阿尔泰山南缘白垩纪以来的剥露历史和古地形恢复

应用裂变径迹技术研究阿尔泰山南缘中新生代以来的隆升剥露历史,并对古地形进行恢复。对所获得的11个磷灰石样品的裂变径迹年龄分析表明,裂变径迹年龄变化分布于99.1~43.7 Ma之间。该地区晚白垩世以来平均视剥露速率为0.050 mm/a。热史模拟结果表明,阿尔泰山南缘自白垩纪以来经历了多期冷却剥露历史:早白垩世至晚白垩世晚期(约120~75 Ma),剥露速率为0.044 mm/a;晚白垩世晚期至始新世(约75-70~50 Ma),剥露速率为0.070 mm/a;中新世以来(约20-15 Ma~现今),剥露速率为0.081 mm/a。对研究区进行古地形恢复显示,自白垩纪(120 Ma)至今,平均剥露幅度达约5 km,古地形海拔降低约0.8 km。阿尔泰山南缘白垩纪以来古地形海拔仅在17.5~50 Ma期间保持基本稳定,其他阶段均有降低的趋势。早白垩世的构造抬升与蒙古-鄂霍茨克海最后阶段的闭合以及西伯利亚板块和中朝-蒙古板块的最终收敛和碰撞有关;晚白垩世晚期至始新世的构造活动则是受拉萨地块、Kohistan-Dras岛弧增生的远距离影响;中新世以来快速隆升可能与印度板块与欧亚板块碰撞的远程效应有关。

Denudation and exposure history and paleotopographic reconstruction of the southern margin of the Altay Mountains since Cretaceous

Altay is an important part of the Central Asian Orogenic Belt,and also the largest accretion orogen in the world.At present,scholars both in China and abroad are focusing their Altay studies on the evolution of various stages of orogenic movements as well as geochemical and geochronological time constraints on the orogenic activities.In contrast,the study of the Mesozoic-Cenozoic cooling history is relatively insufficient.To carry out the researches on the cooling as well as denudation and exposure history of the post-orogenic stage of the Altay orogenic belt is of great importance in the in-depth understanding of the formation of a large mountain range after the evolutionary stage.Researches on the post-orogenic uplift of the cooling as well as denudation and exposure history about Altay orogen are of great significance in that it make researchers obtain in-depth understanding of the formation of a large mountain range after the evolutionary stage.With the improvement of the fission track analysis technology,fission-track thermochronology represented by apatite has become the effective means for revealing the denudation and exposure age and speed of orogenic activity as well as the evolution of tectonic,topographic and thermal history.Low-temperature chronology records rock movement information of the upper crust within a few kilometers and on a million-year scale,and thus offers a good choice for the quantitative recovery of ancient orogenic terrain.This paper discusses uplifting-denudation and exposure history and paleotopographic reconstruction of this region based on fission track technology.AFT data from 11 samples show that the AFTA range from 99.2 Ma to 43.7 Ma.The average denudation and exposure rate has been 0.050 mm/ma since Late Cretaceous.Thermal history modeling results show that there have been multi-phase cooling and denudation and exposure evolutions since the Cretaceous period in the south margin of the Altay Mountains.From Early Cretaceous to Late Cretaceous(about 120～50 Ma),the denudation and exposure rate was 0.044 mm/a,and from Late Cretaceous to Eocene(about 75-70～50 Ma),the denudation and exposure rate was 0.070 mm/a.Since the Miocene(about 20-15 Ma～Present),the denudation and exposure rate has been 0.081 mm/a(the denudation and exposure rate was obtained from three methods,i.e.,the age-closure temperature method,the cooling curve simulation and the age-elevation method).Early Cretaceous tectonic uplift of the Altay Mountains was contemporaneous with the final closure of the Mongol-Okhotsk Ocean,and the convergence and eventual collision of the Siberian and North China(Sino-Korean)/Mongolian continents.Tectonic activities from Late Cretaceous to Eocene were affected by longdistance effects of Lhasa block and Kohistan-Dras arc accretion.Rapid uplift since the Miocene may be related to distant effects of India-Eurasia convergence.Lowtemperature thermochronology can be used to conduct paleotopographic reconstruction,and the specific process is shown as follows: First,the cooling history of rock samples is remolded.Then,the amount of stratigraphic erosion is estimated through temperature gradient.Finally,such affecting factors as balance role,climate and tectonics are considered.Paleotopographic reconstruction of the study area shows that the average denudation and exposure highness from Cretaceous up till now is about 5 km,while paleoelevation has been decreased by about 0.8 km.Cretaceous-Cenozoic paleoelevation of the south range of Altay Mountains has tended to decrease except for a stable period of 50～17.5 Ma.