2. Geophysical survey

In 1989, the Ministry of International Trade and Industry (MITI) conducted an extensive offshore seismic campaign around the Japan Sea and the East China Sea (see Figure 2) by using the M/V

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 87

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 boreholes, respectively.

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

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.

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

In 1989, the Ministry of International Trade and Industry (MITI) conducted an extensive offshore seismic campaign around the Japan Sea and the East China Sea (see Figure 2) by using the M/V

continental fragments is carefully taken into account.

elucidating the transient motion of the marginal sea plate.

2. Geophysical survey

86 Tectonics - Problems of Regional Settings

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

Figure 4. Reflection seismic profile (time migration; SN1-2) on the southwestern shelf of the Japan Sea. See Figure 2 for line locations.

The morphology of the top of the acoustic basement is well preserved in the eastern part of the survey area. Figure 3 (line SN1-1) shows the tilted blocks of the basement, which imply dominant normal faulting in the incipient stage of the back-arc rifting. On occasion, thinly layered younger sediments are tilted and dragged upon the fault scarps reflecting subsequent

Figure 6. Reflection seismic profile (time migration; SN1-7) on the southwestern shelf of the Japan Sea. See Figure 2 for

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

89

line locations. The arrow indicates the location at which the water depth exceeds 300 m.

The most remarkable and traceable event around the survey area is an unconformity in the upper portion of the sediment pile (Figure 5; SN1-6). The east-northeastward (namely arcparallel) axis of the folds coincides with the latest Miocene deformation trend on land [6]. The strong contraction appears to almost reach the northern shelf break (Figure 6; SN1-7). As

Figure 7. Reflection seismic profile (time migration; SN1-8) on the southwestern shelf of the Japan Sea. See Figure 2 for

line locations. The bracket indicates the range of recent normal faulting discussed in the text.

tectonic events (Figure 4; SN1-2).

GECO MY. During the shooting of survey lines, 240 channels of hydrophones (12.5-m intervals) recorded the energy released from a tuned air-gun array (total: 78 l), which was shot at 25-m intervals. Raw field data were stacked and then subjected to a processing sequence for the enhancement of resolution. Regional seismic reflectors were traced throughout the study area and were correlated with the results of a previous seismic survey [5]. Basically, the back-arc shelf consists of an acoustic basement (pre-Neogene metamorphosed sedimentary complex and early Miocene volcaniclastics) and Neogene-Quaternary marine sediments. In the following section, we investigate the characteristic features of the seismic profiles.

Figure 5. Reflection seismic profile (time migration; SN1-6) on the southwestern shelf of the Japan Sea. See Figure 2 for line locations. The bracket indicates the recent high-angle faults discussed in the chapter.

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 89

Figure 6. Reflection seismic profile (time migration; SN1-7) on the southwestern shelf of the Japan Sea. See Figure 2 for line locations. The arrow indicates the location at which the water depth exceeds 300 m.

The morphology of the top of the acoustic basement is well preserved in the eastern part of the survey area. Figure 3 (line SN1-1) shows the tilted blocks of the basement, which imply dominant normal faulting in the incipient stage of the back-arc rifting. On occasion, thinly layered younger sediments are tilted and dragged upon the fault scarps reflecting subsequent tectonic events (Figure 4; SN1-2).

GECO MY. During the shooting of survey lines, 240 channels of hydrophones (12.5-m intervals) recorded the energy released from a tuned air-gun array (total: 78 l), which was shot at 25-m intervals. Raw field data were stacked and then subjected to a processing sequence for the enhancement of resolution. Regional seismic reflectors were traced throughout the study area and were correlated with the results of a previous seismic survey [5]. Basically, the back-arc shelf consists of an acoustic basement (pre-Neogene metamorphosed sedimentary complex and early Miocene volcaniclastics) and Neogene-Quaternary marine sediments. In the following section,

Figure 5. Reflection seismic profile (time migration; SN1-6) on the southwestern shelf of the Japan Sea. See Figure 2 for

line locations. The bracket indicates the recent high-angle faults discussed in the chapter.

Figure 4. Reflection seismic profile (time migration; SN1-2) on the southwestern shelf of the Japan Sea. See Figure 2 for

we investigate the characteristic features of the seismic profiles.

line locations.

88 Tectonics - Problems of Regional Settings

The most remarkable and traceable event around the survey area is an unconformity in the upper portion of the sediment pile (Figure 5; SN1-6). The east-northeastward (namely arcparallel) axis of the folds coincides with the latest Miocene deformation trend on land [6]. The strong contraction appears to almost reach the northern shelf break (Figure 6; SN1-7). As

Figure 7. Reflection seismic profile (time migration; SN1-8) on the southwestern shelf of the Japan Sea. See Figure 2 for line locations. The bracket indicates the range of recent normal faulting discussed in the text.

Figure 8. Reflection seismic profile (time migration; SN1-10) on the southwestern shelf of the Japan Sea. See Figure 2 for line locations.

shown in Figure 5, undulation of the erosional surface and truncation near the surface are indicative of younger tectonic events.

with sporadic geomagnetic anomalies [7]. Together with borehole lithology described in the next

Figure 10. Reflection seismic profile (time migration; SN1-17) around the eastern end of the East China Sea. See Figure 2

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

91

The regional contraction zone abruptly ends as the pivot of a folding fan to the west. Back-arc folds change their azimuth to counterclockwise and converge into the anticlinoria of Tsushima [7]. The islands along the Japan-Korea border are accompanied by a deep half-graben developed beneath an east-dipping thrust (Figure 9; SN1-14) and are regarded as the product of the

In contrast with the strong deformation on the Japan Sea shelf, the easternmost portion of the East China Sea is underlain by intact sediments in shallow horizons (Figure 10; SN1-17). Short-

For the purpose of oil exploration, five deep drilling surveys were performed in the study area. Figure 11 shows their locations (columns 1–5) along with an auxiliary nearby borehole (column 6). Based on detailed stratigraphic assessments [5, 8], lithologic piles penetrated by these boreholes are divided into four units. In ascending order, the X Group corresponds to the acoustic basement and is collectively defined as a mixture of early Miocene nonmarine sediments and pyroclastic rocks and older granitic intrusives. The N Group rests unconformably on the basement and consists of early Miocene marine sediments with numerous tuff intercalations. Nonvolcanic monotonous sediments of the K Group yield foraminiferal assemblages correlated with Blow's [9] zone N14~N16 (late middle Miocene-early late Miocene) and are overlain by sandy clastics of the D Group, the basal part of which is assigned to zone N19 (early Pliocene) of [9]. Thus, the angular unconformity at the K/D boundary is identified to be

section, post-opening volcanic activities may be responsible for the phenomenon.

highest level of transpressive stress around the end of the Miocene.

wavelength deeper undulations are bounded by high-angle ruptures.

3. Stratigraphy

for line locations.

We notice a change in the recent stress regime around the westernmost part of the shelf. Figure 7 (line SN1-8) demonstrates that the latest Miocene unconformable boundary is cut by normal faults. Separation along these faults grows through the Plio-Pleistocene. The areal extent and neotectonic context of this intriguing feature is discussed in the following section.

Deformation trends around the westernmost shelf are partially obscured by strong discontinuous reflectors in shallow horizons (Figure 8; SN1-10). Such disturbances are spatially coincident

Figure 9. Reflection seismic profile (time migration; SN1-14) around the eastern end of the East China Sea. See Figure 2 for line locations.

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 91

Figure 10. Reflection seismic profile (time migration; SN1-17) around the eastern end of the East China Sea. See Figure 2 for line locations.

with sporadic geomagnetic anomalies [7]. Together with borehole lithology described in the next section, post-opening volcanic activities may be responsible for the phenomenon.

The regional contraction zone abruptly ends as the pivot of a folding fan to the west. Back-arc folds change their azimuth to counterclockwise and converge into the anticlinoria of Tsushima [7]. The islands along the Japan-Korea border are accompanied by a deep half-graben developed beneath an east-dipping thrust (Figure 9; SN1-14) and are regarded as the product of the highest level of transpressive stress around the end of the Miocene.

In contrast with the strong deformation on the Japan Sea shelf, the easternmost portion of the East China Sea is underlain by intact sediments in shallow horizons (Figure 10; SN1-17). Shortwavelength deeper undulations are bounded by high-angle ruptures.
