The low-temperature tectonic evolution of the Western Tian Shan (Kazakhstan, Kyrgyzstan, Uzbekistan, Tajikistan)

Abstract

The evolution of intracontinental mountain ranges represent a unique challenge in our understanding of the Earth’s history. One of the largest of these intracontintal mountain ranges on Earth is the Tian Shan in Central Asia. The crustal architecture of the Tian Shan formed during the closure of the Palaeo-Asian Ocean in the Palaeozoic and underwent multiple reactivations in response to distant tectonic events at the Eurasian plate margin, to give rise to the modern expression of the Tian Shan we see today. Most of the previous thermochronological studies in the Tian Shan focused on the core of the mountain range, deciphering a punctuated tectonic history throughout the Mesozoic, with a dominant Cenozoic overprint as a result of the India-Eurasia collision. However, little work has been done on the eastern and western extents of the Tian Shan. These regions are characterised by low relief and record less Cenozoic overprint, providing opportunity to better constrain the Mesozoic thermo-tectonic history of the Tian Shan. In this project, we used multiple thermochronometers (apatite uranium-lead, zircon (UTh- Sm)/He), zircon fission track, apatite fission track, and apatite (U-Th-Sm)/He) on >100 basement samples taken along the western-most extent of the Tian Shan. More specifically, samples from the Karatau–Talas range in the north (Kazakhstan and Uzebkistan), the Chaktal–Kurama range (Uzbekistan and Tajikistan) and the Kyzylkum–Nurata Segment (Uzbekistan and Tajikistan) in the middle, and the Garm region in the south, were analysed to decipher key periods of deformation and mountain building in the Tian Shan. Initially, the western Tian Shan underwent a period of Triassic–early Jurassic fast-cooling and exhumation interpreted as a response to the collision of the Qiangtang block with the Eurasian margin. Subsequently a period of slow-cooling and denudation during the late Jurassic–Cretaceous was recorded, as the western margin of the Tian Shan experienced a hiatus in reactivation. However, the major suture zones in the western Tian Shan still preserve a signal of late Jurassic–Cretaceous fast-cooling and reactivation, as a possible response to extensional tectonics on the Eurasian margin. Finally, much of the Mesozoic Tian Shan is overprinted by the distal effects of the India-Eurasia collision in the Cenzoic. The India- Eurasia collision, the latest and possibly the largest of the Gondwanan fragments to collide with the Eurasian continent, generated a thermal overprint along faults and generated most of the topography that we see today. Using multiple middle to low temperature thermochronometers, this study attempted to unpack the complex and convoluted evolution of the world’s largest intracontinental mountain range. Showing that it is possible to track the evolution, both spatially and temporally, of such a vast orogen from its ancestral foundation through to the more modern controls of continental growth. Furthermore, it has been possible to apply constraints and unpack the tectonic regimes that influence the growth of an intracontinental mountain range, providing an analogue for the definition of other intracontinetal tectonic settings throughout the globe.