Collision tectonics between the Tarim block (basin) and the northwestern Tibet plateau: New observations from a multidisciplinary geoscientific investigation in the Western Kunlun Mountains

New results from deep seismic reflection profiling, wide-angle reflection-r efraction profiling and broadband seismic experiments reveal that a series of south-dipping reflectors occur on the southern margin of the Tarim block (basin). However, it is these south-dipping structures that are intercepte d by another series of north-dipping reflectors at depths from 30 to about 150 km beneath the foreland of the W Kunlun Mountains. No evidence from the above geophysical data as well as geochemical and surface geological data indicate the southward subduction of the Tarim block beneath the W Kunlun M ountains (NW Tibet plateau), forming the so-called "two-sided subduction" m odel for the Tibet plateau as proposed by previous studies. So the authors infer that the tectonic interaction between the Tarim block and the W Kunlu n block was chiefly affected by a "horizontal compression in opposite direc tions", which brought about "face-to-face contact" between these two lithos pheric blocks and led to the thickening, shortening and densifying of the l ithosphere. Hence a "delamination" was formed due to the gravitational inst ability created by the thickening and densifying; then alkaline basic volca nic rocks (mainly shoshonite series) was erupted along the northern margin of the Tibet plateau owing to the delamination. This inference for the form ation of the alkaline basic volcanics has been confirmed by recent geochemi cal and petrological studies in Tibet, indicating that different contacts c ontrol different magmatic activities: the alkali basalts are always develop ed in the "horizontal shortening boundary (contact)" on the northern margin of the Tibet plateau, while the muscovite granite and two-mica granite (le ucogranite) in the "subductional contact" on the southern margin of the Tib et plateau.