The lithospheric electrical structure of Ji'an-Fuzhou profile in the east part of South China
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摘要:
为了研究华南东部地区岩浆活动的深部构造背景，对吉安-福州宽频大地电磁测深剖面数据进行了系统的分析和处理，并利用非线性共轭梯度法进行二维反演，得到了武夷隆起带及周缘地区的岩石圈电性结构；结合区域重磁资料，详细分析了研究区内地壳、上地幔电性结构特征及地质含义.结果表明：华南东部地区岩石圈电性结构存在明显的分区性，并且壳内普遍发育不同成因的高导层，揭示出华南东部地区不同构造单元内的岩浆活动具有不同的成岩构造背景.其中，东南沿海褶皱带深部热侵蚀活跃，岩石圈物质和结构被强烈改造，电阻率普遍较低，软流圈上涌并伴随玄武岩浆底侵，导致岩石圈、地壳剧烈减薄；而武夷隆起带岩石圈电阻率相对较高，印支-燕山早期陆内挤压变形的构造形迹明显，晚中生代岩石圈拉张伸展作用对该地区岩石圈的物质结构有一定的改造.
Abstract:As having experienced multi-stage tectonic magmatic activity, the structure of the lithosphere in the east part of South China is very complicated and the distribution of magma has obvious regularity. In order to study the deep tectonic background of magmatic activity of the different blocks in the east part of South China, this paper has processed a series of qualitative and quantitative analysis based on the Ji'an-Fuzhou magnetotelluric sounding profile data crossing the east part of South China. The subsurface dimensionality was analyzed by the Bahr phase decomposition, the geoelectric strike with different frequencies was obtained by the single-site multi-frequency Groom-Bailey decomposition. Finally, the non-linear conjugate gradients(NLCG)method was used to calculate the 2D resistivity structure in our research area.
The electrical structure model shows that there are significant differences between the two blocks——Wuyi uplift belt and Southeast coastal fold belt. It can be vertically divided into four electrical layers of the high resistivity layer in upper crust, the low resistivity layer in mid-lower crust, the sub-high resistivity layer in the lithosphere mantle, the low resistivity in the asthenosphere. In the upper crust, the high resistivity layer more than 10000 Ωm thick indicates the distribution of granite whose bottom interface is about 15~20 km deep. In the mid-lower crust, the high-conductivity layer in the Wuyi uplift belt is thin and of small scale. It is associated with the thrust faults. However, in the Southeast coastal fold belt, the high-conductivity layer is thicker and larger. It is uplifted in a mushroom shape. The resistivity in lithosphere mantle gradually reduces from inland to coast. Due to the limited detecting depth, the lithosphere-asthenosphere boundary (LAB) is not showed in the Wuyi uplift belt which indicates that the depth of the LAB is more than 100 km. In the Southeast coastal fold belt, the thickness of the lithosphere is reduced to 60km, and the asthenosphere has an uplift tendency.
In the east part of South China, there are a series of discontinuous and different scale high-conductivity layers in the crust. The scale and burial depth of the high-conductivity layers are closely related to the deep tectonic environment and fault distribution. Combining with gravity and magnetic results, we discussed the formation mechanism of high-conductivity layer. It is inferred that the high-conductivity layer in the crust of the Southeast coastal fold belt is the result of partial melting by asthenosphere upwelling and basaltic magma underplating. While in the Wuyi uplift belt, it is resulted from the remelting of the crustal material in a compression environment, and the continuous heating from the deep heat flow in an extensional environment.
The lithosphere structure in the east part of South China has a marked zoning, and the high-conductivity layers with different causes are widespread within the crust. It shows that the magmatic activities in different tectonic units in the east part of South China have different diagenetic tectonic background. In the Southeast coastal fold belt, the deep lithospheric thermal erosion is active, and the lithosphere structure and material are strongly remoulded. The asthenosphere upwelling and the basaltic magma underplating caused the thinning of lithosphere. In the Wuyi uplift belt, the tectonic features of the intra-continental deformation pattern during the Indo-early Yanshanian period are recorded clearly, while the late Mesozoic extension tectonics has transformed the lithosphere material to some degree.
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图 1
(a)研究区大地构造背景图；(b)研究区地质构造及大地电磁测深剖面点位图，岩浆岩及断裂分布(据Li and Li, 2007)
Figure 1.
(a) The study region tectonic map; (b) The location of MT sites. The distribution of magmatic rock is after (Li and Li, 2007)
图 2
华南地区中新生代动力学模式图
Figure 2.
The dynamic model illustrating Meso-Cenozoic evolution in South China
图 4
测点分频段电性走向分布及剖面区域构造走向统计图
Figure 4.
Electrical strike of MT sites at period ranges of 0.1~10 s and 10~1000 s; and the rose diagram of the regional structure strike angle
图 6
TM极化模式的视电阻率、阻抗相位拟合效果
Figure 6.
The fitting effect of TM apparent resistivity and impedance phase
图 7
吉安—福州剖面岩石圈二维电性结构模型及构造解释图
Figure 7.
The lithosphere two-dimensional electrical structure model and geological interpretation along the Ji′an-Fuzhou profile
图 9
布格重力异常4阶小波变换细节图
Figure 9.
The 4th order detailed images of Bouguer gravity anomaly from multi-scale wavelet analysis
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