Paleothermal data from the Cretaceous through Miocene Shimanto Belt reveal important variations in thermal history, both along and across strike within the ancient accretionary prism. Two types of variations are considered: those due to different tectonic settings (e.g., tectonic mélange versus coherent accreted rocks versus slope-basin deposits); and those due to different times of accretion. Within the Cretaceous subbelt on Shikoku, shale samples from mélange and coherent rocks show the same level of illite crystallinity, but samples from a probable slope-basin deposit are significantly less crystalline suggesting that the slope-basin strata experienced less heating than underlying accreted rocks. In contrast,...