THERMAL AND TECTONIC HISTORY OF THE ORDOS BASIN, CHINA - EVIDENCE FROM APATITE FISSION-TRACK ANALYSIS, VITRINITE REFLECTANCE, AND K-AR DATING

Apatite fission track analysis, vitrinite reflectance data, and K-Ar d ating of Permian-Carboniferous and Mesozoic core samples have been suc cessfully integrated to reconstruct the thermal and tectonic history o f the Ordos basin, China, Apatite fission track ages of Carboniferous- Jurassic sedimentary rocks range between 3 and 137 Ma, and are signifi cantly younger than the stratigraphic ages. Confined fission track len gths demonstrate exclusively mixed length distribution, indicating com plex thermal history. The data suggest that the samples-must have all experienced higher paleotemperatures in the past, Mean vitrinite refle ctance values (R(o)) of the Triassic rocks range from 0.61 to 1.06%, g iving a high coalification gradient of 0.36%/km and suggesting a high paleothermal gradient of 57 degrees C/km. Permian-Carboniferous rocks have R(o) values on the order of 1.0-3.0%, and locally up to 4.0-6.0%, Some high R(o) values coincide with positive gravity and magnetic ano malies, K-Ar dating on Permian-Triassic samples reveals distinct illit ization at 170-160 Ma, during which a thermal event occurred due to su bsurface magmatic intrusion related to the early Yanshanian movement, The petroleum source rocks of the Upper Triassic experienced peak temp eratures ranging from 90 to 160 degrees C, corresponding to the oil wi ndow, and Permian-Carboniferous source rocks were heated to more than 150 degrees C, passing through and out of the gas window. Due to rapid uplift and erosion in response to the rise of the Qinghai-Tibet plate au associated with the Asia-India collision and the Himalayan orogeny, cooling has taken place at least since approximately 23 Ma, The diffe rence in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present-da y structural pattern of the basin.