STRUCTURE, WHITE MICAS AND STRAIN ACROSS THE TALAS ALA TAU (NORTHERN TIEN SHAN). INSIGHTS ON THE TECTONIC EVOLUTION OF CENTRAL ASIA

The Talas Ala Tau is a NW-SE trending mountain range located in Northern Tien Shan (Kyrgyz Republic) bounded to the SW by one of the largest faults across Central Asia, the Talas Fergana Fault. The range consists of two large northeast verging units formed by Riphean to Vendian rocks known as the Uzunakhmat and the Karagoin units. Detailed cross sections of the range allowed to build a composite cross section that consists on a well developed imbricated system of possible Baikalian age (Caledonian or older) with subsequent Uralian (Variscan) and Himalayan overprints resulting in local apparent vergence inversions. Most of the thrusts developed low grade phyllonitic rocks generally post-dating a D1 folding event that imparted axial plane cleavage to the rocks. White mica analysis, including illite crystallinity, SEM, TEM and microprobe analysis indicate a consistent low to very low metamorphic grade along the complete cross section and a constant decrease of the metamorphic grade towards the northeast. In addition, there is an important retrogradation related to the D2 thrusting event, related possibly to the different deformation mechanisms involved. Furthermore, the bo parameter indicates a consistent mid to high pressure gradient, which combined with the low to very low metamorphic grade can be interpreted as generated in a fast burial process only possible in a subduction environment. Accordingly with the metamorphic pattern, the strain values measured along the same cross section, depict a similar evolution which is also recognizable using other structural parameters. Accordingly to the presented data, and because of the lack of exposure to the SW due to the large right-lateral displacement of the Fergana Fault, we can interpret that the Baikalian-Caledonian event in North Tien Shan was related, at least partially, to a subduction event towards the SW in present day coordinates.