Mesozoic tectonic evolution and thermal history of the Yuanba area of northeast Sichuan basin-low-temperature thermochronology of apatite and zircon.

By means of the vitrinite reflectance and U-Th/He dating of apatite and zircon in cutting samples from the T x-K j formations in the Yuanba (元坝) area of Northeast Sichuan (四川) Basin, a correlation has been established between the He-derived age and depth/temperature in this area assuming helium closure temperature of apatite in this area being 95 °C. Mesozoic strata (T x-K j) experienced helium closure temperature of apatite approaching 95 °C, but didn't reach the helium closure temperature of zircon (ca. 170-190 °C) although some reached the highest palaeogeothermal temperature of about 170 °C. The Mesozoic strata in the Yuanba area experienced an important uplift and denudation during Paleogene-Neogene periods (0.2-36.4 Ma), the erosion rate being about 109.9 m/Ma. The K j Formation and overlying strata experienced a maximum denudation loss of about 4 000 m. Geotemperatures gradually fell to the helium closure temperature of apatite and then fell further to the current temperature. The thermal evolution history of this area indicates that the maximum palaeogeothermal temperature of Mesozoic strata was close to 170-190 °C, prior to the strata being uplifted. During the period between 176 and 36 Ma, the palaeogeothermal temperature fell to 95-170 °C, and after 36 Ma, it continued to fall to the present geotemperature of less than 95 °C. [ABSTRACT FROM AUTHOR]