札达盆地碎屑裂变径迹揭示的盆山耦合过程

Coupling Process of Sedimentary Basin-Orogenic Belt Induced by Detrital Fission Track Ages of Zanda Basin

札达盆地是中新世9.5Ma以来发育的新生代沉积盆地.沉积厚度、砾石成分和古流向分析显示札达盆地新生代沉积的物源主要来自盆地北部的阿伊拉日居山系.札达盆地系列样品碎屑锆石裂变径迹年龄结构显示存在两个明显的峰值年龄区间,分别为12.6～15.3Ma(P1峰值年龄)与19.8～22.2Ma(P2峰值年龄).锆石裂变径迹年龄的滞后时间(lagtime)与沉积时代对比分析显示,P1和P2峰值年龄为快速冷却事件的静态峰,与北部阿伊拉日居地区基岩U-Pb年代研究揭示的热事件时间具有良好的可对比性.因此,札达盆地碎屑裂变径迹年龄两个峰值年龄区间记录了源区阿伊拉日居的两次构造事件,可能对应于喀喇昆仑断裂在中新世的两次强烈的构造活动.综合碎屑锆石、磷灰石裂变径迹年龄信息,估算源区在32.6～9.5Ma之间的平均冷却速率是15.4℃/Ma,上新世末期-第四纪(3.6～1.4Ma)之间再次发生了一次快速的隆升剥露事件.札达盆地中新生代沉积地层碎屑裂变径迹热年代学结构与喀喇昆仑断裂东南段阿伊拉日居的热事件年龄格局吻合,从碎屑裂变径迹年代学角度揭示了造山带地区的盆山耦合过程.

Zanda basin, formed in 9. 5 Ma, is a Cenozoic sedimentary basin which lies in the converging range between the Himalaya tectonic belt and Karakorum tectonic belt. The Neogene strata in the basin are integrated, continuous and horizontal, showing a steady tectonic condition since Miocene. Analysis of sedimentary section indicates that ancient flow direction is northwest to southeast, while the modern flow direction of river-Xiangquan River is southeast to northwest, opposite to the ancient flow direction. Analysis of sendimentary construction and the composition of gravel displays that the sedimentary source is Ayila Rijyu northwest of Zanda basin. P1, P2 peak ages of Zanda basin are 12.6--15.3 Ma and 19.8--22. 2 Ma respectively, which is consistent with the thermal event age of Ayila Rijyu. Pl, P2 are of static peak ages, reflecting the rapid cooling event such as normal faulting based on the DZFT research of lag time and depositional time by Bernet and Garver. It is found by thermal history analysis of zircon and apatite fission track that the cooling rate of source area is 15.4 ℃/Ma at 32.6--9.5 Ma, and there was a rapid uplift and exhumation at the same time in the source region at 3.6--1.4 Ma reflected by zircon and apatite fission track ages. P1, P2 peak ages of modern river zircon samples of Karakurom belt are 10. 5--10. 8 Ma and 27.7--30. 9 Ma respectively, showing the thermal events of Ayila Rijyu. Comparing the FT ages and depositional information of Zanda basin and orogenetic belt, it is shown that Zanda basin is Cenozoic sedimentary basin determined by southeast of Karakorum fault; Zanda basin and Ayila Rijyu Moutain are a typical coupling process by identical FT thermochronology structure.