关键词: 青藏高原东南缘, 小波多尺度分析, 地壳构造, 物质运动, 孕震环境

Abstract:

Since the Cenozoic, the Qinghai-Tibet plateau is uplifting sharply due to the India-Eurasian violent collision. Its crust has thickened accompanied with north-south shortening and east-west extrusion. The eastward-moving material beneath the southeastern margin of Qinghai-Tibet plateau is stopped by hard blocks such as Alax, Ordos and South China block. Under the interaction, a complex structural belt of north-south direction is formed, called the North-south structural belt. And it is also known as the“North-South Seismic Belt” because of dense distribution and high intensity of earthquakes. Therefore, studies of the crustal structure and mass movement characteristics have scientific significance to reveal the mechanism of tectonic activity and earthquake incubation beneath the southeastern Qinghai-Tibet plateau. It will help improve capabilities of regional earthquake preparedness and disaster mitigation.
Firstly, the simulation test of simple geological body is carried out to verify the effectiveness of wavelet multi-scale analysis method in gravity field separation and the stability of the application program. When designing a simple geological body model, both the superposition effect of geological bodies at different depths and the decomposition effect of the test model are considered. The approximate field source depth of the separated regional field and local field is calculated by radial logarithmic power spectrum, and the calculation results are consistent with the design depth. It is determined that the wavelet base is “bior3.5” and the decomposition order is 5. The test results show that the “bior3.5” wavelet basis can effectively separate the regional and local anomalies in the geological body of the combined model, and obtain an ideal separation effect.
Then, based on the global gravity field model WGM2012 data, the Bouguer gravity anomaly data in the southeastern Qinghai-Tibetan plateau is decomposed in the 5 orders by using multi-scale wavelet analysis, and the radial logarithmic power spectrum is used to analyze the decomposition results, and a subset of the Bouguer gravity anomaly at different depths of this area is obtained. Based on this, this paper discusses the regional crustal structure, mass movement and seismogenic environment. The results show that the small-scale gravity anomaly of the 2^nd and 3^rd order mainly reflects the deformation information of the middle and upper crust, and the approximate field source depth of spectrum estimation is 3.5km and 12.6km. The second-order wavelet details are mainly distributed in strips with positive and negative alternations, while the third-order wavelet details are mainly displayed in tongue shape and trap shape. Compared with the second-order wavelet, the scale of the third-order details is larger and the range is clearer, but the location areas of the positive and negative anomaly distribution of the two are basically the same. The small-scale gravity anomaly indicates that the strong earthquakes mainly occur in the high gravity gradient zone and the boundary of the active block in this area. A comparative analysis of gravity anomalies at various scales reveals that seismogenic environment is not only controlled by the structure of the upper and middle crustal fault blocks but also related to the changes in the density of the lower crust. The lower crust at the epicenter appears as a low anomaly zone. The low-density, low-velocity, and plasticity of the deep medium conditions of the lower crust may have caused the stress of the lower crust in this area not to accumulate and“escape” to the upper crust easily to trigger strong earthquakes. This dynamic process of interaction between shallow and deep crust may be an important condition for earthquakes in the study area. The variation of Bouguer gravity anomaly in the second-order and third-order detail maps is consistent with the geological structure observed on the surface, making this area one of the areas with the most intense Meso-Cenozoic crustal deformation and seismicity; the meso-scale gravity anomaly of the 4^th order mainly reflects the deformation information of the middle and lower crust, and the approximate field source depth of spectrum estimation is 26.2km, showing the existence of a low Bouguer gravity anomaly trap in the Songpan-Garze block. It is consistent with the observation results that there is a thicker low-velocity and low-resistance layer in the crust of Bayan Har block. It may be related to the large thickness of the lithosphere, the higher temperature of the lower crust and the melting of parts of the middle and lower crust at high temperatures. In the Panzhihua area, there is a high gravity anomaly trap, which may be caused by the mass residues in the middle and lower crust by the deep high-density material ascent during the Panxi ancient rift period. As one of the mass eastward migration channels of the Qinghai-Tibet Plateau, part of the materials in the Sanjiang area(Nujiang River, Lancang River and Jinsha River)gushes upward along the fault zone and accumulates in the middle and lower crust, resulting in a low Bouguer gravity anomaly area in the middle and lower crust of the area; the large-scale gravity anomaly of the 5^th order mainly reflects the deformation information of the lower crust, and the approximate field source depth of spectrum estimation is 48.8km. It embodies the characteristics of rigid block. Myanmar microplate passes through western Yunnan in the direction of SEE, and the block shows still a high positive gravity anomaly, reflecting that these blocks have high density and strong rigidity and are not likely to fracture when the Indian plate pushes Eurasia northward, which plays a key role in the formation of the eastern Himalayan tectonic syntax. Large-scale gravity anomaly shows a regional negative anomaly in the Chuan-Dian block, which provides indirect evidence about the existence of “lower crustal flow” in the southeastern Qinghai-Tibet Plateau. Clamped by the Sichuan Basin and the Yunnan-Burma block, the flowing direction and accumulation of low-density mass can be clearly revealed. The low-density mass in the lower crust flows in both directions from north or south. A small part of the mass flows to the north through the Xianshuihe fault zone. Most of the mass flows to the south and is blocked by the southern Yunnan block. One flow goes to Panzhihua-Pu’er. One branch flows in the direction of Dongchuan-Qujing, and has a tendency to flow in the direction of Anshun-Guiyang, causing low-density mass to accumulate in the Lijiang-Daocheng-Panzhihua-Dongchuan-Kunming area.

Key words: southeastern Qinghai-Tibet Plateau, multi-scale wavelet analysis, crustal structure, mass movement, seismogenic environment