Peak lithospheric strength should reside in the rocks that, under the
applied stress, cannot either creep (due to low temperature) or break
(due to high confining pressure). The greatest resistance comes from dry
olivine/pyroxene-rich upper mantle/lowermost crust at Moho conditions
(400-600 degrees C and > 1 GPa). We have conducted laboratory
experiments to investigate the importance of the unbreakable core of the
lithosphere in between its brittle and ductile parts and conclude that
it can control the large-scale lithospheric deformation pattern under
shortening. Regardless of the thickness of the unbreakable core, it acts
as a restraining layer that is easily flexed but is unstretchable. This
eliminates large scale brittle faulting or homogeneous thickening as
available shortening modes and results in irregular wrinkling of the
unbreakable layer. We discuss geodynamic implications of our laboratory
experiments and advocate studies of large scale buckling of the
lithosphere as a relevant shortening mode. (c) 2008 Elsevier B.V. All
rights reserved.