Collaborative Research: Development of Extensional Systems in Regions of Hot, Thick Crust: Insight from Tibet

这个项目将研究现代大陆伸展系统和相关盆地是如何在炎热、厚重的地壳地区发展的。西藏位于地球上最厚的地壳之下，可以说是在地壳中层流动，并正在以超过印度和亚洲南北汇聚总速度三分之一的速度积极地向东西延伸。最近的研究表明，北向青藏正断层及其邻近盆地的特征随拉张强度的变化而有系统地变化，并可能提供递进伸展过程中裂谷盆地演化的序列快照。新生的裂谷以高角度正断层为边界的半地堑盆地为特征。相比之下，更多演化的裂谷被低角度的滑脱断层所包围，其特征是盆地在推测的最大伸展范围内经历了盆地内排水分流的切割。本研究的目的是验证这一假说，即西藏裂谷起源于高角度正断层及其伴生的半地台，它们演化为活动在地壳最上层的拆离断层，在此之上裂谷盆地充填物被抬升和侵蚀，以响应递进的构造卸荷和均衡反弹。这一假设以及几种替代方案将在这个项目中通过地质和构造填图、宇宙成因测年、盆地分析和低温热年代学等手段在西藏中部一口裸露的井中得到验证。该项目是由堪萨斯大学、亚利桑那大学、德克萨斯大学、加拿大达尔豪西大学和中国青藏高原研究所的师生共同开展的多学科合作研究。该项目的发现将产生重大的科学影响，详细记录现代伸展系统如何在热厚的地壳之下的活跃造山作用地区发展。这一结果将检验变质核杂岩和低角度正断层发育的对比模型，并可能为揭示中地壳流动在大陆伸展过程中增强均衡反弹的作用提供新的线索。

This project will investigate how modern continental extensional systems and associated basins are developing in regions of hot, thick crust. Tibet is underlain by the thickest crust on Earth that is arguably flowing at mid-crustal levels, and is actively extending East-West at greater that one-third the total North-South convergence rate between India and Asia. Recent studies show that North-trending Tibetan normal faults and adjacent basins vary systematically in their characteristics as a function of extension magnitude, and may provide sequential snapshots of rift basin evolution during progressive extension. Nascent rifts are characterized by half-graben basins bounded by high-angle normal faults. In contrast, more evolved rifts are bounded by low-angle detachment faults characterized by basins undergoing incision with intrabasin drainage divides in areas of inferred maximum extension. The goal of this research is to test the hypothesis that Tibetan rifts initiate as high-angle normal faults and associated half-grabens that evolve into detachment faults active at uppermost crustal levels and above which rift basin fill is uplifted and eroded, in response to progressive tectonic unloading and isostatic rebound. This hypothesis, along with several alternatives, will be tested in this project along a well exposed in central Tibet by using geological and structural mapping, cosmogenic dating, basin analysis, and low-temperature thermochronology. The project is a multi-disciplinary and collaborative research effort between faculty and students at the University of Kansas, University of Arizona, University of Texas, Dalhousie University in Canada, and the Institute of Tibetan Plateau Research in China.Project findings will have a major scientific impact by documenting in detail how modern extensional systems develop in regions of active orogenesis that are underlain by hot, thick crust. The results will allow testing of contrasting models for the development of metamorphic core-complexes and low-angle normal faults, and may shed new light on the role of mid-crustal flow in enhancing isostatic rebound during continental extension.