祁连山中段白垩纪以来阶段性构造抬升过程的磷灰石裂变径迹证据

摘要

The Qilian Mountain constitutes the northeastern margin of the Tibetan Plateau, and hence characteristics of its tectonic activity recorded by apatite fission track (AFT) analysis play an important role in understanding the uplift and growth of the Tibetan Plateau. 22 samples collected for AFT analysis from the Qilian Mountain belt were located along NS-trending transect across southern Qilian fold belt, Shule Nanshan-Laji Shan suture zone, Central Qilian massif and North Qilian suture zone. AFT ages range from (13±2) Ma to (124±11) Ma, and mean track lengths range from (10.3±1.8)μm to (13.6±2.3)μm. Samples from the same tectonic unit have positive correlation between AFT ages and elevations, whereas samples with younger ages ((60±5) Ma to (13±2) Ma) are clustered around North Central Qilian fault. On the basis of the measured apatite fission track age data, the inversion simulation was used to analyze the thermal history of the Qilian Mountain. The best-fit line of the time-temperature modeling results suggest that at least three cooling periods have occurred since early Cretaceous: 1) Rapid cooling in the Qilian Mountain during early Cretaceous (>(129±14) Ma to (115±17) Ma). The cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan -Laji Shan suture zone were great, suggesting that the Qilian Mountain formed the northeastern margin of the Tibetan Plateau during early Cretaceous; 2) From middle Cretaceous to Miocene ((115±17) Ma to (25±7) Ma), the cooling rates and exhumation rates of South Qilian fold belt and Shule Nanshan-Laji Shan suture zone were quite low, implying that the tectonic activity of the Qilian Mountain was weak during middle Cretaceous to Miocene; 3) Since Miocene time, timing of both thrust activities and regional rapid cooling event shows that the Qilian Mountain experienced north-eastward rise and growth, which is in line with the hypothesis that the Qilian Mountain was formed by thrusting within the Qaidam crust along a large decollement in the lower crust that progressively propagated north-eastward, the Qilian Mountain was uplifted considerably since Miocene, forming basins-mountains tectonic landforms. Early Cretaceous rapid cooling event in the Qilian Mountain probably resulted from the docking of the Lhasa block to the south, and the rapid cooling since Miocene may be the result of the docking of the India-Asia collision, representing the main uplift of the Qilian Mountain.%祁连山作为青藏高原的东北边界，是研究青藏高原隆升和扩展的重要区域，利用磷灰石裂变径迹分析反映的祁连山地区白垩纪以来阶段性隆升和扩展新认识对理解青藏高原的隆升过程有重要的意义。分别采自南祁连陆块、疏勒南山—拉脊山缝合带、中祁连陆块和北祁连缝合带22个样品的磷灰石裂变径迹年龄介于(124±11) Ma与(13±2) Ma之间，平均径迹长度介于(13.6±2.3)μm和(10.3±1.8)μm之间。时间-温度反演模拟结果表明祁连山地区至少经历了3个重要构造活动阶段：1)白垩纪早期(>(129±14)~(115±17) Ma)祁连山隆升，南祁连陆块和疏勒南山—拉脊山缝合带的冷却速率及剥蚀速率均较大，并且祁连山南部可能率先抬升而初步构成高原的东北边界；2)白垩纪中晚期—中新世((115±17)~(25±7) Ma)祁连山构造平静，南祁连陆块和疏勒南山—拉脊山缝合带冷却速率及剥蚀速率均较低；3)中新世以来祁连山由南向北逐渐扩展，构造活动强烈而最终形成盆-山构造地貌格局。祁连山白垩纪早期的快速冷却过程可能是受拉萨地块和羌塘地块碰撞的影响；中新世以来向北扩展则主要是受印度—欧亚板块碰撞的影响。