Controls of the Foreland Deformation Pattern in the Orogen-Foreland
Shortening System: Constraints from High-Resolution Geodynamic Models
Abstract
Controls on the deformation pattern (shortening mode and tectonic style)
of orogenic forelands during lithospheric shortening remain poorly
understood. Here, we use high-resolution 2D thermomechanical models to
demonstrate that orogenic crustal thickness and foreland lithospheric
thickness significantly control the shortening mode in the foreland.
Pure-shear shortening occurs when the orogenic crust is not thicker than
the foreland crust or thick, but the foreland lithosphere is thin
(< 70-80 km, as in the Puna foreland case). Conversely,
simple-shear shortening, characterized by foreland underthrusting
beneath the orogen, arises when the orogenic crust is much thicker. This
thickened crust results in high gravitational potential energy in the
orogen, which triggers the migration of deformation to the foreland
under further shortening. Our models present fully thick-skinned, fully
thin-skinned, and intermediate tectonic styles in the foreland. The
first tectonics forms in a pure-shear shortening mode whereas the others
require a simple-shear mode and the presence of thick (>
~4 km) sediments that are mechanically weak (friction
coefficient < ~0.05) or weakened rapidly
during deformation. The formation of fully thin-skinned tectonics in
thick and weak foreland sediments, as in the Subandean Ranges, requires
the strength of the orogenic upper lithosphere to be less than one-third
as strong as that of the foreland upper lithosphere. Our models
successfully reproduce foreland deformation patterns in the Central and
Southern Andes and the Laramide province.