Home – Home – Jornals – Russian Geology and Geophysics 2003 number 12

The Pamir-Himalayan and Central Asian intracontinental fold belts developed to follow different geodynamic scenarios. In the first case, it was an ultimate version of hard collision, when cratons directly interacted with the Early Precambrian crust and thick lithosphere mantle. The second case was a soft collision, which came to an end in Late Paleozoic-Early Mesozoic time, without reaching the stage when the Siberian, Sino-Korean, and Tarim cratons would be struck. Direct geophysical observations cannot be used for ancient epochs, but the thickness of lithosphere of colliding plates and microplates can be inferred from Sr-Nd isotope characteristics of granitoid batholiths, which depend on average composition and age of the crust, thus indirectly indicating the thickness of a genetically related underlying lithosphere mantle. The upper mantle dynamics has been analyzed for different stages of collisional orogeny. It has been concluded that at the moment of inversion (the beginning of the early collision stage), a slab is detached and an asthenosphere swell appears in the vicinity of the future collisional building immediately beneath the Moho discontinuity. As a result, short-term anomalous temperature gradients appear in the lower crust, large-scale melting occurs, and bimodal volcanic series form, which, on the one hand, still retain suprasubduction geochemical labels but, on the other hand, reflect the composition of the lower crust subjected to advanced melting. Then the collisional orogeny follows the classical scenario of the thickening of the crust and its lithosphere root, covering the period from the end of the early orogeny stage through the late orogeny stage. The time of formation and extent of an orogen depend on the thickness of colliding plates, and the composition of granitoid batholiths is directly correlated with the composition of the geologic environment. The relationship with the mantle, if any, is expressed in specific forms, e.g., in the form of rifts in the orogen foreland during a frontal collision or in the form of feathering convergent and divergent strike-slip faults when the collision is oblique. The dynamics of development of collisional orogens radically changes at the postcollision (taphrogenic) stage. With a greater thickness of the lithosphere root (Pamirs-Himalayas), the density instability causes the lithosphere delamination, and asthenosphere flows move beneath the Moho, thus causing a drastic rise in relief, followed by the orogen's collapse. With a smaller thickness of the lithosphere (arc-arc, arc-seamount, arc-microcontinent, and other collisions), there is no delamination, and the orogen's breakup is due only to gravitation landslides and detachments in the crust. Central Asia is unique for the presence of a large lower-mantle plume. Therefore, the processes of the seemingly classical soft collision actually led there to the initiation of local plumes beneath folded orogens. A model is proposed for the induced plumes that permits the formation of giant granitoid batholiths or their source areas at the postcollision stage, as well as their specific composition combining plume and collision characteristics.