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- 構造地質部会
-.' .. ¸ --ῌ./ ,*** @Structural Geology, No. .., --ῌ./, ,***A Fold structures and basal fault of the Upper Cretaceous Tochiya Formation in the northeastern Kanto Mountains * Shirou Chikashige* Abstract : Geological structures of the Upper Cretaceous Tochiya Formation, once regarded as one of the member of the Atokura Nappe, were carefully investigated in order to clarify the geotectonic evolution of the the Yorii-Ogawa area in the northeastern Kanto Mountains. A macroscopic overturned anticline verging northward was recognized as well as upright folds. Their axes trend N2*ῌ2/W and plunge /ῌ+*. A basal thrust between the allochthonous Tochiya Formation and the underlying Mikabu greenstones cuts the fold structures and exhibits fault-rock fabrics, suggesting a top to the north sense of shear. These evidences for the northward movement suggest that the Tochiya Formation in the study area was a di#erent nappe from the southward-verging Atokura Nappe of the type locality. Key words : Tochiya Formation, Atokura Nappe, overturned fold, basal fault, paleocurrent ῌ !"#$%!&' (%)*+,-./01 2 3# 456789: ;<=3>? @Fig. +A @ ῌ +33, ; ;Bῌ$CDEFGHIJK LB +33/AM N3#&'OPQ R STU/V!"W#VX Y N3ZV$[\](^_3>? @%`" +3/- ; ab`" +30- +300 ; c`" +31* ; +312 ; d +313 ; e&ῌ%f +323 ghAM +33* i j"# ,kῌlmnP 'o Sr (0)pC *YN3#'qr(+#"=3s @ , +32- ; tῌe& +323 ; e&ῌ%f +323 ; e& `" +323 ; -ῌt +33* ; ῌ +33+ ; ῌ +33/ ghAM =#789u./v /v,-.wxy>01/0k1 gz2{F|(}~#3 @I|`" +33* ; 3 +33+ ; 3 +33/ ; Kobayashi, +330 ghA 456 ,*** 1 ± . ²NM ,*** ++ ± +/ ²OM * !m!m³O´m\]µOm¶· Geography Major, Graduate School of Science and Technology, Nihon University, -ῌ,/ῌ.* Sakurajosui, Setgaya-ku, Tokyo +/0ῌ2//*, Japan 789: #g-PFVX56v45 67(8=3>? @abῌe& +332AM ῌ56vk9v=3:;v @i +3/.A 56v)><g= >(g[7-.u./v-.wxgh? (@1>?g?M :;vABῌ VC(!=>(1 '-. DEFg(1sM \]V b`" @+33*A YaG=3[:;vAB ῌ11> 7-.H /vDῌ?^ysM 2 ¡ / I¢£:;v56vg¤2¥E 789 @4:;789:A VXN(+#"gys V8M !ῌ"#$%&' ῌ'¦J§ Fig. , ¨M N V "#%)*+, K ©ª, v )*+,c«¬ , :;v LM,ῌv(AB1>?M N 3#,ῌv®K ©ª, v c«¬, :;v)*+,/ 7891>z21>¯s&,VXY ° 456789: -P1>? ¹ 33 ¹ ¸¹º Fig. + Geological sketch map of the northern Kanto Mountains. Simplified after the Geological Sheet Map + : ,**,*** “Nagano” (+332) and “Utsunomiya” (+33*) of the Geological Survey of Japan. MTL : Median Tectonic Line. ISTL : Itoigawa-Shizuoka Tectonic Line. TTL : Tanagura Tectonic Line. Fig. , Geological outline of the Atokura Nappe in the Yorii-Ogawa area. Simplified after Makimoto and Takeuchi (+33,). ῌ +33, ! ῌ "# +323 WNW-ESE $% N-S $!&'()* +,-./0123ῌ4(56(7ῌ8 9:;)<0 ῌ +33, <= >? (@0/ A4B%C DE; +3/* ; F G +3/. ; HIE; +31* ; 3JE; +33* K LIM3NOP Q ,/+ Ma H +32- K0 ,/, Ma RMῌSG +33* )TU VW. XY;:Z[\ ]&'()%^_`a Hb +313 ; c +32. ; ῌ +33, >?( d=e fdg6(;:<hijklm)K ῌ +33, 6(ng;:oUpqrst lm-3 uv-3Vw. 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U\ B$ { " R 012345 %/7Am$YX " +, km 0Y WNW-ESE 1 2 km ¡ ¢8(U£9 ¤X\]ῌk!, k - km *YRZ $¥ (1 Y N 1*¦2*¦W YR Fig. - §¨©KLª2L«3¬ KLª®¯ ¯ Fig. - ( bB$ ">?@%Wm $? CDEFG%9 $ ° 2L ®¯%W±²$!Y .** m Fig. Geological map of the western part of the Tochiya Formation. and Takeuchi (+33,). ¸ 35 ¸ Modified after Makimoto `¸¹Y 2** m ῌ +** m +** m ,** m !" #$ %&'()$"* +, -.(/ 0%1234 5678 34 9:;<8 * =$ >?:@8A<@B<8"?, % CFig. +, e fD E ῌ C+33,D %F=GH IJK%> ** L1MNO ( +* cm PQ9RS7TQU -* cmV, m - cmV,/ cm %W%( / : + % HX(YZ % . !% + M (!"" # . !% - (!" [\$] M %!"^_$%%`&' Ha((b) [\$]%"? %%*+H,c 1 defg?( 0 hR78i6T8 j-i6T8 Clongtitudal scourD CCollinson & Thompson, +323D klmT i6T8 $ n.H/"?o,p1q NErSW s CFig. .D t%o,p 0uῌv1 C+312D %so,p ESErWNW Cw,D H2 ,H2x" Hyz%ῌ {%%3p p|w P*4%5p 67 |%8}w%5pM~9| :$] % . !% + Fig. . Restored paleocurrent directions on the overturned strata in the Tochiya Formation. Arrows show the directions of paleocurrent. # . !% - (!" ;R78H )<= (>?"**?fg* !"^# @ ?a(M~%#|%&'+ A + ;$^%H Bp| ῌ B7 |ῌ67|(C CFigs. /&0D [\$] H!"#= H# | $ %#% 0%# |ῌ%s D0 Bp |%E #E t%p|% +** m # H B7|(C t%7|%#E +** m # |ῌ#% 0%# /* m FGHI JK 0 %# -* m |ῌ t % B8}% p <9;?8}w N 2* W %3pH +*:;m CFig. /bD H# H +* m L%@(C CFig. +*D t%@ # %~ +,* m #3p ,** m %M<= f CFig. 1D PH%5 p?O)"?T5 CFig. 3D #=< = +-* m #3H /* m N~%~*[O %- ¡¢ CN~ ,-* mD #3p%~ -V/ m %£¤+H # {|ῌ 1qn. #|ῌ#{MP¥ (fgtM CFig. 2D 67|(¦ M t%67| 1qQ§3p%5p #{RV'|8}w+¨© E(' 9" #E(R9|Eª 2/% !S¨« CFig. 3D >? t%68}%¬T! 2 de%<=H#%® p <9;H=$?8}w N 2/W %3p H /:;m CFig. /bD :$] CFig. 0D !"^#¯#3 +**V,** m %ª(#|ῌ# |ῌH% t%ªHp|(C #¯#3 -/* m U°%$% |(±²H$ .** m $%1qn. C/V1 ND H$ +* cm L%4³lE + m L %@HA @#% .* m #|ῌ %"?(?/ B p|#=B7|%E´mTHV W%% B7|%#EµW¶ CFig. 0bD [\$]%8}·X¦ t% º 36 º cd=h i .. j [\]=\^_J_`abBC23cdefg Fig. / Route map in the qarry of the Sankyo area (a) (Locality showing in Fig. -), and cross section (b) along the A-B, C-D sections and attitudes of fold axis. ! "# $%& p '()*+,-./012 34 2*5W 6# -57+89& :;<=> ?Fig. -@ A BCDEFGHI ῌ JKLMNOP/& Q RSῌTU ?+33,@ JV - WXYZBC k 37 k defg Fig. 0 Route map in the Gonotsubo area (a) (Locality showing in Fig. -), and cross section(b)along the A-B, C-D sections and attitudes of fold axis. . ! , + "#$%&' ()*+,-./0 -./1 2345 67 ! 89:;<= > ?@A*+,-./0-./12345 BCDE' N .2FW 1*FS & -./12345 GH .* m IJKLMNO ! -./123 45P/ BCDE'89:;<= >?@&' N 1.FW 2,FS &Q< -./123 45'H +* m RIJKLMNO ! S -./12345 BCD' TUV$%&WX&Y Z/[\Z/]/^ G N .*FW 0,FS BCE&LMNOJ_`/a 7 ! LMNOJ_`/'H .* m bc! h 38 h .. Fig. 1 Photograph of the outcrop of the southward dipping reverse beds, forming the northern limb near the hinge of the overturned anticline in the southern area of the Tochiya Formation. Fig. 2 Photographs of the outcrops of the southern limb of the overturned anticline in the southern area of the Tochiya Formation. (a) Normal strata dipping north in the northern part. (b) Subhorizontal normal strata in the middle part. 39 ®¯° Fig. 3 Outcrop sketch showing the fold profile of the hinge part of the overturned anticline. Fig. +* Fracture zone cutting the overturned north limb at To-- in the southern area of the Tochiya Formation. The overturned strata dipping south in the right hand of fracture zone,the normal strata dipping north in the left hand of fracture zone are observed. Bag in front of the outcrop for scale. !"#$%& ' ()* +* m +, -.* +/ cm /0123456 7N 2,8W ,+8S9 :;"# 7Fig. ++c9 <% 7Fig. ++a b9& 56=>?* -* m @A:B%& /01C DEFGHIJK LM:NO PQ)RSG TUVWXYZ() -K ?4 7top-tothe-north9 [5\E]2^_`-56:N% 7Fig. ++d9& ab /01c;_% N .*8E >?56]defEghE2iYZc_% XZ jI Qklmnopqg]rTUVY Z-K?4\E]2^" 7Fig. ++e9 -s tu]vweqxyQ\E]cz{_%& ,)P | cm J}~cH 234YZ56WX_% 7Fig. +,a b9& -K 6 )Q% 7Fig. +,c d9& `-56 N 108W 228 S N 2-8E 118S +56:)Q#$%& `-56=>* -* m * /* m * 1* m - () 0+* m +,, Q /,/ cm ./01234? N 1*8 1/8W ,*8,.8S 56:N% 7Fig. +-9& 56- ]defEghE56? -*8.*8 ¡O¢" JK /01CDEFGHI TUVWXYZ()-K£¤ NE >??4[5\ E]2^_& Q) . £¤¥z¦§562¨© 5 6?U"# ªO>?¢_%>? ]defEghE2¨«`-56 -% 656:N%c¬5,Q%& ῌ,῍ ῒῐ῏ΐ῎ῑ῏ ± 40 ± z{~ .. pq)qrrstuvw/xyz{$|}U/ Fig. ++ Outcrop of the basal fault of the Tochiya Formation. The hanging wall rock is conglomerate of the Tochiya Formation and the footwall rock, Mikabu greenstone complex. ῌ Whole view of the basal fault. Black line shows the fault trace. ῍ Central part of the basal fault shown in (a). ῎ Close-up view of the fault gauge zone of about +/ cm in width. ῏ Asymmetric imbrication structure made by crushed lens-shaped fragments,showing top-to-the-north shear. ῐ XZ thin section of footwall rock (Mikabu greenstone complex). Asymmetric epidote augen with asymmetric porphyroclast tails indicates top-to-the-north shear (This is a s-type object of mantled porphyroclast (Passchier & Trouw, +330). To-- / m +* m !" # $%&'(ῌ)*+,-,. /012 N /.342*3W 0/3N 5Fig. +*6# $% )&)'( ῌ )*+,-,./012 N 0*3W /23N 789: To-- ;<2# % N 0.3W => To-1 ?7@AA9 5Fig. /a6: ?# To- $ To-1 B 5CD2 N 003W 1*3S6 EFGHIJ,KL< S-C IMNOPQR STU<9$# )& 5V6 W=# & 5WV6 = XYZ;[U/$\]S^9 5Fig. +.6: _# '(ῌ)*`a/$'(ῌ* [b/B# '(ῌ)*./)'(ῌ) *./cdef[U/ghij$\]S ^9kl mn7o9: 41 =mno Fig. +, Photomicrographs of textures near the basal fault. (a) and (b) show the recrystallizied texture in XZ thin section of Mikabu greenstone complex (left : plane-polarized light and right : crossed nicols). Both s-type winged porphyroclasts in the central part and anthetic o#set of displaced grains in lower right corner indicate the sense of the dextral share. (c) and (d) show grain size reduction of quartzes in the matrix of conglomerate in XZ thin section in the Tochiya Formation (left : plane-polarized light and right : crossed nicols). (e) and (f) show the protomylonitized granite pebble in the Tochiya Formation. The boundary between granite pebble and the matrix is viewd left to lower right corner (left : plane-polarized light and right : crossed nicols). + !"#$%&'( )*' +,- ./012 . .3 425678 9:';<=6>?+@A BCDEFGHIJKLMN8OPQ R.ST .U(28VWX NEYSW Z.[T+, \]<^;_`abc WX+CDNGMD dMN#efg_hijkῌlQ6B , p 42 p f{ ¶ .. · ¢1df1³2n23´µ[; Nklo; Fig. +The another outcrop of the basal fault. Black line shows the fault trace. Tochiya Formation Sm : Mikabu greenstone complex S-C structure in the fracture zone cutting the Fig. +. overturned north limb of the Tochiya Formation. Arrows show the sense of shear. Hummer circled for scale. +,*** m N 2/W / ! N 2*W +* " " #$ % #&'()* + ,- ./ ! 012)3456 78( 9:;<= >?)@ AB CDEFG H * N-S IJK > FG >L#$M-ENOP- + "Q>R" S ! ND>. <= > @AB H * N-S IJKTU#$M-ENOP- + M <= >?)@ ABCDEF GV" "IJKT U#$M-EWXSH + ,-* Y-$Z >?)#$M-E )S [;! \]^_`01'ab,N cL(*+ Toc : Conglomerate of the - defg[;hi j ;k l'(mno; npoqr's + tE o;uvUwxyz )QL nk{|}~mno; >L#$M-ENOP- + M o;n uv U - E#! |} > ~( -(* ,-#X"h#$MENOP- + tE ,klo;#zz$1#zz ; @ f %)* ,N " &'?)* ,NHL f;%X 'O' Nf{o Fig. / 2)! " "' N(oM- + ,,N klo;#$N*P + M noqr[;! s\]^_`N )H + ! ¡!E&' "o; ! #$VNOP- + ,"o;Nklo ;V¢£*+H klo; H?EN (¤ ,o;¥¦S [;NNS @)§EWXH + . U¨,fh©ª; b*) «¬ῌ+332 >L E-W (* ENE-WSW 'a b\]^_`FGN-'MG "( <. 's,N+ (?)* + ,- ®) ¯efh E-W 'abFGN"( ° j FG\]^_` *) * ±( + [; ©ª; ®/M-©ª²0;NM-)* ¸ 43 ¸ ¶¼¯( ῌ ! 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The University Printing House, Oxford, ,*1 p. ^}£ῌM¤ ῌ| +3/- ¡tM # !9 ghwjH9 no. -, +ῌ.+9 stuῌ ¥ +33* ¡t¦ , - 9 K 30 -+3ῌ-,,9 +32. ¡tM §¨/C ©ª«¬9 K 3* 0,3ῌ0.,9 ῌ ῌ¡ k +33* ¢ £9 ®r¯*r° ¤ . M! ., +2-ῌ+3+9 i +33- bMO-9 SK r±¤ +** r¥*±²¦ ./,9 i +33/ ¡t¦M8³ !§9 K +*+ 1,3ῌ1-29 Kobayashi, K., +330, Rotation of slip directions of the Atokura Nappe viewed from microstructural analyses of brittle shear zones in the Sambagawa belt, Southwest Japan. Jour, Struct. Geol., +2, /0-ῌ/1+. iῌ +33+ bM !,-\ ¡tgE019 SKr±¤ 32 r¥*±²¦ ,3+9 ¨©V ῌ " ῌ ª « ῌ ῌ ῌ ¬b ®ῌ`¯ῌ#°$ῌῌ±Tῌ z´ῌ +31* ²µ§¨³¶ 8.·ῌ8Kr)H I9 K 10 .,+ῌ .-+9 UV +313 ¡t¦M³¶Nk°´ G¯9 K 2/ +/1ῌ+109 µ¶ +3/. ¡tMK !9 ·%*T ° + +0*ῌ+0/9 & jῌ¬b¸¯ +33, §¨/K9 / KHI¸ >/ '? + K¹ºD K(c» ¹ 44 ¹ V. .. µ /0'e(¶(P±·Q¸V.G¹RS +-0 p ῌ +312 2/ ,*. ῌ ῌ ῌ ! ῌ "# $ῌ%ῌ&' +332 ,* + ( )* +, - . +32- /0 120345 6 K-Ar ! 67 12 -2ῌ-3 Passchier, C. W. and Trouw, R. A. J., +330, Microtectonics. 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