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

该项目将研究现代大陆伸展系统和相关盆地是如何在热厚地壳地区发育的。西藏位于地球上最厚的地壳之下，可以说是在地壳的中层流动，并以超过印度和亚洲之间总南北收敛速度的三分之一的速度积极地向东向西延伸。近年来的研究表明，北向青藏高原正断层及其邻近盆地的特征随伸展幅度的变化而有系统的变化，可能为裂谷盆地在渐进伸展过程中的演化提供了时序快照。新生裂陷以半地堑盆地为特征，以高角度正断层为界。相比之下，在推断的最大伸展区，更演化的裂谷以低角度滑脱断裂为界，以盆地切割为特征，并伴有盆内水系划分。本研究的目的是验证青藏裂谷起源于高角度正断层和相关的半地堑的假设，这些地堑演化为滑脱断层，活动在地壳的最上层，在其之上，裂谷盆地填埋被抬升和侵蚀，以响应渐进的构造卸载和均衡反弹。该项目将通过地质和构造测绘、宇宙成因测年、盆地分析和低温热年代学等方法，对西藏中部的一口井进行验证。该项目是由堪萨斯大学、亚利桑那大学、德克萨斯大学、加拿大达尔豪斯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。通过详细记录现代伸展系统如何在热厚地壳下的活跃造山活动区域发展，项目发现将产生重大的科学影响。这一结果将有助于测试变质核杂岩和低角度正断层发育的对比模型，并可能对大陆伸展期间中地壳流动在增强均衡反弹中的作用提供新的认识。

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.