WIDESPREAD AREAL EXTENSIONAL VOLCANISM ATOP A SHORTENING OROGEN: IMPLICATIONS FOR CRUSTAL EVOLUTION

A widespread area (about 1/4 million square kilometres) became covered by volcanics with an average thickness of 1 km during the 11Ma to the present interval in eastern Turkey. Much of the volcanism occurred during the 11 to 2 Ma ago interval and almost all of it was alkalic in chemistry with some calc-alkalic volcanics in the north. The rocks erupted ranged from trachytes to basalts but subordinate amounts of phonolites have also been recorded. In the Quaternary, volcanicity became more localised around major eruptive centres, but these centres were also not confined to any lines and were randomly distributed in the area.

The region affected by volcanism is a high plateau where intensive north-south shortening after the Arabia/Eurasia collision has been going on since the end of the Oligocene expressed by widespread folding, thrusting and strike-slip faulting. There has been very minor north-south striking normal falting indicating east-west extension. Only one of the youngest stratovolcanoes can be shown to have been nucleated on a north-south striking fissure.

The type of volcanism in eastern Turkey has remained typical of areas of crustal thinning, mantle upwelling, and adiabatic melting, while the underlying crust was actively shortening, thickening and cooling. This anomaly is believed to result from the presence under the volcanic terrain of an initially about 25 km thick accretionary wedge overying a subducting oceanic lithosphere. Following the Arabia/Eurasia collision the oceanic lithosphere progressively steepened from north to south and eventually detached, sucking the asthenosphere towards the underbelly of a thin accretioanry prism. While being thus sucked, the asthenosphere rose and became adiabatically decompressed leading to widespead melting and basalt generation under the accretionary complex. This led to widespread heat introduction into the thickening accretionary complex and to its melting. The "extensional" signature comes from the adiabatic melting of the asthenosphere whereas the observed crustal additions come from this melting. This sort of areal volcanism owing to slab rollback and detachment after collisions is more widespread, especially in Turkic-type orogens, than hitherto reported.