Quantitative assessment of deformation of upper crust based on paleomagnetism of widespread tephra and large-scale pyroclastic flow deposit

本研究对日本本州岛中部广泛分布的火山灰、大规模火山碎屑流沉积和一座第四纪火山进行了野外调查。从39个地点获得了古地磁测量样品。通过旋转磁力计测量剩余磁化强度，并通过渐进热去磁和交流磁场去磁来评估剩余磁化强度的稳定性。为了估算岩石中的铁磁性矿物，采用脉冲磁化仪进行等温剩磁（reversization magnetization，简称reversization）的逐步采集，并通过正交磁化曲线的热退磁验证了估算结果。汇编了现有的场地平均古地磁方向，用于确定高应变区，该高应变区与最近通过全国GPS阵列的大地测量研究发现的变形区相关联。沿着这些研究，对1000万年以来的断裂运动和成盆历史进行了研究。通过对反射地震资料的解释，阐明了日本中部弧-弧结合带（北海道）和孕震俯冲带（日本东北部太平洋沿岸）的构造活动，为认识日本中部的构造提供了重要线索。利用古地磁方法对地壳短期形变进行了精确的估算，该方法通常用于研究板块间长期累积形变。

In this study, field survey was conducted for widespread tephra, large-scale pyroclastic flow deposits and a Quaternary volcano in the central part of Honshu Island, Japan. Samples for paleomagnetic measurements were obtained from 39 sites. Remanent magnetization was measured by a spinner magnetometer, and stability of remanence was evaluated by progressive thermal and alternating-field demagnetization. Progressive acquisition of isothermal remanent magnetization (IRM) was conducted using a pulse magnetizer in order to estimate ferromagnetic minerals in rocks, and the estimation was verified through thermal demagnetization of orthogonal IRM. Available site-mean paleomagnetic directions were compiled for determination of high strain area, which was correlated with a deformation zone recently detected through geodetic studies of nationwide GPS array. Along with these study, fault motion and basin-forming history over 10 m.y. in an arc-arc junction (Hokkaido) and a seismogenic subduction zone (Pacific coast of northeast Japan) were clarified on the basis of interpretation of reflection seismic data, which gave a significant clue to understand tectonics in central Japan. We have precisely assessed short-term crustal deformation utilizing paleomagnetic method, which is generally applied for long-standing cumulative deformation between plates.