Time constraints on the emplacement of klippen in the Longmen Shan thrust belt and tectonic implications

A belt of klippen and nappes was emplaced in the Longmen Shan thrust belt, mostly lying between the Yingxiu-Beichuan and the Pengguan fault zones. Timing of the allochthon emplacement is hotly debated, but quantitative age constraints are rarely documented. This study presents the first isotopic dating of authigenic illite isolated from a 30 m-wide fault gouge zone developed in reverse faults in the leading edge of the largest Baishi-Goujia klippe, a representative of the klippen and nappe belt in the Longmen Shan thrust belt. The results show that the authigenic illite formed 180-215 Ma ago. Combined with apatite and zircon fission track dating of the hanging wall of the Yingxiu-Beichuan fault zone, the emplacement of the Baishi-Goujia klippe is constrained at 171-180 Ma, in the Middle Jurassic. The thickness of a successive strata unit of the Baishi-Goujia klippe measures ~ 3 km and of the Tangwangzhai nappe is over 5 km. This implies that the elevation of the Longmen Shan was at least of 3-5 km a.s.l. during the Indosinian orogeny.