Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite b_o, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.