**3.2. Rapid rifting**

**3. Intra-arc rifting**

**3.1. Incipient rifting**

unconformity prior to the succeeding rapid rifting [21].

**Figure 2.** Index map showing the Eastern Japan Sea Rift System [24].

In response to opening of the Sea of Japan, both Northeast and Southwest Japan Arcs were subjected to intra-arc rifting. Recent studies demonstrated that the two-stage intra-arc rifting occurred along the eastern margin of the Sea of Japan [19-21]. Kano et al. [21] reviewed that Late Eocene and Oligocene (ca. 35-23 Ma) marine sediments distribute sparsely along the eastern margin of the Sea of Japan from Sakhalin to Kyushu through Honshu. It is suggested that rifting started and incipient rift system was formed by the Oligocene time on the back-arc side prior to the opening of the Japan and Yamato basins. During Oligocene-Early Miocene (34-21 Ma) volcanic rocks accumulated in southwest Hokkaido and Oga Peninsula with petrological and geochemical features similar to those of the volcanic rocks from continental rifts, suggesting that the former volcanic rocks were formed under rifting in the Eurasian continental arc during the pre-opening stage of the Sea of Japan [21, 23]. The early phase of incipient rifting during Late Eocene to Oligocene was characterized by slow subsidence (< 800m/m.y.) in the rifted zones [21]. The incipient rifting was interrupted by a regional

156 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

Besides large backarc basins such as the Japan and Yamato basins in the Sea of Japan, the Eastern Japan Sea Rift System [24] was generated along the Sea of Japan coast of the northeast Honshu (Figure. 2) as a series of the NE-SW trending rift basins at around 16 Ma [25], which corresponded to the final phase of the backarc opening. The Eastern Japan Sea Rift System consists of several composite basins, such as the Niigata Basin in the south and the Akita Basin in the north (Figure. 2). In the Uetsu district between the Niigata and Akita basins (Figure. 2), many half grabens trending NNE-SSW to NE developed [26]. Subsidence analysis for syn-rift basins showed that rapid rifting started around 18 Ma and ceased around 15 Ma [26]. The maximum subsidence rate exceeded 1 km/m.y., much faster than that in major continental rifts [26]. Intra-arc rifting in outer arc and in most of inner arc of northeast Honshu ended at around 15 Ma [25-26].

**Figure 3.** Tectonostratigraphic stage division chart of the Akita Basin showing division of lithostratigraphic units, ma‐ jor depositional systems, stacking patterns, uplift and subsidence patterns, tectonic regimes, and volcanic regimes. Compiled from [22, 27-33].

In the center of the Akita Basin as well as of the Niigata Basin, thick piles of basalts accumulat‐ ed in grabens [34-35]. The Middle Miocene (16-13 Ma) northeast Japan was characterized by a distinct bimodal volcanism in which basalts dominated in back-arc side whereas fore-arc side was dominated by felsic rocks [23]. Figure 3 shows a tectonostratigraphic stage division chart of the Akita Basin. The Akita Basin was further subdivided into the coastal basin (basin center) and Yokote and Shinjo marginal basins by an intervening ridge, which have uplifted to form the Dewa Hills at present (Figure. 2). During rapid rifting, syn-rift successions filled horsts and grabens formed in the Akita Basin. Two grabens; the Aosawa graben [34] along the Akita coast and the Kuroko graben in the backbone range [36] were formed in the Akita Basin (Figure. 3). The Dewa Hills and Oga Peninsula constituted the intervening structural highs (horsts). The Aosawa graben is a large-scale graben formed by pull-apart tectonism, and was filled with thick piles (~2,000 m) of graben-fill basalt lavas (Aosawa Formation) [34]. The Kuroko graben is composed of component half grabens, which were filled with thick piles (~1,000 m) of felsic volcanic and pyroclastic rocks interbedded with mudstones (Oishi Formation) [31, 36]. In contrast, the Nishikurosawa and Sugota formations, which are only 30–400 m thick, are distributed on the Oga and Dewa ridges, respectively, with unconformable contacts with the underlying volcanic successions, and consist of shallow-marine sandstones, calcareous sandstones, and conglomerates, which were deposited on a shelf environment (Figure. 3)[22, 27]. These lithofacies and thickness contrasts are likely attributed to differential subsidence between horsts and grabens during the rifting under extensional tectonics [37]. Within the Aosawa and Kuroko grabens, syn-rift volcanism commenced at around 16.5 Ma and lasted until 13.5 Ma, prolonged than that in outer arc and in other inner arc areas in northeast Hounshu [33-34, 38]. Syn-rift volcanism (16.5 – 13.5 Ma) is further divided into early syn-rift volcanism (16.5 ~ 15 Ma) characterized by basaltic volcanism in the Aosawa graben and late syn-rift volcanism (15 ~ 13.5 Ma) characterized by felsic volcanism in the Kuroko graben [33, 38]. The Kuroko ore deposits were formed at the end of syn-rift stage at around 13.5 Ma, which may have been caused by the tectonic conversion from a back-arc to an island-arc setting [33, 36]

Kuroko graben along the eastern margin of the Eastern Japan Sea Rift System with minor amount of basalt activity in the Aosawa graben. The overall evolution of the Sea of Japan rift system may be interpreted as temporal progression from core-complex mode to wide-rift mode to narrowrift mode by a simplified model of crustal extension [39]. Incipient rift system (35 – 24 Ma) may be assigned to core-complex mode while spreading of the Sea of Japan (24 – 15 Ma) is assigned to wide-rift mode. During the late stage of wide-rift mode (18-15 Ma), rifting extended to the Eastern Japan Sea Rift System. Late syn-rift system (15 – 13.5 Ma) developed mainly in the Kuroko graben at the eastern margin of the Eastern Japan Sea Rift System and may be assigned to narrow-

Late Cenozoic Tectonic Events and Intra-Arc Basin Development in Northeast Japan

http://dx.doi.org/10.5772/56706

159

**4. Intra-arc basin development in response to tectonic events: A case study**

This section focuses on the post-rift tectonic events based on a case study of intra-arc basin

**Figure 4.** Index map of the Akita Basin and the Ou Backbone Range showing regional geology. SF: Senya Fault, KWF:

Kawafune-Warikurayama Fault, EMF: Eastern Marginal Fault, KYT: Kitayuri Thrust.

rift mode because the lithosphere became cold and strong [37, 39].

development from the Yuda Basin in the Ou Backbone Range (Figure. 2).

**from the Ou Backbone Range**

#### **3.3. Evolution of the Sea of Japan rift system**

The Sea of Japan rift system referred herein includes all component rift systems such as the Japan and Yamato basins, and the Eastern Japan Sea Rift System associated with the opening of the Sea of Japan. Evolution of the Sea of Japan rift system is summarized as follows. Incipient rift system commenced at around 35 Ma within a narrow zone along the present eastern margin of the Sea of Japan. Geochemical features of volcanic rocks similar to continental rifts suggest a rifting in the Eurasian continental arc. During the late incipient rifting in northeast Japan, the Japan Basin started opening at around 24 Ma followed by the opening of the Yamato Basin at around 21 Ma. The spreading of the Sea of Japan may have occurred in two stages. During the first stage, the Japan and Yamato basins spread from 24 – 21 Ma to 18 ~ 17 Ma. During the second stage, the upper basalts of the Yamato Basin may have accumulated at around 16 Ma [14]. In the meantime, rapid intra arc rifting started in the Eastern Japan Sea Rift System and surrounding areas at around 18 Ma and subsidence rate attained maximum (> 1km/m.y.) at around 16 – 15 Ma with the formation of graben fill basalts in the Akita and Niigata basins. Synrift volcanism and areas of intense subsidence migrated toward east and lasted until 13.5 Ma mainly in the

Kuroko graben along the eastern margin of the Eastern Japan Sea Rift System with minor amount of basalt activity in the Aosawa graben. The overall evolution of the Sea of Japan rift system may be interpreted as temporal progression from core-complex mode to wide-rift mode to narrowrift mode by a simplified model of crustal extension [39]. Incipient rift system (35 – 24 Ma) may be assigned to core-complex mode while spreading of the Sea of Japan (24 – 15 Ma) is assigned to wide-rift mode. During the late stage of wide-rift mode (18-15 Ma), rifting extended to the Eastern Japan Sea Rift System. Late syn-rift system (15 – 13.5 Ma) developed mainly in the Kuroko graben at the eastern margin of the Eastern Japan Sea Rift System and may be assigned to narrowrift mode because the lithosphere became cold and strong [37, 39].

In the center of the Akita Basin as well as of the Niigata Basin, thick piles of basalts accumulat‐ ed in grabens [34-35]. The Middle Miocene (16-13 Ma) northeast Japan was characterized by a distinct bimodal volcanism in which basalts dominated in back-arc side whereas fore-arc side was dominated by felsic rocks [23]. Figure 3 shows a tectonostratigraphic stage division chart of the Akita Basin. The Akita Basin was further subdivided into the coastal basin (basin center) and Yokote and Shinjo marginal basins by an intervening ridge, which have uplifted to form the Dewa Hills at present (Figure. 2). During rapid rifting, syn-rift successions filled horsts and grabens formed in the Akita Basin. Two grabens; the Aosawa graben [34] along the Akita coast and the Kuroko graben in the backbone range [36] were formed in the Akita Basin (Figure. 3). The Dewa Hills and Oga Peninsula constituted the intervening structural highs (horsts). The Aosawa graben is a large-scale graben formed by pull-apart tectonism, and was filled with thick piles (~2,000 m) of graben-fill basalt lavas (Aosawa Formation) [34]. The Kuroko graben is composed of component half grabens, which were filled with thick piles (~1,000 m) of felsic volcanic and pyroclastic rocks interbedded with mudstones (Oishi Formation) [31, 36]. In contrast, the Nishikurosawa and Sugota formations, which are only 30–400 m thick, are distributed on the Oga and Dewa ridges, respectively, with unconformable contacts with the underlying volcanic successions, and consist of shallow-marine sandstones, calcareous sandstones, and conglomerates, which were deposited on a shelf environment (Figure. 3)[22, 27]. These lithofacies and thickness contrasts are likely attributed to differential subsidence between horsts and grabens during the rifting under extensional tectonics [37]. Within the Aosawa and Kuroko grabens, syn-rift volcanism commenced at around 16.5 Ma and lasted until 13.5 Ma, prolonged than that in outer arc and in other inner arc areas in northeast Hounshu [33-34, 38]. Syn-rift volcanism (16.5 – 13.5 Ma) is further divided into early syn-rift volcanism (16.5 ~ 15 Ma) characterized by basaltic volcanism in the Aosawa graben and late syn-rift volcanism (15 ~ 13.5 Ma) characterized by felsic volcanism in the Kuroko graben [33, 38]. The Kuroko ore deposits were formed at the end of syn-rift stage at around 13.5 Ma, which may have

158 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

been caused by the tectonic conversion from a back-arc to an island-arc setting [33, 36]

The Sea of Japan rift system referred herein includes all component rift systems such as the Japan and Yamato basins, and the Eastern Japan Sea Rift System associated with the opening of the Sea of Japan. Evolution of the Sea of Japan rift system is summarized as follows. Incipient rift system commenced at around 35 Ma within a narrow zone along the present eastern margin of the Sea of Japan. Geochemical features of volcanic rocks similar to continental rifts suggest a rifting in the Eurasian continental arc. During the late incipient rifting in northeast Japan, the Japan Basin started opening at around 24 Ma followed by the opening of the Yamato Basin at around 21 Ma. The spreading of the Sea of Japan may have occurred in two stages. During the first stage, the Japan and Yamato basins spread from 24 – 21 Ma to 18 ~ 17 Ma. During the second stage, the upper basalts of the Yamato Basin may have accumulated at around 16 Ma [14]. In the meantime, rapid intra arc rifting started in the Eastern Japan Sea Rift System and surrounding areas at around 18 Ma and subsidence rate attained maximum (> 1km/m.y.) at around 16 – 15 Ma with the formation of graben fill basalts in the Akita and Niigata basins. Synrift volcanism and areas of intense subsidence migrated toward east and lasted until 13.5 Ma mainly in the

**3.3. Evolution of the Sea of Japan rift system**
