**1. Introduction**

The formation of a pull-apart basin is a ubiquitous phenomenon associated with transcurrent faulting in the Earth's upper crust (e.g., [1]). Because an oblique mode of subduction provokes the detachment and transcurrent motion of a forearc crustal sliver [2], numerous pull-apart basins develop on active plate margins.

Southwest Japan is an island arc that has long been under the influence of the oblique sub‐ duction of oceanic plates. Figure 1 delineates its bisecting fault, the Median Tectonic Line (hereafter referred to as MTL). Regional pull-apart basins related to sinistral motion of the MTL during the late Cretaceous were described by Noda and Toshimitsu [3]. Vigorous sinistral faulting also caused regional wrench deformation of the adjoining terranes [4]. The north‐ westward motion of the Philippine Sea Plate, depicted as a positive gravity anomaly (redcolored) portion within the Pacific Ocean in Figure 1, resulted in dextral slips on the MTL since the Pliocene. Although the recent activity of the MTL is key to understanding the neotectonic regime of southwest Japan [5], basin-forming processes along the regional fault system have not been fully described.

The authors attempt to present a quantitative description of sedimentary basins at the western and eastern terminations of the MTL, namely, the Beppu-Iyo Basin and Osaka Basin, respec‐ tively (Figure 1). As shown by negative gravity anomalies, they are enormous depressions filled by recent clastics. Seismic surveys in the study areas became popular in recent decades, but interpretation has not reached a mature stage. Thus, we utilize gravimetric methods for estimating their morphology and volume. Geomagnetic anomaly modeling helps us to identify

© 2013 Itoh et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Itoh et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

subsurface constituents of the sedimentary basins. Additionally, the evolutionary processes of the basins are argued based on geologic information. This is a multidisciplinary case study of basins ahead of a comprehensive visualization of the basin interior with seismic information.

faults constituting a rhomboidal depression surrounded by the faults [7]. These structural

Characteristic Basin Formation at Terminations of a Large Transcurrent Fault — Basin Configuration…

The Iyonada Sea is characterized by a remarkable negative gravity anomaly [8,9], but its origin has not been discussed so far. In contrast with the Beppu Bay basin, ambiguous points remain in the formation mechanism of the extensive Iyonada Sea depression. Paired dextral fault is not identified, and the basin is not regarded as an elongate sag in an area of propagation of lateral fault terminations because the sag is buried by recent sediments (Age assignment of a sedimentary unit upon the basin margin is presented in a following section.) simultaneous

Kusumoto et al. [10] determined a three-dimensional subsurface structure around Beppu Bay on the basis of gravimetric data. We adopt their structural model and estimate the volume of the sedimentary basin. Figure 2 is a compiled map of the Hohi Volcanic Zone. The volume of the basement depression is calculated by the Gauss-Legendre numerical integration [11], from

**Figure 2.** Simplified geology around the Hohi Volcanic Zone including Beppu Bay. Overlain basement structural con‐ tours are based on gravity anomaly modeling after Kusumoto et al. [10]. Grid shows data points for volumetric analy‐

 km3 .

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patterns are characteristic for releasing bend of strike-slip fault.

with those in the western termination of the MTL (Beppu Bay; [7]).

depth data given on the mesh with a 10 km interval, and is 4.1×103

*2.1.1. Beppu Bay*

sis

**Figure 1.** Index map of the studied basins. Base map shows Bouguer gravity anomaly [6]. Bouguer density is 2670 kg/m3. The northern part of the Philippine Sea Plate is depicted as a positive gravity anomaly (red-colored) portion within the Pacific Ocean. Shape of an oceanic plate is generally delineated by positive Bouguer anomaly in ocean, be‐ cause an area of deep water is accompanied by positive gravity values as a result of correction procedures (cf. Y. Itoh, K. Takemura and S. Kusumoto in this book)
