川西重力异常、地壳均衡及其构造学意义

Western Sichuan Province is one of the most famous seismic areas of China. Revealing its main tectonic characteristics and formation mechanism has important scientific significance and disaster reduction value. Our previous work verified the feasibility of constraining the tectonic characteristics of a fault zone by isostasy distributions, and activated the interest in studying the décollement structure beneath the Chengdu plain. In this project we will explore more deeply the gravity anomaly and the isostasy distribution at western Sichuan, and reveal their tectonic implications. Specifically, we will do the following things: 1) Conduct a gravity/GPS observation at middle and southern sections of Longmen Shan fault zone, where the isostasy anomalies are relatively bigger. 2) Based on all those existing gravity/GPS data at western Sichuan Province that observed after the Wenchuan Ms8.0 earthquake and the data we get during above observation, build a precise gravity anomaly field and inverse a crust density model for the study area. 3) Considering the layered density structures of the crust, as well as its lateral changes, calculate the isostasy of the study area, and use it as a constraint to study the main features of the Longquan Shan fault zone. 4) Analyze the effective elastic thickness and the loading mechanism of the lithosphere of the foreland of the Longmen Shan through gravitational admittance method, and then study the mechanism of the uplift of Longmen Shan. This project is expected to update the gravity anomaly field at western Sichuan, obtain a precise distribution of isostasy, and give an answer for the question whether there is a décollement structure beneath the Chengdu plain or not. The research results have important significance for clarifying the eastward evolution of Tibetan Plateau and judging the strong earthquake risk in western Sichuan area.

川西是我国强震多发地之一，揭示其主要构造特征和形成演化规律具有重要的科学意义和减灾价值。项目组的前期工作证明了用地壳均衡分布形态确定断裂带构造特征的可行性，激起了人们对成都平原底部滑脱面的研究兴趣。本项目拟在前期工作的基础上，以川西地区重力异常、地壳均衡及其构造学意义为研究对象，以汶川地震以来的流动重力/GPS 联合观测数据为基础，在龙门山中南段地壳均衡异常较大地区展开补充观测，通过统一平差处理与各类重力改正，构建精密重力异常场，反演地壳密度结构；在考虑地壳密度分层与横向变化的基础上计算区域地壳均衡，分析龙泉山断裂带构造特征；通过重力导纳分析龙门山前陆盆地岩石圈有效弹性厚度与加载机制，研究成都平原底部滑脱面是否存在。项目预期将优化川西地区重力异常场，获取区域地壳均衡精细分布，阐明成都平原底部是否存在滑脱面。项目研究成果对揭示青藏高原东进演化过程、判断川西地区强震危险性具有重要意义。

本项目以川西地区重力异常、地壳均衡及其构造学意义为研究对象，以汶川地震以来的重力/GNSS 联测数据为基础，围绕龙门山断裂带和龙泉山断裂带展开研究，获得如下主要研究成果。（1）基于重力/GNSS联测数据，提出了一种计算岩石圈垂向构造应力的新方法，丰富了岩石圈均衡的表达形式，并依据均衡调整原理使该算法体系化。（2）围绕龙门山断裂带展开重力/GNSS加密观测，结合前期观测结果，通过统一平差处理与重力改正，提高了区域重力异常场和地壳均衡异常场的精度与空间分辨率，并在此基础上反演了三维地壳密度结构，计算了岩石圈垂向构造应力场，给出了川西地区垂向构造应力场与地震分布之间的关系。统计结果表明，地震多发生在岩石圈垂向构造应力高梯度带附近，以及垂向构造应力高值区。（3）基于最小二乘配置方法与“消去-恢复”原理，融合了地表重力观测数据与EGM2008重力模型数据，更新了研究区域自由空气与布格重力异常场。基于融合后重力观测数据，利用重力导纳分析方法，研究了龙门山及周边地区的岩石圈挠曲特性和隆升机制，结果表明，龙门山及其以西高原地区的岩石圈有效弹性厚度为6km，以东盆地地区的岩石圈有效弹性厚度大于30km。相应的加载比结果表明，龙门山地区的岩石圈挠曲完全来自地表加载，下地壳流对龙门山隆升无贡献。（4）基于重力均衡、GNSS地表应变场、震源机制解以及深部探测等多源观测数据，综合分析了龙泉山断裂带总体构造特征，发现龙泉山断裂带是一条深埋的高角度断裂带，现今呈现张性运动特征，其底部延伸至Moho面以下，其顶端分为东西两支，且成都平原底部并不否存在具有明显相对运动特性的滑脱面。总之，本项目优化了川西地区重力异常场，获取了区域地壳均衡精细分布，给出了龙泉山断裂带构造新模型，相关研究结果对阐明青藏高原的东进演化过程、揭示川西地区强震危险性具有重要意义。