龙门山晚新生代均衡反弹隆升的定量研究

Quantitative Research on Longmen Shan Uplift Caused by Late Cenozoic Isostatic Rebound

龙门山位于青藏高原东缘与四川盆地的交接部位,是青藏高原周边山脉中地形梯度变化最大的山脉,其隆升过程和机制一直是国际地学界关注的焦点。晚新生代经过大量的滑坡、泥石流等快速剥蚀作用,龙门山的高程却不断升高。讨论了龙门山构造隆升的3种地球动力学机制,即下地壳通道流机制、地壳挤压缩短变形机制、地壳均衡反弹机制。晚新生代龙门山的隆升与剥蚀引起的均衡反弹作用相关,剥蚀作用使得地壳岩石逐步被移去,剥蚀区重力损失,岩石圈或地壳卸载作用导致山脉顶峰的隆升。结合数字高程模型数据研究表明,巨大地震的长期同震构造变形以及滑坡、泥石流等引起的快速剥蚀所导致的地壳均衡反弹,可能是龙门山晚新生代构造隆升的地球动力学新机制。龙门山地区现今高程受构造作用与剥蚀引起的均衡反弹作用的共同影响,其中剥蚀引起的均衡反弹作用对龙门山隆升的影响贡献率约占30%。

Longmen Shan is located at the boundary between the Sichuan Basin and the eastern margin of Tibetan Plateau, representing the steepest gradient of any edges of the plateau. Its uplifting process and mechanism are important scientific issues studied by international geologists. Especially after the large amount of denudation such as landslides and debris flows since Late Cenozoic, Longmen Shan did not reduce the elevation, but continually uplifted. This article discussed three popular geodynamic mechanisms about Longmen Shan tectonic uplift in Late Cenozoic, i.e. channel flow in the lower crust, crustal shortening deformation and crustal isostatic rebound. The uplift of Longmen Shan in Late Cenozoic is related to isostatic rebound associated with erosion. The erosion denuded crustal rocks gradually, and the space occupied by the original erosional material was replaced by air, which led the lithosphere or crust to produce negative load and caused uplift of the mountain. Combining with digital elevation model data, this research revealed that the coseismic deformation caused by repeated large seis- mic events and isostatic rebound induced by rapid surface erosion, such as earthquake, landslide and debris flow, might be a new geodynamic mechanism to drive ongoing mountain building of Longmen Shan. The elevation of Longmen Shan nowadays may be controlled by tectonic and erosion-induced isostatic rebound effects, and the contribution of erosion-induced isostatic rebound on the uplift of Longmen Shan is about 30%.