4. Discussion

After the abovementioned overview of the structural and stratigraphic characteristics of the southwestern Japan Sea shelf, we now discuss the evolutionary processes of the study area following the back-arc opening event. In this section, the author discusses structural features governed by a unique opening mode, regional inversion provoked by resumed underthrusting of the Philippine Sea Plate, and stress-strain concentration on the shelf under the influence of an emerging simple shear regime, in this order.

#### 4.1. Back-arc opening governed by the divergent rift system

As mentioned earlier, the southern Japan Sea is unique, in that the oceanic basin is studded by a number of submerged highlands composed of massive continental crust. Therefore, in order to force the sea floor to spread, a radial rift system presumably developed on the eastern Eurasian margin from the Oligocene to early Miocene. The normal faults in Figures 3 and 4 are relics of the prevalent extension. Figure 12 shows a paleoreconstruction map in the early Miocene stage. As suggested by [4], such a divergent breakup is endorsed by the presence of the early Miocene rift-margin type volcanism along the normally faulted scarp. In comparison to the pre-rifting paleogeography in Figure 1, the separated southwest Japan block drifted Post-Opening Deformation History of the Japan Sea Back-Arc Basin: Tectonic Processes on an Active Margin… http://dx.doi.org/10.5772/intechopen.71953 93

Figure 12. Paleoreconstruction map of the southern Japan Sea in the syn-rifting stage (early Miocene) modified from ref. [4].

southward and rotated clockwise as a result of differential effective spreading rates determined by rift geometry. Coeval development of N-S high-angle ruptures around the easternmost portion of the East China Sea (Figure 10) is interpreted as dextral faults to compensate for rapid spreading of the Japan Sea back-arc basin [10].

of the late Miocene, which is in good agreement with the formation age of a remarkable folded

Figure 11. Stratigraphy of the offshore boreholes described by [5, 8]. Their localities are also shown in Figure 2. The solid

and broken lines indicate unit boundaries and the 0.6% Ro (vitrinite reflectance) contour, respectively.

After the abovementioned overview of the structural and stratigraphic characteristics of the southwestern Japan Sea shelf, we now discuss the evolutionary processes of the study area following the back-arc opening event. In this section, the author discusses structural features governed by a unique opening mode, regional inversion provoked by resumed underthrusting of the Philippine Sea Plate, and stress-strain concentration on the shelf under the influence of

As mentioned earlier, the southern Japan Sea is unique, in that the oceanic basin is studded by a number of submerged highlands composed of massive continental crust. Therefore, in order to force the sea floor to spread, a radial rift system presumably developed on the eastern Eurasian margin from the Oligocene to early Miocene. The normal faults in Figures 3 and 4 are relics of the prevalent extension. Figure 12 shows a paleoreconstruction map in the early Miocene stage. As suggested by [4], such a divergent breakup is endorsed by the presence of the early Miocene rift-margin type volcanism along the normally faulted scarp. In comparison to the pre-rifting paleogeography in Figure 1, the separated southwest Japan block drifted

zone on land [6].

92 Tectonics - Problems of Regional Settings

4. Discussion

an emerging simple shear regime, in this order.

4.1. Back-arc opening governed by the divergent rift system

Another troublesome but highly intriguing problem is the plate configuration in the Pacific Northwest during the Japan Sea opening. Figure 13 shows two paleogeographic reconstructions around the southern Japan Sea in the Neogene period. Based on the detailed geologic research of the Sundaland, Hall [11] adopted lingering expansion and rotational motion of the Philippine Sea Plate. On the other hand, Itoh et al. [12] advocated an earlier migration of the marginal sea plate. Their kinematic model is dependent on the collision of the easternmost tip of the clockwise-rotating southwest Japan against the Izu-Bonin arc along the eastern margin of the Philippine Sea Plate from 15 to 12 Ma [13].

The rotational processes of the marginal sea plate remain unsettled. Hall [11, 14] argued that the Philippine Sea Plate began to rotate clockwise at the earliest Miocene (ca. 24 Ma) with a relevant sinistral motion around north New Guinea. An incipient spreading center at that time is identified along the northeastern margin of the plate. Based on rapid crustal growth in southwest Japan, Kimura et al. [15] recently insisted that the plate swiftly rotated clockwise nearly simultaneously with the oceanward drift of the Japanese island arc driven by the Miocene Japan Sea opening. On the other hand, the significant rotation phase of the Philippine Sea Plate has been assigned before 25 Ma based on newly obtained paleomagnetic data from the northwestern part of the plate [16]. However, the present author believes that further geochronological information is necessary in order to clarify these processes.

Figure 13. Comparison of two Neogene paleogeographic reconstruction models around the southern Japan Sea. (a: top) Model of lingering (delayed relative to the Japan Sea opening) migration of the Philippine Sea Plate [11]. (b: bottom) Model adopting earlier migration of the Philippine Sea Plate [12].

but also spatial variations in the coupling on the slab surface may be a key to understanding

Figure 14. Spatiotempora distribution of stress-strain regimes in southwest Japan, the southern Japan Sea shelf, and a kinematic model of the Philippine Sea/Eurasian plate convergence since 6 Ma (a to e) compiled from [17, 18]. The red and blue areas represent areas of compressive (contractional) and tensile (extensional) stress (strain), respectively. Modes of

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Seismic data for the westernmost part of the back-arc shelf imply the emergence of an extensional regime during a recent period. The K/D interface as the product of an impulsive contraction is cut by normal faults, which have been active since the Pliocene (Figure 7). The shelf break partly reaches a depth of 300 m (Figure 6) and exceeds the limit of the eustatic sealevel fluctuation. Figure 15 shows a conspicuous depression on the north side of the Tsushima Islands. As mentioned earlier, the border islands originated from a strong transpressive regime around the latest Miocene. However, the depression appears not to have been generated as a foreland basin related to the nearby reverse faulting that became dormant in the early Pliocene. Deformation of superficial sediments in the seismic profiles requires the subsea landforms to originate from neotectonic stress relief. Such a drastic shift from contraction to extension may be linked to an episodic change of the Philippine Sea Plate's motion in the Quaternary. Nakamura et al. [20] suggested that the plate changed its converging direction to be counterclockwise at ca. 2–1 Ma, which inevitably enhanced the right-lateral wrench deformation of southwest Japan and the eventual arc-parallel crustal breakup, such as that at the Median Tectonic Line (MTL in Figure 16). Itoh et al. [21] found an embryotic right-lateral rupture along the Japan Sea margin (Southern Japan Sea Fault Zone; SJSFZ in Figure 16). The area of confined subsidence is coincident with a propagating tensile termination of the lateral fault

4.3. Confined deformation on the back-arc shelf: Emerging Quaternary back-arc break

the Philippine Sea Plate convergence are shown schematically by the length and azimuth of the arrows.

these complicated tectonic processes.

#### 4.2. Extensive inversion: Structural contrast between forearc and back-arc regions

North-South strong contraction is the most notable post-opening event around the southern Japan Sea and southwest Japan. Although the amplitude of the folds tends to diminish toward the intra-arc region [7], arc-parallel gentle undulation was ubiquitous along the late Miocene convergent margin. Based on the spatiotemporal distribution of tectonic events related to contraction/extension found mainly in intra-/forearc areas, Itoh et al. [17] argued that compressive stress propagated progressively westward through the Plio-Pleistocene and attributed the change in the stress-strain state to the shift of the Euler pole of the subducting Philippine Sea Plate. Itoh [18] redefined their Quaternary epochs based on detailed structural analysis of an event sedimentary sequence. Figure 14 shows a series of compiled illustrations depicting variable tectonic regimes around southwest Japan.

Compared to the transient history of southwest Japan, the back-arc shelf appears to have been uniformly deformed throughout its extent, considering the subsurface structures described by Itoh and Nagasaki [7], Itoh et al. [19], and the author of the present study (Figures 5, 6, and 9). The seismic characteristics at the bottom of the D Group do not exhibit clear time-transgressive terminations onto the K/D erosional surface. Thus, the Japan Sea back-arc region appears to have suffered synchronous deformation in a short period.

Nevertheless, it is plausible that resumed convergence of the Philippine Sea Plate was responsible for the regional contraction because frequent igneous intrusions within the upper part of the K Group (Figures 8 and 11) are suggestive of revitalized arc volcanism linked to dehydration of the subducted slab. Not only the change in relative motions of the marginal sea plate Post-Opening Deformation History of the Japan Sea Back-Arc Basin: Tectonic Processes on an Active Margin… http://dx.doi.org/10.5772/intechopen.71953 95

Figure 14. Spatiotempora distribution of stress-strain regimes in southwest Japan, the southern Japan Sea shelf, and a kinematic model of the Philippine Sea/Eurasian plate convergence since 6 Ma (a to e) compiled from [17, 18]. The red and blue areas represent areas of compressive (contractional) and tensile (extensional) stress (strain), respectively. Modes of the Philippine Sea Plate convergence are shown schematically by the length and azimuth of the arrows.

but also spatial variations in the coupling on the slab surface may be a key to understanding these complicated tectonic processes.

#### 4.3. Confined deformation on the back-arc shelf: Emerging Quaternary back-arc break

4.2. Extensive inversion: Structural contrast between forearc and back-arc regions

able tectonic regimes around southwest Japan.

Model adopting earlier migration of the Philippine Sea Plate [12].

94 Tectonics - Problems of Regional Settings

have suffered synchronous deformation in a short period.

North-South strong contraction is the most notable post-opening event around the southern Japan Sea and southwest Japan. Although the amplitude of the folds tends to diminish toward the intra-arc region [7], arc-parallel gentle undulation was ubiquitous along the late Miocene convergent margin. Based on the spatiotemporal distribution of tectonic events related to contraction/extension found mainly in intra-/forearc areas, Itoh et al. [17] argued that compressive stress propagated progressively westward through the Plio-Pleistocene and attributed the change in the stress-strain state to the shift of the Euler pole of the subducting Philippine Sea Plate. Itoh [18] redefined their Quaternary epochs based on detailed structural analysis of an event sedimentary sequence. Figure 14 shows a series of compiled illustrations depicting vari-

Figure 13. Comparison of two Neogene paleogeographic reconstruction models around the southern Japan Sea. (a: top) Model of lingering (delayed relative to the Japan Sea opening) migration of the Philippine Sea Plate [11]. (b: bottom)

Compared to the transient history of southwest Japan, the back-arc shelf appears to have been uniformly deformed throughout its extent, considering the subsurface structures described by Itoh and Nagasaki [7], Itoh et al. [19], and the author of the present study (Figures 5, 6, and 9). The seismic characteristics at the bottom of the D Group do not exhibit clear time-transgressive terminations onto the K/D erosional surface. Thus, the Japan Sea back-arc region appears to

Nevertheless, it is plausible that resumed convergence of the Philippine Sea Plate was responsible for the regional contraction because frequent igneous intrusions within the upper part of the K Group (Figures 8 and 11) are suggestive of revitalized arc volcanism linked to dehydration of the subducted slab. Not only the change in relative motions of the marginal sea plate Seismic data for the westernmost part of the back-arc shelf imply the emergence of an extensional regime during a recent period. The K/D interface as the product of an impulsive contraction is cut by normal faults, which have been active since the Pliocene (Figure 7). The shelf break partly reaches a depth of 300 m (Figure 6) and exceeds the limit of the eustatic sealevel fluctuation. Figure 15 shows a conspicuous depression on the north side of the Tsushima Islands. As mentioned earlier, the border islands originated from a strong transpressive regime around the latest Miocene. However, the depression appears not to have been generated as a foreland basin related to the nearby reverse faulting that became dormant in the early Pliocene. Deformation of superficial sediments in the seismic profiles requires the subsea landforms to originate from neotectonic stress relief. Such a drastic shift from contraction to extension may be linked to an episodic change of the Philippine Sea Plate's motion in the Quaternary. Nakamura et al. [20] suggested that the plate changed its converging direction to be counterclockwise at ca. 2–1 Ma, which inevitably enhanced the right-lateral wrench deformation of southwest Japan and the eventual arc-parallel crustal breakup, such as that at the Median Tectonic Line (MTL in Figure 16). Itoh et al. [21] found an embryotic right-lateral rupture along the Japan Sea margin (Southern Japan Sea Fault Zone; SJSFZ in Figure 16). The area of confined subsidence is coincident with a propagating tensile termination of the lateral fault

[22], as shown in Figure 16. A closer look at the seismic records indicates high-angle faults cutting the sediment surface on the trace of the SJSFZ (see the seismic inset of Figure 16 and the

Figure 17. Subordinate shear deformation developed around the Southern Japan Sea Fault Zone (SJSFZ). Subsea topographic maps were compiled using a multibeam echo sounding system [25]. R and R' with their strike-slip senses (arrows) in the topographic index are the azimuths of the Riedel shear and the conjugate Riedel shear, respectively, provoked by recurrent dextral activities on the SJSFZ. The location of the seismic line (time migration; SN1-10) is also indicated in

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Figure 17 shows another side effect of the SJSFZ's activity. A northwest-trending sinistral rupture, called the Kikugawa Fault [23], extends to the back-arc shelf. The seismic and geologic investigation of the Kikugawa Fault [24] confirmed recurrent slips during the late Pleistocene. Recent sounding of subsea topography [25] delineated an active pull-apart sag on a releasing (i.e., leftward) bend of the rupture. The azimuth and slip sense of the active lineament agree with those of the conjugate Riedel shear provoked by the dextral motion on the arc-bisecting fault, as shown in the figure. The multichannel seismic record shown in Figure 17, acquired parallel to the Kikugawa Fault, is cut by several high-angle faults that can be interpreted as

Thus, the present research demonstrates that the change in the convergence modes of the Philippine Sea Plate triggered a series of episodic deformation around the rim of the overriding plate. The latest mode of highly oblique subduction promotes the development of extensive wrenching in fore-, intra-, and back-arc regions as well as the formation of a crustal sliver sandwiched between the MTL and SJSFZ. This mode also enhances the compartmentalization

The present seismic study has fully described the following tectonic epochs of the Japan Sea

active horst in Figure 5, which is indicated by a bracket).

dextral Riedel shear adjacent to the SJSFZ.

of the Japan Sea back-arc basin.

5. Conclusions

Figure 2.

back-arc basin.

Figure 15. Confined recent depression north of the Tsushima Islands confirmed by seismic profiles. See Figure 2 for line locations.

Figure 16. Neotectonic synthesis around the southwestern Japan arc together with a seismic section (time migration; SN1-4) showing high-angle faults along the trace of the Southern Japan Sea Fault Zone (SJSFZ). EP, NAP, PP, and PSP in the regional inset represent the Eurasian Plate, the North American Plate, the Pacific Plate, and the Philippine Sea Plate, respectively. The location of the seismic section is also shown in Figure 2.

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Figure 17. Subordinate shear deformation developed around the Southern Japan Sea Fault Zone (SJSFZ). Subsea topographic maps were compiled using a multibeam echo sounding system [25]. R and R' with their strike-slip senses (arrows) in the topographic index are the azimuths of the Riedel shear and the conjugate Riedel shear, respectively, provoked by recurrent dextral activities on the SJSFZ. The location of the seismic line (time migration; SN1-10) is also indicated in Figure 2.

[22], as shown in Figure 16. A closer look at the seismic records indicates high-angle faults cutting the sediment surface on the trace of the SJSFZ (see the seismic inset of Figure 16 and the active horst in Figure 5, which is indicated by a bracket).

Figure 17 shows another side effect of the SJSFZ's activity. A northwest-trending sinistral rupture, called the Kikugawa Fault [23], extends to the back-arc shelf. The seismic and geologic investigation of the Kikugawa Fault [24] confirmed recurrent slips during the late Pleistocene. Recent sounding of subsea topography [25] delineated an active pull-apart sag on a releasing (i.e., leftward) bend of the rupture. The azimuth and slip sense of the active lineament agree with those of the conjugate Riedel shear provoked by the dextral motion on the arc-bisecting fault, as shown in the figure. The multichannel seismic record shown in Figure 17, acquired parallel to the Kikugawa Fault, is cut by several high-angle faults that can be interpreted as dextral Riedel shear adjacent to the SJSFZ.

Thus, the present research demonstrates that the change in the convergence modes of the Philippine Sea Plate triggered a series of episodic deformation around the rim of the overriding plate. The latest mode of highly oblique subduction promotes the development of extensive wrenching in fore-, intra-, and back-arc regions as well as the formation of a crustal sliver sandwiched between the MTL and SJSFZ. This mode also enhances the compartmentalization of the Japan Sea back-arc basin.
