华北地区上地幔温度及岩石圈厚度分布研究

上地幔温度是控制地幔流变性和动力学过程的关键参数之一.本文利用高分辨率S波地震层析成像速度结果,基于岩石温度与地震波速度的关系,研究了华北地区上地幔50~300 km深度范围内的温度分布和"热"岩石圈厚度.为了验证结果的可靠性,本文用计算的上地幔60 km深度处的温度作为底面约束,得到了相应的地表热流.计算地表热流与观测地表热流之间符合程度较好,相对误差大部分都在地表热流观测误差范围之内.通过对上地幔的温度分布进行分析,我们研究发现:(1)在上地幔浅部,温度与地表构造之间有很好的对应关系.在小于170 km的深度上,温度呈现东高西低的分布态势.温度较高的区域集中在东部的河淮盆地、渤海湾盆地、华北平原和中部陆块的交界处、西部鄂尔多斯高原北缘的银川―河套地堑以及阴山地区,同时,这些地区的岩石圈厚度也相应较薄,大约为80~100 km;(2)西部的鄂尔多斯高原是整个华北地区岩石圈地幔温度最低的地区,比东部地区低200~400 ℃,岩石圈厚度相应最厚,平均岩石圈厚度达到140~150 km,最厚处超过160 km.(3)在170 km以下的软流圈地幔部分,温度分布发生反转,西部温度高于东部,表明东、西部陆块在地质历史时期经历了不同的深部地幔动力学过程.

Upper-mantle temperature and lithospheric thickness of North China

Temperature is one of the most important key parameters which control viscosity and rheology of the Earth's material and hence the dynamic process of the mantle. Based on the correlation between mineral temperature and seismic velocity structure, we derived the upper mantle temperatures of North China at the depths ranging between 50 and 300 km by high-resolution S-wave tomography model. Defining the depth where the geotherm intersects the mantle adiabat with a potential temperature of 1300 ℃ as the lithosphere-asthenosphere boundary, we estimated the correspondent lithospheric thickness in the North China. Using the inveted temperatures at 60 km depth as the bottom boundary constraints, we calculated the surface heat flow. The calculated heat flow agrees well with the observations at the surface, and the misfits for most regions of North China are within the uncertainty of the heat flow measurements. Three main characteristics can be observed from the distribution of temperature: (1) Upper mantle temperatures at shallow depths are consistent with the tectonic settings. At depths shallower than 170 km, temperatures under the tectonically active eastern part of North China are higher than that in the stable cratonic regions in the west. The regions with high mantle temperature include the Hehuai Basin, the Bohai Bay Basin, the boundary between the North China Plain and the central North China; hotter lithosphere can also be found at the northern margin of Ordos Plateau, including the Yinchuan-Hetao Rift Zone and the Yinshan Orogen. The lithospheric thickness in the regions with warmer lithosphere is about 80~100 km thick; (2) The lowest temperature is located under the Ordos Plateau in western North China, which is about 200 to 400 ℃ lower than that in the eastern North China. The lithosphere in the Ordos block is also the thickest in the North China, which is about 140~150 km on average and 160 km in the thickest areas; (3) At depths below 170 km, the distribution pattern of the thermal structure is almost reverse to the shallower depths, where the temperatures beneath the western North China are higher than that under the eastern part. It may indicate that the two parts of the North China block had undergone different geological processes.