帕米尔东缘晚中新世以来多阶段隆升-剥露过程:来自裂变径迹和(U-Th)/He低温热年代学的制约

Multi-stage uplift and exhumation processes in the eastern Pamir since Late Miocene:Constrained by fission tracks and(U-Th)/He thermochronology

帕米尔东缘发育帕米尔造山带内最大的东西向伸展系统,对理解区域构造演化具有重要意义。然而,该地区的隆升时限和动力学机制等仍缺乏有效的限制。本文选取帕米尔东缘塔什库尔干地区的新生代侵入岩开展磷灰石裂变径迹和锆石、磷灰石(U-Th)/He定年研究,并结合前人的热年代学成果,探讨帕米尔东缘晚新生代以来的构造热演化和动力学机制。低温热年代学数据显示,磷灰石裂变径迹中心年龄为2.5~2.7Ma和8.7~11.5Ma,磷灰石(U-Th)/He年龄为2.3~4.5Ma,锆石(U-Th)/He年龄为3.2±0.3Ma。通过上述样品的剥露速率计算,我们认为帕米尔东缘发生了两期快速冷却期:晚中新世(12~8Ma)和上新世-第四纪(5~1Ma),其对应的剥露速率分别为0.19~0.48mm/yr和0.58~4.00mm/yr。晚中新世期间,帕米尔东缘构造变形、岩浆作用、变质作用几乎同时发生。这些事件可能和帕米尔与南天山造山带在晚中新世初次碰撞有关,该碰撞是由印度板块向北俯冲到帕米尔地体下方引起的。这次碰撞进一步促使地壳增厚,导致帕米尔岩石圈重力分布不均,从而引发重力坍塌。上新世至第四纪的冷却可能受到重力坍塌后持续伸展变形的影响,同时冰川、河流和气候变化引起的侵蚀进一步加剧了剥露作用,导致剥露速率比晚中新世时期显著增加。

The largest east-westward extensional system in the Pamir orogenic belt is developed in the eastern Pamir,which plays a crucial role in understanding regional tectonic evolution.However,there is a lack of effective constraints on uplift timing and dynamic mechanisms of this area.In this paper,we present new data on apatite fission track,zircon and apatite(U-Th)/He analyses from a series samples collected in the Cenozoic intrusion rocks in the Tashkurgan area of the eastern Pamir.Combined with previous thermochronological results,these findings are used to explore the tectono-thermal evolution and dynamic mechanisms of the eastern Pamir since Late Cenozoic.The results of low temperature thermochronological analysis reveal that the apatite fission track ages are ranging from 2.5Ma to 2.7Ma and from 8.7Ma to 11.5Ma,the apatite(U-Th)/He ages range from 2.3Ma to 4.5Ma,while the zircon(U-Th)/He ages are 3.2±0.3Ma.By calculating the exhumation rates of the aforementioned samples,we proposed the occurrence of two periods of rapid cooling in the eastern Pamir,i.e.,the Late Miocene(12~8Ma)and the Pliocene-Quaternary(5~1Ma),and calculated their exhumation rates which is correspondently range from 0.19~0.48mm/yr and 0.58~4.00mm/yr,respectively.During the Late Miocene,the tectonic deformation,magmatism and metamorphism events occurred nearly simultaneously in the eastern part of the Pamir,which may be linked to the initial collision between the Pamir and the southern Tianshan orogenic belt caused by the northward subduction of the Indian Plate beneath the Pamir Terrane during the Late Miocene.This collision contributed to further crustal thickening,resulting in an uneven distribution of gravity within the Pamir lithosphere and triggering gravitational collapse.The cooling process from the Pliocene to the Quaternary was likely influenced by the continuous extensional deformation after the gravitational collapse,while the erosion during this period caused by glaciers,rivers and the climate variations further enhanced the exfoliation,leading to a significantly faster exhumation rate compared to the Late Miocene one.