Transcript
Page 1: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

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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 northeast-

ern Kanto Mountains. A macroscopic overturned anticline verging northward was

recognized as well as upright folds. Their axes trend N2*��2/�W and plunge /��+*�.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

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*

,***� 1± .�²NM ,***� ++± +/�²OM��!m!m³O´m\]µ�Om¶·Geography Major, Graduate School of Science and

Technology, Nihon University, -�,/�.* Sakurajosui,

Setgaya-ku, Tokyo +/0�2//*, Japan

-.�'� � ..¸� --�./� ,*** @Structural Geology, No. .., --�./, ,***A

¹ 33 ¹

Page 2: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

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9��:;)�<0 ������� +33,�� <=� >?(�@0/�� A4B%��C �D�E;� +3/* ; FG� +3/. ; HIE;� +31* ; 3JE;� +33*� K��LIM3NO���PQ�� ,/+ Ma �H��

+32-� K�0� ,/,Ma �RM�SG� +33*� )�TUVW.� ��� �XY;:Z[��\������]&'()��%^_`a� �Hb� +313 ; �c�+32. ; ������ +33,�� >?(�� ��� d�=efd�g�6(;:<�hijklm)K� ������� +33,�� 6(n�g�;:�oUpq�rst�lm-3� uv-3Vw. ���E;� +33* ; D�E;� +33*�� xy.14zQ� Turonian{Cam-panian �=e| 3*{1.Ma� %}~:a/0� �����n�����s��������� +33/�� �1�>?(����3��s��35������V��/0� �Hb� +313 ; �c� +32.�� ���������� ���z�����)K�� �� s�¡¢��£s�m¤�,Q� /3.0 MaVW� ������� +33+�� ��(�� ¥¦���§���(%¨�;:<�©�()K�� �§���(���;:� ¨����=efd�;:<� ������� +33,�� ��(¨��ª��«��������� s�¡¢��£s�m¤�,Q� 0/..MaVW� � ��������%�(789V}��a¬� ��(�4zQ��oU��­®�)K¯�%`a/0� ������� +33,���\�°¤s�± >?(��²� �. ������� +33,�� ³²�´��%µ��� �¶ ��� ��c� +32. ; ������ +33,�� �\�°¤s�±�&O�;: .*,Ma� K-ArQ ·:

Fig. , Geological outline of the Atokura Nappe in the

Yorii-Ogawa area. Simplified after Makimoto and Take-

uchi (+33,).

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.

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» 34 »

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Fig. - Geological map of the western part of the Tochiya Formation. Modified after Makimoto

and Takeuchi (+33,).

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¸ 35 ¸

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��� ������� 2** m����� ��� �������� +** m� ������������� +** m� ����������������� ,** m���� ������� !"�#$� �%&'(�)�$"�*� �+�,��-.���( /� 0%1��234�� 5678�34�� 9:;<8� � *�� ��� ��=$���>?:@8A<@B<8�"?,�%� CFig. +,

e� fD ��E� ����� C+33,D %F���=GH�I���JK%���> �*�*� �L�1MNO( +* cm���� PQ�9RS7TQ�U����� ����������� -* cmV, m����� - cmV,/ cm� ����%W�%�( / : +

��%������ ���H�X�(YZ�� ���%� � .!% +���M������������(!""� # � .!% -�����������(!"���[\$]�� ������������M����������%!"^�_��$%%`&��'Ha(�(b)��� [\$]%��"?������%��%*+H,c� 1defg�?(� 0 ��hR78i6T8� j-i6T8 Clongtitudal

scourD CCollinson & Thompson, +323D� klmTi6T8$� ����n.H/"?o,p�1qNErSW�s� CFig. .D� t%o,p��0u�v1C+312D %s�o,p ESErWNW Cw,D H2�� ,H2x"� ��Hyz����%���������{%�%3p� p|w� P*4�%5p� �67|%8}w%5pM~�9|������:$]�� ���%� � .!% +����

# � .!% -�����������(!"����������������;�R78H�)�<=(>?"�*�*?�fg��*� !"^#������ @�� ?a(�M~�%#�|%&'+A���

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|��67|(C��� CFigs. /&0D�[\$]�� � H!"����#=����������������H�����������#�|$� ������%����#��%�������� 0%# �������������|����%�����s�� D��0 � �Bp|%�E� #E���� t%p|%� +** m# H�B7|(C���� t%7|%#E� +** m�#�|�#��%������������ 0%# � /* m���FGHI� ���JK�� 0%# � -* m���|������ t �%�B8}% p�<9�;�����?8}w�� N 2*�W%3pH +*�:;m��� CFig. /bD� ��H# H� +* mL%@��(C��� CFig. +*D� t%@��# %~�� +,* m#�3p� ,** m%M<=��f������ CFig. 1D �PH������%5p�?O)���"?T��5 CFig. 3D� #=��<=��� +-* m#3��H� /* mN~%~*[O%�-� ¡¢ CN~� ,-* mD �� #�3p%~�� -V/ m%£¤+H� � ��# {��|����� 1qn.� #�|�#��{MP¥��������(fg��tM CFig. 2D ���67|(¦M�� �� t%�67|�� 1qQ§3p%5p#{RV'��|��8}w+�¨©� �E('�9��"� #E(R�9�|��Eª�� 2/�%!S�¨« CFig. 3D� >?� t%�68}%¬T!��� 2de%<=H#­�������%®��� p

�<9�;�H=$��?8}w�� N 2/�W%3pH /�:;m��� CFig. /bD���:$] CFig. 0D ���!"^#¯��#3

+**V,** m%ª������(#�|�#������|����H���%� t%ªHp|(C���� ��#¯��#3� -/* mU°%$%���|(±²�H�$� .** m$%�1qn. C/�V1�ND H�$� � +* cmL%4�³l���E� + mL%@��HA��� @���#%� .* m�#�|����%��"?����������(?/� �Bp|#=��B7|%�E��´m�TH�­��VW���%%� �B7|%#E�µW��¶­� CFig. 0bD� [\$]%8}·X��¦��t%�

Fig. . Restored paleocurrent directions on the over-

turned strata in the Tochiya Formation. Arrows show the

directions of paleocurrent.

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º 36 º

Page 5: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

�������� ����� ������������������� ������ ����������� !� "�#���������� ���$�%�& p'()*+, �-./01�2

34� 2*5W�6�# -57+89��&:;<=> ?Fig. -@ A�� BC�DEF�G�HI �����JK�����LM�NOP/& �Q� RS�TU ?+33,@ �� JV� -WXYZ�BC

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.

[\]=�\^_J_`abBC��23cd�efg�cd=h i ..j

k 37 k

Page 6: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

������� .���� � ��������������������������� !,� � ��+� ��������"#$%&'� ��()*+,-./0� -./12345� ���6���7 ! 89:;<=>?@��A��*+,-./0 -./12345BCD�E'� N .2FW 1*FS&� -./12345�

GH .* m�IJK�LMNO�� ! -./12345 ���P/� BCD�E'89:;<=>?@�&'N 1.FW 2,FS&Q<� -./12345'H +* mR�IJK�LMNO�� !����S-./12345� BCD�'�TUV$%&WX&Y� ���Z/[\Z/]/^�G N .*FW 0,FSBCE&LMNOJ_`/�a7 ! LMNOJ_`/'���H .* mbc!

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.

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h 38 h

Page 7: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

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.

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.

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����������� ���� ��������������� ��� !"#$%& '�� ��()* +* m+,�������������-�.* +/ cm�/0123456� 7N 2,8W

,+8S9 :;"# 7Fig. ++c9 <���% 7Fig. ++a� b9&56��=>?�* -* m@A:B%& /01C�DEFGH�I�JKLM:NO� PQ)�RSG�TUVWXYZ()� -K��?4 7top-to-

the-north9 [5\E]2^_`-56:N% 7Fig.

++d9& ab�/01c;_%��������N .*8E>?�56��]defEghE2iYZc_%XZ

jI��� �Qk��lmnopqg]r�TUVYZ�-K��?4\E]2^" 7Fig. ++e9� -s�tu]vweqxyQ�\E]cz{_%& ,)�P�| cmJ����������}~�c�H���234�YZ�56�����WX_% 7Fig. +,a�

b9& -K���6�����������������)Q% 7Fig. +,c� d9& ��`-56� N 108W 228S��� 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�-�]defEghE�56���?� -*8�.*8�� ¡O��¢"� JK�/01C�DEFGH�I�TUVWXYZ()-K�£¤NE>?�?4[5\E]2^_&�Q)� .���£¤¥z¦§�56�2¨©� 56���?�U"#�� ªO>?��¢_%>?�]defEghE2¨«`-56�� ��������-��%��6���56:N%c¬5,Q%&�,� �������

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.

Fig. 3 Outcrop sketch showing the fold profile of the hinge part of the overturned anticline.

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± 40 ±

Page 9: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

���������� ���� To--�� /

m���������� +* m������ !"�# ���$%� &� '(�)�*�+,-,./�012 N /.342*3W 0/3N 5Fig. +*6# ���$%�)&�)'(�)�*�+,-,./�012N0*3W /23N789:�� To--�;<����2# %� N 0.3W=>�

�� To-1?7@A�A9 5Fig. /a6: ?�# �� To--$�� To-1�B���� 5����CD2 N 003W

1*3S6 E�FGHIJ,K�L< S-CIMNOPQRSTU<9$# )& 5�V6 W=# & 5WV6 �=X�YZ�;�[U/$\]S^9 5Fig. +.6: �� _�# '(�)�*�`a/$ '(� �*�[b/�B�# '(�)�*�./�)'(�)�*�./�cdef�[U/�gh�ij�$\]S^9kl mn7o9:

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. � 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. � 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).

pq��)q�r�rstuvw/�xyz{$|}U/z{�~ � ..�

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Page 10: 日本地質学会 構造地質部会+33, ˘ˇˆ˙˝˛˚˜ ! "# +323 WNW-ESE $%N-S $ !&’()* + ,-./012 3 4(56 (7 ˘8 9 :;)

� �

+� ����������������� �� �������

������������������� !�"�#$��%&'(�����)*'

+,-�./0�12����.�.�3�4�25678��� 9:';<�=6>?+@A�BCDEFGHIJKLMN8OP�Q� R.ST�.U(28VWX�NEYSW�Z.[T+,� \]<��^;_`a�bc�� WX+CDNGMDdMN#efg_hij�k�lQ�6B ,� � ���������

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).

=mno

p 42 p

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Fig. +. S-C structure in the fracture zone cutting the

overturned north limb of the Tochiya Formation. Arrows

show the sense of shear. Hummer circled for scale.

Fig. +- The another outcrop of the basal fault. Black line shows the fault trace. Toc : Conglomerate of the

Tochiya Formation Sm : Mikabu greenstone complex

¢1df�1³2n23´µ�[;�����Nklo;��f{ ¶ ..·

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f�� ������ +332� ¡�t�� ���g � !�� : "�#�$��¢9 �K�� +*.� 20+�2109

Collinson, J. D. and Thompson, D. B, +323, Sedimen-

tary structures ,nd ed. The University Printing

House, Oxford, ,*1 p.

^�}£��M¤ �| ��� +3/-� ¡�t��M����# !9 ��ghwjH�9 no. -, +�.+9

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Kobayashi, K., +330, Rotation of slip directions of

the Atokura Nappe viewed from micro-

structural analyses of brittle shear zones in the

Sambagawa belt, Southwest Japan. Jour,

Struct. Geol., +2, /0-�/1+.

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onics. Springer-Verlag, Berlin Heidelberg, ,23

p.

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