关键词: 长江断裂带, 大容量气枪, 上地壳速度结构, 结晶基底

Abstract: The Yangtze fault zone is a typical tectonic regime transition zone of the eastern China. Tectonically, it is characterized by alternated rifts and uplifts and “several crystalline basements with one sediment cover”. Abundant metal metallogenic deposits are developed. Improvement of the velocity model and basement structure will benefit our understanding and knowledge about the regional tectonics. Large volume airgun sources have been broadly applied to seismic surveys due to significant advantages. For instance, they are environmentally friendly, use lower frequencies, and are repeatable. Several seismic and geological research institutions, such as China Earthquake Administration, carried out a three-dimensional comprehensive sounding using the large volume airgun as the seismic source which was fired at the channel of the Yangtze River in 2015. The source-receiver geometry of this seismic experiment covered the whole Anhui Province which locates at the Middle-Lower Yangtze River. The densest observational area is in the Middle-Lower Yangtze River Metallogenic Belt which is a narrow area along the Yangtze River and consists of the Luzong, Tongling, Ningwu, and Anqing-Guichi ore deposits. The Tanlu fault zone, a giant strike-slip fault of more than 2 000km long, passes through the northwestern margin of this area. Geophysical studies have demonstrated copious geological evidences for the Yangtze fault zone, which is approximately 450km long and crosses central China, extending to the eastern coastal area. The present fault and fold systems are the consequences of the repeated tectonic events since the Mesozoic. We collected and analyzed the seismic data of 20 fixed airgun shot points, then utilized tomography, time term method and head wave traveltime inversion based on ray tracing techniques to model the upper crustal velocity and crystalline basement structure of the Anqing-Maanshan segment beneath the Yangtze fault zone. The profile along the Yangtze River consists of 100 PDS-2 seismometers with a spacing of 2km. We applied the linear and phase weighted stack methods to improve the signal-to-noise ratio of the weak seismic phases from the airgun source. According to the comparison between the linear and phase weighted stack results, the phase weighted stack method significantly improves the quality of the stacked data. We applied the band-pass filter to the stacked data to improve the onset of the first arrival, then picked up the seismic phases and assessed the errors of the picked traveltime. The comprehensive results reveal that the upper crust velocity structure and crystalline basement images show a tectonic feature of alternating rifts and uplifts. The upper crust of the Huaining Basin is the thickest area along the Yangtze River. The basement of the Huaining Basin is around 4.5km and there are Mesozoic lacustrine sedimentary layers whose thickness is about 2km. The crystalline basement depth of the Luzong Basin is 4.1km and the consolidated basin shows clear depression basin shape. This feature of the Luzong Basin reveals that it experienced extensional depression. There is a high-velocity zone beneath the crystalline basement of the Luzong Basin, and the velocity is higher than other areas along the Yangtze River. This high velocity zone shows an arc shape, which agrees with the Paleozoic reflection images by the seismic reflection survey. The profile crosses the Yangtze River in Tongling area and there are obvious velocity differences between the two sides of the Yangtze River. The velocity differences show that the Yangtze faults cut the crystalline basement in Tongling. The upper crust velocity structure of the Tongling area shows clear uplift features and its crystalline basement depth is about 2.2km, which agrees with the arc-reflection structures of the upper crust from the seismic reflection data. This uplift image reveals that the upper crust of the Tongling area has experienced extrusion deformations. The consistency of the seismic reflection imaging results with the near surface geology demonstrates that the large volume air-gun source is applicable to land-based seismic survey.

Key words: Yangtze fault zone, large-volume airgun, upper crustal velocity structure, crystalline basement