To decipher the Miocene evolution of the Shimanto belt of southwestern Japan, structural and paleo-thermal studies we carried out in the western area of Shikoku Island. All units constituting the belt, both in its Cretaceous and Tertiary domains, are in average strongly dipping to the NW or SE, while shortening directions deduced from fault kinematics are consistently orientated NNW-SSE. Peak paleo-temperatures estimated with Raman spectra of organic matter increase strongly across the southern, Tertiary portion of the belt, in tandem with the development of a steeply-dipping metamorphic cleavage. Near the southern tip of Ashizuri Peninsula, the unconformity between accreted strata and fore-arc basin, present along the whole belt, corresponds to a large paleo-temperature gap, supporting the occurrence of a major collision in Early Miocene. This tectonic event occurred before the magmatic event that affected the whole belt at ~15 Ma. The associated shortening was accommodated in two opposite modes, either localized on regional-scale faults such as the Nobeoka Tectonic Line in Kyushu, or distributed through the whole belt as in Shikoku. The reappraisal of this collision leads to reinterpret large-scale seismic refraction profiles of the margins, where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the Shimanto belt. When integrated into reconstructions of Philippine Sea Plate motion, the collision corresponds to the oblique collision of a paleo Izu-Bonin-Mariana Arc with Japan in Early Miocene.