西藏羌塘盆地白垩纪中期构造事件的磷灰石裂变径迹证据

Apatite fission track thermochronology evidence for the Mid-Cretaceous tectonic event in the Qiangtang Basin,Tibet

拉萨与羌塘地块于白垩纪中期的碰撞造山对羌塘原型盆地的热体制和构造演化有着重要影响。运用磷灰石裂变径迹方法,对羌塘盆地隆鄂尼夏里组和托纳木雪山组砂岩分析表明,裂变径迹年龄集中在120～80Ma之间,表明在白垩纪中期,羌塘盆地普遍发生了一次构造抬升事件,该期构造事件的年龄与盆地内早白垩世的岩浆热事件、主要构造变形作用发生在晚白垩世以及雪山组和阿布山组角度不整合的时代(125～75Ma)较一致,是拉萨与羌塘地块碰撞造山事件的记录。热历史模拟表明,白垩纪中期构造事件对羌塘盆地南部和北部的热演化历史有着差异影响,羌塘盆地南部降温速率相对不大,抬升剥蚀厚度约1500m,而北部古地温迅速降温到近地表温度,抬升剥蚀厚度近4000m。这种差异抬升剥蚀可能与班公湖-怒江洋壳向南俯冲使得因拉萨地块构造负载而导致羌塘地块的挠曲有关。

Collision and orogenesis between Lhasa and Qiangtang terrane in Early-Middle Cretaceous have significant impact on the thermal regime and tectonic evolution of the proto-Qiangtang Basin. Apatite fission track dating was applied to analyze the sandstone samples of the Xiali Formation from Longe＇ ni area in southern Qiangtang Basin and of the Xueshan Formation from Tuonamu area in northern Qiangtang Basin. The results show single population grain ages with a single mean age and associated central ages mainly ranging from 120Ma to 80Ma and indicate that there would be a tectonic event in middle Cretaceous. The ages coincide web with the ages of magmatism in the Early Cretaceous, main deformation occurred in the Late Cretaceous, and angular unconformity indicated by the Xueshan and Abushan formations. Therefore, the Mid-Cretaceous tectonic event is the result of collision between Lhasa and Qiangtang terrane. Thermal history modeling suggests a different thermal evolution of southern and northern Qiangtang Basin during the Middle Cretaceous Period. The results indicated that cooling rate in southern Qiangtang Basin with 1500m uplift-denudation is lower than the cooling rate in northern Qiangtang Basin with 4000m uplift-denudation. The different uplift-denudation may be related to flexuring in the Qiangtang terrane caused by tectonic loading of Lhasa terrane during southward subduetion of the Bangong-Nujiang Ocean seafloor.