Are there any fossil seismic zone in an accretionary complex ? -A thermochronologic study-

增生棱镜是由海洋沉积物和海沟充填的大陆沉积物的混合刮离和底沉作用形成的，沿俯冲大陆边缘发育。由于俯冲带是增生棱镜发育的区域，是深度为10-30km的大地震区域，地震辐射的热能可能会对周围的增生复合体造成各种地质事件，这是通过裂变径迹（FT）方法从日本中部Awaji岛野岛断裂带周围的岩石中检测到的。日本西南部九州东北部的岛曼东增生杂岩，根据微化石和岩相划分为4个单元。利用裂变径迹法和镜质组反射率估算了各单元区域变质作用的升温程度。存在于Nobeoka逆冲以南的Mikado组内部，从FT年龄和径迹长度分布来看，热扰动自南向北增大。Nobeoka逆冲断层逆冲断层泥FT年龄表现出比北川组和下Mikado组更高的热扰动。31.2±5.7 Ma的FT年龄可能指示逆冲断层活动的升温时间。沉积与得到的FT年龄之间的时间跨度小于500万年。假设俯冲速度和俯冲角度分别为50毫米/年和13°，逆冲沉积层将在几百万年内到达发生板块边界地震的深度。因此，得到的逆冲活动时间可能在板块边界引起一次大地震。其他断层在FT数据中没有显示出加热的证据。这意味着加热的程度可能沿着断层面变化。

Accretionary prisms develop along subducted continental margins by the offscraping and underplating of the mixture of oceanic sediments and trench fill continental deposits. Because subduction zones, where accretionary prisms develop, is the area of large earthquakes at the depth of 10-30km, the thermal energy radiated by earthquakes could cause various geological events to surrounding accretionary complexes, as is detected by the fission track (FT) method from rocks around the active Nojima Fault, Awaji Island, central Japan.The Shimanto accretionary complex in northeastern Kyusyu, southwest Japan, is divided into four units based on microfossiles and rock facies. The degree of heating by regional metamorphism is estimated for each units using fission track method and vitrinite reflectance. The thermal disturbance within Mikado formation, which exists south of the Nobeoka thrust, becomes higher from south to north according to FT ages and track-length distribution. The FT age of the sample collected from thrust gouge of the Nobeoka thrust fault shows higher thermal disturbance than that of hanging Kitagawa unit and the lower Mikado formation. The FT age, 31.2±5.7 Ma, may indicate the time of heating by the thrust fault activity. The time range between sedimentation and the obtained FT age is less than 5 million years. Provided the velocity and the angle of subduction to be 50 mm/yr and 13°, respectively, underthrusted sediments reach the depth, where plate-boundary earthquakes occur, within a few million years. Hence the obtained time of thrust activity may cause a large earthquake at the plate boundary. Other faults show no evidenceof heating in FT data. This means that the degree of heating may vary along the fault plane.