Tectonic Relationship between Upper and Lower Lithosphere in Guangxi Area: A Comprehension from Lithospheric Density and Magnetic Structures
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摘要: 广西地处华南地块、印支地块与西太平洋板块的汇合部位，因特殊的构造部位，广西区内大地构造单元归属、构造单元边界等许多基础地质问题一直存在争议.自新生代以来的板块构造运动对岩石圈的改造，广西地壳与上地幔在地震波速度及温度结构方面具有显著差异.应用卫星重、磁异常数据以及区域重力和航磁资料对广西地区岩石圈密度和磁化率结构及其与上地壳构造的关系开展了研究，结果显示广西地区地壳密度和上地壳磁性结构与现今地表构造较为契合，但下地壳密度结构与上地幔存在不连续现象；此外，岩石圈磁化率结构指示中下地壳存在不同范围和程度的解耦.对广西岩石圈密度与磁性结构的解读认为，在中生代以来岩石圈被大规模改造的背景下，幔源物质上侵至上地壳的规模和范围都有限，这可能是整个广西地区上地幔结构与地壳构造不对应的主要原因.Abstract: Guangxi is located in the convergent region among South China block, Indo-Chinese block and West Pacific plate. Due to its special tectonic position, there have been disputed on several basic geological problems in Guangxi region, such as the ascription and boundaries of tectonic units. Because the lithosphere has been reconstructed by the plate tectonic movements since the Cenozoic, the crust and upper mantle beneath Guangxi area have obvious differences in seismic wave velocity and temperature structures. It employs satellite gravity and magnetic data as well as regional gravity and aeromagnetic survey data to investigate the relationship between lithospheric density and magnetic susceptibility structure and upper crust structure in Guangxi area. Results suggest that the crustal density and upper-crust susceptibility structures coincide with the surface geological structures, and the density structures of the lower crust and the upper mantle are discontinuous beneath Guangxi area. Susceptibility structure implies that different extents of mid-lower crust decoupling had caused in different parts in Guangxi area. As a result, the scale and scope of mantle-derived materials into the upper crust were limited under the background of large-scale lithospheric reconstruction since Mesozoic. It may be the main reason why the upper mantle structure in Guangxi area is not corresponding to that of the crust. The mid-lower crust decoupling results in that the magmatic activity in Guangxi area is obviously weaker than that in the surrounding area since Cretaceous, especially in the northern region of the Pingxiang-Dali fault zone.
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Key words:
- Guangxi area /
- gravity and magnetic anomalies /
- lithosphere /
- density structure /
- magnetic structure /
- geophysics.
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图 1 广西及邻区卫星重、磁异常及主要区域断裂构造
a.卫星（布格）重力异常；b.卫星重力异常垂直导数；c.卫星磁异常；d.广西区域断裂构造分布图（根据新版广西区域地质志：广西主要区域断裂构造简图汇编），图中红色虚线为推断深大断裂. 图中所列区域断裂分别为：①南丹-昆仑关断裂带；②凭祥-大黎断裂带；③右江断裂带；④峒中-小董断裂带；⑤宜州-柳城断裂带；⑥田林-八马隐伏断裂带；⑦博白-梧州断裂带；⑧桂林-来宾断裂带；⑨那坡断裂带；⑩灵山-藤县断裂带；⑪黑水河断裂带；⑫下雷-灵马断裂带；⑬陆川-岑溪断裂带；⑭栗木-马江断裂带；⑮上思-在妙断裂带；⑯池洞-摩天岭断裂带；⑰和睦-老堡断裂带；⑱资源-溶江断裂带
Fig. 1. Satellite gravity and magnetic anomalies and main regional faults
图 2 广西及邻区上地幔密度与横波波速异常(δVs)
图a，b和c分别为利用卫星重力异常反演得到的不同深度上地幔密度异常；图d，e和f为对应深度上地幔横波波速异常（数据取自SL2013sv模型）
Fig. 2. Density and shear wave velocity vs. anomalies beneath the Guangxi and its surrounding regions
图 3 广西地区岩石圈不同深度的密度异常
图中红色线段为主要区域断裂，虚线段为推测的深大断裂
Fig. 3. Density anomalies of the lithosphere in different depths in Guangxi region
图 4 广西地区岩石圈不同深度的磁化率异常及估算的居里面深度(CPD数据源于Zhi et al., 2018)
图中红色线段为主要区域断裂，黑色线段为垂向切片剖面位置
Fig. 4. Magnetic susceptibility anomalies of the lithosphere in different depths and the estimated depth of Curie point depth (CPD) in Guangxi region (CPD data from Zhi et al., 2018)
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