Gravity inversion for deep crust-mantle interface in Three Gorges region
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摘要: 为了更精细了解长江三峡地区壳幔深部界面起伏特征， 对最新获得的三峡坝区1:20万完全布格重力异常图与以往三峡地区1:50万完全布格重力异常图进行拼接， 结合人工地震测深、 层析成像结果等数据， 利用Parker-Oldenburg位场迭代方法对壳幔主要界面进行了反演研究. 主要新认识有： 巴东—秭归莫霍面深度为42 km左右， 江汉平原及周边中地壳界面深度为21—25 km； 深部界面总形态与地表地质构造呈立交桥式结构； 三峡大坝处于研究区中地壳最薄处， 仅为9 km， 这可能与黄陵隆起中部存在浅层低密度体有关； 三峡地区地震大多发生在本区上地壳内， 与壳幔深部界面的起伏变化密切相关， 如秭归MS5.1地震发生在莫霍面中地壳界面隆陷梯变最大部位， 说明深部作用对区域构造地震活动具有重要影响.Abstract: In order to better understand the fluctuation of deep crust-mantle interface in Three Gorges region, we performed the gravity inversion by using Parker-Oldenburg iterative inversion method. During the inversion, we used the existing map of Three Gorges complete Bouguer gravity anomaly(1:500000) and a new map of Three Gorges dam complete Bouguer gravity anomaly (1:200000), as well as results of deep seismic sounding and tomography. The inversion result shows that the Moho depth between Badong and Zigui is about 42 km, the depth of middle crustal interface in Jianghan plain and its surrounding area lies in the range of 21—25 km, and the overall morphology of the deep interface and the surface geology presents an overpass-like structure. The middle crust beneath Three Gorges dam is about 9 km thick, being the thinnest part of the middle crust in this region, which is possibly related to the shallow low-density body in Huangling anticline area. Earthquakes always occur in the upper crust, so there exists close relationship between seismic activity and deep crust-mantle interface fluctuation. For example, the Zigui MS5.1 earthquake took place in the place where the gradient changes of Moho and middle crustal interface were most significant, indicating that deep structure has important effect on regional seismic activity.
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Keywords:
- inversion /
- gravity anomaly /
- deep structure /
- Three Gorges
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图 2 三峡地区完全布格重力异常图及人工地震测深剖面位置图
(a) 三峡地区1 : 50万布格重力异常图; (b) 拼接后三峡地区布格重力异常图① 奉节—观音垱测深剖面; ② 渔阳关—古夫测深剖面; ③ 麦仓口—五峰测深剖面; ④ 南潭河—袁码头测深剖面; ⑤ 层析成像剖面
Figure 2. Complete Bouguer gravity anomalies and five deep seismic sounding profiles
(a) Map of Bouguer gravity anomaly in Three Gorges area (1 : 500 000); (b) Spliced map of Bouguer gravity anomaly in Three Gorges area Deep seismic sounding profiles: ① Fengjie- -Guanyindang; ② Yuyangguan- -Gufu; ③ Maicangkou- -Wufeng; ④ Nantanhe- -Yuanmatou; ⑤ Tomography profile
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图 3 反演收敛曲线(纵坐标表示界面改进量中误差)
Figure 3. Convergence curves of inversion layer by layer from bottom to top
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图 4 奉节—观音垱剖面测深结果与重力反演结果对比图
Figure 4. Comparison between the crustal interfaces obtained by inverse result and the deep seismic sounding results along the Fengjie- -Guanyindang profile
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图 5 重力残差分布图
(a) 反演B1面后重力异常残差值; (b) 反演B2面后重力异常残差值; (c) 反演B3面后重力异常残差值
Figure 5. Map of gravity anomaly residuals
(a) Gravity anomaly residuals after inversing Moho interface; (b) Gravity anomaly residuals after inversing bottom interface of middle crust; (c) Gravity anomaly residuals after inversing bottom interface of upper crust
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图 6 B1面(a)和B2面(b)深度分布(图中等值线单位： km)
Figure 6. Depth distribution of the interfaces B1 (a) and B2 (b) in Three Gorges region
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图 7 B3面深度分布(图中等值线单位： km)
Figure 7. Depth distribution of the interface B3 in Three Gorges region
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图 8 三峡地区垂直剖面图
(a) 31°N处EW向垂直剖面及其地震震源分布图； (b) 111°E处NS向垂直剖面及其地震震源分布图； (c) 三峡地区轴线NE 向垂直剖面图
Figure 8. Vertical profiles of Three Gorges region
(a) EW vertical profile along 31°N and epicentral distribution； (b) NS vertical profile along 111°E and epicentral distribution; (c) NE- -SW vertical profile from (30°38′N,110°22′E) to (31°17′N,112°30′E)
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