地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
サハリン最北部の地質構造からみた北米・ユーラシアプレート収束境界のテクトニクス
亀田 純木村 学O. A. MELINIKOV早坂 康隆橋本 善孝坂島 隆彦新井 孝志芳野 極鈴木 紀毅
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2000 年 109 巻 2 号 p. 235-248

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The region from Sakhalin in Russia to the eastern margin of the Japan Sea has been regarded as a convergent plate boundary zone between the Eurasia and the North America Plates because large earthquakes and active crustal movements are prominent in this zone. These activities along the eastern margin of the Japan Sea to southern Sakhalin are almost consistent with the expected relative plate motion deduced from the Euler pole which is estimated from the magnetic anomaly lineations in the Atlantic Ocean, but the modern tectonic aspect in northern Sakhalin is inconsistent; e.g., 1995 Neftegorsk Earthquake is one of the typical events.
As a result of a structural and tectonic study in northern Sakhalin, the NE-SW compressive tectonic feature since the Late Miocene was clarified. In the northernmost area of the Schmidt Peninsula, the early Cretaceous ophiolite thrusts upon the Late Cretaceous sedimentary rocks. The ophiolite has an overturned sequence: serpentinites, gabbros, basaltic rocks, and hemipelagic sediments in descending order. The Cretaceous sediments form a map-scale and NW vergent synclinorium. All the deformation structures in outcrop scale, such as micro-folds, minor reverse and normal faults, axial plane cleavages and bedding slips, are consistent with a large scale folding. These structural relationships suggest only one event of deformation, which appears to be linked to overthrusting of the ophiolitic rocks.
A structural investigation of the southeastern part of the Schmidt Peninsula revealed that the Middle Miocene sediments composed of siltstone and sandstone are folded with wavelength of several tens to several hundred meters. Their fold axes trend in the NW direction, which indicates that deformation under the NE compression is the same as the deformation in the northernmost area. The thickness of sedimentary layers are constant everywhere in folding. This fact indicates that the timing of the deformation is after sedimentation, that is, after the Middle Miocene. This deformation event is consistent with modern activity in northern Sakhalin. Therefore, the modern tectonic framework might have started in the Late Miocene time.
A plate tectonic model indicates that the modern relative motion between the Eurasia and the North America Plate started at about 11 m. y. ago. The Late Miocene onset of the modern tectonic framework in northern Sakhalin occurred at almost the same time. The most reliable model to explain the discrepancy of sense of movement from the Eurasia-North America retative motion, may be the “extrusion” of the Okhotsk Block toward the Pacific Ocean. The dextral extrusion boundary in northern Sakhalin may be traced along theeastern coast of Sakhalin to the northern edge of the Kuril Basin, where active seismicityhas been observed although a detailed study of focal mechanisms and other tectonic aspects is needed in the future.

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