重力及重力梯度联合反演青藏高原及邻区岩石圈三维密度结构

Lithospheric 3-D density structure beneath the Tibetan plateau and adjacent areas derived from joint inversion of satellite gravity and gravity-gradient data

本文利用GOCEL2观测重力梯度的五个独立分量(T_(xx),T_(zz),T_(xy),T_(xz),T_(yz)),联合EGM2008地球重力场模型计算垂直重力,反演计算了青藏高原及邻区0~120 km深度岩石圈三维密度结构.将经过低阶项改正、地形效应改正、沉积层界面起伏效应改正得到的剩余重力及重力梯度异常值作为观测值,以改正剩余量归一化权重作为观测权重,基于Tikhonov正则化理论建立反演目标函数.反演过程中,利用地震层析S波速度转换密度作为初始约束,通过非等权最小二乘迭代法计算得到最终反演密度.反演结果表明:(1)40 km深度,青藏高原内部为中地壳,表现为低密度,邻区为中下地壳,表现为高密度.青藏高原内部中地壳强低密度层主要分布在高原边界.其成因是印度板块俯冲和周围坚硬块体阻挡作用导致在高原边界形成的高应变积累闭锁区,为壳内低密度软弱物质的形成提供了条件.(2)80 km深度,青藏高原上地幔顶部显示出低密度的特征.高原内部东、中、西密度特征差异明显,低密度以95°E为中心线呈东西对称分布.以班公一怒江缝合带为中心,在拉萨块体和羌塘块体内从北向南出现了"低-高-低"的密度分布起伏特征.该特征与GRACE得到的莫霍面起伏特征一致,结合大地构造结果,这种起伏特征验证了印度、羌塘块体从南北两侧分别向喜马拉雅、拉萨地块挤入的双向俯冲模式.(3)四川盆地和鄂尔多斯盆地内,地壳高密度异常较地震波速异常明显偏低,表明古老的四川盆地和鄂尔多斯盆地比想象中更冷、更坚硬.塔里木盆地和柴达木盆地内壳、幔高密度的结构特征,对应地幔物质上涌.

A three-dimensional density model of the crust and uppermost mantle beneath the Tibetan plateau and adjacent areas is derived from joint inversion of satellite gravity and gravity-gradient data. In this work, we choose five independent gravity gradients （Txx, Tzz, Txy, Txz, Tyz） of GOCE mission L2 gradient products combined with vertical gravity （Gz） of EGM2008 to perform density inversion. The objective function is given based on the Tikhonov regularization theory. The signal of topographic masses, the effect of density interfaces and the influence of density changes below the uppermost mantle are reduced. Using standard deviation ratios of the residual anomalies, we calculate weights for different components. Seismic S-wave velocities provide initial constraints on the inversion based on a relationship between density and S-wave velocity. The damped least square method is used in the inversion. Final density results suggest some new insights to understanding the underlying geodynamic processes as follows： （1）The lowest density layers are distributed mainly on the boundary of the plateau. This phenomenon supports the existing of the high strain accumulation and locking regions caused by the combined effects of the subduction of the Indian plate and the obstruction of the surrounding rigid blocks along the borders of the plateau. （2）At the depth of 80 km, the top of upper mantle inside the Tibetan plateau shows low density. Significantly different density characteristics are present between eastern, central and western parts of the plateau. Low densities are symmetrically distributed on either side of the central line of 95°E. Taking the Bangong-Nujiang suture zone as the center, ＂low-high-low＂ density characteristize the Lhasa and Qiangtang blocks from north to south. The density fluctuation is consistent with the undulation of the Moho derived from the satellite GRACE. According to tectonics, such an undulation also supports the bidirectional subduction model that the India plate and Qiangtang block underthrust to the Himalaya and Lhasa blocks, respectively.（3）Density anomalies are significantly lower than velocity anomalies in the Sichuan basin and the Ordos basin, implying they are colder and harder than originally thought. The high densities of the crust and upper mantle in the Qaidam and Tarim basins correspond to the upwelling of heavy materials from the upper mantle.