**Post-Opening Deformation History of the Japan Sea Back-Arc Basin: Tectonic Processes on an Active Margin Governed by the Mode of Plate Convergence** Post-Opening Deformation History of the Japan Sea Back-Arc Basin: Tectonic Processes on an Active Margin

DOI: 10.5772/intechopen.71953

Governed by the Mode of Plate Convergence

Yasuto Itoh

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Additional information is available at the end of the chapter Yasuto Itoh

http://dx.doi.org/10.5772/intechopen.71953 Additional information is available at the end of the chapter

#### Abstract

Three-dimensional structure of the Japan Sea back-arc basin is investigated based on an extensive reflection seismic survey. The process of the Oligocene to early Miocene rifting is described in reference to a geologic database, and the most likely paleoreconstruction of rifted continental fragments is presented. The back-arc region has been subjected to intermittent post-opening deformation events, which the author regards as side effects of temporal shifts in the convergence mode of the Philippine Sea Plate (PSP). The southern shelf of the Japan Sea appears to have suffered North-South strong contraction for a short period of time during the latest Miocene. Resumed convergence of the PSP was responsible for the regional tectonic event because frequent igneous intrusions within the upper Miocene series upon the back-arc shelf, which was confirmed by a borehole stratigraphic study, are suggestive of revitalized arc volcanism linked to dehydration of the subducted slab. During the Quaternary period, confined structure in varied forms developed on the shelf, which is related to the dextral wrench deformation of southwest Japan and the eventual arc-parallel crustal breakup along the back-arc region. Simultaneous highly oblique subduction of the PSP provoked the prevailing shear stress and conspicuous neotectonic deformation.

Keywords: convergent margin, back-arc opening, back-arc basin, seismic survey, Japan Sea, Philippine Sea Plate, eastern Eurasia

## 1. Introduction

Back-arc opening specifically occurs on convergent plate margins of the globe. Since the phenomenon inevitably impacts geographical and environmental conditions, numerous researchers have pursued evolutionary processes of back-arc basins.

© 2018 The Author(s). Licensee IntechOpen. This chapter is 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.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Figure 1. (a) Present configuration of southwest Japan and (b) its paleogeographic reconstruction before opening of the Japan Sea following [4]. Bending of the southwestern Japan arc caused by the middle Miocene collision event has been restored referring to paleomagnetic studies, expressed as the straightforward trend of the Median Tectonic Line.

Figure 2. Index map showing geophysical survey tracks (dotted lines) around the southwestern shelf of the Japan Sea. The thick solid lines and larger open symbols are the seismic lines analyzed in the present study and key stratigraphic

Post-Opening Deformation History of the Japan Sea Back-Arc Basin: Tectonic Processes on an Active Margin…

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Figure 3. Reflection seismic profile (time migration; SN1-1) on the southwestern shelf of the Japan Sea. See Figure 2 for

boreholes, respectively.

line locations.

The eastern Eurasian margin has been a site of vigorous basin formation related to long-standing convergence of major oceanic plates since the late Mesozoic [1]. The Japan Sea is located around mid-latitudes on the margin and is interpreted as a late Cenozoic back-arc basin based on the geological affinity between the Japanese Archipelago and the continental rim. In a series of pioneering paleomagnetic studies by Otofuji et al. (e.g., [2, 3]), a fan-shaped opening mode was advocated to explain the large rotation of the rifted block. Figure 1 shows the most probable paleoreconstruction of the southern part of the Japan Sea [4], in which jigsaw fitting of subsea continental fragments is carefully taken into account.

In this chapter, the author focuses on the southwestern shelf of the back-arc basin. The threedimensional architecture of the shelf is visualized by means of detailed seismic data, and its development process is described referring to stratigraphic data of deep boreholes. The spatiotemporal variety in the structural styles reflects intermittent changes in the converging sense of the Philippine Sea Plate (PSP). In other words, deformation records around the arc are a key to elucidating the transient motion of the marginal sea plate.
