Extension during continental convergence, with application to the Tibetan Plateau

The Tibetan plateau is the product of crustal thickening caused by the collision between India and Asia and is the largest active example of extensional tectonics in a zone of continental collision. Throughout most of the Tertiary, the tectonics of the plateau were dominated by north‐south shortening, a significant proportion of which took place on east‐west striking thrust faults. For the last 5 m.y. or so the plateau has been thinning by the mechanism of extension on north‐south trending normal faults. Previous investigations of the collision have been able to account for the large‐scale features of the Tertiary deformation but have failed to explain the transition, in the late Tertiary to Quaternary strain rate field of the plateau, from north‐south compression to east‐west extension. The transition could, in principle, be effected by a reduction in the rate of convergence between India and Asia or by a uniform reduction in strength of the whole continental lithosphere of Asia. It could not, however, be effected by a reduction in strength of the elevated region alone; this produces increased compressional strain rates in the weakened zone. An alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere. The lower part of the lithosphere consists of a thermal boundary layer which, when thickened by horizontal shortening, is colder and denser than its surroundings. Convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere. The convective instability would happen in a time brief compared with the collision time scale (∼50 m.y. in the case of India and Asia) but would only occur after there had already been substantial thickening of the lithosphere. Numerical experiments show that for a range of lithospheric parameters, the increase in surface height (as much as 2 km) and of potential energy (5 to 10 × 10 12 N m −1 ) resulting from convective instability of the lower lithosphere are sufficient for east‐west extension to replace north‐south compression as the dominant feature of the stress field of the Tibetan plateau.