Thermal regime transition in eastern North China and its tectonic implication

The pre-Cenozoic formation in the Cenozoic depositon centres in eastern North China reached its maximum temperature at present, and the earlier paleo-temperature, therefore, has been overprinted, but the record of paleo-temperature by the vitrinite reflectance (R _o) has not been overprinted by the later thermal events in the pre-Cenozoic formation located in the uplift or in the Paleozoic-Mesozoic residual basins out of the Cenozoic depositon centres. The reconstruction of paleo-temperature gradient and paleo-heat flow, based on the vitrinite reflectance in the Paleozoic and the Mesozoic structural layers in boreholes, indicated that the eastern North China was characterized by much higher paleo-temperature gradient (40–55°C/km) and heat flow (>80 mW/m²) during the Middle-Late Mesozoic than those in the Early Mesozoic and at present. The higher paleo-heat flow during the Middle-Late Mesozoic implies that the thickness of the “thermal” lithosphere at that time was just 50–55 km, it had been much thinned relative to the thickness (135–148 km) at the Early Mesozoic. The transition of near-surface thermal regime in eastern North China occurred around 110 Ma, and the corresponding deep tectonothermal processes should take place at ~160 Ma.