パミールにおける新生代の地質構造発達史に関する最近の研究

Abstract

The Pamir is a mountain region in the westernmost part of the Himalayan–Tibetan orogen. It extends 〜300 km from north to south and 〜300–400 km from west to east. The Pamir lies on a double subduction zone, where two buoyant continental plates have subducted several hundred kilometers deep into the asthenosphere. The southern plate is the northward-dipping Hindu Kush slab, and the northern plate is the southward-dipping Pamir slab. The subduction of the Hindu Kush slab began at 〜8 Ma. This narrow part of the Indian plate subducts to a depth of 〜600 km and has a neck at a depth of between 250 and 300 km, causing lithospheric thinning. The Pamir slab, a part of the Asian plate, probably started subduction at 〜25 Ma accompanied by slab rollback (possibly with the back-arc extension), subduction erosion, subduction accretion, and marginal slab-tear faulting. This slab, subducting to depths of 〜400–450 km, forms a broad arcuate shape split at the center due to a vertical tear extending from a depth of 〜200 to 〜400 km. The current geometry of both slabs implies the possible occurrence of tectonic events, such as slab break-off, in the near geological future. The metamorphic and exhumation history of the crystalline basement domes in the Pamir reveal regional tectonic evolution since the Cenozoic India–Asia collision. Prograde metamorphism of the mid–lower crust, driven by crustal shortening/thickening, continued from 45 to 25 Ma. Subsequently, retrograde metamorphism and exhumation of the mid–lower crust began during the period 25–15 Ma. This change suggests a transition from crustal shortening/thickening to crustal extension. A possible cause of this transition is that the Pamir slab obtained sufficient gravitational potential to enable the gravitational collapse of the crust, mainly due to the isostatic uplift of the orogen, which was triggered by the break-off of the Indian plate during 25–15 Ma. Subsequently, activation of extensional exhumation in the southwest and eastern Pamir was synchronous with magmatism in the eastern Pamir at 〜10 Ma. At present, the Pamir region is characterized by an active E–W extension of the Kongur Shan footwall to the east and a westward lateral extension of the western Pamir into the Tajik–Afghan Basin.