Pliocene-Pleistocene sedimentary rocks in the Coastal Range of eastern Taiwan were deposited in a synorogenic collisional basin that developed immediately east of the growing Taiwan fold and thrust belt during active arc-continent collision. Unmetamorphosed mudstones from this stratigraphic sequence, which were buried up to 6 km deep, contain abundant detrital illite and chlorite derived from the adjacent metamorphic orogen. A surprisingly small down-section increase in percent illite layers in mixed-layer illite/smectite provides evidence for a low paleogeothermal gradient (about 14 °C/km), possibly combined with kinetic retardation of the illitization reaction resulting from the short duration of maximum burial. Illite crystallinity increases from the base to the top of the stratigraphic sequence, opposite to the expected trend for burial metamorphism. This reverse trend parallels lithic-fragment detrital modes of sandstones from the same sequence and records progressive unroofing of the nearby prograde metamorphic terrane during collision-related uplift and deformation.