The ocean that tried to swallow itself

Wegener’s famous supercontinent Pangaea lasted for about 200 Ma from the mid Carboniferous to the late Triassic, and formed a ‘slice world’ extending almost from pole to pole. Yet it had a vast spreading embayment on its eastern side around which wrapped two ‘horns’ of continental lithosphere: an ocean dubbed ‘Palaeotethys’. Another peculiarity is that at its core Pangaea is marked by a huge, orogenic belt that seems to have been buckled on a continental scale: the Iberian-Amorican Arc. Such refolded mountain chains are sometimes referred to as ‘oroclines’, and there is considerable debate about how they might have formed. The latest notion is that slab-pull at a north-dipping subduction zone at the northern edge of Palaeotethys not only caused its spreading centre to be consumed, but thereafter continued to suck at the remaining ocean lithosphere (Gutiérrez-Alonso, G. et al. 2008. Self-subduction of the Pangaean global plate. Nature Geoscience, v. 1, p. 549-553). The stresses involved in attempted closure of the wedge-shaped ocean spur on the otherwise elliptical supercontinent would explain several roughly radial rift systems with voluminous magmatism that formed in Pangaea in Permian times, such as the Oslo graben. Ever ready for a bit of fun, New Scientist has referred to Pangaea in terms of an aged, but well-known computer-game object that apparently turned on itself after consuming all lesser objects (Palmer, J. 2008. Pac-Man supercontinent ate itself to pieces. New News Service, 6 July 2008  


Tibetan Plateau reviewed

The roughly 5 km high Tibetan Plateau is not only the largest area of high elevations on Earth, it helps generate the monsoons of southern and SE Asia. Some have argued that it is a major climatic driver and may have been responsible for overall cooling of the Northern Hemisphere by diverting wind patterns once it had reached its present extent. Tibet may even have influenced global cooling through the Cenozoic by encouraging extraction of CO2 from the atmosphere by liberating enormous quantities of silicate minerals for chemical weathering. From a tectonic standpoint the Plateau is especially fascinating. In the mid-1970 Molnar and Tapponnier proposed that the near-doubling of Tibet’s crustal thickness had created unstable conditions and that crust was being extruded eastwards as a result of gravitational collapse: an evolving example of escape tectonics. There are hundreds of papers on or relating to the Tibetan Plateau, its origin and evolution, so a succinct review is welcome (Roydon, L.H. et al. 2008. The geological evolution of the Tibetan Plateau. Science, v. 321, p. 1054-1058). This centres on the development of the escape tectonics idea over 3 decades, and offers an important regional insight. Widening the context to include the evolution of the West Pacific oceanic lithosphere reveals a link between the timing of plate tectonic events there and  changes in crustal collapse far to the west in eastern Tibet and adjacent lands. Soon after India began to collide with Eurasia in the Eocene, the subduction zones of the West Pacific and Indonesia migrated ridgewards, away from Eurasia as a result of trench rollback. This created space into which crustal collapse could spread as the Himalaya and Tibetan Plateau were thrown up. This trench migration stopped during the Miocene, severely interfering with the gravitational possibilities for escape tectonics. Effectively, the escape from Tibet was ‘dammed’, and it is from that date that the phenomenal rise to 5 km elevation has taken place. The authors even link this hindrance to the development of seismically hazardous conditions throughout western China, such as the Longmenshan mountains where the magnitude 7.9 12 May 2008 Wenchuan earthquake occurred (Burchfiel, B.C. et al 2008. A geological and geophysical context for the Wenchuan earthquake or 12 May 2008, Sichuan, People’s Republic of China. GSA Today, v. 18 (July 2008), p. 4-11)

See also: Kerr, R.A. 2008. Pumping up the Tibetan Plateau from the far Pacific Ocean. Science, v. 321, p. 1028-1029

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