Evolutionary process of Beppu Bay in central Kyushu, Japan: a quantitative study of the basin-forming process controlled by plate convergence modes

Abstract

This integrated tectonic study reveals the basin-forming and deforming processes on an active margin. Southwest Japan (SWJ) is an island arc under the influence of oblique subduction of the Philippine Sea plate, which has provoked dextral slips on the arc-bisecting Median Tectonic Line (MTL). Plio-/Pleistocene sediments in Beppu Bay, a tectonic depression at the westernmost portion of the MTL, are categorized into lower (5~0.7 Ma), upper (0.7~0.3 Ma), and an auxiliary uppermost (0.3 Ma~present) unit in ascending order. Detailed seismic interpretation demonstrates that major structures in the deep interior of the basin are an older half-graben under a strong N-S extensional regime and a younger pull-apart sag that developed in a right-stepping part of the MTL as a result of late Quaternary-enhanced strike-slip rates on the fault. Sediments within the pull-apart have been deformed by later inversion events as a contraction phase arose. Conspicuous deformation of the hanging wall of the low-angle detachment of the basin was successfully reproduced by numerical modeling. Based on a discrete element method, this suggests that structural differences in the deformed sedimentary layers are caused by differences in the dip angles of the faults. Remarkable temporal changes in tectonic regimes around Beppu Bay and other areas of SWJ are probably related to transient modes of convergence, including the migration of the Euler pole, of the Philippine Sea plate since ca. 6 Ma.

Dip distribution of Oita–Kumamoto Tectonic Line located in central Kyushu, Japan, estimated by eigenvectors of gravity gradient tensor

“…Although Kusumoto (2015) reported that this dip estimation technique tends to underestimate the actual dip in deep parts when applied to normal faults, seismic reflection surveys have confirmed dips of normal faults distributed in Beppu Bay to be 17° and 7° by seismic reflection surveys (e.g. Itoh et al 2014). Therefore, the dip distribution calculated from the gravity gradient tensor agrees with the observed data.…”

“…The dip of the structure plays an important role in numerical simulations that quantitatively examine tectonics (e.g. Finch et al 2004;Itoh et al 2014;Kusumoto et al 2015).…”

Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan

“…The structural boundary of Zone B is well correlated to a zone with a steep gravity gradient, which is interpreted to be the southern rim of a graben structure (Kamata 1989;Kamata and Kodama 1994). Recent studies suggest that the graben is a pull-apart basin related to the MTL dextral movements partly with volcanic depressions (Itoh et al 1998;Saiga et al 2010;Itoh et al 2014). The altered volcano-clastic rock and hydrothermal water filling the graben are considered to be the cause of the low resistivity at shallow levels.…”

“…The altered volcano-clastic rock and hydrothermal water filling the graben are considered to be the cause of the low resistivity at shallow levels. Although Itoh et al (1998) and Itoh et al (2014) suggest that Zone B is inactive at the present time, GPS data support active shear around this region (Nishimura and Hashimoto 2006;Wallace et al 2009). The stress field estimated from earthquakes also supports aseismic slip at the shear zone (Matsumoto et al 2015).…”

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

“…The eastern Asian and western Pacific margins, including the Japanese islands, are the areas where relative plate motions caused several remarkable tectonic events during the Cenozoic, such as back-arc opening (e.g., Martin 2011;Sasaki et al 2014), arc-arc collision (e.g., Arai and Iwasaki 2014), and severe intra-arc crustal deformation (e.g., Cho and Kuwahara 2013;Itoh et al 2014). The clockwise rotation of Southwest Japan (Otofuji and Matsuda 1983;Torii 1983) is a conspicuous example of such tectonic events.…”