*2.1.2. Neotectonic activity*

**2. Description of major tectonic zones**

192 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

The Median Tectonic Line (MTL) is the largest crustal break in southwest Japan, which bisects the island arc into the old terranes intruded by igneous rocks in the Inner Zone and partly metamorphosed accretionary complexes in the Outer Zone (Figure 1; [9]). It has a period of activity as long as 100 m.y. and highly complicated change in slip direction. In the following sections, we present a chronicle of the MTL activity based on previous research and original

**Figure 1.** An index of the neotectonic regime in southwest Japan. Base map is after Huzita [2]. Gradation on backarc shelf showing onlapping sedimentation pattern and a seismic profile (shown inset) in the area are after Itoh et al. [5]. Influx of crystalline schist gravels is shown by green areas [22-24]. (Right) Bouguer gravity anomaly map. The Bouguer

Compiling reliable paleomagnetic data, Itoh et al. [5] reconstructed the Cretaceous to early Paleogene paleogeography around the eastern Eurasian margin (Figure 2a). They pointed out that the MTL constituted a larger fault zone together with the Central Sikhote Alin Fault, and had a left-lateral slip sense as a result of the quite rapid northerly motion of the Izanagi Plate [10]. Along the fault zone, conspicuous pull-apart basins were developed and buried by the

density is 2670 kg/m3, and contour interval is 10 mGal. Gravity map is generated based on [8]

**2.1. Median Tectonic Line**

interpretation of geophysical data.

*2.1.1. Initiation of the regional fault zone*

It is accepted that the MTL has been reactivated as a right-lateral fault since the late Neogene under the influence of the oblique subduction of the Philippine Sea Plate (e.g., [17]). Nakamura et al. [6] demonstrated that the oceanic plate shifted its convergent motion counterclockwise in the Quaternary, which resulted in vigorous slips on the MTL and westward transportation of the Outer Zone (e.g., [18]). However, when compared with the older stages, geomorpho‐ logical features (e.g., [19]) suggest that the active segment of the MTL shrank during the late Quaternary. No active portion is identified in the eastern part of the Kii Peninsula (Figure 3), in which the geomagnetic anomaly contrast is also obscured. This is in contradiction to the plate subduction regime, and further study of the transient shift of MTL activity is necessary to solve this tectonic paradox. Another noteworthy point is that the MTL trace is characterized by frequent jogs and steps. A sounding survey in the Kii Channel [20] delineated a complex fault pattern that may cause great diversity in basin formation.
