四川盆地深部地壳结构——深地震反射剖面探测

Deep crustal structure in Sichuan basin:Deep seismic reflection profiling

四川盆地位于扬子地块的西北部,被褶皱构造带所围绕,受周缘构造带的侧向挤压作用,盆地卷入了多期次和多边界的构造变形,为开展盆山耦合作用及多边界、多期次构造叠加与复合关系的研究提供了不可多得的理想野外实验室.为揭示四川盆地地壳结构,本文通过对3条不同时间采集的深地震反射剖面数据进行拼接联线处理,获得跨越四川盆地的330 km深地震反射偏移成果剖面,揭示了四川盆地地壳上地幔细结构:沉积层从西北向东南逐渐变薄,在龙门山前沉积层厚度超过15 km,在华蓥山下沉积层减薄到~8 km,且褶皱变形形成华蓥山薄皮褶皱冲断带;莫霍面出现在13~15 s(双程走时),埋深约40~45 km;并发现从下地壳延伸至地幔的东南向的倾斜反射,从13 s向下延伸至18 s,结合四川盆地及其周边地区其他地球物理和地球化学花岗岩同位素年龄等资料,我们认为这些倾斜反射层是扬子克拉通地台西北缘发生的新元古代俯冲的遗迹.

Sichuan basin is located in northwestern Yangtze block and surrounded by adjacent fold belts. The basin subjected to lateral compression by the peripheral tectonic belts, and wasinvolved in structure deformation of multi-stage and multi-boundary. Therefore, the basin provides an ideal area for studying the basin-range coupling, tectonic superposition and complex relationship of multi-border. In order to reveal the crustal structure of the Sichuan basin, we completed data splicing and processing of 3 deep seismic reflection profiles collected at difference times, and obtained the migration section of 330 km long. The section displayed the fine whole crust structure of the basin. The sedimentary layers gradually thinned from northwest to southeast. These sedimentary layers extend to depths in excess of 15 km beneath the front of the Longmenshan. The layers extending to depths of about 8 km beneath the Huayingshan are tightly folded to form the Huayingshan thin-skin fold and thrust belt. The Moho was found about 13~ 15 s （TWT） with the depth of about 40~45 km. We also found that there is a set of southeastern dipping reflectors extending from the lower crust to the upper mantle between 13 s and 18 s （TWT）. Combined with other geophysical and geochemical isotopic ages in the Sichuan basin and its surrounding areas, we propose that these newly revealed reflectors are the remnants of Neo- Proterozoic subduction that occurred along the NW margin of the Yangtze Craton.