Author(s) Teramae, Noriaki; Hayashi, Daigoro.

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1 Title Paleostress analysis around Central Ryukyu, Japan souther Author(s) Teramae, Noriaki; Hayashi, Daigoro Citation 琉球大学理学部紀要 = Bulletin of the College University of the Ryukyus(74): 33-4 Issue Date URL Rights

2 Bull. Fac. Sci., Univ. Ryukyus, No. 74 : (2002) 33 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Japan Noriaki Teramae* and Daigoro Hayashi* 'Department of Physics and Earth Sciences, University of the Ryukyus, Nishihara, Okinawa, , Japan Abstract Meso-scale faults are investigated for reconstruction of paleostress recorded in the Shimajiri Group during the upper Miocene to lower Pleistocene exposed around south ern region of Okinawa-jima (island). Stress field was estimated by means of the stress inversion method. Every fault slip data from the three formations, namely Tomigusuku, Yonabaru and Shinzato Formation, were calculated. The datasets were divided into subgroups by means of histogram which drew angular misfits between observed and predicted slip directions. Newly calculated stresses are classified by the axial directions and stress ratios. As a result, four stress states were obtained for the Shimajiri Group. Stress state A is NW-SE extension, B is NE-SW extension, C is E-W extension and D is strike-slip type. The order of stress state is C and D, B, A from older to newer. These stress states can be interpreted as a result of subduction of the Philippine Sea plate beneath the Eurasia plate and the crustal extension in the Okinawa Trough. 1. Introduction Ryukyu arc is a convergent margin in the northwestern edge of the Philippine Sea, and is extending about 1200 km in length from southern Kyushu to Taiwan. Ryukyu arc-trench system is characterized by the subduction of the Philippine Sea plate beneath the Eurasia plate and by the crustal extension in the Okinawa Trough (Aiba and Sekiya, 1979; Lee et al., 1980; Kimura, 1985; Letouzey and Kimura, 1985, 1986; Sibuet et al., 1987). Okinawajima is an island which is located in the middle of Ryukyu arc (Fig.l). Okinawa-jima is divided into northern part and southern part, the former is composed of Pre-Neogene base ment rocks and the latter is composed of Neogene-Quaternary sedimentary rocks, respec tively. Southern area of Okinawa-jima is composed of the Shimajiri Group which consists of sandstone and mudstone, and the group is unconformably covered by the Ryukyu Group which consists of Quaternary limestone. Many meso-scale faults are found in the Shimajiri Group which offers valuable constraints on the temporal evolution of stress field in the Ryukyu arc - Okinawa Trough region. In this study, 150 striated fault planes were exam ined in 31 sites scattered over the southern part of the Okinawa-jima. With regard to stress field in this area, using fracture analysis, Fournier et al. (2001)

3 Noriaki Tbeamae and Daigoro Hayashi 25'N MIYAKO stud>" DEPRESSION area Jcmfyi. PHILIPPINE SEA 125'E 130-E Fig.1. Index map of the Okinawa -jima (island). After Miki (1995). have obtained three episodes of extension: a late Miocene N40 W to N20 E extension (episode I ), a late Pliocene to early Pleistocene N20 E extension (episode II), and a lat est Pleistocene to present-day N20 W extension (episode M). Although they calculated fault slip data for each fault point, we calculated them for each formation in order to elu cidate the transitional differences of paleostress fields through time. The aim of this paper is to investigate faults in the southern area of the Okinawajima and to reconstruct the paleostress field on the basis of the stress inversion method. 2. Geological setting Shimajiri Group consists mainly of mudstone interbedded by sandstone and tuff, ranges from the upper Miocene to the lower Pleistocene and is divided into three forma tions, Tomigusuku, Yonabaru and Shinzato Formation in ascending order (Fig.2). The group is distributed in the fore-arc area on land and offshore along the Ryukyu islands (Kizaki, 1986) and is considered to be deposited during upper Miocene to lower Pleistocene age (Kizaki, 1986). Thickness of the group tends to decrease westward and to expose in the southern part of Okinawa-jima. Maximum thickness of the group is 2600m with general trend of N50 E to 60 E strike and 10 to 20 SE dip. Figures 3-5 show sampling locali ties, and typical fault structure from each formation.

4 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 35 Epock Formation Holocene Miuatogawa F. Ryukyu Group Pleistocene NahaF. ChinenF. Shinzato F Om Pliocene Yonabaru F. 1100m+ Tomigusuku F. Shimajiri Group Miocene Oroku Sandstone Fig.2. Stratigraphy of the southern Okinawa-jima Tomigusuku Formation The formation distributes in the western part of the study area (Fig.3). Upper part of the formation mainly consists of massive and brownish yellow sandstone which is called "Oroku Sandstone". It is difficult to find fault-slickenline in the formation because the sandstone is weakly consolidated Yonabaru Formation Yonabaru Formation conformably overlies the Tomigusuku Formation, and widely ex posed in the study area (Fig.4). The formation consists of grayish mudstone interbedded with thin sandstone and tuff. The total thickness is about 1400m Shinzato Formation Shinzato Formation conformably overlies the Yonabaru Formation. The formation is dominated in the Chinen Peninsula and the eastern small islands, Miyagi-jima and Hamahiga-jima (Fig.5). The lower part of the formation is called "Shinzato Tuff" whose thickness is 5 to 10 m and is a useful key bed. In contrast to the Yonabaru Formation, sandstone becomes abundant Meso-scale faults in Shimajiri Group Shimajiri Group has many meso-scale faults with few cm to 2 m displacement. Meso-

5 Noriaki Tebamab and Daigoro Hayashi A lower Pliocene Yooabwu Miid«onc and!{. ', ;! >". ' Sanditonc mudstone sandstone Fig.3. A Location map of Tomigusuku Formation. Modified after Fournier et al. (2001). B Meso-scale normal fault in Tomigusuku Formation (Oroku Sandstone, To7).

6 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 37 A B rmudstohe mudstone Fig.4. A Location map of Yonabaru Formation. Modified after Fournier et al. (2001). B Meso-scale normal fault in Yonabaru Formation (Yo 7).

7 Noriaki TERAMAE and Daigoro Hayasiii A lover Pliocene Yoeil>»rn Mudstonc mdttik»eiiju]cn Sandstone B tuff fniidstone r x Fig.5. A Location map of Shinzato Formation. Modified after Fournier et al. (2001). B Meso-scale normal fault in Shinzato Formation (Si 5).

8 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 39 scale faults are mainly normal faults. As the Tomigusuku Formation distributes in popu lous area, it is difficult to measure fault-slip data except 26 faults. The number of meas ured faults in the Yonabaru FormaLion is 96. In comparison with the Tomigusuku and the Yonabaru Formation, Shinzato Formation distributes in a limited area where 28 fault slip data are measured. Total 150 faults are measured in the Shimajiri Group. 3. Paleostress Analysis 3.1. Stress Inversion Method Meso-scale fault is termed as a fault which can be seen in the outcrop scale (Angelier, 1994). Meso-scale fault analysis is a technique to estimate the stress state from the orien tation of fault plane and the slip direction of meso-scale faults (Yamaji, 2001). Although various methods have been developed for stress analysis, the classical stress analysis method using conjugate faults is theoretically incorrect. Standard method used recently is the inversion method developed by Angelier (1979, 1984) and Marrett and Allmendinger (1990) where it is assumed that faults occur along the direction of maximum shear stress (Wallace, 1951; Bott, 1959). A sense of movement along fault is deduced from slickenline on the fault plane. The assumption of homogeneous stress field in a considering area is a weak point of the inversion method because the most fault-slip data is heterogeneous (Yamaji, 2001) Method Authors measured the strike and dip of faults, and trend and plunge of slickenlines on the fault planes within the Shimajiri Group. Fault-slip data are measured for each forma tion to explain the difference of paleostress fields through time. Orientations of three prin cipal stress axes are determined by using the freeware program of stress inversion method "Tectonic VB" which was developed by H.Ortner Result Figure 6 shows the results of paleostress analysis for three formations by the stress in version method. Fig.6 A, B and C show Tomigusuku Formation, Yonabaru Formation and Shinzato Formation, respectively. Great circles are the fault planes that show the strike and dip of faults, and arrows indicate the trend and plunge of slikenlines. N is the number of measured faults in the formation, Stress ratio < > is described as O = (ct2 Os)/{ O\ (7a) and shows the values between 0 and 1. Symbols #,, in the circles show principal stress axes; 0 shows maximum compressive stress axis ( ad, shows intermediate compressive stress axis ( a2) and shows minimum compressive stress axis ( ct3). The black arrows outside the circles indicate the directions of paleostress.

9 40 Noriaki TERAMAfi and Daigoro Hayashi Tomlgusuku Formation A D N=26 m.. m Yonabaru Formation E.1 2 N=96 4>=0.031 Shinzato Formation - N=28 4> =0.018 Fig.6. Fault-slip data from Tomigusuku (A), Yonabaru (B) and Shinzato (C) Formation. Histograms showing angular misfits between observed and predicted slip directions are presented in D-F. Great circles are the fault planes that show the strike and dip of faults. Small arrows on the fault planes indicate the sense of slip. Arrow head means that the sense of slip was determined with full confidence. N is the number of measured faults in the formation. Stress ratio <t> is described as 1 =(o,-a,)/(ci1-o,) and shows the values between 0 and 1. Symbols #,, A in the circles show principal stress axes; # shows maximum compressive stress axis ( o,), M shows intermediate compressive stress axis (O and A shows minimum compressive stress axis (o3). The large divergent arrows outside the nets indicate the directions of extension and the large con vergent arrows indicate the direction of compression.

10 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan Stress field of Shimajiri Group Figures 6A-C show that stress field in the Shimajiri Group is approximately an axial compression with vertical O\ axis. Stress state in the Tomigusuku Formation is E-W exten sion with vertical Oi axis and <D value is shown in Fig. 6A. Stress state of the Yonabaru Formation is NE-SW extension with vertical <7t axis and C> value is as shown in Fig. 6B. Stress state of the Shinzato Formation is NW-SE extension with vertical CTi axis and <D value is shown in Fig. 6C Separation of stresses We separate stresses using the histogram of angular misfits between observed and pre dicted slip directions (Fig. 6D-F). Histogram for the Shinzato Formation is not scattered, which shows that calculated stresses in the formation are reliable. For the Yonabaru Formation, the histogram shows that calculated stresses are fit to the observed fault-slip data, except for few data. Data sets of calculated stress are divided into two subgroups, YOl (small misfit) and YO2 (large misfit). Subgroups YOl and YO2 are bounded at 10 degree. Their optimal stresses in each data set were calculated as shown in Fig. 7. Newly calculated stress for YOl subgroup is similar to the previous stress of the Yonabaru Formation. New stress for YO2 is calculated from 7 faults, and its stress state shows NW-SE extension and C> value is Principal stress direction for YO2 is similar to that of the Shinzato Formation. For the Tomigusuku Formation, direction of deduced shear is mostly scattered in the Shimajiri Group (Fig.6). Data sets of calculated stress for the Tomigusuku Formation are divided into three subgroups, that is, TO1 (small misfit), TO2 (intermediate misfit) and TO3 (large misfit). Subgroups TO1, TO2 and TO3 are separated at 10 degree and at 20 de gree, respectively. The optimal stresses in each data set are calculated as shown in Fig. 8. Recalculated stress for the subset TO1 is similar to the unseparated stress. The subset of TO2 is composed of 5 faults whose new direction of extensional stress is NNE-SSW which is similar to that for YOl and 0 value is Subgroup TO3 consists of 3 faults whose new stress state is triaxial stress state, and <D value is Discussion 4.1. Classification of stress state The obtained six stress states are classified by the direction of principal stress and stress ratios (<X>). NW-SE extension is found in both the Yonabaru Formation (YO2) and the Shinzato Formation. We think these stresses are the same, and labeled this as stress state A. NE-SW extension found in both the Tomigusuku Formation (TO2) and the Yonabaru Formation (YO2) is named as stress state B. E-W extension found in Tomigusuku Formation (TOl) is named as stress state C. Strike-slip type where O\ directs to NNW-SSE and a% directs to NE-SW found in the Tomigusuku Formation (TO3) is called

11 Noriaki Teramae and Daigoro Hayasi-h Yonabaru Formation N=96 =0.031 H YO2 N=S9 <t> =0.032 K 0-10 ll-»0 ll~» <I~M ll-«0 tl-70 TI-10 I1--M Yonabaru Formation 1 Yonabaru Formation 2 Fig.7. Separation of stress for Yonabaru Formation. Symbols are same as in Fig.6.

12 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 43 TO1 N=26 I 2 ' 4> =0.108 Tomigusuku F N-18 * r -. - n n n TO2 TO3 N=5 =0.152 N=3 * =0.459 n. Fig.8. Separation of stress for Tomigusuku Formation. Symbols are same as in Fig.6.

13 44 Noriaki Teeamae and Daigoro Hayasiji Table 1. Classification of stress state in Shimajiri Group. Stress state dataset 7a 0 A Si Yon vertical NW-SE B Yol Ton vertical NE-SW C To I vertical E-W D Tom NNW-SSE NE-SW as stress state D. We have little confidence in the stress state D because the number of faults is only three. Table 1 shows the classification of stress states. Thus the six optimal stress states are reduced to four stress states Age of stresses Figure 9 shows the chronology of stress state. The stress state A is for the Yonabaru and Shinzato Formation. We consider that the stress state A is the latest during the period of the Shimajiri Group. Stress state B is held for the age of the Tomigusuku and Yonabaru Pleistocene Shinzato Formation \ a Yonabaru Formation Pliocene AW B V if Tomigusuku Formation Miocene B c \* Fig.9. Relative chronological relationships between lithological age and obtained stress state.

14 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 45 Formation, thus the stress state B occurred earlier than stress state A. Stress state C and D continued during the period of the Tomigusuku Formation. It is difficult to identify the ages of stress state C and D compared with the stress states A and B. However, we can interpret that these two stress states C and D are the oldest stress state in the age of the Shimajiri Group, because they only recorded in oldest Tomigusuku Formation. Thus the order of stress state is C and D, B, A from older to newer. Nearly zero value of the stress ratio <b for the Shimajiri Group shows that stress state is as^ o Comparison with previous studies As mentioned before, Fournier et al. (2001) have obtained three episodes of extensional stress by using about 450 tectonic joints and striated fault planes measured in 20 localities scattered in our studied area. In comparison with their results, the stress states A and B are similar to their episode II (N20 E extension) and II (N20 W extension), respectively. Stress state C and D are different from their studied stress state. With regard to Miyakojima, Otsubo and Hayashi (2001) have obtained two extensional stress fields in the Shimajiri Group, and one extension and a strike-slip type stress states in the Ryukyu Group. Their two extensions within the Shimajiri Group are similar in direction to the stress states A and B in this study.. For the stress state D, Otsubo and Hayashi (2001) re ported the same type of stress state in the Ryukyu Group, and also Fabbri and Fournier (1999) have obtained it in the Ishigaki-jima, South Ryukyu. 4.4.Tectonic model The four stress states in this study have a genetical relation to the subduction of the Philippine Sea plate beneath the Eurasia plate and the crustal extension in the Okinawa Trough. The Okinawa Trough, which is expanding in present, runs parallel to the western Ryukyu arc in the East China Sea, and is considered the back-arc basin of the Ryukyu arc (Kimura, 1985; Letouzey and Kimura, 1985, 1986; Sibuet et al., 1987). The stress state A, NW-SE extension, is related to the subduction of the Philippine Sea plate which has subducted beneath the Eurasia plate to NW at speeds of 5~7cm/yr (Seno et al.,1993) to produce "trench retreat" (Hu et al., 1996). The stress state B, NE-SW extension, is related to the stretching of the Ryukyu arc in response to the opening of the Okinawa Trough. This extension is considered a consequence of an arc-parallel stretching (Fabbri, 2000). The stress state C, E-W extension, occurred by the effect of the subduction of the Philippine Sea plate. The stress state D is the strike-slip type stress state where <7i directs to NNW-SSE and o3 directs to NE-SW. The sense of the strike-slip stress state is sinistral from the direction of slickenline on meso-scale fault planes. In our opinion, the stress state D occurred by the effect of the rotation of the southern Ryukyu arc. From paleomagnetic study, Miki (1995)

15 Noriaki Teramae and Daigoro Hayashi Stress state A Ryukyu Trench Stress state B Ryukyu Trench Stress state C Stress state D Okinawa-jima Ryukyu Trench J^ ^^ X Ryukyu Trench Fig.10. Tectonic model of four stress states. Stress state A is NW-SE extension which relates to the subduction of the Philippine Sea plate Stress state B is NE-SW extension which is related to the stretching of the Ryukyu arc in re sponse to the opening of the Okinawa Trough. Stress state C is E-W extension. Stress state D occurred by the effect of the rotation of the southern Ryukyu arc.

16 Paleostress analysis around southern area of Okinawa-jima, Central Ryukyu, Jpan 47 reported that the central Ryukyu arc had not experienced any significant rotation between 6-10 Ma, whereas the southern Ryukyu arc had rotated 25 clockwise. Since the stress state D is derived from only three faults, the stress state is not reliable. Further studies for stress state D will give precise tectonic history regarding rotation of central Ryukyu arc. 5. Conclusion Four stress states were obtained from the paleostress analysis in the Shimajiri Group: NW-SE extension (stress state A), NE-SW extension (stress state B)t E-W extension (stress state C) and strike-slip type stress state where Oi directs to NNW-SSE and oa directs to NE-SW (stress state D). The order of stress state in Shimajiri Group is C and D, B, A from older to newer. Stress state in the Shimajiri Group is d *=* Ot. The transition of stress has a relation to the subduction of the Philippine Sea plate beneath the Eurasia plate and the crustal extension in the Okinawa Trough. Acknowledgement We thank S. Baba for critical comments to the research. References Aiba, J., and E. Sekiya., 1979, Distribution and characteristics of the Neogene sedimentary basins around the Nansei Shoto (the Ryukyu Islands). J. Jpn. Assoc. Pet. Tecthnol., 44, (in Japanese with English abstract) Angelier, J., 1979, Determination of the mean principal stresses for a given fault popula tion. Tectonophygics, 56, T17-T26. Angelier, J., 1984, Tectonic analysis of fault slip data sets. Jour. Geophys. Res., 89, Angelier, J., 1994, Fault slip analysis and paleostress reconstruction. In: Hancock, P.L.(Ed), Continental deformation, Pergamon Press, Oxford, Bott, M. H. P., 1959, The mechanics of oblique slip faulting. Geol. Mag., 96, Fabbri, O., 2000, Extensional deformation in the northern Ryukyu arc indicated by mesoscale fractures in the middle Miocene deposits of Tanegashima Island, Japan. Jour. Geol. Soc. Japan., 106, Fabbri, O., and Fournier, M., 1999, Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back arc rifting. Tectonic, 18, Fournier, M., Fabbri, 0., Angelier, J and Cadet, J.P., 2001, Regional seismicity and onland deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough. J. Geophys Res., 106, Hu.J.C, Angelier, J., Lee.J.C, Chu.H.T., Byrne, D., 1996, Kinematics of convergence, de formation and stress distribution in the Taiwan collision area: 2-D finite-element nu merical modeling. Tectonophysics. 255,

17 48 Noriaki Tbramae and Daigoro Hayashi Kimura, M., 1985, Back-arc rifting in the Okinawa Trough. Mar. Pet. Geol. 2, Kizaki, K., 1986, Geology and tectonics of the Ryukyu Islands, Tectonophysics, 125, Lee, C.S., Shor, G.G., Bibee, L.D., Lu, R.S., and Hilde, T.W.C., 1980, Okinawa Trough, ori gin of a back-arc basin. Mar. Geol., 35, Letouzey, J., and Kimura, M., 1985, Okinawa Trough Genesis: structure and evolution of a backarc basin developed in a continent. Mar. Pet. Geol, 2, Letouzey, J., and Kimura, M., 1986, The Okinawa Trough: genesis of a back-arc basin de veloping along a continental margin. Tectonophysics, 125, Marrett, R. and Allmedinger, R.W., 1990, Kinematic analysis of fault-slip data. Jour. Struct. Geol., 12, Miki, M., 1995, Two-phase opening model for the Okinawa Trough inferred from paleomagnetic study of the Ryukyu arc. J. Geophys. Res., 100, Otsubo, M. and Hayashi, D., 2001, Miocene to Pleistocene stress field transitions, around the Miyako-jima Island, South Ryukyu, Japan. Bull. Fac. ScL, Univ. Ryukyus, 72, Seno. T., STEIN, S. and GRIPP, A. E.( 1993, A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res., 98, Sibuet, J. C, et al., 1987, Back arc extension in the Okinawa Trough, J. Geophys. Res. 92, 14, , 063. Wallace, R.E., 1951, Geometry of shearing stress and relation to faulting. Jour. Geol., 59, Yamaji, A., 2001, Review on fault striation analysis. Jour. Geol. Soc. Japan, Vol.107, No.7, (in Japanese with English abstract)