Analog Model of Gravitational Collapse and Surface Extension during Continental Convergence

MOST mountain belts occur where continents collide, so it is not surprising that the dominant form of surface deformation is shortening. But there is also abundant geological evidence for extension in the central parts of many mountain belts, at the same time as shortening occurs elsewhere. Previous models for extension require temporal changes in the thermal structure of the lithosphere(1,2), the rate of convergence(2,3), the strength of the crust(3) or the geometry of accretion(4). Here we present a simple model in which no such changes are required for surface extension during the convergent thickening of a viscous 'crustal' layer. Convergence is driven by the motion of two plates at the base of the layer. In the area where one plate 'subducts' under the other, the surface begins to extend soon after the start of convergence, eventually stretching by more then 60 per cent. Extension occurs because gravity drives horizontal flow faster at the free surface than in the centre of a viscous layer that is fixed at its base. In real mountain belts, mid-crustal weaknesses may allow the depth-dependent motion required for surface extension.