Kinematic Analyses and Radiometric Dating of the Large‐Scale Paleogene Two‐Phase Faulting Along the Median Tectonic Line, Southwest Japan

Abstract

The Median Tectonic Line (MTL) in Southwest Japan, a major east-west trending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and the Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. Based on the detailed fieldwork and microstructural studies of fault rocks, we reconstruct the kinematic history along the MTL during the Paleogene, which can be divided into the Ichinokawa and pre-Tobe phases. While the Ichinokawa phase is defined by large-scale, top-to-the-north normal faulting, the pre-Tobe phase is represented by large-scale, high-angle right-stepping en échelon faults almost parallel to the MTL in the Upper Cretaceous Izumi Group. We found that left-handed en échelon folds have developed along the right-stepping faults, which contain 25-60 m wide cataclasite and fault gouge. Both map scale en échelon folds and microstructures (e.g., composite planar structures) in the fault rocks suggest that they were formed by sinistral-reverse faulting with top-to-the-SW kinematics. Furthermore, based on the new K-Ar age dating of authigenic illite from the fault gouge along the MTL and right-stepping faults, it can be concluded that the MTL was activated in two discrete stages at approximately 59 Ma (Ichinokawa phase) and 47-46 Ma (pre-Tobe phase). Based on these results, we reappraise the kinematic framework of the MTL in the Paleogene, which can be interpreted as the record of the movements of the subducting oceanic plate relative to the continental plate.

Quartz Microstructures from the Sambagawa Metamorphic Rocks, Southwest Japan: Indicators of Deformation Conditions during Exhumation

“…Amphibole and phengite K-Ar (or 40 Ar/ 39 Ar) ages range from 94 to 65 Ma (e.g., [28,29], reproduced in Figure 1d), probably indicating the ages of exhumation. The Sambagawa metamorphic rocks were finally juxtaposed against the Late Cretaceous Ryoke metamorphic rocks and granitoids, and the unconformably overlying Upper Cretaceous Izumi Group by large-scale north dipping normal faulting at c. 59 Ma in the earliest Paleogene [30,31]. [32].…”

“…D3 resulted in the formation of open and upright folds with horizontal and E-W to WNW-ESE trending axes, the wavelength of which varies from a microscopic (thin section, e.g., [53]) to macroscopic (geological map) scale (Figure 1b). The D3 stage folds are considered to have formed after substantial exhumation [54], in relation to the left-lateral displacement along the MTL [31,36]. The size of recrystallized quartz grains was first systematically analyzed with an intercept method by [20] for 25 quartz schist samples from the chl to ol-bt zones of the LSL along the Asemi-River.…”

Trench and drilling investigation of the Median Tectonic Line in Shikoku, southwest Japan: implications for fault geometry

“…The MTL has a long tectonic history since the Cretaceous, and has experienced several slip senses of dextral, sinistral, normal, and reverse faulting with changes in regional stress (e.g., Kobayashi 1941;Ichikawa 1980;Takagi and Shibata 1992;Aoya et al 2013;Kubota et al 2020). The current MTL displacement shows dextral movements in response to the oblique subduction of the Philippine Sea Plate (Nishimura et al 2018).…”

“…The Kawakami Fault bounds the Pliocene-Pleistocene half-graben Okamura Group and the Cretaceous Izumi Group. The presence of slickensides and the asymmetric structure of the fault zone indicate predominant strike-slip movement at the latest slip surface (Kubota et al 2020). Based on 14 C dating at the Kawakami Fault, the most recent seismic event was traced to the Keicho earthquake of 1596 (Ikeda et al 2014(Ikeda et al , 2019.…”

“…The Median Tectonic Line (MTL; Naumann 1886; Tsutsumi and Okada 1996;Goto 2018;Kubota et al 2020) is a trench 1000 km in length parallel to a dextral strike-slip fault, dividing southwest Japan into outer and inner zones (Fig. 1a).…”