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

本研究以古地磁学为基础，对本州弧中央地区的第四纪构造运动进行了评价。由于弧-弧碰撞，本州岛中部山脉的上地壳具有相当高的应变和抬升速率。我们识别了指示活动断裂的地形线状构造，并沿着穿过目标断裂的横断面收集了古地磁样品。样品来自Nyukawa和Kamitakara火山碎屑流沉积，分别侵位于第四纪早期和晚期。在本州岛中部附近已经发现了与之相关的广泛存在的苔藓。Nyukawa火山碎屑流矿床中保存的磁矢量的详细分析表明，已发育了一个与高度复杂的地壳绕垂直轴旋转有关的第四纪盆地，反映了受断层运动影响剪切的地壳块体的应变松弛过程。对Kamitakara火山碎屑流沉积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，能够显着地检测到少量的现代形变。与上述火山碎屑流沉积相关联的广泛分布的特氏岩的稳定碎屑剩磁作用，圈定了所研究活动断裂的地震活动格局。还指出，上地壳的第四纪累积形变与近十年来根据卫星研究准确确定的瞬时应变分布相似。

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.