Plate Tectonics

Active collisional orogens represent the most impressive topographic features on Earth, with enormous masses of rocks being uplifted, exposed to surface erosion and sculptured into lofty mountains and deep valleys that show vast diversity in terms of climate, biodiversity, natural resources. In this chapter the modern Himalaya is presented in more detail, together with the Alpine system and the older Grenvillan orogenic belt that provides a deeper level of orogenic erosion. Differences are discussed and related to tectonic setting, duration of collisional phase, rigidness of colliding continents, synorogenic crustal heating and precollisional accretionary history. This chapter demonstrates that while the convergent Himalayan-Tibetan system generates a regime of thrust tectonics along its margins, the Tibetan plateau is dominated by extensional tectonics and strike-slip deformation. This is related to orogenic collapse and lateral extrusion linked to flow of partially molten middle to lower crust. Geophysical data are presented that give support to this interpretation. The Scandinavian Caledonides are presented as an example of a relatively short lived but major collisional orogen with deep continental subduction and a strong phase of extensional collapse.