Scale dependence, strain compatibility and heterogeneity of three-dimensional deformation during mountain building: a discussion

Plate motion at convergent margins causes crustal shortening and orogenic thickening. Relative motion that is oblique to the plate margins is an inevitable consequence of plate kinematics on a sphere and results in non-coaxial three-dimensional deformation that cannot be approximated to simple shear. Models of mountain building that include a smoothly varying component of pure shear shortening allow strain compatibility to be maintained by deforming the upper free surface of the Earth without disrupting the material continuum. However such models do not reflect accurately the nature of deformation in many areas of high strain in the upper crust, which are characterized by interconnected arrays of kinematically linked faults that can be active on several scales of magnitude simultaneously. As brittle deformation increases, the coherence of the material continuum is highly reduced. In such situations, strain compatibility is maintained by partitioning the deformation amongst structures of varying kinematic significance over a wide range of scales and not by smooth variations in strain magnitude acting on a single scale across a material continuum. There is a marked tendency for such partitioned domains to be oriented parallel or sub-parallel to the orogenic grain. Alignment of domains in this way represents a strong structural anisotropy, which acts as a highly significant boundary condition that controls deformation at subordinate scales. Finite strain observed within an individual domain at a given scale need not therefore display the same magnitude or orientation as bulk finite strain at the plate scale and consequently data must be collected from as large an area as possible to relate outcrop-scale structures to global-scale tectonics.