Pliocene-quaternary orogeny in the Central Tien Shan

Formation of mountains of the Central Tien Shan is usually explained by the isostatic response of the crust to its shortening caused by the India-Eurasia collision. The rise of the region in the period from Oligocene to Late Pliocene (2 myr ago) reached ∼700 m on average, which corresponds to the isostatic response. For the last 2 myr (Late Pliocene–Quaternary), the rate of rise increased by an order of magnitude. This is proved by the coarsening of Cenozoic molasse up the sections, acceleration of cutting of drainage systems into ridges, and formation of new ridges within basins. In the Quaternary, most of intermontane basins underwent uplifting, though not so intense. The average rate of lateral crustal shortening increased ∼2–2.5 times only, and the contribution of this process to the Late Pliocene-Quaternary orogeny was no more than 10%. The acceleration of rise was caused mainly by the convective replacement of the mantle lithosphere by the less dense asthenosphere. This was due to the quick softening of the mantle lithosphere as a result of the infiltration of active fluid from the lower mantle. Such accelerations of crustal uplifting took place in the Pliocene and Quaternary in many continental regions. This evidences that mantle processes, first of all, the full or partial replacement of the mantle lithosphere by the asthenosphere, played an important role in the formation of mountains.