The timing and cause of mountain building in central Asia

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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.