Obduction

Abstract: The Tibetan Plateau, between the Kunlun Shan and the Himalayas, consists of terranes accreted successively to Eurasia. The northernmost, the Songban Ganzi Terrane, was accreted to the Kunlun (Tarim-North China Terrane) along the Kunlun-Qinling Suture during the late Permian. The Qiangtang Terrane accreted to the Songban-Ganzi along the Jinsha Suture during the late Triassic or earliest Jurassic, the Lhasa Terrane to the Qiangtang along the Banggong Suture during the late Jurassic and, finally, Peninsular India to the Lhasa Terrane along the Zangbo Suture during the Middle Eocene. The Kunlun Shan, Qiangtang and Lhasa Terranes are all underlain by Precambrian continental crust at least a billion years old. The Qiangtang and Lhasa Terranes came from Gondwanaland. Substantial southward ophiolite obduction occurred across the Lhasa Terrane from the Banggong Suture in the late Jurassic and from the Zangbo Suture in the latest Cretaceous-earliest Palaeocene. Palaeomagnetic data suggest successive wide Palaeotethyan oceans during the late Palaeozoic and early Mesozoic and a Neotethys which was at least 6000 km wide during the mid-Cretaceous. Thickening of the Tibetan crust to almost double the normal thickness occurred by northward-migrating north-south shortening and vertical stretching during the mid-Eocene to earliest Miocene indentation of Asia by India; Neogene strata are almost flat-lying and rest unconformably upon Palaeogene or older strata. Since the early Miocene, the northward motion of India has been accommodated principally by north south shortening both north and south of Tibet. From early Pliocene to the Present, the Tibetan Plateau has risen by about two kilometres and has suffered east-west extension. Little, if any, of the India Eurasia convergence has been accommodated by eastward lateral extrusion.

Abstract: The Zagros orogen provides a unique opportunity within the Alpine system to evaluate the interplay between a young Tertiary collision and earlier subduction/obduction processes. Within the Crush zone and the Sanandaj–Sirjan (internal) zone separating the Zagros Fold belt from Central Iran, we document several major tectonic events taking place at the end of the Cretaceous, of the Eocene and from the Mio–Pliocene onwards (ca. <20–15 Ma). Contrary to recent interpretations, our data (cross-sections and description of the overall deformation style) strongly suggest that the Main Zagros Thrust (MZT) is deeply rooted, possibly to Moho depths, and that the suture zone effectively runs along the MZT. Field observations show that the final resorption of the oceanic domain took place slightly after 35 Ma and that collision must have started before ca. 23–25 Ma in northern Zagros. The shortening rate across the Crush zone since the Mid-Miocene (20–15 Ma) is estimated at a minimum 3–4 mm/year. Shear movements in the Crush zone during the Eocene–Oligocene period and extensional/strike-slip movements in the internal zones during the late Cretaceous point to an oblique setting early in the convergence history. A geotectonic scenario for convergence from the time of obduction to the present is finally proposed.

Abstract: This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia-Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj-Sirjan, Kermanshah and Urumieh-Dokhtar magmatic arcs), the P-T-t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW-NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115-85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene-Eocene (60-40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30-0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj-Sirjan Zone: 20-15 Ma, High Zagros: ~12-8 Ma; Simply Folded Belt: 5-0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.