3. Stratigraphy

shown in Figure 5, undulation of the erosional surface and truncation near the surface are

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

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

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

neotectonic context of this intriguing feature is discussed in the following section.

indicative of younger tectonic events.

90 Tectonics - Problems of Regional Settings

line locations.

for line locations.

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

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.

of the late Miocene, which is in good agreement with the formation age of a remarkable folded zone on land [6].

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

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

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

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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

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.

rapid spreading of the Japan Sea back-arc basin [10].

of the Philippine Sea Plate from 15 to 12 Ma [13].
