Thermal and Tectonic History of the Ordos Basin, China: Evidence from Apatite Fission Track Analysis, Vitrinite Reflectance, and K-Ar Dating

Abstract

Apatite fission track analysis, vitrinite reflectance data, and K-Ar dating of Permian-Carboniferous and Mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the Ordos basin, China. Apatite fission track ages of Carboniferous-Jurassic sedimentary rocks range between 3 and 137 Ma, and are significantly younger than the stratigraphic ages. Confined fission track lengths demonstrate exclusively mixed length distribution, indicating complex thermal history. The data suggest that the samples must have all experienced higher paleotemperatures in the past. Mean vitrinite reflectance values (R0) of the Triassic rocks range from 0.61 to 1.06%, giving a high coalification gradient of 0.36%/km and suggesting a high paleothermal gradient of 57°C/km. Permian-Carboniferous rocks have Ro values on the order of 1.0-3.0%, and locally up to 4.0-6.0%. Some high R0 values coincide with positive gravity and magnetic anomalies. K-Ar dating on Permian-Triassi samples reveals distinct illitization at 170-160 Ma, during which a thermal event occurred due to subsurface magmatic intrusion related to the early Yanshanian movement. The petroleum source rocks of the Upper Triassic experienced peak temperatures ranging from 90 to 160°C, corresponding to the oil window, and Permian-Carboniferous source rocks were heated to more than 150°C, passing through and out of the gas window. Due to rapid uplift and erosion in response to the rise of the Qinghai-Tibet plateau associated with the Asia-India collision and the Himalayan orogeny, cooling has taken place at least since approximately 23 Ma. The difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present-day s ructural pattern of the basin.