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Tectonic significance of Holocene marine terraces in the Coastal Range, Eastern Taiwan
121
Tectonophysics, 183 (1990) 121-127 Elsevier Science Publishers B.V., Amsterdam
Tectonic significance of Holocene marine terraces in the Coastal Range, eastern Taiwan Ping-Mei Liew, Moon-Lung Hsieh and Cheng-Kuo Lai Institute of Geology, National Taiwan University, Taipei (Taiwan, China) (Received April 10.1989;
revised and accepted October 6.1989)
ABSTRACT Liew, Ping-mei, Hsieh, Moon-lung and Lai, Cheng-kuo, 1990. Tectonic significance of Holocene marine terraces in the Coastal Range, eastern Taiwan. In: J. Angelier (Editor), Geodynamic Evolution of the Eastern Eurasian Margin. Tectonophysics, 183:121-127.
Well-developed marine terraces on the coast of eastern Taiwan between Chengkung and Hualien can be divided into three morphotectonic sectors: (I) Chengkung-Kueian, (II) Shuilien and its vicinity and (III) north of Shuilien. Sector I displays at least five terrace levels and has undergone northward tilting and uplift. The available 2%/230Tb and 14C ages suggest that one of its Holocene terraces rises to about 40 m and locally to 70 m above MSL. The average uplift rate for the Holocene in this sector decreases northward from 9-10 mm/yr to about 2-3 mm/yr. Few traces of former shorelines or marine terraces higher than 5 m are found in Sector II. North of Shuilien, in Sector III, terrace elevation increases gradually and indicates an uplift rate of about 2-3 mm/yr. The uplift pattern agrees with geodetic data obtained between 1985 and 1988. The high uplift rates in Sector I may represent compression and shortening of the northwestern comer of the Philippine Sea Plate; the low rates in Sector II may correlate with the region where the Ryukyu Trench meets the eastern coast of Taiwan.
Introduction The Coastal Range of eastern Taiwan is a product of active arc-continent collision. This collision is marked by the formation of intra- and fore-arc basins (Teng 1987; Huang, 1988), the unroofing of the sedimentary cover (Teng, 1979; Dorsey, 1985), a dramatic increase in sedimentation rate on both foreland and fore- and intra-arc basins since 4 Ma (Chi et al., 1985) and rapid uplift in both the Coastal and the Central Ranges since the Pliocene (Peng et al., 1977; Liu, 1982). Continuing activity is indicated both by frequent earthquakes and by geodetic data (Tsai et al., 1981; Yu and Tsai, 1982). On the basis of coastal morphology, Hsu (1962) recognized variations in uplift along the Coastal Range. Geomorphological work on the surfaces of the areas shows that there are mainly three superimposed surfaces along the coast. Recent uplift rates have been derived for the Coastal 0040-1951/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
Range by many authors (Lin, 1969; Hashimoto, 1972; Konishi et al., 1968; Peng et al., 1977; Liew et al., 1988) from 14C and 234U/230Th dates on fossil shells or corals. Peng et al. (1977) reviewed these data and concluded that in most parts of the Coastal Range the uplift rate during the Holocene has been about 5 mm/yr. This paper focuses on the Holocene marine terraces in the middle and northern parts of the Coastal Range, between Chengkung and Hualien, and uses geomorphological and chronological significance.
data to explore their tectonic
The number and altitude of steps of terraces and other sea-level indicators define three sectors in the area: Sector I refers to the area from Chengkung to south of Kueian (Fig. 1, sections l-6) in which both the number and the altitude of the terraces are at their highest. Although both the altitudes and number of marine terraces decrease from south to north, overall this sector represents
PING-ME1
122
Fig. 1. Distribution
a rapidly uplifting area. Sector II refers to the vicinity of Shuilien (section 7, Fig. 1) where marine terraces 8, Fig. Hualien, as high heights
are poorly represented. Sector III (section 1) represents the area from Shuilien to where marine terraces are not as many or as in Sector I. Throughout this paper, are given above mean sea level.
and profiles
LIEW
ET AL.
of terraces.
The terraces
and their correlation
Sector I
North of Chengkung, the coast is bordered by three major surfaces (Fig. 2) which contain at least
123
HOLOCENE MARINE TERRACES, COASTAL RANGE, TAIWAN
Fig. 2. Distribution of Holocene terraces in the southern
part of Sector I; I to its left.
five distinct terraces. The uppermost, with its inner edge at about 65-70 m, is capped by very thick marine sands, is separated by a distinct scarp from an extensive composite terrace which may represent at least two stillstands, and ranges in altitude from 25 to 45 m. There are two other lower terraces at altitudes of about 20 and 7 m as well as irregular benches indicating brief still stands below 6 m.
Near Changping, a large fan delta is topped by a marine platform at about 50 m which can be morphologically correlated with the 65 m terrace immediately to the south and can be traced to Pahsientung. Peats within the sands that cap it have been dated by 14C to 5130 f 60 yrs B.P. Near Pahsientung, the coast is characterized by several superimposed notches in the volcaniclastic Tuluanshan Formation. Middle Holocene trans-
1
Near Lot. 1
Chomgan
40
Taiwan
Academia
Lab., N.J., U.S.A.
National
of Earth Sciences,
Age determination
Lab., Institute
Isotopes
(4) Geochemistry
(3) Teledyne
Lab., Geology
(2) Radiocarbon
Department,
Lab., Physics Department,
(1) Radiocarbon
1970f40 University.
Sinica.
University.
1040 + 100 (2)
980+80
14C Coral,
18
Shell,
(4)
Coral pinnacle, 5100+600
“‘Th,
3930 f 100 (3)
3690 f 100 (3)
3760 + 100 (3)
Coral,
3560 + 100 (3)
i4C Shell,
26
(3)
2
Kueian
Fengping
26
6
5
Age
28
37
Shihmen
6
24
38
Shihti
present
Shell,
Taiwan
36
beach sands
Peat within
Peat on terrace
534Ok260
5050 f 60 (2)
37 4970*250(l)
x30*60
Chingpu
50
4
Pahsientung
ages in years before
3
(1)
in metres;
Changping
Heights
2
(2)
two major terraces.
5.2
National
45
70
Chengshan
of the upper
14
(2)
correlation
Height
Lowest surface
Age
Height
Surface II
Age
60
Wushipi
dates and the proposed
Surface I Height 60
Available
TABLE
HOLOCENE
MARINE
TERRACES,
COASTAL
RANGE,
TAIWAN
gressive sediments rise to 37 m. As some of them fill the notches, these notches may be relic features of the Pleistocene. Peats within the beach sands have been dated to 4970 f 250 and 5340 f 260 yrs B.P. North of Pahsientung (Figs. 1 and 2) the main terrace with an altitude of about 38 m is bordered by benches at 19-28 m, 15 m, 11 m and 6 m. Near Shihti, however, the 38 m surface is represented only by a notch and its associated small abrasion platform (Shih et al., 1988). 14C ages of four fossil corals and shells at about 26 m indicate an age of 3600-3900 yrs B.P. (Table 1). 234U/230Th dating of corals pinnacles at 23-24 m gives a value of 5100 f 600 yrs B.P. (Lin, 1989) and may indicate an earlier, higher shoreline. North of Shihti, the terraces are poorly developed because the mountains are close to the sea, but a former shoreline corresponding to the 38 m terrace of Chingpu is found at 30-35 m and north of a small fan delta, near Tawan, at 25-30 m. South of Fengping, there is well-developed terrace with an altitude of 25 m which may form,part of the same shoreline. North of Fengping, the altitude of the highest well-developed terrace falls from 25 m to 23-18 m. There are also narrow steps at 12 m, 7 m and 4 m. Sector II
North of Kueian there are few signs of marine terraces higher than 5 m, and the only clear platform has an altitude of 2-3 m. Sector II1
North of Shuilien, the best section is at Takeng where at least three distinct terraces capped by beach sediments can be seen. The 20-13 m terrace is the most extensive. The altitude of the terraces increases slightly northward (Shih et al., 1988). Correlation
Correlation of terraces is based here on both geomorphological characteristics and radiometric
125
dating (Table l), radiometric dating only being available within Sector I. In the southern part of the study area (from Chengkung to Changping), although radiometric ages are not available for the 65 m terrace in Chengkung it represents, nevertheless, the top of the thick transgressive sequence and morphologically corresponds to the 50 m terrace further north in Changping which has been dated by the radiocarbon method to the middle Holocene. The 50 m terrace north of Changping can in turn be correlated with the 14C-dated 37 m terrace north of Pahsientung. They all are the highest, well-preserved surfaces along the coast. North of Pahsiengtung, correlation of terraces is further substantiated by their continuity, development and spacing. In contrast with the terrace sequence to the south, they are represented by two relatively extensive surfaces of which the lower may contain at least two distinct shorelines. Below these two prominent surfaces there are usually two or three distinct though narrower steps. Dating constraints on the southern and middle parts of Sector I north of Changping and in Pahsientung respectively provide a cross-check with the results from morphological correlation as discussed above. From Shihti northward, the terraces are poorly developed and it was necesary to define the upper limit of the Holocene transgression from marine beds. This indicates that a well-developed surface with an altitude of about 25-30 m could be the middle Holocene surface. In Fengping, the widest terrace is still at about 25-30 m, but farther north it lies below 20 m. Terraces farther north are poorly developed, but north of Kueian a 2-3 m terrace is found. We believe that the Holocene terrace is low in this part of the coast because (1) the topography near Shuilien indicates a very low uplift rates in the recent past and (2) the geodetic data show this same trend of low uplift (Liu and Yu, 1988). The terraces of Sector III are also characterized by a wide upper surface and two or three lower narrow steps. The altitude of the best-developed topographic surface of the marine terraces is about 20-13 m in Takeng area. This terrace comprises thick beach sands and could be morphologically correlated with the highest well-developed surface of Sector I.
126
PING-ME1
LIFW ET AL
m SECTOR
I
DISTANCE
1-p
SECTOR
0
Fig. 3. Distribution
II _-~-SECTOR
50 km and correlation
of the highest
Holocene
surface
(Surface
III
4
100 km I, stippled).
Broken
lines indicate
probable
extent
of
lower terraces.
Discussion If our correlation
is correct,
the uppermost
terrace, which is locally dated to about 5-5.5 ka and represents the highstand of the Holocene transgression, falls in level from 65-70 m near
artifacts dating from 3-4 ka are found among beach cobbles at about 46 m (Hsieh, 1990). The observed northward decrease in the uplift rate may be due to the relative age of collision (i.e., the relative maturity of accretion of the Luzon arc onto the Eurasian continent). The collision
Chengkung to about 50 m north of Changping and 37 m near Pahsientung and Chingpu (Fig. 3). More dates are required to establish whether the lower terraces were formed by episodic uplift or by sea-level fluctuations since 5.5 ka. In their study of the age and height distribution of Holocene transgressive deposits of New Zealand, Ota et al. (1988) noted that an age slightly older than the general culmination time of the Flandrian
started in the Pliocene and propagated (Suppe, 1984) southward. The trend has been recorded by the stratigraphy of basin deposits (Huang, 1988) and Rb/Sr dating of volcanic rocks (Chen, 1989) (the volcanic rocks in the Chengkung-Changping area are younger than those to the north). The southern part of Sector I would represent recent active convergence between the two plates, with the abrupt variation in the uplift rate within Sec-
transgression may exist in the deposits which have undergone the greater uplift rate. Taking this into account, the age of the highest Holocene surface in different parts of Sector I may thus be slightly variable, probably ranging from 7500 to 5000 yrs B.P. due to different uplift rates. The average uplift rate in Sector I is probably 9-10 mm/yr or more in the Chengkung-Changping area, reducing to about 2-3 mm/yr northward. Even this high value is only about half that obtained by levelling between 1985 and 1988 (i.e., 20 mm/yr or more in the same area (Liu and Yu, 1988)). Nevertheless, the geodetic data support the distinction drawn here between the three sectors, with the highest uplift rate in Sector I and the lowest rate in Sector II. It is worth adding that south of the study area
tor I probably being due to block faulting. Bathymetric maps of the Shuilien area show that the nearest deep kink in contours is located off Sector II, which might imply a possible link with the Ryukyu Trench (see Fig. 1). Analysis of the principal stress directions and focal mechanisms across this area favour this interpretation (Lin et al., 1985). Both seismic refraction studies (Hagen et al., 1988) and a ray-tracing study of the crustal structure in eastern Taiwan (Wang and Chen, 1988) indicate that the Ryukyu Trench can be traced into Taiwan near 23”30’N (see Fig. 1). The very low uplift rate in Sector II may be related to the proximity of the trench system. The medium rate of uplift in Sector III would then indicate that compression north of the Shuilien
HOLOCENE
MARINE
TERRACES,
COASTAL
RANGE,
TAIWAN
area is not so intense as in the south. Acknowledgements We thank Dr. Wang of the Institute of Earth Sciences, Academia Sinica, and Miss C.F. Lin of the Geological Institute for the 2%-230Th ages. We also thank Dr. Yu, Dr. Liu and Mr. Kuo of Academic Sinica for the levelling data. References Chen, C.H., 1989. The Rb/Sr geochemistry of the volcanic rocks of eastern Taiwan and offshore islands. Ph.D. Thesis, National Taiwan Univ. Chi, W., Namson, J. and Suppe, J., 1985. Stratigraphic record of plate interactions in the Coastal Range of eastern Taiwan. Mem. Geol. Sot. China, 4: 155-194. Dorsey, R., 1985. Petrography of the Neogene sandstones from the Coastal Range of eastern Taiwan : Response to arccontinent collisions. Pet. Geol. Taiwan, 21: 187-215. Hagen, R.A., Buennebier, F.K. and Hsu, V., 1988. A seismic refraction study of the crustal structure in the active seismic zone east of Taiwan. J. Geophys. Res., 93: 4785-4796. Hashimoto, W., 1972. Studies on the younger Cenozoic deposits in south and east Taiwan. Geol. Palaeontol. Southeast Asia, 10: 265-303. Hsieh, M.L., 1990. The study of late Quaternary deposits, marine terraces and n~t~to~srn of the coastal area between Ha&en and Taituug, eastern Taiwan. M.S. Thesis, National Taiwan Univ. (Unpubl.). Hsu, T.L., 1962. A study on the coastal geomorphology of Taiwan. Proc. Geol. Sot. China, 5: 29-46. Huang, C.Y., 1988. Arc collapse and subsidence: Consequence of arc-continent collision in Coastal Range, eastern Taiwan. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Program Abstr., p. 59. Konishi, K., Gmura, A. and Kimura, T., 1968. 234U-230Th dating of some late Quatemary coral limestones from southern Taiwan {Formosa). Geol. Palaeontol. Southeast Asia, 5: 211-224. Liew, P.M., Lai, C.K. and Hsieh, M.L., 1988. Holocene marine terraces and their tectonic significance of the Coastal Range, eastern Taiwan. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Program Abstr., p. 78. Lin, C., Yeh, Y. and Tsai, Y., 1985, Determination of regional principal stress directions in Taiwan from fault plane solutions. Bull. Inst. Earth Sci., Acad. Sin., 5: 67-85. &in, CC., 1969. Holocene geology of Taiwan. Acta Geol. Taiwan, 13: 83-126.
127
Lin, J.F., 1989. Studies of 234U/230Th dating on the Holocene corals of eastern and southern Taiwan. M.S. Thesis, National Taiwan Univ. Liu, C.C. and Yu, S.B., 1988. Vertical crustal deformation in eastern Taiwan and its tectonic impactions. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Pro8tam Abst., p. 79. Liu, T.K., 1982. Tectonic implication of fission track ages from the Central Ranges, Taiwan. Prcc. Geol. Sot. China, 25: 22-37. Ota, Y., Berryman, K.R., Hull, A.G., Miyauchi, T. and Iso, N., 1988. Holocene transgressive deposits in eastern North Island, New Zealand. Palaeogeogr, Palaeoclimatol., Palaeoecol., 68: 135-151. Peng, Z., Li, Y. and Wu, F., 1977. Tectonic uplift of Taiwan Island since the early Holocene. Mem. Geol. Sot. China, 2: 57-69. Shih, T., Teng, K., Hsu, M. and Yang, G., 1988. A geomorphological study of marine terraces on Huatung coast of Taiwan. Geogr. Res. Inst. Geogr., Nat. Taiwan Normal Univ., 14: l-50 (in Chinese). Sung, W.H., 1969. Changping culture, the first pre-ceramic culture of Taiwan. Neslett. Chin. Ethnol., 9: l-27 (in Chinese). Suppe, J., 1984. Kinematics of arc-continent collision, flipping of subduction, and back-arc spreading near Taiwan. Mem. Geol. Sot. China, 6: 21-33. Teng, L.S., 1979. Petrographical study of the Neogene sandstones of the Coastal Range, easter Taiwan (1, northern part). Acta Geol. Taiwan., 20: 1299156. Teng, L.S., 1987. T~tonostrati~ap~~ facies and geologic evolution of the Coastal Range, eastern Taiwan. Mem. Geol. Sot. China, 8: 229-250. Tsai, Y.B., Liaw, Z.S., Lee, T.Q., Lin, M.T. and Yeh, Y.H., 1981. Seismological evidence of an active plate boundary in the Taiwan area. Mem. Geol. See. China, 4: 143-154. Wang, C. and Chen, C., 1988. A study of the crustal structure in eastern Taiwan using the ray tracing method. Proe. Taiwan Symp. Geophys. (Taipei), 2nd. Inst. Earth Sei., Acad. Sin., pp. 165-175. Yu, S.B. and Tsai, Y.B., 1982. A study of microseismicity and crustal deformation of the Kuangfu-Fuh area in eastern Taiwan. Bull. Inst. Earth Sci., Acad. Sm., 2: l-18. Yu, S.B., Liu, C.C. and Kuo, L.C., 1988. Crustal deformation before and after the Huahen earthquake of November 14, 1986. Proc. Taiwan Symp. Geophys. (Taipei), 2nd. Inst. Earth Sci., Acad. Sin., pp. 312-323. Zhoa, S.T., 1984. Studies on the Evolution of China’s Coast. Sei. Tech. Publ. Fukein, 194~~. (in Chinese).
Tectonophysics, 183 (1990) 121-127 Elsevier Science Publishers B.V., Amsterdam
Tectonic significance of Holocene marine terraces in the Coastal Range, eastern Taiwan Ping-Mei Liew, Moon-Lung Hsieh and Cheng-Kuo Lai Institute of Geology, National Taiwan University, Taipei (Taiwan, China) (Received April 10.1989;
revised and accepted October 6.1989)
ABSTRACT Liew, Ping-mei, Hsieh, Moon-lung and Lai, Cheng-kuo, 1990. Tectonic significance of Holocene marine terraces in the Coastal Range, eastern Taiwan. In: J. Angelier (Editor), Geodynamic Evolution of the Eastern Eurasian Margin. Tectonophysics, 183:121-127.
Well-developed marine terraces on the coast of eastern Taiwan between Chengkung and Hualien can be divided into three morphotectonic sectors: (I) Chengkung-Kueian, (II) Shuilien and its vicinity and (III) north of Shuilien. Sector I displays at least five terrace levels and has undergone northward tilting and uplift. The available 2%/230Tb and 14C ages suggest that one of its Holocene terraces rises to about 40 m and locally to 70 m above MSL. The average uplift rate for the Holocene in this sector decreases northward from 9-10 mm/yr to about 2-3 mm/yr. Few traces of former shorelines or marine terraces higher than 5 m are found in Sector II. North of Shuilien, in Sector III, terrace elevation increases gradually and indicates an uplift rate of about 2-3 mm/yr. The uplift pattern agrees with geodetic data obtained between 1985 and 1988. The high uplift rates in Sector I may represent compression and shortening of the northwestern comer of the Philippine Sea Plate; the low rates in Sector II may correlate with the region where the Ryukyu Trench meets the eastern coast of Taiwan.
Introduction The Coastal Range of eastern Taiwan is a product of active arc-continent collision. This collision is marked by the formation of intra- and fore-arc basins (Teng 1987; Huang, 1988), the unroofing of the sedimentary cover (Teng, 1979; Dorsey, 1985), a dramatic increase in sedimentation rate on both foreland and fore- and intra-arc basins since 4 Ma (Chi et al., 1985) and rapid uplift in both the Coastal and the Central Ranges since the Pliocene (Peng et al., 1977; Liu, 1982). Continuing activity is indicated both by frequent earthquakes and by geodetic data (Tsai et al., 1981; Yu and Tsai, 1982). On the basis of coastal morphology, Hsu (1962) recognized variations in uplift along the Coastal Range. Geomorphological work on the surfaces of the areas shows that there are mainly three superimposed surfaces along the coast. Recent uplift rates have been derived for the Coastal 0040-1951/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
Range by many authors (Lin, 1969; Hashimoto, 1972; Konishi et al., 1968; Peng et al., 1977; Liew et al., 1988) from 14C and 234U/230Th dates on fossil shells or corals. Peng et al. (1977) reviewed these data and concluded that in most parts of the Coastal Range the uplift rate during the Holocene has been about 5 mm/yr. This paper focuses on the Holocene marine terraces in the middle and northern parts of the Coastal Range, between Chengkung and Hualien, and uses geomorphological and chronological significance.
data to explore their tectonic
The number and altitude of steps of terraces and other sea-level indicators define three sectors in the area: Sector I refers to the area from Chengkung to south of Kueian (Fig. 1, sections l-6) in which both the number and the altitude of the terraces are at their highest. Although both the altitudes and number of marine terraces decrease from south to north, overall this sector represents
PING-ME1
122
Fig. 1. Distribution
a rapidly uplifting area. Sector II refers to the vicinity of Shuilien (section 7, Fig. 1) where marine terraces 8, Fig. Hualien, as high heights
are poorly represented. Sector III (section 1) represents the area from Shuilien to where marine terraces are not as many or as in Sector I. Throughout this paper, are given above mean sea level.
and profiles
LIEW
ET AL.
of terraces.
The terraces
and their correlation
Sector I
North of Chengkung, the coast is bordered by three major surfaces (Fig. 2) which contain at least
123
HOLOCENE MARINE TERRACES, COASTAL RANGE, TAIWAN
Fig. 2. Distribution of Holocene terraces in the southern
part of Sector I; I to its left.
five distinct terraces. The uppermost, with its inner edge at about 65-70 m, is capped by very thick marine sands, is separated by a distinct scarp from an extensive composite terrace which may represent at least two stillstands, and ranges in altitude from 25 to 45 m. There are two other lower terraces at altitudes of about 20 and 7 m as well as irregular benches indicating brief still stands below 6 m.
Near Changping, a large fan delta is topped by a marine platform at about 50 m which can be morphologically correlated with the 65 m terrace immediately to the south and can be traced to Pahsientung. Peats within the sands that cap it have been dated by 14C to 5130 f 60 yrs B.P. Near Pahsientung, the coast is characterized by several superimposed notches in the volcaniclastic Tuluanshan Formation. Middle Holocene trans-
1
Near Lot. 1
Chomgan
40
Taiwan
Academia
Lab., N.J., U.S.A.
National
of Earth Sciences,
Age determination
Lab., Institute
Isotopes
(4) Geochemistry
(3) Teledyne
Lab., Geology
(2) Radiocarbon
Department,
Lab., Physics Department,
(1) Radiocarbon
1970f40 University.
Sinica.
University.
1040 + 100 (2)
980+80
14C Coral,
18
Shell,
(4)
Coral pinnacle, 5100+600
“‘Th,
3930 f 100 (3)
3690 f 100 (3)
3760 + 100 (3)
Coral,
3560 + 100 (3)
i4C Shell,
26
(3)
2
Kueian
Fengping
26
6
5
Age
28
37
Shihmen
6
24
38
Shihti
present
Shell,
Taiwan
36
beach sands
Peat within
Peat on terrace
534Ok260
5050 f 60 (2)
37 4970*250(l)
x30*60
Chingpu
50
4
Pahsientung
ages in years before
3
(1)
in metres;
Changping
Heights
2
(2)
two major terraces.
5.2
National
45
70
Chengshan
of the upper
14
(2)
correlation
Height
Lowest surface
Age
Height
Surface II
Age
60
Wushipi
dates and the proposed
Surface I Height 60
Available
TABLE
HOLOCENE
MARINE
TERRACES,
COASTAL
RANGE,
TAIWAN
gressive sediments rise to 37 m. As some of them fill the notches, these notches may be relic features of the Pleistocene. Peats within the beach sands have been dated to 4970 f 250 and 5340 f 260 yrs B.P. North of Pahsientung (Figs. 1 and 2) the main terrace with an altitude of about 38 m is bordered by benches at 19-28 m, 15 m, 11 m and 6 m. Near Shihti, however, the 38 m surface is represented only by a notch and its associated small abrasion platform (Shih et al., 1988). 14C ages of four fossil corals and shells at about 26 m indicate an age of 3600-3900 yrs B.P. (Table 1). 234U/230Th dating of corals pinnacles at 23-24 m gives a value of 5100 f 600 yrs B.P. (Lin, 1989) and may indicate an earlier, higher shoreline. North of Shihti, the terraces are poorly developed because the mountains are close to the sea, but a former shoreline corresponding to the 38 m terrace of Chingpu is found at 30-35 m and north of a small fan delta, near Tawan, at 25-30 m. South of Fengping, there is well-developed terrace with an altitude of 25 m which may form,part of the same shoreline. North of Fengping, the altitude of the highest well-developed terrace falls from 25 m to 23-18 m. There are also narrow steps at 12 m, 7 m and 4 m. Sector II
North of Kueian there are few signs of marine terraces higher than 5 m, and the only clear platform has an altitude of 2-3 m. Sector II1
North of Shuilien, the best section is at Takeng where at least three distinct terraces capped by beach sediments can be seen. The 20-13 m terrace is the most extensive. The altitude of the terraces increases slightly northward (Shih et al., 1988). Correlation
Correlation of terraces is based here on both geomorphological characteristics and radiometric
125
dating (Table l), radiometric dating only being available within Sector I. In the southern part of the study area (from Chengkung to Changping), although radiometric ages are not available for the 65 m terrace in Chengkung it represents, nevertheless, the top of the thick transgressive sequence and morphologically corresponds to the 50 m terrace further north in Changping which has been dated by the radiocarbon method to the middle Holocene. The 50 m terrace north of Changping can in turn be correlated with the 14C-dated 37 m terrace north of Pahsientung. They all are the highest, well-preserved surfaces along the coast. North of Pahsiengtung, correlation of terraces is further substantiated by their continuity, development and spacing. In contrast with the terrace sequence to the south, they are represented by two relatively extensive surfaces of which the lower may contain at least two distinct shorelines. Below these two prominent surfaces there are usually two or three distinct though narrower steps. Dating constraints on the southern and middle parts of Sector I north of Changping and in Pahsientung respectively provide a cross-check with the results from morphological correlation as discussed above. From Shihti northward, the terraces are poorly developed and it was necesary to define the upper limit of the Holocene transgression from marine beds. This indicates that a well-developed surface with an altitude of about 25-30 m could be the middle Holocene surface. In Fengping, the widest terrace is still at about 25-30 m, but farther north it lies below 20 m. Terraces farther north are poorly developed, but north of Kueian a 2-3 m terrace is found. We believe that the Holocene terrace is low in this part of the coast because (1) the topography near Shuilien indicates a very low uplift rates in the recent past and (2) the geodetic data show this same trend of low uplift (Liu and Yu, 1988). The terraces of Sector III are also characterized by a wide upper surface and two or three lower narrow steps. The altitude of the best-developed topographic surface of the marine terraces is about 20-13 m in Takeng area. This terrace comprises thick beach sands and could be morphologically correlated with the highest well-developed surface of Sector I.
126
PING-ME1
LIFW ET AL
m SECTOR
I
DISTANCE
1-p
SECTOR
0
Fig. 3. Distribution
II _-~-SECTOR
50 km and correlation
of the highest
Holocene
surface
(Surface
III
4
100 km I, stippled).
Broken
lines indicate
probable
extent
of
lower terraces.
Discussion If our correlation
is correct,
the uppermost
terrace, which is locally dated to about 5-5.5 ka and represents the highstand of the Holocene transgression, falls in level from 65-70 m near
artifacts dating from 3-4 ka are found among beach cobbles at about 46 m (Hsieh, 1990). The observed northward decrease in the uplift rate may be due to the relative age of collision (i.e., the relative maturity of accretion of the Luzon arc onto the Eurasian continent). The collision
Chengkung to about 50 m north of Changping and 37 m near Pahsientung and Chingpu (Fig. 3). More dates are required to establish whether the lower terraces were formed by episodic uplift or by sea-level fluctuations since 5.5 ka. In their study of the age and height distribution of Holocene transgressive deposits of New Zealand, Ota et al. (1988) noted that an age slightly older than the general culmination time of the Flandrian
started in the Pliocene and propagated (Suppe, 1984) southward. The trend has been recorded by the stratigraphy of basin deposits (Huang, 1988) and Rb/Sr dating of volcanic rocks (Chen, 1989) (the volcanic rocks in the Chengkung-Changping area are younger than those to the north). The southern part of Sector I would represent recent active convergence between the two plates, with the abrupt variation in the uplift rate within Sec-
transgression may exist in the deposits which have undergone the greater uplift rate. Taking this into account, the age of the highest Holocene surface in different parts of Sector I may thus be slightly variable, probably ranging from 7500 to 5000 yrs B.P. due to different uplift rates. The average uplift rate in Sector I is probably 9-10 mm/yr or more in the Chengkung-Changping area, reducing to about 2-3 mm/yr northward. Even this high value is only about half that obtained by levelling between 1985 and 1988 (i.e., 20 mm/yr or more in the same area (Liu and Yu, 1988)). Nevertheless, the geodetic data support the distinction drawn here between the three sectors, with the highest uplift rate in Sector I and the lowest rate in Sector II. It is worth adding that south of the study area
tor I probably being due to block faulting. Bathymetric maps of the Shuilien area show that the nearest deep kink in contours is located off Sector II, which might imply a possible link with the Ryukyu Trench (see Fig. 1). Analysis of the principal stress directions and focal mechanisms across this area favour this interpretation (Lin et al., 1985). Both seismic refraction studies (Hagen et al., 1988) and a ray-tracing study of the crustal structure in eastern Taiwan (Wang and Chen, 1988) indicate that the Ryukyu Trench can be traced into Taiwan near 23”30’N (see Fig. 1). The very low uplift rate in Sector II may be related to the proximity of the trench system. The medium rate of uplift in Sector III would then indicate that compression north of the Shuilien
HOLOCENE
MARINE
TERRACES,
COASTAL
RANGE,
TAIWAN
area is not so intense as in the south. Acknowledgements We thank Dr. Wang of the Institute of Earth Sciences, Academia Sinica, and Miss C.F. Lin of the Geological Institute for the 2%-230Th ages. We also thank Dr. Yu, Dr. Liu and Mr. Kuo of Academic Sinica for the levelling data. References Chen, C.H., 1989. The Rb/Sr geochemistry of the volcanic rocks of eastern Taiwan and offshore islands. Ph.D. Thesis, National Taiwan Univ. Chi, W., Namson, J. and Suppe, J., 1985. Stratigraphic record of plate interactions in the Coastal Range of eastern Taiwan. Mem. Geol. Sot. China, 4: 155-194. Dorsey, R., 1985. Petrography of the Neogene sandstones from the Coastal Range of eastern Taiwan : Response to arccontinent collisions. Pet. Geol. Taiwan, 21: 187-215. Hagen, R.A., Buennebier, F.K. and Hsu, V., 1988. A seismic refraction study of the crustal structure in the active seismic zone east of Taiwan. J. Geophys. Res., 93: 4785-4796. Hashimoto, W., 1972. Studies on the younger Cenozoic deposits in south and east Taiwan. Geol. Palaeontol. Southeast Asia, 10: 265-303. Hsieh, M.L., 1990. The study of late Quaternary deposits, marine terraces and n~t~to~srn of the coastal area between Ha&en and Taituug, eastern Taiwan. M.S. Thesis, National Taiwan Univ. (Unpubl.). Hsu, T.L., 1962. A study on the coastal geomorphology of Taiwan. Proc. Geol. Sot. China, 5: 29-46. Huang, C.Y., 1988. Arc collapse and subsidence: Consequence of arc-continent collision in Coastal Range, eastern Taiwan. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Program Abstr., p. 59. Konishi, K., Gmura, A. and Kimura, T., 1968. 234U-230Th dating of some late Quatemary coral limestones from southern Taiwan {Formosa). Geol. Palaeontol. Southeast Asia, 5: 211-224. Liew, P.M., Lai, C.K. and Hsieh, M.L., 1988. Holocene marine terraces and their tectonic significance of the Coastal Range, eastern Taiwan. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Program Abstr., p. 78. Lin, C., Yeh, Y. and Tsai, Y., 1985, Determination of regional principal stress directions in Taiwan from fault plane solutions. Bull. Inst. Earth Sci., Acad. Sin., 5: 67-85. &in, CC., 1969. Holocene geology of Taiwan. Acta Geol. Taiwan, 13: 83-126.
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Lin, J.F., 1989. Studies of 234U/230Th dating on the Holocene corals of eastern and southern Taiwan. M.S. Thesis, National Taiwan Univ. Liu, C.C. and Yu, S.B., 1988. Vertical crustal deformation in eastern Taiwan and its tectonic impactions. Int. Symp. Geodyn. Evolut. Eastern Eurasian Margin (Paris), Pro8tam Abst., p. 79. Liu, T.K., 1982. Tectonic implication of fission track ages from the Central Ranges, Taiwan. Prcc. Geol. Sot. China, 25: 22-37. Ota, Y., Berryman, K.R., Hull, A.G., Miyauchi, T. and Iso, N., 1988. Holocene transgressive deposits in eastern North Island, New Zealand. Palaeogeogr, Palaeoclimatol., Palaeoecol., 68: 135-151. Peng, Z., Li, Y. and Wu, F., 1977. Tectonic uplift of Taiwan Island since the early Holocene. Mem. Geol. Sot. China, 2: 57-69. Shih, T., Teng, K., Hsu, M. and Yang, G., 1988. A geomorphological study of marine terraces on Huatung coast of Taiwan. Geogr. Res. Inst. Geogr., Nat. Taiwan Normal Univ., 14: l-50 (in Chinese). Sung, W.H., 1969. Changping culture, the first pre-ceramic culture of Taiwan. Neslett. Chin. Ethnol., 9: l-27 (in Chinese). Suppe, J., 1984. Kinematics of arc-continent collision, flipping of subduction, and back-arc spreading near Taiwan. Mem. Geol. Sot. China, 6: 21-33. Teng, L.S., 1979. Petrographical study of the Neogene sandstones of the Coastal Range, easter Taiwan (1, northern part). Acta Geol. Taiwan., 20: 1299156. Teng, L.S., 1987. T~tonostrati~ap~~ facies and geologic evolution of the Coastal Range, eastern Taiwan. Mem. Geol. Sot. China, 8: 229-250. Tsai, Y.B., Liaw, Z.S., Lee, T.Q., Lin, M.T. and Yeh, Y.H., 1981. Seismological evidence of an active plate boundary in the Taiwan area. Mem. Geol. See. China, 4: 143-154. Wang, C. and Chen, C., 1988. A study of the crustal structure in eastern Taiwan using the ray tracing method. Proe. Taiwan Symp. Geophys. (Taipei), 2nd. Inst. Earth Sei., Acad. Sin., pp. 165-175. Yu, S.B. and Tsai, Y.B., 1982. A study of microseismicity and crustal deformation of the Kuangfu-Fuh area in eastern Taiwan. Bull. Inst. Earth Sci., Acad. Sm., 2: l-18. Yu, S.B., Liu, C.C. and Kuo, L.C., 1988. Crustal deformation before and after the Huahen earthquake of November 14, 1986. Proc. Taiwan Symp. Geophys. (Taipei), 2nd. Inst. Earth Sci., Acad. Sin., pp. 312-323. Zhoa, S.T., 1984. Studies on the Evolution of China’s Coast. Sei. Tech. Publ. Fukein, 194~~. (in Chinese).