The Mesozoic thermotectonic evolution of the western Tian Shan (Kazakhstan, Uzbekistan, Tajikistan)

Abstract

The crustal architecture of Central Asia formed during the closure of the Palaeo-Asian Ocean 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 study the full extent of the Mesozoic thermo-tectonic history of the Tian Shan. In this study, we used multiple thermochronometers (zircon fission track, apatite fission track, and apatite (U-Th- Sm)/He) on >100 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) in the middle, and the Kyzylkum–Nurata Segment (Uzbekistan and Tajikistan) 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 closure of the Palaeo-Asian Ocean. Subsequently a period of slow-cooling and denudation during the late Jurassic–Cretaceous was recorded, as the western margin of the Tian Shan was sheltered from much of the reactivation experienced by the Tian Shan. However, the major suture zones in the Western Tian Shan still preserve a signal of late Jurassic–Cretaceous fast-cooling and reactivation. Apatite fission track age–elevation and age-length “boomerang” plots will be presented that provide evidence for this multi-phase and differential Mesozoic thermal history. This study highlights that the Cretaceous deformation history of the Tian Shan was focused on its major fault zones, while the hinterland only records slow cooling. This pattern suggests that the Cretaceous Tian Shan was formed by extensional tectonics, where fault reactivation records rapid cooling by footwall exhumation. Our hypothesis is supported by recent geodynamic (Gplate) models that suggest Central Asia was under extension during the middle Cretaceous. Therefore, we envisage the Cretacoeus Tian Shan landscape and deformation style to be comparable to the present-day Basin and Range province in the south-western USA.