Set Map.MinimumResolution. By default the map sets it to the resolution of the last tile in the tiled layers. However you can override this by explicitly setting it. Note that this will make these tiles stretch beyond what they were meant for. You can in turn avoid this by setting myTiledlayer.MinimumResolution to the resolution of its lowest LOD. That way the tiled layer will turn off when you pass this point, and the dynamic layers will be the only thing visible at this point.