Quantitative assessment of the Quaternary tectonic movement based on paleomagnetism of widespread tephra and large-scale pyroclastic flow deposit

本研究的目的是在古地磁的基础上评估本州弧中部周围的第四纪构造运动。本州岛弧中部山区的特点是相当高的应变和隆起速率的上地壳作为弧-弧碰撞的结果。我们确定了指示活动断层的地形线性构造，并收集了沿穿越目标断层的沿着断面的古地磁样本。样品取自Nyukawa和Kamitakara火山碎屑流矿床，分别侵位于第四纪早期和晚期。在本州中部发现了与之相关的广泛分布的火山岩。对牛川火山碎屑流存款中保存的磁矢量的详细分析表明，第四纪盆地的形成与高度复杂的地壳绕垂直轴旋转有关，反映了在断层运动影响下剪切的地壳块体的应变松弛过程。通过对Kamitakara火山碎屑流存款中保存的磁矢量的详细分析表明，变形一直持续到晚第四纪。令人惊讶的是，在古地磁方法的基础上，可以显着检测到少量的现代变形。与上述火山碎屑流沉积物相关的广泛分布的火山岩的稳定碎屑反射磁化，描绘了所研究的活动断裂的地震活动模式。第四纪上地壳的累积变形与近十年来卫星观测的瞬时应变分布相吻合。

This study aimed at assessment of the Quaternary tectonic movements around the central part of the Honshu arc on the basis of paleomagnetism. Mountainous range of the central Honshu arc is characterized by quite high strain and uplift rate of the upper crust as a result of an arc-arc collision. We identified topographic lineaments indicative of active faults, and collected paleomagnetic samples along transects crossing the target faults. Samples are taken from Nyukawa and Kamitakara Pyroclastic Flow Deposits, which emplaced in the early and late Quaternary, respectively. Their correlative widespread tephras have been discovered around the central Honshu. Detailed analysis of magnetic vector preserved in the Nyukawa Pyroclastic Flow Deposit demonstrated that a Quaternary basin had been developed associated with highly complicated crustal rotation around vertical axes reflecting relaxation process of strain in crust blocks sheared under the influence of fault movements. Detailed analysis of magnetic vector preserved in the Kamitakara Pyroclastic Flow Deposit demonstrated that the deformation has continued through the late Quaternary. It is amazing that small amount of recent deformation can be significantly detected on the basis of paleomagnetic approach. Stable detrital remanent magnetization of widespread tephras correlative with the above-stated pyroclastic flow deposits delineates pattern of seismological activity of the studied active faults. It is also noted that the Quaternary cumulative deformation of the upper crust resembles with instanteneous strain distribution exactly determined on the basis of satellite research during the last decade.