Rock assemblages in central Asia and the evolution of the Southern Asian Margin

By looking in detail at the rock record in Central Asia, it is possible to map rock assemblages that are diagnostic of particular plate settings. Simplest of the assemblages include the rocks of the Tadjik Depression, where Mesozoic and Paleogene sediments record the development of a south facing passive continental margin. Analysis of igneous activity in Central Asia and of the timing of deformation of these miogeoclinal rocks indicates that the Paleotethyan ocean to the south was, in the Mesozoic, subducting to the south beneath a migratory arc consisting of parts of Iran, Afghanistan, and the Pamir. Only in the late Tertiary does the Tadjik Depression develop a peripheral foreland assemblage which indicates any deformation of the miogeocline. This presumably occurred as a result of the closing of the Paleotethyan ocean and accretion of this portion of the arc to the southern margin of Asia. It appears unlikely that the Depression passive margin sequence developed in a normal rift margin setting. Analysis of assemblages in the Hercynian orogen (Tien Shan) to the north indicates that, unlike the Uralian section of the orogen, the deformation here occurred due to the closure of a marginal basin and overthrusting of its basement rocks over a south facing arc. Thus considering Depression basement composition, it appears that the passive margin assemblage is superimposed over either a late Paleozoic forearc or Paleozoic ocean floor. Paleozoic rocks exposed north of the Herat suture in Afghanistan may require the forearc basement interpretation. Noting that the geometry of subduction and the timing of Paleotethyan suturing seem to have varied along strike in the orogen, a more complicated model of tectonic accretion is required.