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

本文对日本本州岛中部广泛分布的火山、大型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本。用旋转磁力计测量了剩磁强度，并用渐进热退磁和交变退磁评价了剩磁稳定性。为了估计岩石中的铁磁性矿物，利用脉冲磁化器对等温剩余磁化强度进行了渐进式采集，并通过正交等温剩余磁化强度的热消磁验证了估计结果。编制了可用的站点平均古磁方向，用于确定高应变区，该高应变区与最近通过全国GPS阵列大地测量研究发现的变形区相关。同时，在反射地震资料解释的基础上，阐明了北海道弧-弧交汇带和日本东北太平洋沿岸发震俯冲带10多年来的断层运动和盆地形成历史，为认识日本中部构造提供了重要线索。我们利用古地磁法精确地评估了短期地壳形变，这种方法通常应用于板块间长期累积变形。

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.