The timing and cause of mountain building in central Asia

板块构造论（认为地球表面是由一些刚性碎片组成的，这些碎片只在其边缘形成狭窄的变形带的观点）作为对地球表面海洋部分行为的准确描述，已经被接受了近40年。与此同时，同样清楚的是，大陆不像海洋板块那样运动。大陆的变形并不局限于其边缘的狭窄地带，而是在其内部蔓延数百公里。一个主要的例子是印度与亚洲的碰撞，这导致了从喜马拉雅山脉到西伯利亚绵延3000公里的山脉的形成。大陆构造学研究的一个基本挑战是理解引起观测到的变形的力——大陆变形的“动力学”研究。在我们研究造山的动力学之前，我们必须首先了解变形是如何演变成我们今天所看到的状态的。因此，对造山开始的时间及其空间演化提供精确的限制，对于理解大陆变形的机制至关重要。印度和亚洲之间的碰撞开始于4500万年前，喜马拉雅和青藏高原的变形在那之后不久就被记录下来了。众所周知，碰撞带北部主要山脉的起源要比南部年轻得多，但这些北部山脉的起源和随后的生长所受到的限制数量太少，无法对整个地理范围进行研究，因此，印度-欧亚大陆碰撞发展的每个阶段的可能原因也就无从考证了。我建议对哈萨克斯坦和蒙古西部的天山、准噶尔阿拉塔山和阿勒泰山进行勘察研究。这些山脉构成了印度大陆与亚洲大陆碰撞的最北端，目前对它们的发展知之甚少。构造演化的制约因素将来自裂变径迹和U-Th/He热时。这些技术记录了地球顶部1-3公里处岩石的冷却情况，将与A. Carter博士合作进行。特别令人感兴趣的问题是，为什么中亚山脉的变形开始得比喜马拉雅和西藏晚得多。一种可能的解释是，变形带的宽度在逐渐增大，山脉的起始年龄向北变小。其他主要的说法是，1000万到1500万年前青藏高原的快速上升给亚洲大陆带来了力量，导致了山脉的建造向北移动，或者是亚洲太平洋边缘力量的变化导致了大陆内部变形的重组。通过揭示哈萨克斯坦和蒙古西部的造山历史，并对其历史上每个阶段的时间进行严格的年龄限制，我将能够区分这些不同的情景，这些情景负责产生我们今天看到的山脉分布。我们之所以选择关注哈萨克斯坦和蒙古西部的造山史，是因为：(1)人们对该地区山脉的构造史知之甚少，但对理解亚洲大陆的变形机制可能非常重要；(2)山区交通便利，我们曾于2011年7月在哈萨克斯坦进行过一次勘察访问，因此我们有信心在一个野外季节成功完成所有采样；(3)从蒙古西部阿尔泰地区采集的样品的初步测量表明，存在构造上有用的冷却信号。

Plate tectonics (the idea that the Earth's surface consists of a few rigid fragments that deform only in narrow bands round their edges) has been accepted for almost 40 years as an accurate description of the behaviour of the oceanic parts of the Earth's surface. At the same time, it has been equally clear that the continents do not behave like the oceanic plates. Deformation in continents is not restricted to narrow bands at their edges, but is spread many hundreds of kilometres through their interiors. A major example is the the collision of India with Asia, which has led to the formation of mountains stretching from the Himalaya to Siberia, over a distance of 3000 km. A fundamental challenge in the study of continental tectonics is to understand the forces that give rise to the observed deformation - the study of the 'dynamics' of continental deformation. Before we can investigate the dynamics of mountain-building we must first develop an understanding of how the deformation has evolved to the state in which we see it today. Providing precise constraints on the times of initiation of mountain-building, and on its spatial evolution, are thus central to understanding the mechanics of continental deformation. The collision between India and Asia began more than 45 Million years ago, and deformation within the Himalaya and Tibetan Plateau is recorded very soon after that time. The initiation of the major mountain ranges in the northern part of the collision zone is known to be much younger than in the south, but the existing constraints on initiation and subsequent growth in these northerly ranges are too few in number for the full geographic extent, and hence the likely cause, of each stage in the development of the evolving India-Eurasia collision to be examined. I propose a reconnaissance study of the Tien Shan, Dzungar Alatau and Altay mountains of Kazakhstan and western Mongolia. These ranges constitute some of the northernmost parts of the collision between the continents of India with Asia, and very little is known of their development at present. Constraints on the tectonic evolution will come from fission-track and U-Th/He thermochronometry. These techniques, which record the cooling of rocks through the top 1-3 km of the Earth undertaking, will be performed in collaboration with Dr. A. Carter. Of particular interest is the question as to why deformation in the mountain ranges of central Asia started much later than in the Himalaya and Tibet. One potential explanation is that the deformation zone is gradually increasing in width, with the initiation ages of mountain ranges younging to the north. Other leading suggestions are that a rapid rise of the Tibetan plateau 10-15 Million years ago introduced forces in the Asian continent that caused mountain-building to step northwards, or that changes in the forces at the Pacific margin of the Asia caused reorganization of deformation within the interior of the continent. By unravelling the history of mountain building in Kazakhstan and western Mongolia, and by placing firm age constraints on the timing of each stage in their history, I will be able to distinguish between these differing scenarios for the forces responsible for producing the distribution of mountains we see at the present day.We have chosen to focus on the history of mountain building in Kazakhstan and western Mongolia because: (1) The tectonic history of the mountain ranges in this area is poorly understood but is potentially very important for understanding the mechanics of continental deformation in Asia; (2) The mountains are easily accessible, and we have experience of travel in Kazakhstan through a reconnaissance visit in July 2011, such that we are confident that all sampling can be completed successfully in one field-season; (3) Our initial measurements from samples collected from the Altay of western Mongolia indicate that tectonically useful cooling signals are present.