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

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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.