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Re-deﬁnition of the Hayachine Tectonic Belt of Northeast Japan anda proposal of a new tectonic unit, the Nedamo Belt

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Masayuki Ehiro* and Noritoshi Suzuki**
Abstract : The Hayachine Tectonic Belt, Northeast Japan is characterized by theextensive distribution of the Hayachine Complex, which consists of basic to ultrabasic
rocks and considered to be an Ordovician arc ophiolite. It borders the South Kitakami
Belt southward and the Kuzumaki-Kamaishi Belt northward. The South Kitakami Beltis composed of the pre-Silurian basements, such as the Matsugadaira-Motai Metamorphic Rocks and Hikami Granites, and their covering strata ranging in age from Silurian
to Early Cretaceous, whereas the Kuzumaki-Kamaishi Belt Jurassic accretionary complexes. The Hayachine Tectonic Belt was originally called the Hayachine-Goyozan
Tectonic Belt and deﬁned as a fault zone accompanied with basic to ultrabasic intrusives. Later the belt was extended to the east. However, this eastern extension, the“phyllite sub-belt”, consists of Jurassic accretionary complexes and should belong to
the Kuzumaki-Kamaishi Belt. Recent ﬁeld survey and laboratory work on the Nedamo
Complex in the northwestern part of the Hayachine Tectonic Belt in eastern Morioka
revealed that the complex di#ers in both lithology and geologic age from the main rockunit of the Hayachine Tectonic Belt. The Nedamo Complex is a probable Late Paleozoic
accretionary complex, composed mainly of deformed clastic rocks and greenstones
with bedded chert, and the greenstones are considered to be OIB and MORB of
Devonian age. From these lithological and chronological di#erences, the distribution
area of the Nedamo Complex should be separated from the Hayachine Tectonic Belt as
an independent tectonic unit, the Nedamo Belt. The re-deﬁned Hayachine Tectonic Beltis composed mainly of the Ordovician Hayachine Complex, associated with preSilurian metamorphic rocks in the Miyamori-Hayachine district and Paleozoic shallow
marine deposits in the Kamaishi district. The belt is narrow and extends southeastward from Morioka through Mt. Hayachinesan to Mt. Goyozan. Its northwesternextension is shifted to the south to the Miyamori district by a sinistral strike-slip fault(the Hizume-Kesennuma Fault). The newly proposed Nedamo Belt is characterized by
the Nedamo Complex, which is probably a Late Paleozoic accretionary complex, andoccupies a northwesterly expanding triangular area on the east of Morioka. The
Kitakami Massif is thus divided into the South Kitakami, Hayachine Tectonic, Nedamo,
Kuzumaki-Kamaishi and Akka-Tanohata Belts from southwest to northeast.
key words : Hayachine Tectonic Belt, Hayachine-Goyozan Tectonic Belt, Nedamo Complex,
Nedamo Belt, tectonic division, Kitakami Massif, Northeast Japan,**- ) + * ,* +,-. ,**- ) - * +3 +,/.
* 0123435678
The Tohoku University Museum, Sendai 32*ῌ2/12, Japan** 0123239/3:;<=>3?@73AB
Institute of Geology and Paleontology, Tohoku University,
Sendai 32*ῌ2/12, Japan  

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Ehiro, M. and Kanisawa, S., +333, Origin and evolution of the South Kitakami Microcontinent
during the Early-Middle Palaeozoic. In Metcalfe, I., ed., Gondwana dispersion and Asian accretion : IGCP -,+ Final results volume, A. A. Balkema, Rotterdam, ,2-ῌ,3/.
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Research Group, +33*, Southern Kitakami Terrane. In Ichikawa, K., Mizutani, S., Hara, I.,
Hada, S. and Yao, A., eds., Pre-Cretaceous Terranes of Japan, Nippon Insatasu Shuppan Co.
Ltd, Osaka, ,.3ῌ,00.
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Mori, K., Okami, K. and Ehiro, M., +33,, Paleozoic
and Mesozoic sequences in the Kitakami Mountains (,3th IGC Field Trip A*/). In Adachi, M.
and Suzuki, K., eds., ,3th IGC Field Trip Guide
Book Vol. +, Paleozoic and Mesozoic Terranes :Basement of the Japanese Islands Arcs, Nagoya
University, Japan, 2+ῌ++..
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Onuki, H., +30-, Petrology of the Hayachine ultramaﬁc complex in the Kitakami Mountainland,
northern Japan. Sci. Rep. Tohoku Univ., ser. -,2, ,.+ῌ,3/.
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northern Japan. Sci. Rep. Tohoku Univ., ser. -,3, ,+1ῌ,10.
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Ozawa, K., +32., Geology of the Miyamori ultramaﬁc complex in the Kitakami Mountains,Northeast Japan. Jour. Geol. Soc. Japan, 3*,
031ῌ1+0.
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Shibata, K. and Ozawa, K., +33,, Ordovician arc
ophiolite, the Hayachine and Miyamori complexes, Kitakami Mountains, Northeast Japan :isotopic ages and geochemistry. Geochem.
Jour., ,0, 2/ῌ31.
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