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

本研究旨在基于古地磁学评估本州岛弧中部周围的第四纪构造运动。本州岛弧中部山脉的特点是由于弧-弧碰撞而导致上地壳具有相当高的应变和隆升率。我们识别了表明活动断层的地形轮廓，并沿着穿过目标断层的横断面收集了古地磁样本。样品取自 Nyukawa 和 Kamitakara 火山碎屑流沉积物，分别位于第四纪早期和晚期。在本州中部周围发现了与其相关的广泛分布的火山灰。对入川火山碎屑流沉积物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的旋转有关，反映了在断层运动影响下剪切的地壳块体应变的弛豫过程。对 Kamitakara 火山碎屑流沉积物中保存的磁矢量的详细分析表明，变形一直持续到第四纪晚期。令人惊奇的是，根据古地磁方法可以显着地检测到少量的近期变形。与上述火山碎屑流沉积相关的广泛分布的火山灰的稳定碎屑剩磁描绘了所研究的活动断层的地震活动模式。还指出，上地壳第四纪累积变形与近十年卫星研究精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.