*4.3.1. Stage I (Syn-rift stage; 16-13.5 Ma)*

This stage was characterized by a rapid subsidence (600 m/m.y.) with accumulation of thick volcanic and volcaniclastic successions in the Yuda Basin (Figure. 11). The amount of tectonic subsidence attains no less than 1,000 m even if altitude of sea-level rise at around 15 Ma [47] is subtracted (Figure 11). The Eastern Marginal Fault and Kawafune-Warikurayama Fault systems may have been activated as normal faults and formed half grabens in the eastern and western sectors, respectively as a result of crustal stretching under extensional tectonics during Early-early Middle Miocene rifting (Figure 12) [40].

#### *4.3.2. Stage II (Post-rif transition stage; 13.5–12 Ma)*

This stage was characterized by the cessation of syn-rift volcanism and accumulation of hemipelagic sediments at a slower rate (10 cm/k.y.) in the Yuda Basin (Figures. 8 & 12). The subsidence reconstruction of the Yuda Basin (Figure. 11) indicates subsequent reduction of tectonic subsidence rate although precise estimation is difficult due to unreliable estimation of paleo-depth under bathyal environment.

#### *4.3.3. Stage III (partial inversion stage: 12–9Ma)*

This stage was represented by temporal uplift of the Ou Backbone Range and associated unconformity in the Yuda Basin. The amount of tectonic uplift in the Yuda Basin at around 12-11 Ma was estimated at more than 500 m. This uplift was followed by a cessation of

**Figure 11.** The total and tectonic subsidence curves of the Yuda Basin with tectonic stages (I-VI), inferred stress field

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[30] and a eustatic curve proposed by [47]. Modified from [31, 43].

space possibly because of the uplift of surrounding backbone range. In addition, this uncon‐ formity marks the major changes from uniform sedimentation rate in north-south direction in the Kurosawa Formation to differential sedimentation rate in the Hanayama Formation (Figure. 7). Those changes in depositional style may have resulted from tectonic conversion

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

The angular unconformity between the Hanayama and Yoshizawa formations is the most significant structure among three unconformities described herein. The Kurosawa and Hanayama formations form a syncline with dips of 20 – 30 ° and are unconformably overlain by almost flat Yoshizawa Formation (Figure. 6). The formation of syncline and subsequent unconformity may have reflected uplift of the eastern and western sectors of the Ou Backbone Range during and after deposition of the Hanayama Formation. The unconformity was formed

Evolution of the Yuda Basin since Middle Miocene is summarized here, based on correlation of tuff beds and unconformities (Figures 7 & 10) and basin subsidence analysis (Figure. 11). Tectonic history of the Yuda Basin and the surrounding Ou Backbone Range was divided into

This stage was characterized by a rapid subsidence (600 m/m.y.) with accumulation of thick volcanic and volcaniclastic successions in the Yuda Basin (Figure. 11). The amount of tectonic subsidence attains no less than 1,000 m even if altitude of sea-level rise at around 15 Ma [47] is subtracted (Figure 11). The Eastern Marginal Fault and Kawafune-Warikurayama Fault systems may have been activated as normal faults and formed half grabens in the eastern and western sectors, respectively as a result of crustal stretching under extensional tectonics during

This stage was characterized by the cessation of syn-rift volcanism and accumulation of hemipelagic sediments at a slower rate (10 cm/k.y.) in the Yuda Basin (Figures. 8 & 12). The subsidence reconstruction of the Yuda Basin (Figure. 11) indicates subsequent reduction of tectonic subsidence rate although precise estimation is difficult due to unreliable estimation

This stage was represented by temporal uplift of the Ou Backbone Range and associated unconformity in the Yuda Basin. The amount of tectonic uplift in the Yuda Basin at around 12-11 Ma was estimated at more than 500 m. This uplift was followed by a cessation of

six tectonic stages according to the basin subsidence pattern (Figure. 12)[31].

from extension to compression as discussed later.

after 3 Ma although the precise age has not been dated.

**4.3. Evolution of the Yuda Basin since Middle Miocene**

*4.3.1. Stage I (Syn-rift stage; 16-13.5 Ma)*

Early-early Middle Miocene rifting (Figure 12) [40].

*4.3.2. Stage II (Post-rif transition stage; 13.5–12 Ma)*

of paleo-depth under bathyal environment.

*4.3.3. Stage III (partial inversion stage: 12–9Ma)*

*4.2.3. < 3 Ma unconformity*

**Figure 11.** The total and tectonic subsidence curves of the Yuda Basin with tectonic stages (I-VI), inferred stress field [30] and a eustatic curve proposed by [47]. Modified from [31, 43].

sector, and was accompanied by the westward tilting of the eastern sector (Figure 12). The eastern sector began to emerge and became a sediment source to the surrounding basins by subaerial erosion. The uplift of the eastern sector of the backbone range may be attributed to a subsequent inversion of a half graben formed during Stage I because of an increase in horizontal compressional stress (Figure. 12)[31]. The western sector of the backbone range also uplifted from middle bathyal to shelf environments, although the bounding normal fault does not seem to have been inverted. This stage was also characterized by reduced volcanic activity around the basin as suggested by scarcity of tuffs intercalated in the lower Kurosawa Forma‐

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This stage was characterized by slow subsidence and deposition of sand in shallow marine environments in the Yuda Basin. The subsidence resumed at around 9 Ma and shallow-marine sandstones were deposited over the unconformity in the eastern margin of the Yuda Basin, while the eastern sector of the backbone range remained as a sediment source. The lower Kurosawa Formation thins in intensely uplifted and truncated areas such as west of Yugawa (Figures. 6 & 7), which suggests that the Kurosawa Formation onlaps these uplifted topo‐ graphic highs. While subsidence rates in the eastern part of the Yuda Basin were uniform in north-south direction (Figure. 7) within the magnitude of eustatic sea-level fluctuation (Figure. 11), the western sector subsided more rapidly. Total tectonic subsidence in the western Yuda Basin was estimated at least 600 m during this stage from the thickness of the Sannai and upper Kurosawa Formations in the core 41PAW-1 (Figure. 7)[31]. This stage was also characterized by increased felsic volcanism, as represented by increased felsic tuffs such as the Tsukano and Torasawa Tuff beds in the Kurosawa Formation in the Yuda Basin (Figure. 7). Occurrence of Northeast-Southwest-trending minor normal faults in the Kurosawa Formation indicates Northwest-Southeast-trending extensional stress field [30], which resulted in regional slow

Stage V represents differentiation of uplifted and subsided areas within the Yuda Basin. Subsidence pattern changed from the preceding stage IV, and the northern Yuda Basin turned into an uplifted area (Figure. 11). Moreover, conglomeratic deposits within the basin indicate uplift of the surrounding mountains. The high frequency depositional sequences were developed in this stage, suggesting that supply of coarse sediments began to exceed the accommodation space. The western sector may have also uplifted because marine environments gradually retreated from the Yuda Basin by ~4 Ma [30], which indicates the emergence of the western sector by that time. The Northeast-Southwest trending minor normal faults found in the Kurosawa Formation disappeared at the base of the Hanaya‐ ma Formation (~6 Ma) in the Yuda Basin [30]. After 6 Ma, only North-South-trending reverse faults were formed in the Yuda Basin. The observation indicates that stress changed from extension to E-W compression at the beginning of this stage. The compressive stress field

tion in the western sector (Figure. 7).

subsidence.

*4.3.4. Stage IV (subsidence stage: 9–6.5 Ma)*

*4.3.5. Stage V (basin inversion and compression stage: 6.5 – 3~2 Ma)*

**Figure 12.** A cartoon illustrating the tectonic evolution of the Ou Backbone Range (modifed from [31]). K-W Fault: Kawafune-Warikurayama Fault. See text for explanation.

subsidence and uplift within the basin until about 9 Ma (Figure. 11), leading to a hiatus in the eastern margin of the basin. Correlation of key beds and unconformities (Fig. 7) clearly demonstrates that intensely uplifted areas at the eastern margin of the basin were more subject to intense truncation. The uplift was more intense in the eastern sector than in the western sector, and was accompanied by the westward tilting of the eastern sector (Figure 12). The eastern sector began to emerge and became a sediment source to the surrounding basins by subaerial erosion. The uplift of the eastern sector of the backbone range may be attributed to a subsequent inversion of a half graben formed during Stage I because of an increase in horizontal compressional stress (Figure. 12)[31]. The western sector of the backbone range also uplifted from middle bathyal to shelf environments, although the bounding normal fault does not seem to have been inverted. This stage was also characterized by reduced volcanic activity around the basin as suggested by scarcity of tuffs intercalated in the lower Kurosawa Forma‐ tion in the western sector (Figure. 7).

#### *4.3.4. Stage IV (subsidence stage: 9–6.5 Ma)*

This stage was characterized by slow subsidence and deposition of sand in shallow marine environments in the Yuda Basin. The subsidence resumed at around 9 Ma and shallow-marine sandstones were deposited over the unconformity in the eastern margin of the Yuda Basin, while the eastern sector of the backbone range remained as a sediment source. The lower Kurosawa Formation thins in intensely uplifted and truncated areas such as west of Yugawa (Figures. 6 & 7), which suggests that the Kurosawa Formation onlaps these uplifted topo‐ graphic highs. While subsidence rates in the eastern part of the Yuda Basin were uniform in north-south direction (Figure. 7) within the magnitude of eustatic sea-level fluctuation (Figure. 11), the western sector subsided more rapidly. Total tectonic subsidence in the western Yuda Basin was estimated at least 600 m during this stage from the thickness of the Sannai and upper Kurosawa Formations in the core 41PAW-1 (Figure. 7)[31]. This stage was also characterized by increased felsic volcanism, as represented by increased felsic tuffs such as the Tsukano and Torasawa Tuff beds in the Kurosawa Formation in the Yuda Basin (Figure. 7). Occurrence of Northeast-Southwest-trending minor normal faults in the Kurosawa Formation indicates Northwest-Southeast-trending extensional stress field [30], which resulted in regional slow subsidence.

#### *4.3.5. Stage V (basin inversion and compression stage: 6.5 – 3~2 Ma)*

subsidence and uplift within the basin until about 9 Ma (Figure. 11), leading to a hiatus in the eastern margin of the basin. Correlation of key beds and unconformities (Fig. 7) clearly demonstrates that intensely uplifted areas at the eastern margin of the basin were more subject to intense truncation. The uplift was more intense in the eastern sector than in the western

**Figure 12.** A cartoon illustrating the tectonic evolution of the Ou Backbone Range (modifed from [31]). K-W Fault:

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

Kawafune-Warikurayama Fault. See text for explanation.

Stage V represents differentiation of uplifted and subsided areas within the Yuda Basin. Subsidence pattern changed from the preceding stage IV, and the northern Yuda Basin turned into an uplifted area (Figure. 11). Moreover, conglomeratic deposits within the basin indicate uplift of the surrounding mountains. The high frequency depositional sequences were developed in this stage, suggesting that supply of coarse sediments began to exceed the accommodation space. The western sector may have also uplifted because marine environments gradually retreated from the Yuda Basin by ~4 Ma [30], which indicates the emergence of the western sector by that time. The Northeast-Southwest trending minor normal faults found in the Kurosawa Formation disappeared at the base of the Hanaya‐ ma Formation (~6 Ma) in the Yuda Basin [30]. After 6 Ma, only North-South-trending reverse faults were formed in the Yuda Basin. The observation indicates that stress changed from extension to E-W compression at the beginning of this stage. The compressive stress field resulted in basin inversion and uplift of both the eastern and western sectors of the Ou Backbone Range (Figure. 12).

**5.1. Stage 0 (Incipient rift stage; 35 – 20 Ma)**

**5.2. Stage I (Syn-rift stage: 20 - 13.5 Ma)**

*5.2.1. Substage IA (20 – 15 Ma)*

*5.2.2. Substage IB (15 – 13.5 Ma)*

Stage 0 represents formation of the incipient rift system along the eastern margin of the Sea of Japan as already noted in section 3. Rift basins in this stage were relatively small and marine incursion was limited within a narrow zone along the present Sea of Japan coast [21]. The timing of marine incursion varied among rift basins [21], suggesting that individual marine basin was relatively short lived. The incipient rift basins had not directly developed into later syn-rift large basins, but were interrupted by regional unconformity during the late Stage 0

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Syn-rift stage has been divided into two substages: IA and IB on the basis of tectonic conversion from wide rift mode to narrow rift mode [39], associated with a stress change from regional

After the incipient rifting stage, dacitic volcanics with some amount of basaltic volcanics together with conglomerates and sandstones deposited in terrestrial environments at around 20 Ma in the Akita Basin [22, 34]. These stratigraphic units may represent slow subsidence [22] prior to rapid rifting after 18 Ma as described in section 3.2. They were unconformably overlain by non-marine to marine successions deposited during rapid rifting. The stratigraphic units deposited during the rapid rifting (ca. 18 – 15 Ma) represent regional marine transgression in northeast Japan as a result of rapid subsidence under extensional tectonics associated with rifting [26, 69]. The equivalent stratigraphic units during Substage IA also deposited within rotated half grabens in the eastern margin of the Sea of Japan [49]. Rapid subsidence associated with rifting and half grabens also took place in the fore-arc side of northeast Japan [56, 66, 70] and in the Kanto Plain (Figure 1)[5, 71]. In terms of igneous activity, this stage was assigned to a backarc basin volcanic period (Figures, 3, 13)[33, 58] and was characterized by intense

basaltic volcanism within rift grabens in the Akita and Niigata basins [34, 36, 58].

Substage IB is characterized by shrinkage of rift zones and by uplift with a notable uncon‐ formity in fore-arc side and fore-arc basins in northeast Japan. Formation of half grabens in the Kanto Plain (Figure 1) was suddenly terminated by rapid uplift with formation of a notable unconformity (the Niwaya unconformity) at 15.3 – 15.2 Ma [5, 71]. Marine fine-grained sediments with low sedimentation rates unconformably overlie the successions in rotated half grabens [71]. This change in the style of subsidence and deposition was attributed to tectonic conversion from extensional to strong compression stress, followed by relatively quiet tectonics [71]. Sedimentation rates in the post rift successions over the Niwaya unconformity in the northern Kanto Plain had been suppressed until 14 – 12.5 Ma, suggesting compression lasted until ca. 13 Ma [64]. The Joban forearc basin (Figure 1) was also inverted and uplifted at around 15 Ma (Figure 13) with a notable unconformity being formed [68]. This tectonic change was accompanied by NW – SE trending folding along the coast of Sendai [56] and in

extension to coexistence of extension and compression at around 15 Ma.

#### *4.3.6. Stage VI (Intense compression stage; 3~2 Ma - Present)*

Stage VI represents the uplift of the whole Backbone Range and the formation of an angular unconformity between the Hanayama and Yoshizawa Formations within the Yuda Basin (Figures 11, 12). The uplift of the Backbone Range during this stage resulted from "pop-up" of the sectors of the Backbone Range by activation of the Senya Fault system as well as reactivation of the Eastern Marginal Fault and Kawafune-Warikurayama Fault systems under intense compression (Figure. 12)[40].
