关键词: 腾冲火山, 地壳各向异性, 地壳厚度, 泊松比, 岩浆囊

Abstract:

Tengchong volcanoes are not extinct but a group of dormant volcanoes with magma underground. The Tengchong volcanic area is a unique geological condition integrating magma activity, earthquakes, and hot springs. Crustal thickness and Poisson’s ratio are two important parameters that characterize crustal structure and material composition and are crucial for accurately detecting the location and scale of magma chambers in the Tengchong volcanic area. However, previous studies on obtaining crustal thickness and Poisson’s ratio in the Tengchong volcanic area only used nine volcanic network stations, which had insufficient resolution and could not effectively constrain the position of magma chambers. The traditional H-κ method is adopted to an isotropic crust with a flat Moho. The crust maybe anisotropic and the Moho is dipping. In the presence of a complex crustal structure with azimuthal anisotropy or dipping Moho, the H-κ results may be biased. So, we extracted 4 268 P receiver functions from teleseismic wave data recorded at 23 digital seismic stations. A H-κ-c method with harmonic corrections is used to obtain crustal thickness and Poisson’s ratio in the Tengchong volcano area. Before the harmonic corrections to the P receiver functions, we perform the incident moveout corrections and back azimuthal binning of 5°. The H-κ-c method can correct the influence of crustal anisotropy and dipping interfaces on receiver functions by harmonic transformation, can acquire more stable and reliable crustal thickness and wave velocity ratios, and can obtain information on the inclination of the Moho and crustal azimuthal anisotropy. Based on previous research, we discussed the crustal deformation mechanism of the Tengchong block and revealed the corresponding relationship between crustal structure, heat flow, earthquakes, and magmatism.

Results show the fast-wave polarization directions with a dominant NW-SE orientation in the north and change to a dominant NE-SW orientation in the south, and delay times varying between 0.06 and 0.80s, with a mean of 0.40s. It is consistent with the Tengchong block undergoing clockwise rotation around the EHS. There is an inclined Moho surface and strong azimuthal anisotropy in the intersection of the Tengchong Fault, Yingjiang Fault, and Longchuan Fault. The strong azimuthal anisotropy in the Tengchong block maybe related to the strong influence of the upwelling of the deep thermal material from the upper mantle. The fast wave polarization direction parallel to the Longling-Ruili fault indicates that the observed anisotropy may be related to the fracture of the fault. The crustal thickness ranges from 32 to 39km, and the Poisson’s ratio ranges from 0.235 to 0.326. There exist three Moho-uplifting centers, one in Gudong-Qvshi-Mazhan-Tengchong, the other in Qingshui-Xinhua, another in Zhen’an-Longxin-Xiangda. The very high Poisson’s ratio(σ>0.3) is consistently located within these three locations. We speculated that the Moho-uplifting and higher Poisson’s ratio at the three sites denote the existence of three magma chambers. The horizontal scale of the three magma chambers is respectively 20km×35km, 20km×20km, 25km×25km, and separately controlled by the Tengchong Fault and Longchuanjiang Fault, Tengchong Fault and Longchuan Fault, Nujiang Fault and Longling-Ruili Fault. The locations of the magma chambers are different from that obtained by the same receiver function method, which the different seismic stations and stacking methods may cause. The locations of the magma chambers are not exactly the same as those of the geothermal anomaly areas and the mantle-derived volatile release anomaly areas measured by the surface hot springs. The reason for this difference may be the ground temperature and the mantle-derived volatile component, which are the measurement results of the surface hot spring. The Poisson’s ratio value we calculated is the average value of the entire crust. Under the four Holocene volcanic craters of HeikongShan, Dayingshan, Laoguipo, and Ma’anshan, there is an interconnected magma chamber, the most vigorous volcanic activity since the Holocene. There are almost no earthquakes occurring in the crust at the center of the Moho uplift; most earthquakes are distributed in the crust around the Moho-uplifting centers. This may be because the hot magma heated the crust, resulted in the rocks in the crust being plastic, and it is difficult to accumulate large strains. Our results have important reference value and guiding significance for earthquake and volcanic activity monitoring, earthquake prevention and disaster reduction in the Tengchong volcanic area.

Key words: Tengchong volcano, crustal azimuthal anisotropy, crustal thickness, Poisson’s ratio, magma chambers