If you remove the triangle strips between two clusters of triangles, and represent them as one big polygon, it's "just" a retriangulation problem.

Alternatively, for each cluster, render the edge triangles of the cluster at the neighbouring cluster's resolution with Z-write disabled, then render the cluster (overdrawing the real edge triangles), then render the Z-write of the edge. That'll reasonably fill the cracks with little overhead.

May be completely wrong but it looks as if triangles are subdivided to approximate the next lod level while avoiding cracks.