Development of joints in the Sandankyo Area, Hiroshima Prefecture, Southwest Japan Yasuyuki Hirayama*, Takaya Hayashi, Shinichi Nakai, Atsuo Tsush

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1 Development of joints in the Sandankyo Area, Hiroshima Prefecture, Southwest Japan Yasuyuki Hirayama*, Takaya Hayashi**, Shinichi Nakai**, Atsuo Tsushima*** and Yukimasa Oho* * Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, Japan ** Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, Japan *** Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Japan Abstract Joints are markedly developed in the acidic welded tuff of the Sandankyo Area, Hiroshima Prefecture, Southwest Japan. Six sets of joint are distinguished in this area, namely N-S set, NE- SW set, NNE-SSW set, NW-SE set, NNW-SSE set, and horizontal set. It is considered that NE- SW, NNE-SSW, NW-SE, and NNW-SSE sets of joint were formed by the simple shear due to NE-SW strike-slip faulting with right-lateral shear sense. Detailed microscopy shows that N-S and NE-SW sets of joint contain narrow quartz vein with newly recrystallized muscovite, and cut by the other sets. This suggests that, preceding the NE-SW faulting, WNW-ESE extention accompanied with granite porphyry intrusion formed N-S and NE-SW sets of joint. ( 1000 m) ( m) ( 100 m) ( 1972), , 1985

2 N Mochinoki fault Misumi Index map Sandankyo area Hiroshima Yokogawa fault Mochinok i Yokogawa river Yoshigahara Shiwagi river Kurofuchi Shiwagi Rhyolitic pyroclastic rocks Granite porphyry Strike and dip of major joints Fault Granite km Itagatani fault

3 Fig mm0.2-1 mm 10-30μm1995 NE-SW 1959Fig K-Ar N NE-S W,ENE-WS W set s N-S set NE-SW set N ENE-WSW set NW-SE,NN W-SS E set s Horizontal N-S set NW-SE set Horizonta l se t NNW- SSE set 5% 4% 3% 2% 1% km

4 (Figs. 1, 2, 3) Fig. 3 Fig. 3 N-SNE- SWENE-WSWNW-SENNW-SSE Fig. 2 m 10 m cm1 m 20 cm cmfig. 4 N20 W N20 E 60 NS 70 E 10 cm m cm Fig. 4N-S 1 km Fig. 3N50 E 85 SE 10 cm cm N60 E N E N70 E SE 10cm N50 W 85 NE 1 km 10 cm N30 W N30 W 70 SW NNW-SSE 30 cm1 m 20 cm 1 mcm Type-1 Type-2 Type-3 Type-1

$Fig. 4.1: N-S set of joint, striking N11 W and dipping to 78 E. Space between the two original fracture surfaces of joints is 2 to 10 cm. Fig. 4.2: N-S and horizontal sets of joint.$

5 広島県三段峡地域における節理の形成過程 Horizonta l set N-S set N-S set 50c m 3 NE-S W set 10c m 50c m 4 5 NNW-SS E set Horizonta l set 50c m NW-SE set NE-S W set 50c m Fig. 4. Joints in the welded acidic tuﬀ of the Sandankyo Area. Fig. 4.1: N-S set of joint, striking N11 W and dipping to 78 E. Space between the two original fracture surfaces of joints is 2 to 10 cm. Fig. 4.2: N-S and horizontal sets of joint. N-S set strikes N3 W and dips 74 E, and horizontal set strikes N3 E and dips 8 E. Fig. 4.3: NE-SW set of joint, striking N45 E and dipping 78 S. Fig. 4.4: NNW-SSE set of joint, striking N43 W, dipping 82 S. Dashed line shows joint, striking N23 E and dipping 72 E (N-S set?). Fig. 4.5: NE-SW, NWSE and horizontal sets of joint. NE-SW set strikes N47 E and dips 81 S. NW-SE set strikes N61 W and dips 84 N. Horizontal set strikes N30 E and dips 12 N.

平 28 山恭之林隆也中井真一津島淳生於保幸正 2 1 Type- 1 Type- 1 0.5mm 1mm 3 4 Type- 2 0.05mm 0.5mm 6 5 Type- 3 Type- 3 Type- 1 1mm Type- 2 0.5mm Fig. 5. Photomicrographs of three types of joint, Type-1, Type-2 and Type-3.

6 平 28 山恭之林隆也中井真一津島淳生於保幸正 2 1 Type- 1 Type mm 1mm 3 4 Type mm 0.5mm 6 5 Type- 3 Type- 3 Type- 1 1mm Type mm Fig. 5. Photomicrographs of three types of joint, Type-1, Type-2 and Type-3. Fig : Type-1 joint. Type-1 joints inﬁlled by quartz vein, cutting quartz phenocryst and matrix. Highlight platy minerals, which are muscovite, crystallize along Type-1 joint. Fig. 5.3: subgrains in quartz vein. Fig. 5.4: Type-2 joint, fracturing quartz phenocryst. The boundary part consists of micro-fractures. Fig. 5.4: Type-3 joint deﬁned by empty ﬁssures. Fig. 5.6: Type-1, Tpye-2 and Type-3 joints. Type-1 is intersected by both Type-2 and Type-3. Fig and Fig. 5.6: Crossed polars, Fig. 5.5: Plane-polarized light. る石英脈の組み合わせとして特徴づけられる Fig. は石英やカリ長石の斑晶および基質を切って連 Type-1の節理は５地点でのみ観察され続的に発達している破断面の形態は全体として南北あるいは北東南西方向の走向を持ちほぼみると直線あるいは緩やかな曲線を示す石英脈垂直に傾斜している Fig. 6 このタイプの節理の幅は mm程度でその幅は一つの連続し

7 N Type 1 Type 2 Type mm μm Fig. 5.3 Fig. 5.2 mm μmtype-1 Type-2 Fig. 5.4 Type-2 Fig. 6 μm mm 10 μm 100 μm Type-2 Type-1Fig. 5.6 Type-3 Fig. 5.5 Fig μm 0.3 mm Type-1Fig. 5.6Type-3 Type-2 Type-2 Type-3 Type

8 PTXRR' Nicolas, Fig. 7(1) NE- SWNNW-SSENW-SEENE-WSW NE-SW XR'R Fig. 7(2)PT PNNE-SSW NE-SWN-S T E-W NE-SWNNW-SSENW-SE ENE-WSW (1) Pattern of second order fractures associated with a dextral fault T R (2) Joint system of Sandankyo area NNE-SSW? X NW-SE NNW-SSE N-S P fault R NE-SW ENE-WSW E-W? NE-SW fault N-S Type-1 Fig. 5.2

9 Type-1 Type-1 Type-1 Fig. 6N-S NE-SW Type-1 Type-2 Type-3 Type-2 Type-3 Type-3 Type-2 Type-3 N-S Type-1Type-2 Type-3 Type-1 Fig. 5.6 Type-1 Type-1 N-SNE-SW Fig. 6 Type-1 N-SNE-SW 1985 N-S Type-1 N-S NE-SW N-S N30 E N60 E N20 W N50 W Fig. 8 N-SNE-SW Fig. 3 Type-1 N-SNE-SW N

10 1997 NE-SW NW-SE 1989 NW-SENE-SW N-S 1983 N-S N-S NE-SWNW-SE Kitagawa & Nishido K-Ar 31- Ma Type-1 N-S NE-SW NE-SWENE-WSWNW-SE NNW-SSE N-SNE-SWENE-WSWNW-SE NNW-SSE 20 cm cm N-SNE-SW cm Type-1 Type-2 Type-3 Type-1N-SNE-SW Type-2 Type-3 Type-1N-SNE-SW Type-2 Type-3 NE-SWENE- WSWNW-SENNW-SSE Type-1 N-S NE-SW NE-SW Type-1 Type-2 Type-3

11 ( ) vol. 39, Kitagawa, R. and Nishido, H., 1994, Orientation Analysis and Formation Ages of Fractures Filled with Clay Minerals (Clay Veins) in Hiroshima and Shimane Prefectures, Southwest Japan. Jour. Japan Soc. Eng. Geol., Vol. 35, No. 2, K-Ar -, Vol. 38, No. 5, Nicolas, A., 1984 ed., Principles of Rock Deformation. D. Reidel Publishing Company, Dordrecht, 208p ed., 226p Structure Contour Map ---III

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untitled 2 3 4 kw kw 5 500 112 6 7 8 9 10 11 0.060 0.060 0.040 0.020 0.040 0.020 0.000 0.003 0.002 0.003 0.002 0.000 0.003 0.001 0.002 0.002 12 13 N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW N NNE NE ENE