藏东南老君山地区河流侵蚀和拉分断陷导致的岩石剥露历史的对比

Quantitative analysis of internal agency- and external agency-driven exhumation history are crucial for an improved understanding the role of different dynamic processes in landscape evolution, enhancing the accuracy of data interpretation of low temperature thermochronology, and thus better constraining the tectonic activities and landscape evolution of orogenic belt. River incision (external agency) and pull-aparting (internal agency) are the main driving forces that control the exhumation of southeastern Tibetan Plateau. In order to decipher these two forces-driven exhumation history, we will aim at the Laojun Mountain area (>2400m relief in 50km2) to carry out detailed geological investigation and geomorphic comparison, and then design 3 traverses for apatite sample collection (1 across the Jinsha river gorge and 2 across the Weixi-Qiaohou and Longpan-Qiaohou strike-slip faults, respectively). Apatite (U-Th)/He and fission track techniques will be conducted to reconstruct the distinct exhumation history around the Laojun Mountain, including timing, rate, episode and amplitude. Further analysis and interpretation for these data will be made to reveal the differences between the river incision and fault-related exhumation processes. On the basis of mutual dating constraint of the two distinct exhumation, we will analyze river development and fault activities in the Laojun Mountain area and further discuss the possible geodynamics that dominate the crustal deformation and landscape evolution of southeastern Tibetan Plateau.

定量分析不同内外动力主导的岩石剥露历史的差异，对深入理解不同动力过程在地貌演化中的作用，提高低温热年代学数据解释的准确性，更好的约束造山带构造活动和地貌演化具有重要意义。以河流侵蚀为主的外动力和断裂走滑-拉分断陷为主的内动力是控制青藏高原东南缘岩石剥露的重要地质作用。本项目拟以老君山地区典型河流侵蚀地貌和拉分断陷地貌为研究对象，对两种地貌进行详细的野外调查和比对；设定3条剥露过程单一(或河流侵蚀或拉分断陷)、不同高程处可采集到磷灰石样品的剖面；以磷灰石(U-Th)/He和裂变径迹定年为手段，揭示老君山地区河流下切和拉分断陷两种不同动力过程造成的岩石剥露历史，包括时间、速率、期次和幅度；深入剖析不同剥露方式之间的异同及原因。在两种不同剥露历史相互制约的基础上，分析研究区断裂活动历史和河流地貌演化，并探讨控制青藏高原东南缘地壳变形和地貌演化的可能地球动力学机制。

对藏东南地壳抬升的时间和机制长期存在着争论. 本项目使用磷灰石(U-Th)/He方法对长江第一湾地区的长英质岩石进行了定年. 结果显示在渐新世-早中新世(~30-20 Ma)存在河流的快速下切过程. 结合前人大量的地质研究结果，我们认为藏东南长江第一湾地区的地壳缩短、地壳抬升和河流快速下切可能发生在前中新世而不是之前所认为的晚中新世. 藏东南在渐新世-早中新世时期的地壳抬升可能是由印-亚碰撞后侧向挤出引起的地壳缩短造成的，也可能是下地壳流作用的结果.