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The Hayachine Tectonic Belt (HTB) is a boundary zone between the North Kitakami Terrane (NKT) and the South Kitakami Terrane (SKT). It is characterized by intense tectonic deformation and slicing by numerous faults, along which serpentinite bodies were emplaced. The following eight geologic units constitute the HTB and its vicinity. The predominant lithology and contained fossils of these units are as follows: Unit A mudstone, bedded chert (with Triassic conodonts) and chert-laminite. Unit B mudstone, sandstone and bedded chert (with Jurassic radiolarians). Sometimes show olistostromal occurrences. Unit C basalt, basaltic volcaniclastics, massive chert and muddy/siliceous laminite. Unit D foliated serpentinite with tectonic blocks-slices of amphibolite, basalt, chert and pelitic schistose rocks. Unit E ultramafic mass occasionally containing gabbroic layers. Unit F basalt, dolerite and gabbroic rocks with minor amount of quartzdioritic rocks. Unit G volcaniclastic sandstone, mudstone (with Silurian brachiopods), limestone and acid tuff. Unit H quartzofeldspathic sandstone, mudstone and conglomerates with abundant Middle Silurian fossils. Unit A apparently is equivalent to the NKT, Units F-H are assigned to the SKT. The HTB is represented by Units B-E. The NKT and SKT are discontinuously sutured by HTB. Three different fault systems are recognized within the HTB: (1) north-dipping thrust faults, (2) WNW-trending high-angle faults, and (3) NNW-trending high-angie faults, in formative order. The thrust faults are recognized only beneath the Hayachine Ultramafic Mass, which is the largest one in the HTB. The WNW-trending faults dominate the other two, and are characteristic tectonic components of the HTB. These are associated with foliated serpentinite bodies often accompanying tectonic block-slices. The NNW-trending faults comprise both the western and eastern sides of the HTB, showing strike-slip dislocation. Formation of the NNW-trending faults occurred during the Early Cretaceous. The lithologic assemblage of geologic units of the HTB is very similar to that of the Motai Group, which has been considered generally as pre-Devonian or pre-Silurian metamorphic basement in the Motai-Matsugadaira Belt (MMB). These two belts formed a continuous geologic body prior to sinistral dislocation by such NNW-trending faults as the Hizume-Kesen'numa Fault. The geotectonic view of the HTB is presented hypothetically as follows. At first, an accretionary complex including olistostromal facies with Triassic-Jurassic chert blocks was formed at convergent margin. This convergent process possibly persisted until the end of Jurassic. The SKT collided with the margin to form an allochthonous terrane. Formation of the nappe structure might have been associated with the collision event. Subsequently, WNW-trending high-angle faults formed, with possible transcurrent dislocation and intense deformation. This dislocation rearranged the accretionary complex to form the HTB as the boundary tectonic belt between the NKT and SKT. Finally, an Early Cretaceous NNW-trending strike-slip fault system further dissected the HTB. Present distribution of the HTB, SKT, NKT and MMB occurred prior to the Early Cretaceous (110-120 Ma) granitic rocks that were commonly intruded into these terranes.
