The Kaikoura Diastrophism and the Fossa Magna Disturbance

Abstract

In the middle Tertiary, two similar orogenic movements have occurred on the western margin of the Pacific, one in New Zealand (Kaikoura Diastrophism), the other in Japan (Fossa Magna Disturbance). In the Miocene times, New Zealand was subjected to the Principal Horizontal Stress coming from the East East South and the Alpine Arc of the Crystalline schist series, the backbone of the Is. was wrenched off near wairau into the two blocks, the North and the South Is. which are separated by the Cook Strait. At that time the volcanoes were erupted in the North Is., which extend to the north far away into the Kermadec and Tonga submarine ridges, whose eastern side is limited by the trench of the same name. The Tonga Kermadec Trench reaches 3, 500 km in length, whose south end comes to the Banks Peninsula of the South Is. The fault whose plane dips west penetrates the bottom of the trench, forming a Deep earthquake zone in the depth of above 300 km. According to Kingma, the zone might have been built up at the time of the Kaikoura Diastrophism.
In the Oligocene-Miocene times, the outer zone of Southwestern Japan was subjected to the Principal Hrizontal Stress coming from the south, and with the inner zone it was wrenched off at the Fossa Magna into the two blocks, Northeast Honshu and Southeast, which were separated temporarily by a strait connecting the Japan Sea and the Pacific. At that time volcanoes erupted in the strait, which extended to the south far away into the Shichito submarine ridges. The eastern side of the Shichito submarine ridges is limited by the Shichito Trench running northsouth, whose bottom is penetrated by the fault whose plane dips west, and in the depth of above 300 km the fault plane forms a Deep earthquake zone. The fault line runs parallel to the trench but on the way turns the course to the northwest, little before the mouth of the Sagaminada which opens into the Shichito Trench ; and after passing the Fossa Magna and crossing the Japan Sea it reaches the Asiatic continent. The fault may reach about 2, 500 km in length. As in the case of the Kaikoura Diastrophism, in Japan the Deep earthquake zone might have been built up by the Fossa Magna Disturbance. The course of the zone might have been interrupted by the granodiorite batholith which lies in Northeast Honshu occupying the Kitakami and Abukuma mountain land ; in avoiding the great massifs it seems to have taken the present course of the Fossa Magna.
In the Fossa Magna Disturbance, the Akaishi sphenoid which occupies a member of the outer zone has stood up at a right angle from the former position. So how the Paleo-Akaishi might have been may be recalled to our view by bringing back the sphenoid to the former eastwest trend of the outer zone. In the study of the Paleo-Akaishi, the Kii Peninsula and Shikoku we may notice the outthrusting of several formations from the south toward the Median Line, which increases its saliency with its approach to the east. The outthrusting must have taken place in the Laramide Epoch, prior to both the deposition of the Muro series (Paleogene) of the Kii Peninsula, and the Naharigawa series of Shikoku, and immediately after that of the Izumi sandstone group (Upper Cretaceous).
In the Shikoku and Kinki districts the Median Line is subjected to the lateral thrust from the north, but it may have been a movement posterior to the Koyasan outthrusting of the Laramide Epoch.