攀西地区重力场特征及地壳密度结构

Gravity field characteristics and crust density structure in the Panxi region,China

攀西地区位于峨眉山大火成岩省中西部,构造和岩浆特征显著,地震活动强烈.通过对野外重力测量得到的云县—会东和普洱—七甸两条剖面的高精度重力观测数据进行处理和分析,构建了沿剖面的二维地壳密度结构,其中普洱—七甸剖面与孟连—马龙宽角地震剖面部分位置重合.同时结合区域重力异常特征及下地壳视密度填图结果,得到如下初步认识:红河断裂带是南北地震带南段地区重要的构造分界线,断裂带南北向密度结构和莫霍面分布形态存在较大差异,沿走向构造变化.云县—会东剖面上大姚—会东段下地壳底部存在密度较高的壳幔过渡层,结合研究区下地壳底部壳幔过渡层的密度分布特征,认为该过渡层不是攀西裂谷下的"裂谷垫",而是由岩浆底侵作用造成的.

The Panxi region is located in the mid-west of the Emeishan Large Igneous Province,with significant features of geological structures,magmatic activity and strong earthquakes.Many gravity studies have been conducted in this area.However,due to the limitations of gravity data and techniques,some understandings from gravity data interpretation are different from those by other methods.Therefore,it is necessary to do further gravity research in this area.Our gravity data were collected under the control of absolute gravity measurement,along the Yunxian-Huidong and Puer-Qidian profiles crossing the Panxi region between April and May in2011.Data acquisition was made along roads with two LCR-G gravimeters.GPS synchronized measurement was also conducted with the relative gravity survey,using static measurement method.We applied the following reductions to get the Bouguer gravity anomalies along the two profiles：（1）earth tide reduction,（2）Normal reduction,（3）height reduction,（4）topographic mass reduction and（5）terrain reduction.And then,we built the 2Dcrustal density structure along both two profiles via 2.5Dgravity modeling.We also analyzed the characteristics of the regional gravity anomalies in the Panxi region,and obtained the apparent density distribution of the lower crust beneath this region through anomaly separation and density mapping. There are obvious differences of Bouguer gravity anomaly characteristics on both sides of the Red River faults zone.In the Yunxian-Huidong profile density model,the average crust density on the western side of the Red River fault zone is lower than those on the eastern side.The faults at depth show a certain degree of Moho undulation,which is likely associated with the tectonic extension in the early Cenozoic.The average crust density of the Puer-Qidian profile between both sides of the Red River fault zone are almost the same,no significant change of the Moho discontinuity.The structural differences between north and south of the Red River fault zone may be related with the distribution of earthquakes. More earthquakes have occurred in the northwestern section,while few happened in the southeast with shallower hypocenters.Seismic profiles and new gravity data suggest that the high gravity anomalies in the eastern section of the Yunxian-Huidong profile is not caused by the Moho uplift.There is a high-density crust-mantle transition layer at the bottom of the lower crust.Previous research drew the similar conclusion by using seismic tomography,which thought that the Panxi region has a 20 km thickness mantle material additional layer at the bottom of the lower crust and the top of upper mantle.In order to further study the density distribution characteristics of the crust-mantle transition layer beneath the Panxi region,we obtained the apparent density distribution of the lower crust in this region through anomaly separation and density mapping. The distribution range of crust-mantle transition layer in the Panxi region is far beyond the scope of the Panxi rift,thus we infer that the transition layer is caused by the magmatic underplating rather than the rift cushion.There are large differences of density structure and Moho relief between the north and south sides of the Red River fault zone,where structure changes along the strike.A high-density crustmantle transition layer is present at the bottom of the lower crust in the profile of YunxianHuidong.Based on the understanding of the density distribution in the lower crust of this region,the transition layer is considered to be caused by the magmatic underplating rather than the rift cushion below the Panxi area.