Differences of structure in mid-lower crust between the eastern and western blocks of the Sichuan basin

作者：Xiong Xiao-Song^*; Gao Rui; Zhang Ji-Sheng; Wang Hai-Yan; Guo Liang-Hui

来源：Acta Geophysica Sinica, 2015, 58(7): 2413-2423.

DOI：10.6038/cjg20150718

• Sichuan basin
• Middle-lower crust
• Basement
• Continental nucleuse
• Gravity anomaly

The Sichuan basin is the main part of the middle-upper Yangtze craton, which has been experienced a long-term tectonic evolution since Archean. It is regarded as a stable block until the collision with the Cathaysia block in late Neoproterozoic. As one of the largest and most complex Archean cratons in China, the Yangtze craton consists of Archean-Paleoproterozoic crystalline basement (mostly covered) outcropped in the western and southwestern margins and the three George area surrounded by late Mesoproterozoic to early Neoproterozoic fold belts, which are locally unconformably overlain by middle Neoproterozoic weakly metamorphosed strata and late Neoproterozoic unmetamorphosed Sinian and Phanerozoic cover. These strata limit the understanding of the nature and distribution characteristics of the middle-lower crust beneath the sedimentary rocks.
Gravity anomalies can reveal the overall regional structure. In this paper we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho. Gravity anomalies measured on the surface have many possible origins, including deposits, structures, mass excess or defect caused by interfaces in the crust. First, we obtain the Moho depth and sedimentary thickness contours from resampling and interpolating the deep seismic profiles collected. Then we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho.
The middle-lower crust of the Sichuan basin has gravity anomalies ranging form 32 to 206 mGal, unevenly distributed. It can be divided into western and eastern parts bounded by the Chongqing-Huaying line. In the western part, there are two high gravity anomaly closures, centered at Neijiang-Suining and Nanchong-Dazhou, respectively. The maximum anomaly value reaches 170 mGal. In the eastern part, there is only one anomaly closure with 170 mGal as the highest value. Therefore, the gravity anomaly structures of the Sichuan basin on either side of the Chongqing-Huaying line are different.
Combined with the deep seismic reflection profiling and other geochemical and geophysical data, we propose that the boundary of mid-lower crust between the eastern and western parts is along the Chongqing-Huaying line, which may indicate that there are two continental nucleuses beneath the Yangtze block.

• 出版日期2015-7
• 单位中国地质大学（北京）; 中国地质科学院