鄂尔多斯盆地深部热结构特征及其对华北克拉通破坏的启示

基于二维稳态热传导方程,利用有限元数值模拟方法,选取东西向横穿鄂尔多斯盆地地质与地球物理解释大剖面进行了深部温度场数值模拟研究,得到了华北克拉通西部的鄂尔多斯盆地下伏岩石圈热结构特征.地幔热流变化范围:21.2~24.5mW·m-2,体现为东高西低特征.壳幔热流比(Qc/Qm)介于1.51~1.84之间,为"热壳冷幔".与华北东部地幔热流对比表明,西部的鄂尔多斯盆地相对处于稳定的深部动力学环境.在岩石圈热结构研究基础上,对克拉通地震岩石圈与热岩石圈厚度差异进行了对比,研究表明:鄂尔多斯盆地西部地震岩石圈与热岩石圈厚度差异约达140km,而东部的汾渭地堑,渤海湾盆地二者差异逐渐减小.华北克拉通自西向东,地震岩石圈厚度与热岩石圈厚度差异不断减小,意味着华北克拉通岩石圈下部的软流圈地幔黏性系数自西向东逐渐降低,本文从地热学角度可能印证了太平洋俯冲脱水作用对华北克拉通的影响.

Lithospheric thermal structure of the Ordos Basin and its implications to destruction of the North China Craton

Recently, the destruction of the North China Craton has become a center of active discussion in earth sciences. While its mechanism remains unclear and debatable. Moreover, geothermal research on this subject is relatively few. The Ordos Basin, located in the west of the North China Craton, is a typical intraplate tectonic unit with stable sedimentation since Paleozoic. Jurassic to Cretaceous is an important period of the formation and evolution of this basin, which were affected by the Tethyan and circum-Pacific tectonic domains. In an attempt to understand the deep dynamics of lithospheric destruction of the North China Craton, the thermal structure of a two-dimensional profile (named AB) across the Ordos Basin from west to east has been studied to provide geothermic evidence to addressing this issue. Based on the two-dimensional steady-state heat conduction equation and using the finite element algorithm, thermal modeling along the profile(named AB) across the Ordos Basin has been carried out, resulting in the lithospheric thermal structure in the Ordos Basin west of the North China Craton. Furthermore, in the process of simulated calculation, we constantly adjust the heat flow at the bottom of the model to calculate surface heat flow (named calculated surface heat flow), which will be used to fit the measured heat flow on the surface of the earth. So the simulation results are convinced because they are constrained by both thermal physical parameters and the measured surface heat flow. The results are as follows: (1) The Moho temperature along the profile ranges from 610 ℃ to 700 ℃. The temperature in the east is higher than that in the west. (2) Mantle heat flow values in different tectonic units in the Ordos Basin range from 21.2 to 24.5 mW·m^-2. In the eastern Ordos Basin mantle heat flow values are high while the values in the western region are relatively low. But mantle heat flow values are smooth and not high overall, showing a stable deep thermal background in the west of the North China Craton. (3)The heat flow ratios of crust to mantle(Q_c/Q_m)along the profile are between 1.51 and 1.84, which indicates a thermal state of relatively hotter crust while colder mantle. (4) In the west of Ordos Basin, the thermal lithospheric thickness is about 160km, while in the east it is about 140km. They both indicate that the Ordos Basin has a thick thermal lithospheric root. By the lithospheric thermal structure study, we find that the Ordos Basin, located in the west of North China Craton, is in a relatively stable deep dynamic environment. Moreover, we focus on the disparity in thickness between the thermal and seismic lithosphere. The difference between seismic and thermal lithosphere thicknesses in the western Ordos Basin is about 140km, which decreases gradually from the Fenwei graben in the eastern Ordos Basin to the Bohai Bay Basin farther east. That is to say the differences between seismic and thermal lithosphere thicknesses decrease gradually from the west to the east of North China Craton. The simulation results imply that viscosity of the asthenosphere under the North China Craton decreases gradually from west to east, confirming that dehydration of the Pacific subduction plate likely has a great effect on the North China Craton. Combining previous results and this study, we suggest that convection erosion and peridotite melting in the big mantle wedge formed by the Pacific subduction under the eastern North China Craton are the dynamic mechanisms of the destruction of the North China Craton.