What controls the geometry of large collisional orogens: insights from numerical modelling

English Abstract:

When continents collide mountain ranges with high topographies and complex geometries are formed. Compressional stresses during ongoing convergence result in crustal thickening, localized deformation, and material transport. Thus, crustal material is transported and redistributed within the orogen.

We use numerical thermo-mechanical models to investigate the physical processes of continent collision zones and its implications on crustal scale deformation and architecture. We demonstrate that compression of two continental blocks, separated by a rheologically weak suture zone can result in (i) double-vergent or (ii) single-vergent orogens, with distinct geometries, deformation and exhumation patterns. The transition between these different modes of collision is controlled by the rheology of the continental lithosphere and its temperature. Coupled crustal layers form double vergent orogens, while decoupled crustal layers result in single-vergent orogens.

Double vergent orogens are formed in response to the gradual accretion of crustal material along pro- and retro-shears (fore- and back-thrust). In these models, deformation is highly localized on the retro-side, where large off-sets are recorded. At Moho depth, the progressive decoupling of crustal material from the mantle lithosphere, leads to subduction of mantle lithosphere and accumulation of lower crust material at the plate interface. Typical examples include the collision recorded by the Swiss Alps and the Pyrenees.

In contrast, single-vergent orogens are characterized by large-scale lower plate deformation and are accompanied by the subduction of lower continental crust. In this situation, no significant retro-shear formation is observed, which is in agreement with recent physical modelling studies on deformation of the continental lithosphere. The resulting orogen is highly asymmetric and almost exclusively oriented towards the foreland. Natural examples of such single vergent orogens are common in the Mediterranean (Carpathians, Dinarides, Apennines, Betics) or the SE Asia subduction zones.

We conclude that deformation and exhumation in continent-continent collision zones may occur in foreland or hinterland settings, depending on the rheological structure of the continental lithosphere.