**4.4. Basin migration and Pliocene foredeep**

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

## **4.5. Post-orogenic sedimentation**

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

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 Pliocene ages.
