Abstract

A series of finite element models of contractional deformation are presented that examine the role preexisting mechanical heterogeneities within the lithosphere play in controlling the partitioning of stress and strain during orogenesis. The models focus particularly on orogens that develop near previously rifted continental margins, although many of the observations are relevant to other fold and thrust belts. The models indicate that orogen evolution is strongly influenced by crustal architecture during the first 150 to 200 km of shortening. Major decollements develop at midcrustal and lower crustal levels, partitioning strain into upper crustal, lower crustal, and mantle strain domains. The magnitude and spatial distribution of strain within each domain are nearly uniform but differ significantly between domains. The decollements extend throughout the entire width of the orogen, providing a successively increasing degree of decoupling between deformation in the shallow crust and deeper levels. The midcrustal decollement terminates beneath the leading edge of the thrust front, but the deep crustal decollement extends more than 500 km beyond the foreland. This allows up to 5%–10% strain to develop well in front of the orogenic belt. In the interior of the orogen, deformation extends through all levels of the crust beginning during the earliest stages of contraction. Deformation propagates continentward with increasing shortening, with the foreland regions undergoing the most intense shortening in the shallow crust. The loci of maximum total shortening within the upper and lower crust are laterally offset, with the thickest crust lying continentward of the greatest elevations and the site of most intense surface deformation. This pattern of crustal thickness variation is a primary feature of orogenesis in the models and is not related to postorogenic collapse. After 150 to 200 km of shortening, sufficient topography develops to cause the balance between rock strength, gravitational forces, and shear stress on the basal detachment to dominate the style of deformation. Preexisting mechanical heterogeneities thereafter exert little influence on orogenic processes.

First Page Preview

First page PDF preview
You do not currently have access to this article.