渭河盆地岩石圈热结构模拟及其对地热系统热源机理的启示

Simulation of Lithospheric Thermal Structure and Its Implications for Heat Source Mechanism of Geothermal System in Weihe Basin

深部温度场与岩石圈热结构特征是认识地热系统深部热源机理的重要途径。本文在系统分析渭河盆地及其邻区现今大地热流特征基础上,基于旬邑—西峡宽角反射/折射地震测深剖面揭示的地壳分层结构,采用二维有限元方法,对渭北隆起、渭河盆地以及北秦岭构造带的深部温度场和岩石圈热结构开展数值模拟研究,在此基础上分析渭河盆地地热系统深部热源机理。结果表明,旬邑—西峡剖面上大地热流介于57.6~75.7 mW/m^(2)之间,平均为(70.4±4.7)mW/m^(2);地幔热流在29.5~38.6 mW/m^(2)之间,平均值为34.1 mW/m^(2);莫霍面温度变化范围约在600~740℃之间;“热”岩石圈厚度约为95~110 km。从渭北隆起—渭河盆地—秦岭造山带,大地热流、莫霍面温度和地幔热流值表现出低→高→低的变化规律,相应地“热”岩石圈厚度则表现出厚→薄→厚的变化趋势。渭河盆地地壳厚度减薄明显,莫霍面温度显著高于渭北隆起和秦岭造山带,暗示着渭河盆地地壳活动性显著。然而,从渭北隆起—渭河盆地—秦岭造山带,“热”岩石圈厚度变化范围不大,且渭河盆地内的壳幔热流比接近1.0,表明深部活动并不强烈,这符合被动裂谷的深部温度场和岩石圈热结构特征。在印度板块和欧亚板块碰撞的远场效应下,渭河地区地壳伸展减薄,在重力均衡作用下上地幔隆升,高温的幔源物质沿深大断裂上涌加热上地壳,与地壳放射性产热一起为地热系统提供热源。

The deep temperature field and lithospheric thermal structure characteristics are important for understanding the deep heat source mechanism of geothermal system.In this paper,on the base of a systematic analysis of the present terrestrial heat flow characteristics in Weihe Basin and its adjacent areas,a numerical simulation study was carried out on the deep temperature field and lithospheric thermal structure of Weibei Uplift,Weihe Basin and North Qinling Tectonic Zone using a two-dimensional finite element method.The stratified structure of the crust was revealed by the wide angle reflection/refraction seismic bathymetry section along Xunyi-Xixia.Afterwards,the deep heat source mechanism of the geothermal system was analyzed.The results showed that the present terrestrial heat flow along the Xunyi-Xixia profile ranged from 57.6 to 75.7 mW/m^(2)with an average of(70.4±4.7)mW/m^(2),while the mantle heat flow ranged from 29.5 to 38.6 mW/m^(2),with an average of 34.1 mW/m^(2).Furthermore,the temperature on the Moho surface ranged from approximately 600 to 740°C,while the thickness of the thermal lithosphere was approximately 95~110 km.From the Weibei Uplift,Weihe Basin to the Qinling orogenic belt,the present terrestrial heat flow,temperature on Moho surface and mantle heat flow values showed a low→high→low variation trend,while the corresponding thermal lithosphere thickness showed a thick→thin→thick variation trend.The crustal thickness of Weihe Basin thined significantly,and temperature on the Moho surface was significantly higher than that of Weibei Uplift and Qinling orogenic belt,which suggested significant crustal activity in Weihe Basin.However,from Weibei Uplift,Weihe Basin to the Qinling orogenic belt,the thickness of thermal lithosphere did not vary much and the crust/mantle heat flow ratio within the Weihe Basin was close to 1.0,indicating that deep activity was not very strong.It was consistent with the passive rift on the deep temperature field and lithospheric thermal structure.On the far-field effect of the collision between the Indian plate and the Eurasian plate,the crust in Weihe area was extended and thinned,and the upper mantle was uplifted under the action of gravity equilibrium.The material with high temperature derived from mantle upwelled along the deep fault and heated the upper crust.Together with the crustal radioactive heat source,it provided heat source for the geothermal system.