青藏高原西北缘盆山过渡带陡坡地貌的形成时代与成因——来自西昆仑山北缘裂变径迹分析的证据

Ages and causes of steep slope relief between Tarim Basin and northwestern margin of the Tibetan Plateau:The evidence from apatite fission track dating of north margin of west Kunlun Mountains

平均海拔大于4 500 m的青藏高原,是通过高原边缘的陡坡地貌与海拔低于1 500 m的周缘盆地或平原相连接的,这些围绕高原的陡坡地貌是何时、如何形成的呢?本文通过对西昆仑山中段北缘主逆冲断层上盘陡坡地貌区9件磷灰石样品的裂变径迹年龄与长度分析表明:在海拔3 900～4 635 m的陡坡地貌中的裂变径迹样品年龄为6.2±1.4 Ma～0.9±0.3 Ma,呈现"上新下老"的反序分布特征;而通过热历史模拟显示约5 Ma,约3～2 Ma,约2～1 Ma和约1 Ma该地区出现多阶段的隆升与剥露。结合前人研究成果和野外地质的观察认为,现今青藏高原西北缘陡坡地貌的形成是中新世晚期以来高原边界叠瓦状断裂系经历了约8 Ma、约5Ma、约3～2 Ma、约2～1 Ma和约1 Ma多阶段后展式逆冲运动的结果,这为青藏高原周缘陡坡地貌的形成和青藏高原的隆升时代与型式提供了关键的热年代学约束。

Tibetan Plateau ,whose average elevation is more than 4 500 m ,is joined the basin or plain whose elevation is less than 2 000 m through the steep slope relief of the edge of the Tibetan Plateau. When,how does the steep slope relief formed？ Nine Apatite fission track ages and length of the steep slope relief from the main thrust fault hanging wall in the middle of the northern margin of west Kunlun Mountains are analyzed in this paper,showed that fission track ages from the steep slope relief area of elevation 3 900 ～4 635 m are ranging from 6.2±1.4 Ma -0. 9±0. 3 Ma,fission track ages show as ＂up new under old＂ anti-order distribution. Thermal history modeling shows multi-stage uplifting and exhumation of ～ 5 Ma,～ 3 ～ 2 Ma, ～ 2 ～1 Ma and since - 1 Ma. Combined with previous studies results and geological observations,think that steep slop relief what we now see in the northwestern margin of Tibetan Plateau is overstep propagation thrust results of west Kunlun thrust fault system since late Miocene,it experienced the multi-stage overstep propagation thrusting movement of ～8 Ma, ～ 5 Ma, ～ 3 ～ 2 Ma, N 2 ～ 1 Ma and since～ 1 Ma. It provides an important chronology constraints for age and causes of the steep slope relief forming in the margin of the Tibetan Plateau and uplifting of age and type of Tibetan Plateau.