SEDIMENTOLOGICAL EVIDENCE FOR EARLY MESOZOIC CRUSTAL SUTURING OF THE PAMIR

The Pamir Mountains form the western syntax of the Himalayan-Tibetan orogeny and are composed of three tectonic terranes: the Northern Pamir, the Central Pamir, and the Southern Pamir that are separated by the Tanymas suture and Rushan-Pshart suture, respectively. The Northern Pamir is part of the Paleozoic-Late Triassic southern margin of Asia and before accretion of the Central Pamir it was bounded to its south by the northward-subducting Paleotethys Ocean basin. The Central and Southern Pamir are Gondwanan terranes that rifted as crustal fragments during the Carboniferous. One outstanding question about the region pertains to the timing of suturing between the Gondwana-derived terranes and the southern margin of Asia. Two models have been proposed to account for the timing of suturing of the Gondwanan fragments: 1) The timing of collision between the Northern Pamir and the Central Pamir along the Tanymas suture was coeval with the collision between the Central and Southern Pamir along the Rushan-Pshart suture during the Late Triassic to Early Jurassic Cimmerian Orogeny or 2) The Tanymas suture formed during the Late Triassic to Early Jurassic, whereas the Rushan-Pshart suture formed during the Late Jurassic. In the Southern Pamir, terrigenous deposits of the Hettangian Darbasatsh formation cap the regional post-Cimmerian unconformity, and provide key information for resolving the debate. New detrital zircon analyses of the Darbasatash formation yield a dominant Permo-Triassic age population and a subordinate early Paleozoic (Silurian-Devonian) population. The only Permo-Triassic arc signature within the region is found in the Karakul Mazar terrane of the Northern Pamir, and the Early Paleozoic signature is characteristic of the Kunlun terrane of the Northern Pamir. Our results suggest: 1) collision between the southern margin of Asia and all Gondwanan terranes of the Pamir was completed by the Early Jurassic (Hettangian) and 2) the Northern Pamir were topographically elevated relative to the Central and Southern Pamir upon closure of the Paleotethys Ocean basin.