西昆仑山北缘新生代隆升历史的裂变径迹证据

摘要

自新生代以来,西昆仑地区发生强烈的构造变形和隆升,其初始隆升和末次快速隆升的时限仍是有待探讨的重大问题.本文沿西昆仑北缘采集一系列砂岩样品,利用裂变径迹分析方法探讨了西昆仑北缘新生代的冷却历史.结合裂变径迹年龄和径迹长度分布进行分析,可以将6个磷灰石样品分为2组.3个磷灰石样品的径迹年龄远小于所在地层的年龄.平均径迹长度为(12.0±2.3) ～(12.6±1.3) μm,呈不对称单峰形态,反映样品缓慢地通过部分退火带;另外3个磷灰石样品径迹年龄与各自地层的沉积年龄接近,平均径迹长度介于(10.7±2.3)～(11.4±1.3)μm,呈现双峰或混合分布的特征,表明沉积后发生部分退火.热史模拟显示,自晚白垩世以来,西昆仑山北缘共经历了3期抬升冷却事件.晚始新世(40～30 Ma),受早期印度板块向古亚洲大陆板块俯冲碰撞的影响,西昆仑山北缘已经开始隆升;晚渐新世一早中新世(25～15 Ma)是西昆仑乃至青藏高原重要的隆升时期;最后一轮强烈隆升则发生在距今5 ～3 Ma以来,冷却速率最高达15℃/Ma,剥蚀速率相当于600m/my.电子自旋共振测试揭示了早中新世(15 Ma)和晚上新世以来(2.6～0.63 Ma)两期强烈的构造变形和热液活动,更进一步限定了西昆仑最后一期强烈隆升在2.6 Ma以来.%The Western Kunlun Mountains (WKL),which located in the northwestern margin of the Qinghai-Tibet plateau,has experienced intense tectonic deformation and uplift science the Cenozoic.The timing of the initial and the last rapid uplift is still a major issue to be explored.In this paper,we collected a series of sandstone samples along the northern WKL and discussed the Cenozoic thermal history according to analysis of fission track.Analysis of apatite fission track ages combined with track length distribution suggests that 6 apatite samples can be divided into two groups.Apatite fission track (AFT) ages of the 3 samples are much younger than their stratigraphic ages and mean track lengths range from (12.0 ± 2.3) Ｌm to (12.6 ± 1.3) μm with single peak distribution,showing that the samples cooled slowly through the partial annealing zone (PAZ).The other 3 AFT ages are very close to their stratigraphic ages and mean track lengths range from (10.7 ± 2.3) μm to (11.4 ± 1.3)μm with bimodal or mixed distribution,showing that these samples suffered partial annealing after their deposition.AFT ages and thermal history modeling results reveal that the northern WKL have experienced three rapid uplifting periods since Late Cretaceous.The first uplift event occurred in the Late Eocene (40 ～30 Ma),which was related to the initial collision event of the Indian plate with the Eurasian plate in the Cenozoic.Late Oligocene-Early Miocene (25 ～ 15 Ma) is an important uplifting period of the WKL and even Qinghai-Tibet plateau.The last rapid uplift event probably started at 5 ～3 Ma with a relatively high cooling rate of 15.67 ℃/Ma,and inferred exhumation rate of 626.8 m/my.Meanwhile,electron spin resonance dating ages indicate that the northern WKL have experienced strong metamorphism and hydrothermal activity in the Early Miocene (15 Ma)and Late Pliocene -Pleistocene (2.6 ～ 0.63 Ma),corresponding well with the last two uplift events revealed by AFT modeling.This ESR age set a limit of the last rapid uplift of WKL to 2.6 Ma.