Well, it's the change of seismic wave speed representing a change in the physical properties of the rocks at the base of the crust. I wonder if the transition is not sharp here, rather a gradational transition. Maybe the closer we look at these things, the more diffuse they become.

Sorry this is short, I've more thoughts on this and I haven't even read the article yet. That and I don't have all my resources at hand. I'm up in the Colorado Rockies right now, and I would be out biking if not for the solid rain and snow. Yeah, snow up above timberline. The trees are already turning color, another week to ten days and the color will be I peak, I guess. A bit early for the aspens to be turning.