A TALE OF TWO OROGENS: TECTOSTRATIGRAPHIC EVOLUTION OF ZAGROS AND HIMALAYA

Tectonostratigraphic comparisons of Zagros (1800 km long) and Himalaya (2400 km long) reveal certain fundamental similarities as well as distinct characteristics in the tectonic evolution and anatomy of these two high mountain belts bordering, respectively, the Iranian and Tibetan plateaus. Both Zagros and Himalaya are part of the larger Alpine-Himalayan belt which originated from the closure of Neo-Tethys and collision of Gondwana continental plates with Eurasia. The plate collision of Arabia (Oligocene) and India (Eocene) took place diachronously in a NW-SE (counterclockwise) direction. The continental collision in both Zagros and Himalaya was preceded by a Late Cretaceous-Eocene Andean-type magmatic arc: Urmieh Dokhtar Belt in Iran and Kohistan-Lhasa Belt in southern Tibet. The foreland propagating thrusts in both Zagros and Himalaya followed the same polarity of Neo-Tethys slab subduction. However the post-collisional styles of deformation also vary between and within the two orogens. In Zagros, the High Zagros (up to 80 km wide) is still a thin-skinned imbricated thrust wedge while the Simply Folded Zagros (up to 300 km wide) is dominated by ‘whale-back’ anticlines and localized thrust faults. The Infracambrian Hormoz Salt or equivalent shale has served as an underlying detachment horizon for the deformation in both tectonic zones. In Himalaya (250-300 km wide), the Higher Himalayan Cyrstalline Zone is a thick-skinned thrust wedge bounded by a basement rupture, Main Central Thrust (MCT), to the south, and an orogen-parallel, MCT synchronous extensional structure, South Tibetan Detachment, to the north which marks the southern boundary of Tethyan Himalayan (Cambrian-Eocene) sediments. Miocene-age crustal melts form leucogranite summits between the Higher Himalaya and Tethyan Himalaya. The Lesser Himalaya (Proterozoic metasediments) to the south of MCT is thrust over the Siwalik (Neogene) foreland sediments along the Main Boundary Thrust. Unlike Himalaya, Zagros foreland basin was superimposed on Neo-Tethyan shelf sediments – hence its rich petroleum fields. Basement reactivation and greater elevation and crustal thickness in Himalaya (70 km, compared to 55 km in Zagros) are likely because of the relatively faster motion of Indian plate (5 cm/yr) compared to the Arabian plate (2 mm/yr).