OROGENIC EVOLUTION ABOVE A STRIKE-LIMITED SUBDUCTION ZONE: INSIGHT INTO SYNTAXIS DEVELOPMENT

Interactions between tectonics and surface processes has been a fertile research field for the last twenty years and significant insight into convergent orogen evolution has been derived from numerical models that couple surface and mechanical processes. Vertical section 2-D plane-strain models have been particularly useful in applications to the dominantly orthogonal portions of convergent orogens, where significant out-of-plane motion is not expected. However, most convergent orogens exhibit considerable 3-D deformation, near the collision/subduction zone edges and/or due to oblique convergence, which cannot be fully examined using 2-D models. We present 3-D numerical experiments from a recently developed thermo-mechanical model that simulate the deformation of a layered viscous-plastic continental crust driven by kinematic boundary conditions that result in subduction of the underlying mantle lithosphere. These experiments provide insight into the tectonic evolution of this system subject to surface processes, complex velocity boundary conditions and isostasy.

We consider two cases where the boundary conditions result in normally directed mantle-lithosphere subduction beneath a strike-limited region. The mantle detachment location prior to subduction ('s-line') is either fixed in space or advances into the over-riding plate with time, simulating an advancing subduction zone, such as in the Himalayan-Tibetan orogen. In the absence of surface erosion, the central part of both orogens grows as a linear range above the s-line interface, as in standard 2-D models. Topographic development near the end of the range is also similar for the two orogens, but the advancing s-line experiments have increased curvature and produce a trailing topographic tail. The tail develops along the shear zone interface between the two plates (similar to the Himalayan syntaxes) and grows in the direction opposite to the subducting plate vergence. Models with surface processes and erosion show curvature near the end of the orogen that is more pronounced, but the formation of the orogen tail is less clear. Overall, these results suggest that strike-limited s-line motion likely plays an important role in the development of orogenic syntaxes, such as those in the Himalaya, which may be enhanced by oblique convergence.