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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 inorder to clarify the geotectonic evolution of the the Yorii-Ogawa area in the northeastern Kanto Mountains. A macroscopic overturned anticline verging northward wasrecognized 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 thesouthward-verging Atokura Nappe of the type locality.
Key words : Tochiya Formation, Atokura Nappe, overturned fold, basal fault, paleocurrent


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Geography Major, Graduate School of Science andTechnology, Nihon University, -ῌ,/ῌ.* Sakurajosui,
Setgaya-ku, Tokyo +/0ῌ2//*, Japan

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¹ 33 ¹¸¹ºFig. +
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,).

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and Takeuchi (+33,).

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Restored paleocurrent directions on the overturned strata in the Tochiya Formation. Arrows show thedirections of paleocurrent.

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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.

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Fig. 0
Route map in the Gonotsubo area (a) (Locality showing in Fig. -), and cross section(b)along the A-B, C-Dsections and attitudes of fold axis.

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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 thesouthern area of the Tochiya Formation. (a) Normal strata dipping north in the northern
part. (b) Subhorizontal normal strata in the middle part.

 39 

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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.

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Fig. ++
Outcrop of the basal fault of the Tochiya Formation. The hanging wall rock is conglomerate of the Tochiya Formationand 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. ῏ Asymmetricimbrication structure made by crushed lens-shaped fragments,showing top-to-the-north shear. ῐ XZ thin section of footwallrock (Mikabu greenstone complex). Asymmetric epidote augen with asymmetric porphyroclast tails indicates top-to-the-northshear (This is a s-type object of mantled porphyroclast (Passchier & Trouw, +330).

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# $%/ 012 N /.342*3W 0/3N 5Fig. +*6# $%)& )'( ῌ )* +,-,./ 012 N
0*3W /23N 789:
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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 thecentral 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).

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p 42 pf{ ¶ .. ·¢1df1³2n23´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.

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Collinson, J. D. and Thompson, D. B, +323, Sedimentary structures ,nd ed. The University Printing
House, Oxford, ,*1 p.
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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+.
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Passchier, C. W. and Trouw, R. A. J., +330, Microtectonics. Springer-Verlag, Berlin Heidelberg, ,23p.
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