How Do Intracontinental Faults Terminate? The Altyn Tagh Fault and Its Connection With a Possible S-Directed Backthrust System in Western Tibet as a Type Example

目前关于大陆变形有两种互不相容的观点：一种是大陆通过分裂成微板块而非均匀变形，另一种是大陆以连续的形式均匀变形。区分这些观点的中心是确定陆内断层连接形成微板块-边界系统的程度，该系统在数千万年至数亿年内保持活跃。印度-亚洲碰撞提供了一个很好的机会来了解断裂联系的长期演化，因为它在整个新生代都很活跃，并被一些主要断裂切割。左侧阿尔金-塔格断裂是这些构造中最大的，自碰撞早期以来一直活动。了解这一断层是如何终止的，对于确定它是否构成了长期存在的微板块-边界系统的一部分至关重要。一项基于野外的构造和热年代学研究正在验证阿尔金断裂西端与早新生代南向背冲带相连的假设。由40Ar/39Ar分析得出的基底钾长石的野外填图和冷却历史表明，三条断裂是这一假想的南向背冲带内的关键构造。无论这三个断裂系统的构造演化是否与背冲假设的预测一致，正在确定每个断裂系统的以下几个方面：1)运动的几何和历史，2)变形的时间，3)新生代总偏移量，4)这些断裂如何与阿尔金塔格系统相连。研究的重点是卡拉卡克斯、玉龙和洛克宗系统。卡拉卡克斯断层目前是一条活动的北倾走滑断层，但似乎有较早的反向/逆冲运动历史。玉龙断裂系统切割了第三纪火山岩，并可能在南向逆冲过程中抬升了6800米的山峰。在洛克宗断裂系统内，白垩系和第三系局部受南向构造变形。如果逆冲推力假说是正确的，阿尔金-塔格断层构成了一个微板块-边界系统的一部分，该系统自印度支那-亚洲碰撞的最早阶段就存在了。这一结果将支持印支-亚洲碰撞带的微板块模型，并与大陆变形在长时间尺度上基本均匀的观点相矛盾。

There are currently two mutually exclusive views of continental deformation: either continents deform heterogeneously by breaking into microplates or they deform homogenously as continua. Distinguishing between these views centers on determining the extent to which intracontinental faults link to form microplate-boundary systems that remain active for tens to hundreds of millions of years. The Indo-Asian collision presents an excellent opportunity to understand the long-term evolution of fault linkages since it has been active throughout the Cenozoic and is cut by a number of major faults. The left-lateral Altyn Tagh fault is the largest of these structures, and has been active since the early stages of the collision. Understanding how this fault terminates is critical for determining if it has formed part of a long-lived microplate-boundary system. A field-based structural and thermochronologic study is testing the hypothesis that the western end of the Altyn Tagh fault linked with an early Cenozoic, south-directed backthrust belt. Field mapping and cooling histories derived from 40Ar/39Ar analyses of basement K-feldspars together indicate that three faults are key structures within this hypothesized south-directed backthrust belt. Whether the structural evolution of these three fault systems is consistent with predictions of the backthrust hypothesis, the following are are being determined for each fault system: 1) the geometry and history of motion, 2) the timing of deformation, 3) the total Cenozoic offset, and 4) how these faults link with the Altyn Tagh system. The study focuses on the Karakax, Yulung, and Loqzung systems. The Karakax fault is presently an active, north-dipping strike-slip fault but appears to have an earlier history of reverse/thrust motion. The Yulung fault system cuts Tertiary volcanics and may have uplifted a range of 6800 m peaks during south-directed thrusting. Within the Loqzung fault system, Cretaceous and Tertiary strata are locally deformed by south-directed structures. If the backthrust hypothesis is correct, the Altyn Tagh fault formed part of a microplate-boundary system that has existed since the earliest stages of the Indo-Asian collision. This result would support the microplate model for the Indo-Asian collision zone and contradict the idea that continental deformation is fundamentally homogeneous at long time scales.