一种基于温度历史和瞬态地温场模拟的剥露历史反演方法

根据低温热年代学数据,提取岩石从深部剥露到地表的信息,对理解诸多地质问题(如造山带演化、地表过程及其相互作用等)具有重要意义.本文提出一种基于岩石温度历史(可利用古温标、热年代计等方法制约),并考虑剥露过程对地温场扰动的剥露历史反演计算方法.基于假定的与真实数据的正反演模拟和参数敏感性分析表明:热扩散率的变化对剥蚀量计算影响不大,在常规岩石热扩散率变化范围内(20～35km2/Ma),总剥蚀量变化小于10%;传统计算方法低估了剥蚀总量,对于现今地温梯度小于20℃/km、冷却速率大于2～3℃/Ma,或现今地温梯度大于30℃/km、冷却速率大于5～10℃/Ma的地区,需要考虑热平流对剥蚀量计算的影响;匀速冷却的热历史指示剥蚀速率持续减小,而非匀速剥蚀.本文将该方法应用到龙门山南段和四川盆地,反演计算显示龙门山南段15Ma以来的剥蚀总量为8km,四川盆地中部80Ma以来剥蚀总量约为3km、东部约5km.

An inversion method of denudation history based on temperature history and exhumation-induced transient thermal advection

The denudation history is of great significance for understanding many geological problems, such as orogen evolution, surface processes and their interactions. This contribution describes a new inversion method, which considers the thermal advection associated with rock exhumation, to solve rock exhumation history from temperature histories constrained by other methods (such as paleo-temperature index and thermochronology methods). A combination of forward and inversion modeling, parameter sensitivity analysis of real and synthetic data suggests that (1) variability of thermal diffusivity of most rocks (20~35 km²/Ma) has a limited effect (<10%) on the calculation of total erosion. (2) Conventional methods (total denudation=cooling/modern thermal gradient) underestimates the total denudation, because of the increase of thermal gradient during exhumation. If using 10~15% of the total as an acceptable uncertainty for estimating denudation, our study suggests that thermal advection has to be considered for regions where the current thermal gradient is less than 20 ℃/km and the cooling rate is greater than 2~3 ℃/Ma and where the current thermal gradient is greater than 30 ℃/km and the cooling rate is greater than 5~10 ℃/Ma. (3) Different from conventional understandings, a thermal history with constant cooling rate in fact results from decreased rates of exhumation. Finally, the new method was applied to quantify rock exhumation histories of the southern Longmen Shan and the Sichuan Basin. The results indicate the total exhumation is ~8 km since 15 Ma in the southern Longmen Shan, and ~3 km and ~5 km since 80 Ma in the central and eastern part of the Sichuan Basin.