哀牢山-红河剪切带渐新世的构造体制转换与剥露历史:来自哀牢山南段磷灰石裂变径迹的证据

哀牢山-红河剪切带是东南亚重要的构造边界,其记录了青藏高原东南缘新生代以来的陆内变形和地貌演化。本次研究对该剪切带哀牢山南段开展了基于LA-ICPMS法测试的磷灰石裂变径迹低温年代学分析。磷灰石裂变径迹年龄数据和热史反演模拟揭示哀牢山段存在晚始新世-早中新世(40～20Ma)的快速剥露事件,而早中新世(大约20Ma)之后处于稳定的慢速剥露过程。磷灰石裂变径迹年龄-海拔分布曲线特征暗示:快速剥露机制存在差异,早期阶段(40～26Ma)的剥露过程受控于伸展为主的左旋走滑体制影响;晚阶段(26～20Ma)的快速剥露归因于简单剪切为主的左旋走滑剪切体制,上述结果暗示哀牢山-红河构造带在晚渐新世发生了一次重要的构造体制转换,即从走滑伸展变形转换为简单剪切变形。哀牢山杂岩带北段、中段、南段冷却路径对比,表明北-中段可能存在两阶段快速冷却作用,而南段只发生单一快速冷却作用;结合青藏高原东南缘低温热年代学数据,暗示自中-晚中新世,青藏高原中、下地壳物质可能向东南缘扩展,并已到达哀牢山中段,同时诱发哀牢山杂岩带以北广大地区的抬升和快速冷却。

Tectonic transformation and its exhumation history of the Ailao Shan-Red River shear zone in Oligocene: Evidences from apatite fission track thermochronology of the southern segment of the Ailao Shan range

The Ailao Shan-Red River shear zone is an important tectonic boundary in Southeast Asia, recording the intracontinental deformation and geomorphic evolution of the southeastern region of the Tibet since the Cenozoic. In this study, low temperature thermochronology analysis of the apatite fission track based on LA-ICPMS test is carried out in the southern Ailao Shan range of the shear zone. These apatite fission track data and thermal history inversion modeling results reveal that the southern Ailao Shan experiences a rapid exhumation event from the Late Eocene to the Early Miocene (40~20Ma), and a stable slow exhumation process after the Early Miocene (about 20Ma). The distribution curve of elevation-apatite fission track ages suggests that there is difference in the rapid exhumation mechanism. The exhumation process in the early stage (40~26Ma) was controlled by the extension-dominated left-lateral strike-slip deformation, and the rapid exhumation in the late stage (26~20Ma) is attributed to the simple left-lateral strike-slip shear deformation. The above results suggest that an important tectonic system transformation occurs in the Ailao Shan-Red River shear zone during the Late Oligocene, namely, the transition from transtensional deformation to simple shear deformation. The comparison of cooling paths between the northern, the middle and the southern segments of Ailao Shan complex belt shows that there may be a two-stage rapid cooling in the north-middle segment, and only a single rapid cooling in the southern segment. Combined with the low temperature thermal chronology data of the southeastern region of the Tibet, it is suggested that the extension of the middle or lower crustal materials of the Tibet to the southeast may have reached the middle segment of Ailao Shan, and have induced the uplift and rapid cooling of the large area north of Ailao Shan belt since the Middle-Late Miocene.