海嶺衝突とその地質的影響 : 西南日本の後期白亜紀を例として  [in Japanese] Active ridge-forearc collision and its geological consequences : an example from Late Cretaceous Southwest Japan  [in Japanese]

Some melange-like units in the Shimanto Supergroup of Ryukyu and Southwest Japan, such as Naze Formation (Amami-Oshima), Makimine Formation (east Kyushu), Okitsu melange (west Shikoku), Mugi Formation (east Shikoku), Ryujin Formation (Kii Peninsula) and Inui Group (Akaishi Mountains), contain MORB-type rocks which were extruded on the deep-sea floor where terrigenous clastic sediments had been accumulating. Fossil ages from the strata containing these in-situ basaltic rocks become younger eastward from Amami-Oshima to Akaishi Mountains over a distance of 1300 km, implying that the site of igneous activity in the trench are amigrated eastward along the Ryukyu and Southwest Japan margins between Cenomanian-Turonian to Late Campanian-Paleocene time. This suggests that the basaltic rocks were emplaced in connection with Kula-Pacific ridge-forearc collision. The migration of the ridge-trench-trench triple junction along the convergent margin of Southwest Japan produced the following serious changes in the mechanical and thermal state of the arc-forearc region. The approach of the triple junction led to an activation of igneous activity in the Inner zone of Southwest Japan anda trenchward migration of the front of igneous activity. Consequently, a great volume of coarse clastics derived from the acidic igneous rocks accumulated in the trench area prior to ridge collision. The thick trench deposits composed of coarse clastics would be accreted and incorporated into the forearc by the processes of offscraping and underplating. The underplating of coarse clastics probably resulted in the exhumation of the Sanbagawa metamorphic rocks. The approch of the triple junction also led to an uplift of the forearc region owing to the subduction of more buoyant oceanic plate, and the resultant eastward extinction of a shelf basin in the Chichibu Belt. After the ridge collided, the magmatic front in the Inner zone of Southwest Japan migrated northward (to the Japan Sea side) from the Ryoke Belt with a diminution in the dimensions of igneous activity.