Tectonic stresses associated with collision mountain ranges are studied by finite element modelling. Large horizontal deviatoric tension is produced in the strong elastic core of the upper crust by the topographical loading and the associated crustal root. Equally significant is the effect of the dense downbulge of cool lithospheric mantle produced by plate convergence, as recently observed beneath the Alpine region. This dense slab with its associated downflexure produces compression. This compression is 25% or less of the theoretical value computed from the density-moment function because of the finite width of the slab. The actual state of upper crustal stress is a superimposition of root-produced tension and slab-produced compression. An asymmetrically located deep slab produces large compressions at one edge of a mountain belt co-existing with large tensions towards the other side of the range, leading to mountain building at one edge and extensional tectonics elsewhere. The stresses produced by the dense slab support and accentuate the crustal root, in opposition to the self-inflicted root erosion. When the upper crust is thrust-faulted, the sinking slab produces a collision slab pull force of about 1.2 × 1012 N/m (depending on slab size) which is an essential factor with ridge push in initiating and developing mountain ranges. Surface downflexure produced by slab downpull may give rise to wide borderland sedimentary basins such as the Po basin. © 1990.
