青藏高原北缘中生代伸展构造^(40)Ar/^(39)Ar测年和MDD模拟

Mesozoic Extension in Northern Tibetan Plataeu:^(40)Ar/^(39)Ar Analyses and MDD Modeling

沿着青藏高原北缘的阿尔金山脉东段 ,发育了长度大于 30 0km、EW走向的拉配泉断裂。中美合作阿尔金课题组的地质填图结果表明 ,该断层实际上是一条南倾的正断层 ,局部倾角可以低至 30°以下。沉积学和热年代学研究控制了拉配泉断裂的活动时代 :早—中侏罗统地层可以解释为断裂上盘的伸展盆地沉积 ;下盘岩石中钾长石40 Ar/ 3 9Ar测年和MDD模拟给出 2个阶段的冷却事件 ,早期事件出现在约 2 2 0～ 187Ma之间 ,晚期事件出现在早白垩世晚期 (约 10 0Ma)。早期事件代表了拉配泉断裂正断作用的主要阶段。

The >300 km long, east striking Lapeiquan fault lies in the eastern Altyn Tagh range along the northern margin of the Tibetan plateau. Geological mapping conducted by Sino America Altyn Group led by An Yin (UCLA) shows that the fault is in fact a south dipping normal fault with its dip angle locally as shallow as <30° Sedimentary and thermochronologic evidence constrains the age of Lapeiquan fault: the Early Middle Jurassic strata can be interpreted as deposits in an extensional basin above the fault; 40 Ar/ 39 Ar thermochronologic analyses and MDD modeling of K feldspars from the footwall of the fault indicate two preminent cooling events. The older event occurred in the latest Triassic earliest Jurassic between ～220 and 187 Ma, while the younger event occurred in the latest Early Cretaceous at ～100 Ma. The older event is suggested to represent its main stage of the normal faulting. Mesozoic extension in Tibet and other parts of eastern Asia is attributed to back arc extension during subduction of the Tethys ocean in the south and the Pacific plate in the east.