Structural evolution of metamorphic rocks in the Talas Alatau, Tien Shan, Central Asia: Implication for early stages of the Talas-Ferghana Fault

The Talas Alatau Ridge consists of three tectonic units separated by thrusts. The central (Talas parautochthon) and northeastern (Kumyshtag sheet) parts consist mainly of unmetamorphosed terrigenous and carbonate deposits. The southwestern part of the Talas Alatau (Usunakhmat sheet) consists of greenschists and is bounded by the Central Talas Thrust (CTT) and Talas-Ferghana Fault (TFF). The latter is a major strike-slip fault with post-Permian dextral displacement estimated at as much as 200 km. A structural study shows that elongation axes of strain ellipsoids, stretching lineation and map-scale fold axes, as well as the trend of major faults including the CTT and the TFF, are parallel to each other. The natural deviatoric strain value gradually increases from the CTT to the TFF. The shape of the strain ellipsoid gradually varies within the Usunakhmat sheet from oblate at the frontal thrust to prolate in internal parts of the unit. The magnitude of shortening increases towards the CTT, whereas natural deviatoric strain decreases towards the CTT implying structural thickening of internal parts of the Usunakhmat sheet. Strong parallelism of the TFF, CTF and other map- and outcrop-scale structures and their close connection with strain patterns show that they were formed during the same tectonic event. They were likely formed during Middle to Late Ordovician collision and built up Kyrgyzian microcontinent as a thrust fold belt whereas reactivation of the TFF as a strike-slip fault occurred later, in Late Paleozoic.