Characteristics of flexural isostatic gravity anomalies in the Tibetan Plateau and its geological significances
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摘要:
自新生代印度板块的块持续碰撞与俯冲作用下，青藏高原经历了快速隆升与复杂的岩石圈改造过程，但高原现今的垂向动力学机制和地壳形变特征仍然存在争议。基于非均一有效弹性厚度的挠曲模型，利用地形和地球重力场模型数据，计算了青藏高原及邻区的挠曲均衡重力异常。结果显示，青藏高原的均衡重力异常在-120~90 mGal之间，高原中部为明显的正异常特征，边缘为显著的均衡负异常。极小值出现在青藏高原西北部及其相邻的帕米尔高原，极大值则出现在与之紧邻的喜马拉雅块体西北部。此外，在青藏高原北面和东面，塔里木盆地和四川盆地显示出大片的均衡正异常。这些特征说明青藏高原及邻区地壳现今处于非均衡的状态，在板块碰撞挤压作用下，老的块体地壳整体发生抬升，导致了均衡正异常特征；而年轻的造山区域，地壳形变主要表现为地表抬升与下地壳强烈增厚，形成了均衡负异常。在高原中部和北部，均衡调整方向与地壳垂向运动趋势相一致；但在高原南面（喜马拉雅块体）和东面（四川盆地），均衡调整方向与地表形变观测结果相反。这说明印度板块碰撞与俯冲仍然控制着青藏高原南部、东部及其相邻块体的地壳形变过程，然而在更北的区域，地壳正通过均衡调整恢复均衡状态。
Abstract:The Tibetan Plateau underwent rapid uplift and complex lithospheric modification during the Cenozoic under the continuing collision and subduction of the Indian Plate, but the present-day vertical dynamic motion and crustal deformation of the plateau remain controversial. This paper calculates the flexural isostatic gravity anomalies on the Tibetan Plateau and its neighboring blocks based on the flexural model with variable effective elastic thickness and using topographic data and Earth's gravity field model. The results show that the isostatic gravity anomalies on the Tibetan Plateau range from -120 to 90 mGal, with the central part characterized by distinct positive anomalies and the margins by significant negative anomalies. Minimal values occur in the northwest Tibetan Plateau and its adjacent Pamir Plateau, while maximal values occur in the northwest part of the Himalaya block. In addition, to the north and east of the Tibetan Plateau, the Tarim Basin and Sichuan Basin show large areas of positive isostatic anomalies. These features suggest that the crust of the Tibetan Plateau and its surroundings is now in a nonisostatic state, with the overall uplift of the older block under the effect of plate collision and compression, leading to positive isostatic anomalies. In younger orogenic regions, crustal deformation is primarily characterized by surface uplift and strong thickening of the lower crust, resulting in negative isostatic anomalies. In the central and northern parts of the plateau, the direction of isostatic adjustment is consistent with the trend of crustal vertical motion. However, south and east of the plateau (e.g., the Himalaya block and Sichuan Basin), the direction of isostatic adjustment is opposite to that observed for surface deformation. This suggests that the Indian Plate collision and subduction still control the crustal deformation processes in the southern and eastern parts of the Tibetan Plateau and its neighboring blocks. However, further north, the crust regains an isostatic state through isostatic adjustment.
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图 2 青藏高原重力异常及地形重力效应
a. 自由空间重力异常；b. 地形重力效应；c. 布格重力异常
Figure 2. Distribution of the gravity anomalies and terrain gravity effects over the Tibetan Plateau
图 3 青藏高原及邻区的有效弹性厚度分布
Figure 3. Variations in the effective elastic thicknessover the Tibetan Plateau
图 4 均衡补偿深度与其他莫霍面数据的对比
Figure 4. Comparison of isostatic compensation depth with other Moho models
表 1 本研究计算过程中需要的所有参数
Table 1. Values of constants and parameters assumed in calculations
参数 符号 数值 万有引力常数/(m3·kg-1·s-2) G 6.672 59×10-11 杨氏模量/GPa E 100 泊松比 υ 0.25 平均地幔密度/(kg·m-3) ρm 3 270 平均地壳密度/(kg·m-3) ρc 2 670 空气或海水密度/(kg·m-3) ρf 0, 1 030 重力加速度/(m·s-2) g 9.81 
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