腾冲火山区地壳详细结构的地球物理探测研究

We plan to carry out a high-resolution geophysical research in the Tengchong Holocene volcanic area, southwest China. According to the research plan, this project includes following works. (1) A seismic observation along a 30km long profile between Tenghong and Gudong, in which 15-20 broadband seismic instruments will be installed with average station spacing of 1000-1500m. These instruments will be continous to operate at least for one year. The receiver function technique will be applied to invert the S-wave velocity structure beneath these stations. (2) A magnetotelluric survey along the Tengchong-Gudong seismic observation profile with average sample spacing of 500-1000m. The obtained data will be used to invert geoelectric structure of the crust and uppermost mantle. (3) A three-dimentional magnetotelluric array around the Maanshan or Dayingshan volcanic crater. In this array, we plan to use a dense sample spacing of 200m to obtain high-resolution geoelectric images. Based on above results, firstly, we are going to determine tectonic features of the volcanic area between Tengchong and Gudong by analyzing seismic and geoelectric structures along the profile; we also try to find deep conditions of magma generation, storage, transport, and their realations with surface volcanic craters and fault systems. Next, we are going to search evidence of magma chambers and heat flow channels beneath the Maanshan or Dayingshan crater and to determine their solidification level and potential activity. The aim is to establish a reasonable volcanic crustal model.

在腾冲北部全新世火山较为集中的地区实施高分辨地球物理探测，按照拟定的研究计划分阶段开展以下工作：（1）在腾冲－固东之间布设长约30km的天然地震观测剖面，以1000－1500m的台站间距设置15-20台宽频带地震仪，进行为期1年的连续记录，利用接收函数方法反演台站下方的S波速度结构和泊松比。（2）沿腾冲－固东天然地震观测剖面进行500－1000m点距的大地电磁测深，利用电磁场数据反演地壳和上地幔顶部的电阻率结构。（3）在马鞍山或打鹰山布设密集点距（200m）的大地电磁观测台阵，获取高分辨的三维电性结构图像。在完成上述工作的基础上，综合地震波速度和电阻率结构的变化分析腾冲－固东之间串珠状火山分布的区域构造特征，判明壳内岩浆生成、存储和运移的深层介质条件，建立与地表火山分布和断裂通道的内在联系；寻找马鞍山或打鹰山下方岩浆囊和热流通道的痕迹，确认固结程度和潜在的活动性，建立合理的火山地壳模型。

腾冲是青藏高原东南缘的第四纪火山活动区域，全新世以来的火山主要集中在腾冲盆地的中央，由北向南形成一个串珠状的火山链。为了深入研究火山区的深部结构和岩浆活动特征，我们在腾冲盆地开展了宽频带地震观测和大地电磁测深。研究结果表明，腾冲北部火山区的壳内热流活动主要分布在三个区域，其中打鹰山，大－小空山和黑空山共享一个相互联通的岩浆囊，它的深度为6-15km，南北方向宽约16km；火山湖下方有一个相对独立或收缩的岩浆囊，它的深度为9-16 km, 南北方向小于8km；老龟坡与毗邻的马鞍山可能共有一个岩浆存储系统，也许与腾冲南部热海地热区的岩浆活动有关，并受到大盈江断裂与腾冲火山断裂相互交汇的影响。壳内的热流活动以及幔源物质的侵入是火山区地壳平均波速比和泊松比偏高的主要原因，地幔上涌对火山区莫霍面的局部抬升产生了一定影响。部分台站的壳幔边界具有过渡带性质并呈开放状，热流有可能沿此通道从上地幔进入下地壳。.上述研究通过开展密集点距的地球物理观测，清晰地展示了腾冲北部火山区地壳详细结构，确定了岩浆囊的数量、深度范围和分布特征，建立了壳内热流活动与地表火山分布的隶属关系，为完善腾冲火山区的热流模型和成因机制提供了新的依据。