**6. Conclusions**

in the Chapter 5, changes in proximity of sediment source and/or in lithologic compositions

Pliocene gravelly deposits crop out on the northwestern side of the Hidaka Basin and inter‐ finger with shallow marine to shelfal deposits. Their distribution, distinct from that of the Miocene basin fill, suggests a major foreland-ward shift of the depocenter. Although the sedimentary processes of the Pliocene deposits are poorly known, the spatiotemporal distri‐ bution of the gravelly body suggests cyclic progradation of shelf-type fan-delta systems toward

In the Tenpoku Basin, the MTD dominated coarse clastic wedge is overlain by mud-prone interbeds and subsequently developed basin-plain muddy deposits, suggesting rapidly declining tectonic activity in the northern area of the collision zone by the early Late Miocene. Subsequently accumulated siliceous/diatomaceous muddy deposits buried the "abandoned foredeep." A recent study suggested that the accommodation space of the late Miocene age was maintained as a pull-apart depression along a right-stepping dextral fault system transformed from the former transpressional thrust system [38]. The siliceous/diatomaceous deposits are interpreted as siliceous muddy turbidites [39]. The declining thrust activity resulted in starvation of terrigenous sediment input to the Tenpoku Basin. Siliceous tests, originating from siliceous phytoplankton blooms in the photic zone, were alternately trans‐

Notably, diatom productivity dramatically increased in the North Pacific and paleo-Japan Sea during the late Miocene [40]. As a result, syn-orogenic turbiditic deposits in the Ishikari and Hidaka Basins are also accompanied by similar muddy deposits during the late Miocene to

**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‐

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

in the orogen also caused a decrease in the fine-grained fraction.

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

the west [32] or south to southwest (Takano, personal communication).

ported by laterally induced muddy turbidity currents [38, 39].

**4.4. Basin migration and Pliocene foredeep**

**4.5. Post-orogenic sedimentation**

Pliocene ages.

sion and resultant basin deformation.

This paper provides an introduction to a tectonically controlled foreland basin stratigra‐ phy at the arc-arc collision zone of Miocene age in Hokkaido, northern Japan. Spatial

differences in the degree of tectonic disturbance caused variations in stratigraphic/sedimen‐ tary architecture between the separately developed depressions in the foreland basin area. Limited tectonic activity resulted in trapping of coarse-grained deposits in the inner foredeep setting. Thick muddy deposits filled the sediment-starved and abandoned axial foredeep. Moderate to high thrust activity formed turbiditic successions several thousand meters thick in the axial foredeep. Progressed thrusting caused an increase in coarsergrained sediment-input to the foredeep and the sedimentary style changed from the basinal turbidites by efficient-flows to the slope-apron turbidites by poorly-efficient flows. The longterm shallowing of the foreland basin area can be explained by lithospheric flexural rebound caused by the isostatic readjustment for a thinning orogen and/or decreased horizontal compressional stress due to a gradual or stepwise decline of thrust activities in central Hokkaido.

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