Palaeogeothermal response and record of Late Mesozoic lithospheric thinning in the eastern North China Craton

The palacotemperature recorded by vitrinite reflectance (R.) in the pre-Cenozoic uplifted stratigraphic strata, and in Palaeozoic-Mesozoic remnant basins outside the Cenozoic depocentres, has not been overprinted by later thermal events in the eastern North China Craton (NCC). Based on downhole R. data from the Palaeozoic and the Mesozoic subsections, we reconstruct the temperature gradients when the subsections reached their maximum palaeotemperatures in the Middle Triassic and the Cretaceous, and calculate the corresponding heat flow histories since the early Mesozoic. The temperature gradient and heat flow were much higher in the Cretaceous (35-43 degrees C km(-1) and 73-83 mW m(-2), respectively) than in the Middle Triassic and at the present. The high palaeo-heat flow during the Late Mesozoic implies that the thickness of the 'thermal' lithosphere at that time was c. 65 km, about half the thickness of c. 135 km estimated for the Early Mesozoic. The change from a stable thermal regime to an active thermal regime took place during the Late Jurassic-Early Cretaceous (c. 110 Ma). This tectonothermal event was accompanied by extensive surface erosion, and is also evidenced in the areas adjacent to the NCC, such as the South Yellow Sea and East China Sea basins. Our study provides not only geothermal evidence for the Late Mesozoic lithospheric thinning, but also additional constraints on the thinning mechanism, which is currently being debated.