Mesozoic tectonics of the Gondwanan terranes of the Pamir plateau

Abstract

The Central and Southern Pamir terranes represent Gondwanan crustal fragments within the Pamir plateau, and are bounded by the Tanymas, Rushan–Pshart, and Wakhan-TBZ suture zones from north to south. Stratigraphic and geochronologic data suggest that collision of the Central and Southern Pamir terranes with the southern margin of Asia was broadly coeval during the latest Triassic–earliest Jurassic Cimmerian orogeny (∼200Ma). The Cimmerian orogeny in the Pamir region does not appear to have resulted in significant crustal thickening of the Central and Southern Pamir as both terranes are overlain by Lower to Middle Jurassic carbonate platform deposits. Post-suturing latest Triassic–earliest Jurassic sandstones with a strong arc signature in the Southern Pamir suggest derivation from the Karakul–Mazar terrane to the north, implying the Northern Pamir were a region of elevated topography relative to the Gondwanan terranes immediately after closure of the Palaeotethys and Rushan ocean basins.A key question in the post-Cimmerian tectonic evolution of the Pamir is the extent and magnitude of Mesozoic deformation of the Central and Southern Pamir. Three lines of evidence suggest that much of the post-Jurassic crustal shortening documented in the Central and Southern Pamir is related to Mesozoic tectonic activity, rather than the Cenozoic India–Asia collision as generally interpreted: (1) The Southern Pamir does not record post-Jurassic marine deposition indicating the terrane elevated above sea level by the Early Cretaceous, most likely due to crustal shortening and thickening; (2) Coarse terrigenous deposits distributed throughout the Central and Southern Pamir interpreted to be syn-tectonic with regional folding and thrust faulting are poorly dated with many studies interpreting them to be Jurassic to Cretaceous rather than Paleogene; (3) Strong Cretaceous crustal thickening and shortening documented to the south in the Karakoram terrane, and to the north in the Northern Pamir, suggest that the Central and Southern Pamir were similarly affected. These relationships suggest that most of the upper crustal shortening documented in the Central and Southern Pamir is related to retroarc deformation north of the Jurassic–Cretaceous Andean margin that developed within the Karakoram and Southern Pamir terranes during subduction of the Neotethys Ocean. Thus, the Pamir plateau may have been a westward continuation of the pre-Cenozoic “Lhasa-plano” of the Tibetan plateau.