**2. Geological setting**

The present Hokkaido Island is located on a junction between the Northeast Japan Arc and the Chishima (Kuril) Arc (Figure 1), which originated from an interaction between the Eurasian and Okhotsk plates that commenced in the Late Eocene [4]. A right-lateral oblique collision between the two arcs during the Miocene age formed a west-vergent fold-thrust belt and a subsiding foreland area nearly 400 km long in central Hokkaido [4].

Because of the right-lateral oblique collision and the subsequent westward migration of the Chishima forearc sliver [4], the collision event has endured longer in the south. In the northern region, the thrust activity had declined by the late Miocene, but in the southern region, it continues up untill the present-day. Therefore, the degree of total crustal shortening and exhumation by the collision event is larger in the southern region [5]. For instance, the amount of crustal shortening in the Sorachi-Yezo Belt varies from 12 km in the north, to 50–64 km in the south [5, 6].

An exhumed basement in the eastern orogen (the eastern zone of the Sorachi-Yezo Belt and the Hidaka Belt) consists of Cretaceous to Early Paleogene accretionary complexes (Figure 1) formed by the arc-trench system along the eastern margin of the Eurasian continent [7]. A metamorphic core of granulite facies crops out only in the southern area of the orogen (the Hidaka metamorphic rocks) [8]. The cooling ages of the metamorphic core indicate the middle to late Miocene ages (20–10 Ma), and are synchronized with the evolution of the foreland basin ([9] and references therein).

Before the foreland basin subsidence, relatively shallow sea environments were widespread in the area during the Early to early-Middle Miocene ages corresponding roughly to the sealevel rise during the Mid-Neogene Climatic Optimum [10]. However, the detailed tectonic control of this shallow marine basin remains unclear. It is evident that initial deep depressions were formed around 15–16 Ma, due to rapid subsidence (Figure 2). On close inspection, the central part of the foreland basin area is constricted, and this is probably owing to the existence of a projection on the foreland lithosphere (the Kabato basement high (KBH) of Rebun-Kabato Belt in Figure 1). The Rebun-Kabato Belt is defined by the Cretaceous arc volcanics/plutonics and forearc basin-fills [11] which are overlain by the thick sedimentary succession of Paleo‐ gene–Quaternary in the Ishikari Lowland.

Slightly right-stepping deep depressions developed in both the northern and southern parts of the foreland basin area. These deep depressions, known as the Tenpoku, Haboro, Ishikari, and Hidaka basins (from north to south) [3, 12] (Figure 1), are filled with Miocene–Pliocene deposits up to 6000 m thick (Figure 2), sourced from the eastern orogen [9, 12–16]. Most of the basin fills consist of parallel-sided, non-channelized turbiditic sand and basin-plain mud interbeds ("basinal turbidites": [17]) and coarser-grained turbiditic deposits of immature

facies. These are settled in various areas from the outer to the inner foredeep settings. In this paper, the terms "outer", "axial" and "inner foredeep" follow the previous work [17, 18], (i.e., the outer foredeep is the most distal part of the foredeep above the foreland ramp; the axial foredeep is the central and deepest part of foredeep with relative flat basin floor; and the inner foredeep is the proximal part of foredeep along the thrust front characterized by steep slope

**Figure 1.** Simplified geologic map of central Hokkaido, northern island of Japan. The Miocene foreland basins were formed in the western zone of the Sorachi-Yezo Belt. The central part of the foreland basin area is constricted by the projection (Kabato basement high: KBH). Tenpoku Basin (TP), Haboro Basin (HB), Ishikari Basin (IS), and Hidaka Basin

Foreland Basins at the Miocene Arc-Arc Junction, Central Hokkaido, Northern Japan

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(HD) are major depressions separated by topographic highs (after [3]).

Foreland Basins at the Miocene Arc-Arc Junction, Central Hokkaido, Northern Japan http://dx.doi.org/10.5772/56748 133

reviews the stratigraphic architecture and depositional system in each depression, and then

The present Hokkaido Island is located on a junction between the Northeast Japan Arc and the Chishima (Kuril) Arc (Figure 1), which originated from an interaction between the Eurasian and Okhotsk plates that commenced in the Late Eocene [4]. A right-lateral oblique collision between the two arcs during the Miocene age formed a west-vergent fold-thrust belt and a

Because of the right-lateral oblique collision and the subsequent westward migration of the Chishima forearc sliver [4], the collision event has endured longer in the south. In the northern region, the thrust activity had declined by the late Miocene, but in the southern region, it continues up untill the present-day. Therefore, the degree of total crustal shortening and exhumation by the collision event is larger in the southern region [5]. For instance, the amount of crustal shortening in the Sorachi-Yezo Belt varies from 12 km in the north, to 50–64 km in

An exhumed basement in the eastern orogen (the eastern zone of the Sorachi-Yezo Belt and the Hidaka Belt) consists of Cretaceous to Early Paleogene accretionary complexes (Figure 1) formed by the arc-trench system along the eastern margin of the Eurasian continent [7]. A metamorphic core of granulite facies crops out only in the southern area of the orogen (the Hidaka metamorphic rocks) [8]. The cooling ages of the metamorphic core indicate the middle to late Miocene ages (20–10 Ma), and are synchronized with the evolution of the foreland basin

Before the foreland basin subsidence, relatively shallow sea environments were widespread in the area during the Early to early-Middle Miocene ages corresponding roughly to the sealevel rise during the Mid-Neogene Climatic Optimum [10]. However, the detailed tectonic control of this shallow marine basin remains unclear. It is evident that initial deep depressions were formed around 15–16 Ma, due to rapid subsidence (Figure 2). On close inspection, the central part of the foreland basin area is constricted, and this is probably owing to the existence of a projection on the foreland lithosphere (the Kabato basement high (KBH) of Rebun-Kabato Belt in Figure 1). The Rebun-Kabato Belt is defined by the Cretaceous arc volcanics/plutonics and forearc basin-fills [11] which are overlain by the thick sedimentary succession of Paleo‐

Slightly right-stepping deep depressions developed in both the northern and southern parts of the foreland basin area. These deep depressions, known as the Tenpoku, Haboro, Ishikari, and Hidaka basins (from north to south) [3, 12] (Figure 1), are filled with Miocene–Pliocene deposits up to 6000 m thick (Figure 2), sourced from the eastern orogen [9, 12–16]. Most of the basin fills consist of parallel-sided, non-channelized turbiditic sand and basin-plain mud interbeds ("basinal turbidites": [17]) and coarser-grained turbiditic deposits of immature

discusses the relationship between basin evolution and tectonics.

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

subsiding foreland area nearly 400 km long in central Hokkaido [4].

**2. Geological setting**

the south [5, 6].

([9] and references therein).

gene–Quaternary in the Ishikari Lowland.

**Figure 1.** Simplified geologic map of central Hokkaido, northern island of Japan. The Miocene foreland basins were formed in the western zone of the Sorachi-Yezo Belt. The central part of the foreland basin area is constricted by the projection (Kabato basement high: KBH). Tenpoku Basin (TP), Haboro Basin (HB), Ishikari Basin (IS), and Hidaka Basin (HD) are major depressions separated by topographic highs (after [3]).

facies. These are settled in various areas from the outer to the inner foredeep settings. In this paper, the terms "outer", "axial" and "inner foredeep" follow the previous work [17, 18], (i.e., the outer foredeep is the most distal part of the foredeep above the foreland ramp; the axial foredeep is the central and deepest part of foredeep with relative flat basin floor; and the inner foredeep is the proximal part of foredeep along the thrust front characterized by steep slope and rough topography due to thrust propagation). The turbiditic deposits are covered with siliceous/diatomaceous and/or shelfal muddy deposits of the late Miocene to Pliocene ages (Figure 2).

**Figure 3.** Cross section of the Tenpoku Basin (after [21]). Primary basin geometry is well preserved because of restrict‐

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In the easternmost outcrop, a middle Miocene turbiditic succession (the Masuporo Formation) is characterized by abundant mass-transport deposits (MTDs), such as slumped sand/mud interbeds and chaotic sand to gravel beds, bearing many intrabasinal blocks [20, 22–25] (Figure 4). It is noteworthy that the MTDs at the base of the Masuporo Formation rest directly on the shallow marine sandy deposits of the early Middle Miocene (the Onishibetsu Formation), which settled prior to the foreland basin subsidence. In the uppermost horizon of the turbiditic succession, mud-prone basinal turbidites and basin-plain mudstones are predominant, and

The basin fill fines also foreland-ward drastically, and the axial foredeep is filled mainly with basin-plain muddy deposits (2000–3000 m thick) [26]. As an exception, a 200 m thick slumped interval occurs in the upper part of this muddy succession [26]. This interval is characterized by muddy chaotic deposits containing granule-grade grains, although no detailed sedimentary

The entire part of the basin was covered by basin-plain muddy deposits in the late-Middle Miocene, after which siliceous/diatomaceous muddy deposits (>1000 m thick) were accumu‐

Although the sediment dispersal pattern in the Tenpoku Basin is not clearly understood, a clastic composition of basin fill, which is rich in granite and hornfels clasts, indicates a sediment

ed later tectonic disturbances. See Figure 1 for legend in the index map.

the succession thus shows an overall fining-upward trend.

features are described.

lated basin-wide during the late Miocene.

supply from the Hidaka Belt in the east.

**Figure 2.** Stratigraphy of the middle Miocene to early Pliocene deposits in central Hokkaido. Depositional ages have been determined by diatom biostratigraphy and chronostratigraphic data. Diatom biostratigraphic zonation after [19]. Columns show the successions of inner (orogen-ward, proximal) and outer (foreland-ward, distal) areas of each depression.
