**5. Tectonically controlled basin geometry and stratigraphic architecture**

It is supposed that along the collision zone in central Hokkaido, the spatial variation in geodynamic properties of the foreland lithosphere is small, because of the assumption of a uniform geological composition, geotectonic history, and geothermal structure throughout the region. Thus, the variations in basin geometry and stratigraphic/sedimentary architecture for each depression are attributed to the regional difference in the degree of horizontal compres‐ sion and resultant basin deformation.

The wider geometry of the Tenpoku Basin is the result of restrictive basin deformation. Many of the turbiditic deposits are confined to the inner foredeep and the axial foredeep is filled with muddy deposits. This condition is common to other depressions at the initial stage of basin evolution. Conversely, the thick accumulation of turbiditic deposits in the axial foredeep requires persistently high-relief basin physiography from the hinterland to the basin plain and high-volume sediment input.

Consistent basin infilling and/or thrust propagation results in shallowing of the foredeep and a transition in sedimentary style [37]. In addition, synchronous changes in the detrital composition in the Ishikari and Hidaka Basins suggest a close relationship between the sedimentary style in the foredeep and structural deformation in the hinterland (see discussion of [41]). In the Ishikari Basin, the compositional change in the detritus sug‐ gests lateral growth of the orogen (extension from the Hidaka Belt to the Sorachi-Yezo Belt). The proximity between the depocenter and the newly emergent source area resulted in an increase in the coarser-grained fractions and generation of relatively poorly-efficient flows. In contrast, the evolution of the detritus composition in the Hidaka Basin implies exhuma‐ tion of deep-seated crustal rocks. Thus, the increase in coarse-grained deposits through poorly-efficient dense flows is attributed to an increase in the distributional area of crystalline rocks in the Hidaka Belt (Hidaka metamorphic rocks). Although the detrital compositional signal is unclear in the Haboro Basin, subaerial erosion of the basin fills in the eastern area suggests syn-depositional thrusting near the basin margin.

A similar succession, consisting of lower basinal turbidites and upper coarser-grained turbiditic deposits, is well documented from the Miocene foredeep turbidites in the Northern Apennines [e.g., 42]. In that area, the change in sedimentary style was control‐ led by the narrowing and closure (shallowing) of the foredeep due to thrust propagation [42]. In Hokkaido, in contrast, the stratigraphic architecture does not show obvious narrowing of the foredeep depressions. The coarse-grained slope-apron turbidites occur basin-wide and their thickness is approximately the same as that of the basinal turbidites. In addition, they are covered with relatively thick siliceous/diatomaceous muddy depos‐ its. Nevertheless, the depressions appear to shallow gradually upward, as indicated by the dominance of bioturbation, plant debris, and shell fossils in the Late Miocene basin fills. Despite the migration of the depocenter in the Hidaka Basin, the depth of the depocenter gradually decreases foreland-ward until the Pliocene. Initial regional shallowing occurred around 13–14 Ma, beginning in the northern foreland area. An eustatic sea-level fall [43] is not sufficient to explain such a long-term gradual shallowing of the basin; however, a flexural rebound of underlying lithosphere [44] can explain such shallowing. The re‐ bound was probably caused by isostatic readjustment for a thinning orogen or decreased horizontal compressional stress corresponding to a gradual or stepwise decline of thrust activities in central Hokkaido [45, 46].
