渭河盆地岩石圈热结构与地热田热源机理

岩石圈热结构是盆地现今地温场研究的重要延伸和扩展,是了解大陆岩石圈构造变形及演化等大陆动力学问题的重要窗口,更是地热田热源机理研究的核心问题.本次工作,在系统分析渭河盆地现今地温场和水动力系统基础上,编制了渭河盆地大地热流分布等值线图;通过实测生热率等热物性参数,利用一维稳态热传导方程计算了研究区岩石圈热结构,并分析了渭河盆地岩石圈热结构特征和地热田热源机理.结果表明,渭河盆地现今大地热流值分布范围为62.5~80.2mW·m-2,平均为70.8±4.8mW·m-2,西部明显高于东部,西安坳陷最高,咸礼凸起次之;渭河断裂并不是控热断裂,其沟通作用引起的水热循环一定程度上影响了浅部热量再分配,对渭河盆地地温场并没有起到明显的控制作用.西安坳陷—咸礼凸起地壳热流介于32.2~37.5mW·m-2之间,平均为34.6mW·m-2;地幔热流分布范围为33.8~38.9mW·m-2,平均为36.0mW·m-2;地壳热流和地幔热流的总体变化趋势一致,西安坳陷高于咸礼凸起,分析认为西安坳陷沉积层厚度大于后者,且沉积层放射性生热率更大,是造成西安坳陷地壳热流高于咸礼凸起的原因,而西安坳陷相比咸礼凸起更高的地幔热流,表明西安坳陷深部活动性强于咸礼凸起.西安坳陷和咸礼凸起地壳/地幔热流比值相近,介于0.93~1.01之间,平均为0.96,"热"岩石圈厚度约为95~101km.渭河盆地岩石圈热结构特征与鄂尔多斯盆地在很大程度上具有相似性,暗示着二者具备相似的深部稳定性,这与渤海湾盆地为代表的中国东部中—新生代主动裂谷盆地岩石圈热结构特征截然不同,表明渭河盆地为被动伸展裂陷.从鄂尔多斯盆地、渭河盆地、山西裂谷到华北盆地,"热"岩石圈厚度的有序变化表明太平洋板块俯冲引起的地幔对流对华北地块深部动力学行为的影响主要发生在太行山以东,而太行山以西的鄂尔多斯盆地和渭河盆地则影响甚微,这种空间差异影响从侧面暗示着华北克拉通破坏过程的有序性.综合分析渭河盆地地质—地球物理资料认为,岩石圈表层伸展破裂、深部重力均衡调整进而引起软流圈被动上涌,其产生的相对高地幔热流的热传导和深大断裂沟通的水体热对流相互叠加作用,共同构成了渭河盆地中—低温地热田的热源机理.

The thermal structure of the lithosphere and heat source mechanism of geothermal field in Weihe Basin

Study on the thermal structure of lithosphere, as an important expansion of geothermal pattern at present, is not only a key to understand the continental lithospheric evolution and tectonic deformation, but also the issue of heat source mechanism in geothermal field. In this work, 13 newly measured high-quality terrestrial heat flow data based on systematical well-logging temperature data were reported. Then, the thermal structure of lithosphere was calculated using a one-dimensional steady-state heat conductive equation. Furthermore, the heat source mechanism of Weihe rift was analyzed.#br#The results showed that the present-day heat flow ranged from 62.5 to 80.2 mW·m^-2 with an average of 70.8 ± 4.8 mW·m^-2. The pattern of the heat flow contour map showed the following characteristics. (1) The heat flow in western Weihe rift basin was significantly higher than the east. (2) The highest heat flow values over 74 mW·m^-2 were confined to the Xi'an Depression, which was followed by Xianli Uplift. (3)The Weihe fault did not control the heat flow pattern of the Weihe basin. To some extent, hydrothermal circulation by fracture affected only the heat redistribution in shallow layers. The terrestrial heat flow consisted of crustal heat flow varying from 32.2 to 37.5 mW·m^-2 with a mean of 34.6 mW·m^-2 and mantle heat flow varying from 33.8 to 38.9 mW·m^-2 with an average of 36.0 mW·m^-2 along Xi'an Depression-Xianli Uplift. Along this profile, the crustal heat flow changed consistently with mantle heat flow. We believed that the thicker and greater radioactive heat generation rate of sediments brought out higher crustal heat flow in Xi'an Depression than Xianli Uplift. The higher mantle heat flow suggested stronger activity in Xi'an Depression.#br#The ratio of the crustal/mantle heat flow was similar between 0.93 to 1.01, with an average of 0.96 along Xi'an Depression-Xianli Uplift profile. The thickness of thermal lithosphere was about 95~101 km, indicating the similar thermal structure of the lithosphere and stability between Weihe Basin and Ordos Basin. It was inferred that the Weihe basin was a passive rift, which was much different from the active rift, such as Bohai Bay Basin in eastern China. The thickness of thermal lithosphere changed regularly from the Ordos Basin, Weihe basin, Shanxi rift systems and North China Basin, which indicated varying impact of mantle convection caused by the subduction of the Pacific plate on the dynamic behavior of the North China block.#br#Comprehensively geological-geophysical analysis of the Weihe basin suggested that the relatively higher mantle heat flow caused by the stretching crust and passively upwelling asthenosphere was the most important heat source in Weihe Basin. In addition, hydrothermal circulation by fracture affected the heat redistribution in shallow layers. Thus, the Weihe geothermal field was a conductive-convective geothermal fields with middle-low temperature fluid.