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| THE PRIMARY INVESTIGATION ON THE ORIGIN OF THE OKINAWA TROUTH |
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| Abstract: |
| The seismic data show that the topography on the east of Okinawa Trough is not symmetric with that on the west. The topography of each side has its own characteristics. The rough topography on the west was formed by the upward intrusion of the rock magama of the Pliocene which was small both in scale and in depth, while that on the east, caused by the upthrust of the large and thick mantel which took place earlier than the uplift of the batholith west of the trough, is steep and smooth. The acoustic wave stratum breaks into two units, the upper horigontal stratum and the lower stratum, the boundary between which is the plane of unconformity. The normal fault occurred at the base of slope east of the seismic section of the trough. Moreover, the simultaneity folds which were the result of the sea-floor spreading occurred locally in the upperstratum on the east and west sides of the trough. The developement of Okinawa Trough may be divided into four stages and in two directions. The four stages are: the strong differential settlement of the lithosphere, causing tension fissures; the disappearance of the lithosphere under the Philippine Sea towards the continent, resulting in the thinning of the crust of Okinawa Trough; the uplift of the mantel in large scale, leading to the rift of the lithosphere; the upward intrusion of the magama, bring about the widening of the trough. The two directions are: the eastward movement of the trough following the moving of the Ryukyu Island Arch; the westward extension of the trough as a result of the successive upward intrusion of the magama. |
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