Coupling of strain rate, orogen width and plate convergence speed in continental collisions

Most models that describe the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary. Yet a growing body of evidence in the Tibetan orogen suggests that deformation occurred early in the orogen's history at the far northern extent of the modern plateau and thus, our current mechanical understanding of orogenic plateau development is incomplete. Regardless of whether or not high topography was built simultaneously as a result of this deformation, early Cenozoic - present deformation in northern Tibet signifies that the modern limit of the orogen has been held fixed since continental collision began. Therefore strain has not significantly propagated further away from the plate boundary in time and the orogen width has narrowed as convergence continued since collision. Using a uniform average strain rate across the plateau derived from geodetic measurements, I predict changes in plate convergence rate through time as the orogen narrows. Observed decreases in plate convergence speed of India with respect to Eurasia through the Cenozoic (Molnar and Stock, in press) can be explained by the simple condition of a fixed orogen boundary and constant strain rate. This same relationship can also be derived for the Arabian-Eurasia collision however fewer geologic data and fewer plate velocity measurements are available to constrain such a model compared to Tibet.