpho-stratigraphy and uplift rates. Data from Yonaguni and Okierabu Islands which were not included in Konishi's interpretation, are also considered in our discussion. The upper diagram of Fig. 11 is of Chilean-type subduction, which is characterized by the presence of a shallow dipping Wadachi-Benioff Zone and associated strong compression as well as great earthquakes. This type of subduction is considered representative of the northern part of the Ryukyu Trench (Carr et al., 1973; Konishi, 1980). The Kikai-type is interpreted to result from recent and intense tectonic activity on the fore-arc rise of the overriding plate along the Chilean-type subduction. The change from island submergence to uplift of the Daito-type may reflect the slow trenchward migration of the Daito Islands over a swell in the subducting Philippine Sea Plate during much of the Quaternary. We can assume that the Daito Islands were at least about 220 km, probably more than 300 km, away from the present location when the atolls emerged, judged by the velocity of the plate motion (60 mm/kyr) and the minimum age for Daito Dolostone (ca. 3.6-5.3 Ma). Mariana-type subduction is inferred for the southern part of the Ryukyu Trench (Konishi, 1980; lower diagram of Fig. 11), where a steeply-dipping WadachiBenioff Zone without great earthquakes exists. This type of subduction results in an extension behind the subduction zone and only small amount of uplift. The formation of Hateruma-type sequence of coral reefs may, therefore, be the result of a slow uplift on the overriding plate with a Mariana-type subduction setting. In addition, relative plate motion might be more oblique in the southern part of the Ryukyu Trench, judged by a change in direction of the trench (Fig. 1). This could explain the low uplift rate on Hateruma also. Other islands such as Yonaguni and Okierabu Islands may also belong to a Hateruma-type setting, if we only consider the uplift rate and number of preserved terraces younger than Stage 5e as the criteria. However, it must be recognized that evolution of coral reef terraces on these islands is quite different from Hateruma Island, judged by the presence of many tilted blocks bounded by normal faults on Yonaguni,
bu/ge
Pacific Ocean
Mariana-type subduction zone
w
~ 0
~C ~
W ~1~ rr
~. slow up#tt - r
b~ck-arc I ) a ~ r ¢ ~
__
no bulge P a c i f i c O c e a n
FIG. 11. Schematic diagrams showing the location of the four islands in the two types of subduction zone tectonic settings. Tectonic models are from Ota and Kaizuka (1991), after Uyeda (1982). Solid circles show approximate location of four islands discussed in the text.
and presence of fringing reefs around the central former islands on Yonaguni and Okierabu. In other parts of the Pacific, similar patterns of terraces to those on the Ryukyu and Daito Islands exist. Coral reef terraces on Santo and Malekula of New Hebrides (Jouannic et al., 1980) and those on the Huon Peninsula, Papua New Guinea (Chappell, 1974), are similar to Kikai Island, judged by their high uplift rate, presence of terraces corresponding to each high sea-level stand, multiple Holocene terraces, and the presence of many active faults. However, in contrast to Kikai Island, all the coral reef terraces on the Huon Peninsula are emergent fringing reefs. Therefore, the detailed evolutionary history must be different from Kikai Island. Coral reef terraces on islands of the southern Tonga group illustrate a very small amount of uplift (Taylor and Bloom, 1977), suggesting the similarities with the Daito-type terrace sequence. However, there are no emergent atolls in the Tonga Islands and their tectonic setting, located on the overriding plate of
28
Y. Ota and A. Omura
a Mariana-type subduction setting, is certainly different from Daito Islands setting. These comparisons suggest that many other factors, in addition to uplift rate, control the evolution of coral reef terrace sequences.
CONCLUSION The chronology of Late Quaternary coral reef terraces on Kikai, Hateruma and Yonaguni on the overriding Eurasian Plate and Kita- and Minami-Daito Islands on the subducting Philippine Sea Plate have been established, based on the morpho-stratigraphical study on coral reef terraces and 23°Th/234U ages of corals comprising terraces. Uplift rates since the last interglacial (Isotope Stage 5el are very different from island to island, reflecting different tectonic settings of these islands. Kikai Island (uplift rate 1.8 m/kyr) on the fore-arc rise along a Chilean-type subduction zone compares with 0.3 m/kyr on Hateruma Island, located on the fore-arc rise along a Mariana-type subduction zone. On Kita-Daito and Minami-Daito Islands uplift rates 0.03-0.05 m/kyr occur, and these islands are located on a subduction-related swell in the subducting plate. Uplift rate is the chief controlling factor in the number of terraces preserved, particularly the presence of terraces formed during high sea level stillstands younger than Isotope Stage 5e. Three distinct patterns of coral reef terrace formation and morphology are apparent. The Kikai-type is characterized by many terraces, westward tilting, and active faults disrupting the terrace surfaces. The Hateruma-type has a concentric distribution of a limited number of terraces. Emergence of both types have started in the Late Quaternary as the emergent table reefs of Stage 5e and 7, respectively, and indicate recent active tectonics on the overriding plate. The Daito-type is characterized by very slow uplift but a long geologic record of coral reef development. Non-uniform uplift rate through time is obtained for Kikai and Hateruma Islands. This requires us the further reconsideration of an assumption of uniform rate of uplift during the Late Quaternary which has been used as a fundamental concept for the estimation of paleo sea level height. Judged by non-uniform uplift rate on these two islands, it seems likely that the tectonic processes responsible for uplift, formation and preservation of coral reef terraces has changed through time.
ACKNOWLEDGEMENTS This work was financially supported by Grant-in-aid-for scientific research, provided by the Ministry of Education, Science and Culture, Japan at various stages. This paper is improved by the critical reading by A.L. Bloom, P. Pirazzoli, K.R. Berryman and A.R. Nelson.
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