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Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
Journal of China Universiry of Geosciences, Vol. 18, No.1, p.1-10, March 2007 Printed in China
ISSN 1002-0705
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough Wu Shiguo" (%Ffm) Key Lab of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China; College of Georesources and Information, Petroleum University of China (Huadong), Qingdao 266555, China Ni Xianglong (!%#&), Guo Junhua ($BFq) Key Lab of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 26607 1, China; Graduate School, Chinese Academy of Sciences, Beijing 100049, China ABSTRACT On the basis of the multi-channel seismic data and the other data, using 2DMove software, the tectonic evolution in three seismic profiles was restored since Pliocene. The tectonic restoration results show that: (1) the initial active center lay in the west slope and then was transferred to east and south via trough center during the evolution process; (2) several main normal faults controlled the evolution of the southern Okinawa Trough; (3) since Late Pliocene, the southern Okinawa Trough has experienced two spreading stages. The early is depression in Early-Middle Pleistocene and the late is back-arc spreading in Late Pleistocene and Holocene, which is in primary oceanic crust spreading stage. KEY WORDS: balanced cross section, tectonic evolution, back-arc basin, Okinawa Trough.
INTRODUCTION The Okinawa Trough (OT) lays the boundary between Eurasian plate and Philippine Sea plate (PSP). It is a nascent back-arc basin of the trench-arc-basin system in West Pacific margin. It is a key spot for continental margin rifting, especially for the Chinese continent and its adjoining areas. Many scholars have paid much attention to it (Liu et al., 2005; Zeng et al., 2003; Jin and Yu, 1987; Kimura, 1985; Lee et al., 1980; Herman et al., 1978). This paper is supported by the Knowledge Innovation Project
of Chinese Academy of Sciences (Nos. KZCX3-SW-219, KZCX3-SW-224) and the Taishan Scholarship Project of Shandong Province. *Corresponding author: swu 0ms .qdio .ac.cn Manuscript received September 28,2006. Manuscript accepted December 20,2006.
The OT tends NE in whole, and is divided into three parts by Tokara fracture zone and Miyako fracture zone (Fig. 1). The north part tends NNE, with depth about 600-800 m. The middle part is NE tending, with 1 000-1 200 m deep, and the south part NEE, mostly over 2 000 m deep (Fig. 2). In this area, the crust is thinned, with strong tectonic movements and high heat flux (Fan and Yang, 2004; Li et al., 2001; Jin and Yu, 1987). The Miyako fracture zone is similar to transformation fault. To its north, the faults are right-strike slip, whereas in the south are left-strike slip (Jin and Yu, 1987; Fig. 1). There remains dispute on the rifting stage. Depending on seismic strata, bore data, and the strata development features in this area, the OT mainly experienced three tectonic periods: Late Miocene, Late Pliocene-Early Pleistocene, and Late Pleistocene-Holocene. Late Miocene is the rifting stage of OT (Jiang et al., 2003; Zeng et al., 2003). Herman et al. (1978) thought the rifting occurred
2
Wu Shiguo, Ni Xianglong and Guo Junhua
between Late Miocene and Early Pleistocene, when the Ryukyu islands broke from the continent, based on the nonconformity between upper and lower sediment layers. Kimura (1985) suggest that the opening of OT began 6-4 Ma BP, based on the sedimentary sequences in middle OT. However, Park et al. (1998) thought that during 6-2 Ma BP there was no spreading, it began about 2 Ma BP; whereas Miki (1995) thought it began spreading since 10-6 Ma BP. There are also different opinions on the structural property of present
O T some scholars thought the south and middle parts of OT are on "oceanic-crust" spreading (Yang et al., 2004; Jiang et al., 2003; Jin and Yu, 1987), whereas others thought there is no spreading (Liu, 2001). Therefore, further study about the formation and evolution of OT is necessary. On the basis of seismic interpretation and geological analysis of OT (Guo, 2004), the quantity described on the formation and evolution process using balanced cross section technique is discussed in this article.
32"N
30"
28"
26"
24"
Figure 1. Geological map of East China Sea (modified after Jin and Yu, 1987). 1. Hupijiao rise; 2. Haijiao Qiantang I rise; 3. Yushan rise; 4. Wuyishan low rise. @ Fujiang depression; @ Yangtze depression; @ Xihu I depression; @ Taipei I depression. depression; @ Oujiang depression; @
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
3
Figure 2. Map of water depth in Okinawa Trough with seismic lines. A, B, C are the seismic lines. SEISMIC SEQUENCES The data used were collected by R N “KEXUE I” Cruises during 1982-1984. After Jin and Yu (1987), Park et al. (1998) and Liu et al. (2005), five seismic sequences, which have various velocity models (Table l), were identified and modified. Three seismic profiles in southwest OT (Fig. 2) were chosen to restore by further process and interpretation. The profiles show no fold, and all strata are disconformity contact or conformity contact. This
means that in the whole trough, there is no compression, and the OT is extensional structural environment. Several main faults controlled sedimentary development in the OT. During Late Pleistocene, many small contemporaneous faults developed. So this period had strong tectonic activity with high deposition rate (Fig. 5). Some diapirs also developed, which may have been formed by magma upwelling (Fig. 3).
Figure 3. Interpretation map of section A (position in Fig. 2).
4
Wu Shiguo, Ni Xianglong and Guo Junhua CDP
0 I -2 $3 + 4 5
6
CDP
- 3000
4000
6000
5000
7000
8000
9000
10000
11000
Figure 4. Interpretationmap of section B (position in Fig. 2). oCDP 500
1100
1700
2300
2900
3500
4100
4700
5300
5900
6500
7100
5900
6500
7100
7700
1 2 h
w
v
E 3
4 5 6 CDP 500
1100
1700
2300
2900
3500
4100
4700
5300
"I
7700 '
I
I
6'
Figure 5. Interpretationmap of section C (position in Fig. 2). METHOD AND RESULTS OF BALANCED CROSS SECTION Balanced cross-section technique is a quantifying tool for geological structures. Dahlstrom (1969) first advanced the concept, and also made some applications. From then on, it developed and improved continuously. At present, it is mainly applied for checking result of seismic interpretation, quantity study on tectonic deformation, and tectonic evolution of basin (Zhou, 2005; Diao and Cheng, 2004; Mao et al., 1998). Section keeping balance is a base geological rule. On the basis of the law of conservation of matter, the
balanced cross-section technique is advanced, following the laws of conservation, the agreement of layers' length, displacement, and shortening. Zhang and Chen (1998), and Diao and Cheng (2004) concluded the steps by applying this technique: firstly, construct proper geologic column model; secondly, establish the structural setting, this technique is more suitable for simple compaction or extension area; thirdly, collect aspect of profiles, the optimum aspects parallel the greatest structure migratory direction, for it can evade sliding movement; finally, recover the tectonic compaction.
5
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
Table 1 Seismic sequences in the Okinawa Trough Period
Date(Ma)
Reflected waves
Sequences in shelf
Sequences in insular slope
Sequences in trough horizon velocity (WS)
I, 1.6-2.0
I
11, 2.23-2.40
/
Holocene I
T; -
0.01
Pleistocene
Middle-Late
t-
0.7 2
III, 2.23-2.40
IV-2
Pliocene I
5.3
- 0
I
Miocene
1 I !
2.51-3.13
IV-1
I
I
V
V
v-2
3,67459
v-1
Oligocene
23.1
G
36.0 57.8 66.4
lT i-4 Mesozoic
NW 0.
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
-E
0
-
0
2 4
E 2 4 8
0 h
E
5 4
.x
8
-
0
E r
3 4 8
1
Holocene
Middle-Late Ple,stocene
Early Pleistocene
Pliocene
Pre-miocene
I
Figure 6. Restoration of section A. Following the above steps, the profiles A, B, and C (Fig. 2) were restored by 2Dmove software. Because of shortage of data, the Pliocene and Miocene strata were not restored. So, Early Pleistocene,
Middle-Late Pleistocene, Holocene and the present of the above three seismic profiles were restored and the results are shown in Figs. 6-8.
6
Wu Shiguo, Ni Xianglong and Guo Junhua
Figure 8. Restoration of section C. The restoration maps of line A in four periods are in Fig. 6. The Pliocene strata developed at the northern continent slope (Fig. 6). Old Miocene strata only occurred in the shelf. Normal faults developed on shelf, and a big normal fault controlled the sedimentations. Two subsequence stages can be
divided in the profile. The early extension occurred in Early Pleistocene and accepted 1 000 m sediment, and the late stage was in the Middle-Late Pleistocene, whereas the Holocene was about 2 000 m, with 4% extension rate. The whole extension rate was 7% greater. Depocenter transferred from the shelf in
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
Pliocene to the island slope in Holocene, completing the south transference of depocenter. The tectonic restoration of line B is on Fig. 7. Four sediment layers overlaid basement. A few inverse faults developed on the continent slope. But many normal faults developed mainly on the continental slope, trough center, and the islands slope. And the faults were active in the trough floor. So the remainder of Miocene was conserved. The active faults controlled two depocenters in Early Pleistocene. The two depocenters had 1 700 m and 2 050 m thick sediments, respectively. During Middle-Late Pleistocene the depocenter migrated to the trough. And the depocenters accepted 1 700 m and 2 050 m respectively. Ascribing to sedimentary compaction, the Early Pleistocene became 750 m and 1 500 m thick in the two depocenters respectively. And compaction amplitude in center was bigger than in east. This may be because the Middle-Late Pleistocene sediments in center were heavier than in east. During Holocene, the sediments deposited in center and east part with thickness of 1 600 m. The compaction of Early Pleistocene was small, changing into 700 m and 1 450 m. Whereas the Middle-Late Pleistocene was big, from 2 200 m and 1 600 m, changing into about 1 200 m and 900 m. But the extension rate was very small, only about 3%. The tectonic restoration of line C is on Fig. 8. The sedimentary feature is the same as Fig. 7. There also were two depocenters, trough center and off islands slope. By Middle-Late Pleistocene, the Early Pleistocene deposited 3 050 m and 1 900 m separately in two depocenters. Whereas by Late Pleistocene, Middle-Late Pleistocene deposited about 1 900 m and 1400 m, making the Early Pleistocene sediment compact to 3 000 m and 1 590 m. Obviously, the compaction was not big, so regional subsidence might have been intense. The Holocene depositing into this area was the same as the line B, with 1 300-1 500 m thickness. Ascribing to sedimentary compaction, the Early Pleistocene became 2 750 m in west slope and 1 550 m in middle part, whereas the Middle-Late Pleistocene became 1 600 m and 900 m. From the map, the stretching rate of line C was about 4%, amounting to 3 000 m. From the above, the stretching rate and
7
compaction rate are different in different stages, and mainly in Early Pleistocene and Late Pleistocene to Holocene, the two spreading stage of OT. The line A is the biggest in stretching rate among the three lines. Maybe in the southwestern OT there appears oceanic-crust spreading. That is to say, OT is at the initial stage of oceanic-crust spreading.
DISCUSSION The faults develop widely in this area (Fig. l), and most are normal faults with high angle (Figs. 3-5). The north subduction of Philippine Sea plate and Huatong basin makes tensional stress flow back to the Ryukyu Islands. Then the upwelling of materials from mantle forms the OT. In this tensional environment, normal faults develop widely, whereas some inverse faults develop locally because of compression. It is easier to see the formation and the controlling of faults in this area from the restoration maps. Most of the faults in this region formed before Early Pleistocene. The faults are mostly normal faults and they control the development of sedimentary strata in OT. Only a few inverse faults occur in the west slope of OT. So, the southern part of OT is in tensional environment in this period. The basement is composed of Pliocene and Miocene sedimentary layers and volcanic rocks. It is thus evident that the southern OT began to form and accept sediments. During Pleistocene, there are two depocenters in the trough floor and off island slope, which accumulated thick sediments. But to continent shelf and slope, there is no Pleistocene sediment, maybe because the sea level decreased, the slope was steep and tectonic activity was strong. During this period, many normal faults dipping to trough are formed in the slope induced by subduction of Philippine Sea plate. The dip angle is high and sediment distorts very intensely. Holocene sediments cover the whole area, and are thick in the two depocenters. Few faults can be discerned from the profiles. It means that, the tectonic movement was not active during Holocene. From the above analyses, the development of faults controlled the migration of depocenters. The depocenter migrated the continent slope to off island slope. Since the Pliocene, the profiles are stretching (Table 2). Especially on Fig. 7, the extension arrives to
8
Wu Shiguo, Ni Xianglong and Guo Junhua
4 000 m, with stretching rate up to 7%. On Figs. 8-9, this phenomenon is not obvious, with rate of 3%-4%, eroding excluded. The difference in rate may be caused by their places. Line A lies in most southern of
OT, whereas lines B and C lie off Miyako fracture zone. The OT rotates clockwise during spreading, and rotation axis is Miyako fracture zone. Comparing rates, this rotation may have happened in Holocene.
Table 2 Stretching rate of the section along seismic lines Seismic line
A
B
C
Extension amount (&/rate (%) in Holocene Extension amount (m)/rate(%) in Middle-Late Pleistocene Extension amount (m)/rate(%) in Early Pleistocene
2 000/3.33
1 000/1.32
1000/1.32
1600/2.10
1400/1.84
100011.67
70010.92
900/1.18
Total extension amount (m)/rate (%)
4 000/6.67
3 300/4.34
3 300/4.34
~~
East China Sea
Okinawa Trough
ooo/l .67
Ryukyu Islands Arc
Front of Arc
Ryukyu
--+4-bt-------b'-bW 0 h
g
2
5 2 4 L3
6
0 E c 2 5 a 8 4
h
6
h
E
c2
5
24
L3
6 Miocene
Pliocene
I.*:.:,:.[
Early Pleiocene
~ ~ ~ ~ ~ e & n ~ a t Holocene e
Figure 9. Evolution model of Okinawa Trough in Cenozoic. The evolution model of OT can be constructed as shown in Fig. 9, according to the seismic interpretation, which includes depositing, uplift and erosion, rifting, and initial spreading. During the Miocene the southern OT accepted the huge thickness
of sediments (Fig. 9a), Pliocene strata were distributed in both slopes, but absent in the trough basin. It is inferred that, the Pliocene originated from the Chinese mainland deposited in this region, and the trough did not open. In the Late Pliocene the southern OT started
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
to rise, and thinning in the center subducted by Gagua Ridge. So the region uplifted and the Pliocene and the early deposits were eroded because of this movement. The Pliocene in the trough area were nearly eroded and Miocene remained only a little as the residual deposition. The acoustic basement was exposed in some areas. Then nonconformity occurred. From the seismic profiles, the Miocene pinched out northwestward probably because of subsidence. After the last movement, the trough area began to rift. The Early Pleistocene concentrated into the trough area and filled the depressions. With the sea level ascending, the Middle and Late Pleistocene and Quaternary deposited. The neonatal submarine volcanoes suggest that the southern OT is in the initial seafloor spreading stage. This agreed with the preceding studies (Yang et al., 2004; Jiang et al., 2003; Jin and Yu, 1987).
9
Fan, D. J., Yang, Z. S., 2004. Development and Distribution of Natural Gas Hydrate in the Okinawa Trough. Acta Petrolei Sinica, 25 (3): 11-17 (in Chinese with English Abstract) Guo, J. H., 2004. Seismic Sequences and the Tectonic Restoration
of
the
Southern
Okinawa
Trough:
[Dissertation]. Institute of Oceanology, Chinese Academy
of Sciences, Qingdao (in Chinese with English Abstract) Herman, B. M., Anderson, R. N., Truchan, M., et al., 1978. Extensional Tectonics in the Okinawa Trough. AAPG Memoir, 29: 199-208 Jiang, W. W., Liu, S. H., Hao, T. Y., et al., 2003. Character of Geophysical Field and Crustal Structure of Okinawa Trough and Adjacent Region. Progress in Geophysics, 18(2): 287-292 (in Chinese with English Abstract) Jin, X. L., Yu,P. Z., 1987. The Tectonic Features and Evolution of Okinawa Trough. Science in China (Se,: B), 2: 196-203 (in Chinese with English Abstract) Kimura, M., 1985. Back-Arc Rifting in the Okinawa Trough. Ma,: Pet. Geol., 2: 222-240
CONCLUSIONS The results of balanced cross-section restoration and seismic interpretation show that, since Pleistocene the depocenters have migrated to east; several large faults controlled the development of minor faults and strata; the OT has experienced two spreading since Pliocene, the early was fault depression in Early-Middle Pleistocene, the late was back-arc spreading from Late Pleistocene to Holocene. And at present, it is in the initial stage of oceanic crust spreading.
Lee, C. S., Shor, G G, Bibee, L. D., et al., 1980. Okinawa Trough: Origin of a Back-Arc Basin. Marine Geology, 35: 2 19-24 1 Li, W. R., Yang, Z . S., Wang, Q., et al., 2001. Tenigenous Transportation through Canyou and Sedimentation of Submarine Fan in the Okinawa Trough. Chinese Journul
of Oceanology and Limnology, 32(4): 371,380
(in
Chinese with English Abstract) Liu, J. H., 2001. Features of Seismic Reflection Wave in the South Okinawa Trough and the Geological Interpretation. Donghai Marine Science, 19(1): 19-26 (in Chinese with English Abstract)
ACKNOWLEDGMENT The research was supported by the Knowledge Innovation Project of Chinese Academy of Sciences (Nos. KZCX3-SW-219, KZCX3-SW-224) and the Taishan Scholarship Project of Shandong Province. The authors would like to thank Prof. Yu Puzhi for advice on revising this article.
Liu, J. H., Lin, C. S., Gao, J. Y., et al., 2005. Analyses on the Basement Constitution of the Southern Okinawa Trough, Chinese Journal of Oceanology and Limnology, 36(3): 261-267 (in Chinese with English Abstract) Mao, X. P., Wu, C. L., Yuan, Y. B., 1998. Three Dimensional Structural Modeling: Volume-Balance Technique. Earth Science-Jouml
of China University of Geosciences,
24(5): 506-508 (in Chinese with English Abstract)
REFERENCES CITED Diao, B., Cheng, S. Y., 2004. Efficacious Methods to Improve the Applied Values of the Balanced Cross Sections. Science Journal of Northwest University Online, 2(3) (in Chinese with English Abstract) Dohlstrom, C. D. A., 1969. Balanced Cross Sections. Canadian Journal of Earth Science, 6: 743-757
Miki, M., 1995. Two-Phase Opening Model for the Okinawa Trough Inferred from Paleomagnetic Study of the Ryukyu Arc. Jou,: Geophys. Res., 100: 8169-8184 Park, J. O., Hidekazu, T., Masanao, S., et al., 1998. Seismic Record of Tectonic Evolution and Backarc Rifting in the Southern Ryukyu Island Arc System. Tectonophysics, 294: 21-37
Wu Shiguo, Ni Xianglong and Guo Junhua
10
Yang, W. D., Zhang, Y, B., Shi, J., 2004. A Study on Modem Tectonic Tension Fault in the Okinawa Trough. Offshore Oil, 24(4): 15-20 (in Chinese with English Abstract)
Geophysical Prospecting, 33(4): 532-540 (in Chinese with
English Abstract) Zhou, J. X., 2005. The Balanced Cross-Section Method for
Zeng, Z. G , Zhai, S. K., Du, A. D., 2003. 0 s Isotopic
Restoration of Structural Evolution in Compressional
Compositions of Seafloor Massive Sulfides from the Jade
Basins with Synkinematic Sedimentation and Its
Hydrothermal Field in the Okinawa Trough. Chinese
Application. Acta Geoscientica Sinica, 26(2): 151-156 (in
Journal of Oceanology and Limnology, 34(4):407-414 (in
Chinese with English Abstract)
Chinese with English Abstract) Zhang, M. S., Chen, F. J., 1998. Application Condition of Balanced-Section Technique and the Case Analysis. Oil
ISSN 1002-0705
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough Wu Shiguo" (%Ffm) Key Lab of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China; College of Georesources and Information, Petroleum University of China (Huadong), Qingdao 266555, China Ni Xianglong (!%#&), Guo Junhua ($BFq) Key Lab of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 26607 1, China; Graduate School, Chinese Academy of Sciences, Beijing 100049, China ABSTRACT On the basis of the multi-channel seismic data and the other data, using 2DMove software, the tectonic evolution in three seismic profiles was restored since Pliocene. The tectonic restoration results show that: (1) the initial active center lay in the west slope and then was transferred to east and south via trough center during the evolution process; (2) several main normal faults controlled the evolution of the southern Okinawa Trough; (3) since Late Pliocene, the southern Okinawa Trough has experienced two spreading stages. The early is depression in Early-Middle Pleistocene and the late is back-arc spreading in Late Pleistocene and Holocene, which is in primary oceanic crust spreading stage. KEY WORDS: balanced cross section, tectonic evolution, back-arc basin, Okinawa Trough.
INTRODUCTION The Okinawa Trough (OT) lays the boundary between Eurasian plate and Philippine Sea plate (PSP). It is a nascent back-arc basin of the trench-arc-basin system in West Pacific margin. It is a key spot for continental margin rifting, especially for the Chinese continent and its adjoining areas. Many scholars have paid much attention to it (Liu et al., 2005; Zeng et al., 2003; Jin and Yu, 1987; Kimura, 1985; Lee et al., 1980; Herman et al., 1978). This paper is supported by the Knowledge Innovation Project
of Chinese Academy of Sciences (Nos. KZCX3-SW-219, KZCX3-SW-224) and the Taishan Scholarship Project of Shandong Province. *Corresponding author: swu 0ms .qdio .ac.cn Manuscript received September 28,2006. Manuscript accepted December 20,2006.
The OT tends NE in whole, and is divided into three parts by Tokara fracture zone and Miyako fracture zone (Fig. 1). The north part tends NNE, with depth about 600-800 m. The middle part is NE tending, with 1 000-1 200 m deep, and the south part NEE, mostly over 2 000 m deep (Fig. 2). In this area, the crust is thinned, with strong tectonic movements and high heat flux (Fan and Yang, 2004; Li et al., 2001; Jin and Yu, 1987). The Miyako fracture zone is similar to transformation fault. To its north, the faults are right-strike slip, whereas in the south are left-strike slip (Jin and Yu, 1987; Fig. 1). There remains dispute on the rifting stage. Depending on seismic strata, bore data, and the strata development features in this area, the OT mainly experienced three tectonic periods: Late Miocene, Late Pliocene-Early Pleistocene, and Late Pleistocene-Holocene. Late Miocene is the rifting stage of OT (Jiang et al., 2003; Zeng et al., 2003). Herman et al. (1978) thought the rifting occurred
2
Wu Shiguo, Ni Xianglong and Guo Junhua
between Late Miocene and Early Pleistocene, when the Ryukyu islands broke from the continent, based on the nonconformity between upper and lower sediment layers. Kimura (1985) suggest that the opening of OT began 6-4 Ma BP, based on the sedimentary sequences in middle OT. However, Park et al. (1998) thought that during 6-2 Ma BP there was no spreading, it began about 2 Ma BP; whereas Miki (1995) thought it began spreading since 10-6 Ma BP. There are also different opinions on the structural property of present
O T some scholars thought the south and middle parts of OT are on "oceanic-crust" spreading (Yang et al., 2004; Jiang et al., 2003; Jin and Yu, 1987), whereas others thought there is no spreading (Liu, 2001). Therefore, further study about the formation and evolution of OT is necessary. On the basis of seismic interpretation and geological analysis of OT (Guo, 2004), the quantity described on the formation and evolution process using balanced cross section technique is discussed in this article.
32"N
30"
28"
26"
24"
Figure 1. Geological map of East China Sea (modified after Jin and Yu, 1987). 1. Hupijiao rise; 2. Haijiao Qiantang I rise; 3. Yushan rise; 4. Wuyishan low rise. @ Fujiang depression; @ Yangtze depression; @ Xihu I depression; @ Taipei I depression. depression; @ Oujiang depression; @
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
3
Figure 2. Map of water depth in Okinawa Trough with seismic lines. A, B, C are the seismic lines. SEISMIC SEQUENCES The data used were collected by R N “KEXUE I” Cruises during 1982-1984. After Jin and Yu (1987), Park et al. (1998) and Liu et al. (2005), five seismic sequences, which have various velocity models (Table l), were identified and modified. Three seismic profiles in southwest OT (Fig. 2) were chosen to restore by further process and interpretation. The profiles show no fold, and all strata are disconformity contact or conformity contact. This
means that in the whole trough, there is no compression, and the OT is extensional structural environment. Several main faults controlled sedimentary development in the OT. During Late Pleistocene, many small contemporaneous faults developed. So this period had strong tectonic activity with high deposition rate (Fig. 5). Some diapirs also developed, which may have been formed by magma upwelling (Fig. 3).
Figure 3. Interpretation map of section A (position in Fig. 2).
4
Wu Shiguo, Ni Xianglong and Guo Junhua CDP
0 I -2 $3 + 4 5
6
CDP
- 3000
4000
6000
5000
7000
8000
9000
10000
11000
Figure 4. Interpretationmap of section B (position in Fig. 2). oCDP 500
1100
1700
2300
2900
3500
4100
4700
5300
5900
6500
7100
5900
6500
7100
7700
1 2 h
w
v
E 3
4 5 6 CDP 500
1100
1700
2300
2900
3500
4100
4700
5300
"I
7700 '
I
I
6'
Figure 5. Interpretationmap of section C (position in Fig. 2). METHOD AND RESULTS OF BALANCED CROSS SECTION Balanced cross-section technique is a quantifying tool for geological structures. Dahlstrom (1969) first advanced the concept, and also made some applications. From then on, it developed and improved continuously. At present, it is mainly applied for checking result of seismic interpretation, quantity study on tectonic deformation, and tectonic evolution of basin (Zhou, 2005; Diao and Cheng, 2004; Mao et al., 1998). Section keeping balance is a base geological rule. On the basis of the law of conservation of matter, the
balanced cross-section technique is advanced, following the laws of conservation, the agreement of layers' length, displacement, and shortening. Zhang and Chen (1998), and Diao and Cheng (2004) concluded the steps by applying this technique: firstly, construct proper geologic column model; secondly, establish the structural setting, this technique is more suitable for simple compaction or extension area; thirdly, collect aspect of profiles, the optimum aspects parallel the greatest structure migratory direction, for it can evade sliding movement; finally, recover the tectonic compaction.
5
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
Table 1 Seismic sequences in the Okinawa Trough Period
Date(Ma)
Reflected waves
Sequences in shelf
Sequences in insular slope
Sequences in trough horizon velocity (WS)
I, 1.6-2.0
I
11, 2.23-2.40
/
Holocene I
T; -
0.01
Pleistocene
Middle-Late
t-
0.7 2
III, 2.23-2.40
IV-2
Pliocene I
5.3
- 0
I
Miocene
1 I !
2.51-3.13
IV-1
I
I
V
V
v-2
3,67459
v-1
Oligocene
23.1
G
36.0 57.8 66.4
lT i-4 Mesozoic
NW 0.
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
0
8
16
24
32
40
48
56
-E
0
-
0
2 4
E 2 4 8
0 h
E
5 4
.x
8
-
0
E r
3 4 8
1
Holocene
Middle-Late Ple,stocene
Early Pleistocene
Pliocene
Pre-miocene
I
Figure 6. Restoration of section A. Following the above steps, the profiles A, B, and C (Fig. 2) were restored by 2Dmove software. Because of shortage of data, the Pliocene and Miocene strata were not restored. So, Early Pleistocene,
Middle-Late Pleistocene, Holocene and the present of the above three seismic profiles were restored and the results are shown in Figs. 6-8.
6
Wu Shiguo, Ni Xianglong and Guo Junhua
Figure 8. Restoration of section C. The restoration maps of line A in four periods are in Fig. 6. The Pliocene strata developed at the northern continent slope (Fig. 6). Old Miocene strata only occurred in the shelf. Normal faults developed on shelf, and a big normal fault controlled the sedimentations. Two subsequence stages can be
divided in the profile. The early extension occurred in Early Pleistocene and accepted 1 000 m sediment, and the late stage was in the Middle-Late Pleistocene, whereas the Holocene was about 2 000 m, with 4% extension rate. The whole extension rate was 7% greater. Depocenter transferred from the shelf in
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
Pliocene to the island slope in Holocene, completing the south transference of depocenter. The tectonic restoration of line B is on Fig. 7. Four sediment layers overlaid basement. A few inverse faults developed on the continent slope. But many normal faults developed mainly on the continental slope, trough center, and the islands slope. And the faults were active in the trough floor. So the remainder of Miocene was conserved. The active faults controlled two depocenters in Early Pleistocene. The two depocenters had 1 700 m and 2 050 m thick sediments, respectively. During Middle-Late Pleistocene the depocenter migrated to the trough. And the depocenters accepted 1 700 m and 2 050 m respectively. Ascribing to sedimentary compaction, the Early Pleistocene became 750 m and 1 500 m thick in the two depocenters respectively. And compaction amplitude in center was bigger than in east. This may be because the Middle-Late Pleistocene sediments in center were heavier than in east. During Holocene, the sediments deposited in center and east part with thickness of 1 600 m. The compaction of Early Pleistocene was small, changing into 700 m and 1 450 m. Whereas the Middle-Late Pleistocene was big, from 2 200 m and 1 600 m, changing into about 1 200 m and 900 m. But the extension rate was very small, only about 3%. The tectonic restoration of line C is on Fig. 8. The sedimentary feature is the same as Fig. 7. There also were two depocenters, trough center and off islands slope. By Middle-Late Pleistocene, the Early Pleistocene deposited 3 050 m and 1 900 m separately in two depocenters. Whereas by Late Pleistocene, Middle-Late Pleistocene deposited about 1 900 m and 1400 m, making the Early Pleistocene sediment compact to 3 000 m and 1 590 m. Obviously, the compaction was not big, so regional subsidence might have been intense. The Holocene depositing into this area was the same as the line B, with 1 300-1 500 m thickness. Ascribing to sedimentary compaction, the Early Pleistocene became 2 750 m in west slope and 1 550 m in middle part, whereas the Middle-Late Pleistocene became 1 600 m and 900 m. From the map, the stretching rate of line C was about 4%, amounting to 3 000 m. From the above, the stretching rate and
7
compaction rate are different in different stages, and mainly in Early Pleistocene and Late Pleistocene to Holocene, the two spreading stage of OT. The line A is the biggest in stretching rate among the three lines. Maybe in the southwestern OT there appears oceanic-crust spreading. That is to say, OT is at the initial stage of oceanic-crust spreading.
DISCUSSION The faults develop widely in this area (Fig. l), and most are normal faults with high angle (Figs. 3-5). The north subduction of Philippine Sea plate and Huatong basin makes tensional stress flow back to the Ryukyu Islands. Then the upwelling of materials from mantle forms the OT. In this tensional environment, normal faults develop widely, whereas some inverse faults develop locally because of compression. It is easier to see the formation and the controlling of faults in this area from the restoration maps. Most of the faults in this region formed before Early Pleistocene. The faults are mostly normal faults and they control the development of sedimentary strata in OT. Only a few inverse faults occur in the west slope of OT. So, the southern part of OT is in tensional environment in this period. The basement is composed of Pliocene and Miocene sedimentary layers and volcanic rocks. It is thus evident that the southern OT began to form and accept sediments. During Pleistocene, there are two depocenters in the trough floor and off island slope, which accumulated thick sediments. But to continent shelf and slope, there is no Pleistocene sediment, maybe because the sea level decreased, the slope was steep and tectonic activity was strong. During this period, many normal faults dipping to trough are formed in the slope induced by subduction of Philippine Sea plate. The dip angle is high and sediment distorts very intensely. Holocene sediments cover the whole area, and are thick in the two depocenters. Few faults can be discerned from the profiles. It means that, the tectonic movement was not active during Holocene. From the above analyses, the development of faults controlled the migration of depocenters. The depocenter migrated the continent slope to off island slope. Since the Pliocene, the profiles are stretching (Table 2). Especially on Fig. 7, the extension arrives to
8
Wu Shiguo, Ni Xianglong and Guo Junhua
4 000 m, with stretching rate up to 7%. On Figs. 8-9, this phenomenon is not obvious, with rate of 3%-4%, eroding excluded. The difference in rate may be caused by their places. Line A lies in most southern of
OT, whereas lines B and C lie off Miyako fracture zone. The OT rotates clockwise during spreading, and rotation axis is Miyako fracture zone. Comparing rates, this rotation may have happened in Holocene.
Table 2 Stretching rate of the section along seismic lines Seismic line
A
B
C
Extension amount (&/rate (%) in Holocene Extension amount (m)/rate(%) in Middle-Late Pleistocene Extension amount (m)/rate(%) in Early Pleistocene
2 000/3.33
1 000/1.32
1000/1.32
1600/2.10
1400/1.84
100011.67
70010.92
900/1.18
Total extension amount (m)/rate (%)
4 000/6.67
3 300/4.34
3 300/4.34
~~
East China Sea
Okinawa Trough
ooo/l .67
Ryukyu Islands Arc
Front of Arc
Ryukyu
--+4-bt-------b'-bW 0 h
g
2
5 2 4 L3
6
0 E c 2 5 a 8 4
h
6
h
E
c2
5
24
L3
6 Miocene
Pliocene
I.*:.:,:.[
Early Pleiocene
~ ~ ~ ~ ~ e & n ~ a t Holocene e
Figure 9. Evolution model of Okinawa Trough in Cenozoic. The evolution model of OT can be constructed as shown in Fig. 9, according to the seismic interpretation, which includes depositing, uplift and erosion, rifting, and initial spreading. During the Miocene the southern OT accepted the huge thickness
of sediments (Fig. 9a), Pliocene strata were distributed in both slopes, but absent in the trough basin. It is inferred that, the Pliocene originated from the Chinese mainland deposited in this region, and the trough did not open. In the Late Pliocene the southern OT started
Balanced Cross Section for Restoration of Tectonic Evolution in the Southwest Okinawa Trough
to rise, and thinning in the center subducted by Gagua Ridge. So the region uplifted and the Pliocene and the early deposits were eroded because of this movement. The Pliocene in the trough area were nearly eroded and Miocene remained only a little as the residual deposition. The acoustic basement was exposed in some areas. Then nonconformity occurred. From the seismic profiles, the Miocene pinched out northwestward probably because of subsidence. After the last movement, the trough area began to rift. The Early Pleistocene concentrated into the trough area and filled the depressions. With the sea level ascending, the Middle and Late Pleistocene and Quaternary deposited. The neonatal submarine volcanoes suggest that the southern OT is in the initial seafloor spreading stage. This agreed with the preceding studies (Yang et al., 2004; Jiang et al., 2003; Jin and Yu, 1987).
9
Fan, D. J., Yang, Z. S., 2004. Development and Distribution of Natural Gas Hydrate in the Okinawa Trough. Acta Petrolei Sinica, 25 (3): 11-17 (in Chinese with English Abstract) Guo, J. H., 2004. Seismic Sequences and the Tectonic Restoration
of
the
Southern
Okinawa
Trough:
[Dissertation]. Institute of Oceanology, Chinese Academy
of Sciences, Qingdao (in Chinese with English Abstract) Herman, B. M., Anderson, R. N., Truchan, M., et al., 1978. Extensional Tectonics in the Okinawa Trough. AAPG Memoir, 29: 199-208 Jiang, W. W., Liu, S. H., Hao, T. Y., et al., 2003. Character of Geophysical Field and Crustal Structure of Okinawa Trough and Adjacent Region. Progress in Geophysics, 18(2): 287-292 (in Chinese with English Abstract) Jin, X. L., Yu,P. Z., 1987. The Tectonic Features and Evolution of Okinawa Trough. Science in China (Se,: B), 2: 196-203 (in Chinese with English Abstract) Kimura, M., 1985. Back-Arc Rifting in the Okinawa Trough. Ma,: Pet. Geol., 2: 222-240
CONCLUSIONS The results of balanced cross-section restoration and seismic interpretation show that, since Pleistocene the depocenters have migrated to east; several large faults controlled the development of minor faults and strata; the OT has experienced two spreading since Pliocene, the early was fault depression in Early-Middle Pleistocene, the late was back-arc spreading from Late Pleistocene to Holocene. And at present, it is in the initial stage of oceanic crust spreading.
Lee, C. S., Shor, G G, Bibee, L. D., et al., 1980. Okinawa Trough: Origin of a Back-Arc Basin. Marine Geology, 35: 2 19-24 1 Li, W. R., Yang, Z . S., Wang, Q., et al., 2001. Tenigenous Transportation through Canyou and Sedimentation of Submarine Fan in the Okinawa Trough. Chinese Journul
of Oceanology and Limnology, 32(4): 371,380
(in
Chinese with English Abstract) Liu, J. H., 2001. Features of Seismic Reflection Wave in the South Okinawa Trough and the Geological Interpretation. Donghai Marine Science, 19(1): 19-26 (in Chinese with English Abstract)
ACKNOWLEDGMENT The research was supported by the Knowledge Innovation Project of Chinese Academy of Sciences (Nos. KZCX3-SW-219, KZCX3-SW-224) and the Taishan Scholarship Project of Shandong Province. The authors would like to thank Prof. Yu Puzhi for advice on revising this article.
Liu, J. H., Lin, C. S., Gao, J. Y., et al., 2005. Analyses on the Basement Constitution of the Southern Okinawa Trough, Chinese Journal of Oceanology and Limnology, 36(3): 261-267 (in Chinese with English Abstract) Mao, X. P., Wu, C. L., Yuan, Y. B., 1998. Three Dimensional Structural Modeling: Volume-Balance Technique. Earth Science-Jouml
of China University of Geosciences,
24(5): 506-508 (in Chinese with English Abstract)
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