利用接收函数研究四川威远地区地壳结构特征

Crustal structure characteristics beneath the Weiyuan area,Sichuan revealed by receiver functions

利用在威远地区布设的40台宽频带流动地震仪记录的远震波形数据,提取了径向P波接收函数.接收函数中可以看到清晰的Moho面转换波,并且在此震相之前存在一组正负相间的震相.通过合成测试,判断负震相C2为沉积层基底的PpSs+PsPs震相.通过拾取C2震相的到时,与建模合成的接收函数进行对比,得到了研究区沉积层厚度的分布.正震相C1可能是沉积层基底PpPs震相或康拉德界面转换波.在这两种情况下,C1震相的到时随射线参数的变化趋势相反,前者表现为负斜率,后者表现为正斜率.利用这一特征,对各台站C1震相的成因进行了分析,威远背斜之外主要为正斜率,推断存在Conrad界面.然后通过H-κ叠加估计了研究区地壳厚度和平均波速比,并使用共转换点(CCP)叠加方法获得了研究区Moho面和壳内界面的起伏形态.结果显示,研究区沉积层厚度在3.0~7.0 km之间,威远背斜较薄,自贡台凹较厚;研究区地壳厚度40.9~49.2 km,平均约44.1 km,整体上表现出背斜区域厚,向斜等区域薄的特征.在背斜之外的区域,Conrad界面埋深大约为19 km.本文推断威远背斜上、下地壳在挤压环境下均发生增厚,上地壳顶部沉积盖层在隆升过程中遭受剥蚀,而下地壳得到了相对完整的保存.研究区波速比横向变化较大,在1.71~1.92之间,平均约1.79,与全球大陆平均值相同.研究区存在两个高波速比区(>1.85),威远背斜东段与李家场褶皱交汇处可能存在小规模的地幔物质上涌;而自贡台凹在受到地壳顶部较厚沉积盖层影响的同时,中下地壳可能受到了川滇块体壳内热物质的影响,使地壳平均波速比明显增高.威远背斜核部和自流井背斜东南区域地壳平均波速比相对较低,其地壳成分可能主要为长英质或中性岩石.

The Weiyuan area is located in the southwest of the Sichuan basin,in which the Weiyuan anticline is the main surface tectonic unit.Based on the teleseismic waveform data recorded by 40 three-component broadband stations in the Weiyuan area,the P-wave receiver functions(RFs)are extracted.Clearly Moho Ps phase can be found in RFs,and positive phase C1 and negative phase C2 also exit in 1~4 s.After the synthetic testing,we believe that the C2 phase indicates the sedimentary PpSs+PsPs phase.Firstly,we pick up the C2 phases and compare them with the modeling synthesis RFs to obtain the sedimentary thickness of each station.And C1 phase may indicate the sedimentary PpPs phase or the Ps phase of the Conrad interface.For the former phase,the arrival time of C1 decreases with increasing ray parameters,which means this phase shows a negative slope.However,the Conrad Ps phase shows a positive slope.Based on this character,we calculate the slope of the C1 phase at each station and infer the existence of the Conrad interface in areas outside the Weiyuan anticline because of its positive slope.Then the crustal thickness and the VP/VS ratio are estimated by H-κstacking.The Common Conversion Point(CCP)stacking method is used to reveal the Moho and Conrad interfaces.The results show that the sedimentary thickness ranges from 3.0 to 7.0 km,and the sedimentary layer of the Weiyuan Anticline is thin,while the sedimentary layer of the Zigong sag is thick.The crustal thickness in the study area is about 40.9~49.2 km,with an average of 44.1 km.The crustal thickness is characterized by thick beneath the anticline core and thin beneath the syncline.The Conrad interface depth is about 19 km outside the Weiyuan anticline.According to the depth of the sedimentary basement,the Moho,and the Conrad interface,we speculate that the uplift of the Weiyuan anticline is caused by the thickening of both the upper and lower crust under the compressional environment.However after the uplift,the top of the upper crust has suffered erosion,and the lower crust has been preserved relatively intact.The VP/VS ratio varies from 1.71 to 1.92 with an average of 1.79 in the study area,which is the same as the average value of the global continent.There are two high-value regions in the study area.There may be a small-scale upwelling of mantle material at the intersection of the eastern Weiyuan anticline and the Lijiachang fold.While the Zigong sag is affected by the thick sedimentary at the top of the upper crust,and meanwhile,the middle-lower crust may be affected by the high-temperature material originating from the Sichuan-Yunnan block.The crustal composition of the Weiyuan anticline and southeast artesian anticline is mainly felsic or intermediate rocks,because of the lower VP/VS ratio.