East-West Asymmetry of the Deep Lithosphere beneath Northern Tibet Revealed by a 3D Gravity Inversion

The Tibetan Plateau, produced by collision between the Indian and Asian plates in Cenozoic, shows an asymmetric surface topography in the east-west direction, but the mechanism of lithospheric deformation controlling the uplift of plateau is still unclear. This study proposes a new lithospheric density model obtained in a 3D gravity inversion constrained by a tomography model converted to density variations. The results show that there is a significant change in the density structure of the upper mantle in the east-west direction beneath the vicinity of the Jinsha suture (34–35°N). Taking 89°E as the dividing line, the western region shows the obvious low-density anomaly, which may be caused by removal of lower lithosphere and upwelling of asthenosphere material. In contrast, the eastern part shows a high-density anomaly dipping to the south, which may reflect the subduction of the Asian lithospheric mantle beneath Tibet. In summary, the deformation of the deep lithosphere beneath northern Tibet is asymmetric in the east-west direction, in which lithospheric root loss and intracontinental subduction may play important roles in the west and east, respectively.

青藏高原由新生代时期印度板块与亚洲板块碰撞形成，其地表地形在东西向呈现不对称分布，但控制高原隆升的岩石圈变形机制仍未明确。本研究构建了一种全新的岩石圈密度模型，该模型通过以经密度变化转换的层析成像模型为约束开展三维重力反演得到。研究结果显示，在金沙江缝合带（北纬34°~35°）附近下方，上地幔的密度结构沿东西向存在显著差异。以东经89°为界，西侧区域呈现显著的低密度异常，该异常可能由岩石圈下部拆沉与软流圈物质上涌所致；与之相对，东侧区域则呈现向南倾伏的高密度异常，这或反映了亚洲岩石圈地幔向青藏高原下方俯冲的过程。综上，青藏高原北部下方的深部岩石圈变形沿东西向呈不对称分布，其中岩石圈根柱消减与陆内俯冲分别在西侧与东侧发挥了关键作用。