Deformation within a steady-state compressional orogen, i.e., where
tectonic accretion is, on geological time scales, balanced by surface
erosion, can best be described by a stationary velocity field.
Instantaneous deformation results from spatial gradients in the velocity
field, whereas total accumulated strain results from the integration of
this instantaneous deformation along material paths following the flow
lines defined by the velocity field. We have synthesized the net strain
distributions for rocks exposed at the surface of such an orogenic
system using simple, linear velocity fields corresponding to (a) simple
shear within a dipping shear zone and (b) pure vertical shear. In both
cases we demonstrate the development of surface patterns of finite
strain accumulation that do not reflect the geometry of the assumed
velocity field in a simple manner. Large gradients in finite strain
arise as a consequence of the geographic variation in particle residence
time imposed by the surface boundary, even for the limiting case where
no instantaneous strain gradient exists. Such patterns of deformation
are often recognised in exhumed orogenic systems, but have commonly been
assumed to reflect more complex velocity fields resulting from
nonlinear, localizing crustal rheologies. We therefore demonstrate that
caution should be exercised in interpreting observed strain patterns
because a proportion of the observed strain localization must be
attributed to this purely kinematic (or geometric) effect - and this
proportion may be significant in many systems. Such kinematic effects
should be quantified, and subtracted from observed strain distributions
before they are used to infer the rheological behavior of crustal rocks.
We also suggest that in a simple shear (thrust) setting, kinematic
strain localization may in fact nucleate strain softening on the side of
the deforming region that is stable or fixed with respect to the Earth's
surface and thus be responsible for the asymmetry that characterizes the
large majority of thrust systems. (C) 2010 Published by Elsevier B.V.