ABNORMAL TECTONOTHERMAL EVOLUTION AND ITS DYNAMIC MECHANISM IN THE EASTERN ORDOS BASIN: RESPONSE TO LITHOSPHERIC THINNING AND DESTRUCTION OF THE NORTH CHINA CRATON

Using various thermal indictors and basin model, a new abnormal tectonothermal model was obtained. AFT analysis indicate that the eastern Ordos Basin experienced two major tectonothermal events since the Late Paleozoic, including deep depositional thermal metamorphism stage and magmatic thermal metamorphism stage, dividing into five sub-stages: (a) slow burial and heating stage (245–175 Ma), (b) rapid burial and magmatism stage (175–125 Ma), (c) rapid cooling and thermal subsidence stage (125–105 Ma), (d) rapid cooling and thermal subsidence stage (125–105 Ma) and (e) rapid cooling stage (20–0 Ma). Fluid inclusions data show the lower homogenization temperatures record the peak temperature of the deep burial thermal metamorphism and the first peak of hydrocarbon generation, the higher homogenization temperatures record the metamorphic temperature of magmatic thermal metamorphism and the second peak of hydrocarbon generation.

Hydrocarbon generation started as early as 237 Ma with its first peak at 205Ma and second peak at 162 Ma, and expulsion started at around 230 Ma. Source rocks in the Taiyuan and Shanxi Formations reached gas generation stage (1.3% < R_o < 2.0%) showing great wet gas generation potential in south and north regions, while the source rocks in central region appear dry gas generation stage (R_o >2.0%).

The abnormal tectonic thermal evolution of the eastern Ordos Basin is mainly influenced by the integrated thermal effects of the destruction of the NCC and the lithospheric thinning caused by plate tectonics, demonstrating two major stage. (a) Deep depositional thermal metamorphism stage. Due to the inactive tectonism of the NCC from the Carboniferous to the Middle Jurassic, the eastern Ordos Basin was deposited stably, resulting in that the thermal evolution of source rocks increased gradually due to the huge heat generated by the larger buried depth. (b) Magmatic thermal metamorphism stage. From the Late Jurassic to the Early Cretaceous, the eastern Ordos Basin experienced an intense tectonic thermal event coupled with a large-scale uplifting process with magmatic thermal metamorphism, which greatly promoted the hydrocarbon generation and expulsion of source rocks in the eastern Ordos Basin.