**4.1. Primary wedge-top to inner foredeep**

A sedimentary succession within the primary inner foredeep can be observed in the eastern Tenpoku Basin (Figure 4). The sedimentary fill typically includes syn-orogenic packages of chaotic MTDs. The MTDs rest directly on the pre-orogenic shallow marine sandstones and often bear abundant intrabasinal blocks. Thus, the primary inner foredeep setting was quite unstable because of rapid syn-depositional subsidence and thrusting. The laterally discontin‐ uous distribution of MTDs and sandy/gravelly turbiditic deposits with locally presented sand/ mud interbeds suggests the ponding of frequently generated dense flows in small accommo‐ dation spaces of the irregular basin floor. As discussed in the next subsection, a large collapse event also resulted in a thick accumulation of cohesive debris flow deposits in the axial foredeep setting.

In the Hidaka Basin, a thick gravelly body is locally present in the boundary zone between the inner and the axial foredeep. Its basin-axial paleoflow direction suggests the development of a sediment conduit controlled by the thrust fault itself (cf. [34]). The gravelly body rapidly passes down-dip into the basinal turbidites of the axial foredeep and is interpreted as slopefan deposits fed by a northern point source [30].

The sedimentary features of primary wedge-top basin (i.e., basins on thrust sheets and located at proximal side of inner foredeep) during the early syn-orogenic phase of foreland basin evolution are not evident. Non-marine to shallow marine deposits of middle to late Miocene age occur adjacent to the Tenpoku and Haboro Basins, but most of the deposits were formed during the post-orogenic phase in the northern collision zone.
