The D″ region beneath North Central Asia is investigated using a weighted migration based on the Generalised Radon Transform. We image complex structure including a D″ discontinuity with variable topography and velocity contrast. We also present observations of an area where the D″ reflector stops. Using synthetic data sets generated for distributions of point scatterers we show that this is not due to deficiencies in the data coverage, but requires that the D″ discontinuity is not a continuous surface. The termination of the discontinuity appears to restrict it to a region high seismic velocity. This could be interpreted as confirmatory evidence for the existence and behaviour of the post-perovskite phase transformation. However, compositional variations may also play a role.