阿尔金断裂带年代学和阿尔金山隆升

对阿尔金山断裂带内变形的中生代剪切带中眼球状片麻岩、糜棱岩化花岗片麻岩和新生代剪切带中强变形绿片岩系列样品中同构造新生云母矿物的⁴⁰Ar-³⁹Ar法定年,获得了164.3~178.4Ma和26.3~36.4Ma两组年龄。结合阿尔金走滑断裂水平错距的研究,认为164.3~178.4Ma的年龄代表阿尔金走滑断裂带的起始活动时间(早侏罗世末—中侏罗世),尔后分别在距今100~85Ma,40~25Ma和10~8Ma发生多次脉冲式走滑活动。新生代沉积物研究表明上干柴沟组和下干柴沟组砂岩骨架矿物成份含量明显不同,随时间变化,石英含量减少,岩屑组份和不稳定矿物含量增加,分选性逐渐变差。这表明阿尔金山在渐新世(下干柴沟组时期)开始发生明显的隆升。地震和柴达木—塔里木盆地沉积速率资料显示在晚中新世山体也发生了一次强烈的隆升。七个泉组(上新世末—早更新世初)和下伏沉积物(上新世中期)之间的角度不整合反映了最后一次强烈隆升事件发生在晚上新世。多数隆升事件和阿尔金断裂带新生代脉冲式活动的同位素年龄完全一致,表明阿尔金山的隆升和断裂带的活动具有密切的成因联系。

GEOCHRONOLOGY OF THE ALTUN FAULT ZONE AND RISING OF THE ALTUN MOUNTAINS

The ~ 40 Ar-~ 39 Ar dating of the syntectonic growing micas, separated from the deformed augen gneiss and mylonitized granitic gneiss in the Mesozoic shear zone and strongly mylonitized greenschist in the Cenozoic shear zone from the transect across the Altun fault zone, yielded two age groups of 164.3~178.4 Ma and 26.3~36.4 Ma. Combining with the offset studies of the strike-slipping of Altun fault, it suggested that the ages of 164.3~178.4 Ma should represent the initial activity age (the latest Early Jurassic-Middle Jurassic) of the Altun strike-slip fault. After that, the strike-slipping activities along the fault occurred 85~100 Ma, 25~40 Ma and 8~10 Ma ago, respectively. The studies on the Cenozoic sediments showed the different contents of the sandstone framework components between the Xiaganchaigou (E_ 3 g) and Shangganchaigou (N_ 1 g )Formations in the western Qaidam Basin. With the sedimentary process, the quartz component decreased, the clastic component increased with more unstable minerals and worse sorting degree, which suggested that the Altun Mountains began to rise in Oligocene (sedimentary period of the Xiaganchaigou Formation). A strong uplifting happened in Late Miocene according to the evidence from the seismic section of the western Qaidam Basin and sedimentary velocity of the Qaidam and Tarim Basins. The last and strongest rising event occurred in the end of Early Pleistocene according to the unconformity between the Qigequan Formation (from the end of Pliocene to the beginning of Early Pleistocene) and the overlying sediments (Middle Pleistocene). Most of uplifting events were consistent with the isotopic ages recorded in the fault belt, indicating that the mountain uplifting should be related to the activities of strike slipping of the Altun fault zone.