Evolution of Riedel-shear pop-up structures during cumulative strike-slip faulting: A case study in the Misayama-Godo area, Fujimi Town, central Japan

Abstract

Strike-slip faulting often develops en échelon faulting resulting from Riedel shears. When faulted basement rocks are covered by thick unconsolidated deposits, this en échelon faulting leads to the subsequent evolution of a series of pop-ups along the fault during cumulative displacement. Although previous sandbox experiments and numerical simulations have examined the evolutionary processes for Riedel-shear pop-up structure formation, few field examples have been reported until now concerning (1) the time-series of evolutionary processes of pop-up structure generation and (2) time periods necessary to reach the stage of a “throughgoing fault zone”. The Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone is among the largest and fastest-slipping active fault zones in Japan, and a series of tectonic pop-ups related to left-lateral strike-slip displacement have developed in the Fujimi area in the central to southern part of the fault zone. This area is covered by thick (~820 m) unconsolidated deposits, which have resulted in Riedel-shear pop-up structures. We conducted a geomorphic analysis, an extensive drilling survey, and pit excavation survey on a pop-up in the Misayama-Godo area of Fujimi, and concluded that (1) the pop-up structures in the Misayama-Godo area have been built up by two marginal faults between ~100 ka and ~10–20 ka, and (2) the fault displacement at depth has been concentrated on the faults that run across the pop-up structures (corresponding to the “throughgoing fault zone”) since ~10–20 ka. One of the faults that run across the pop-up structures probably slipped during the most recent earthquake in the central to southern part of the ISTL fault zone. This study is presented as a robust field example of the evolution of Riedel-shear pop-up structures during cumulative strike-slip faulting.