So I'm at LPSC and went to a bunch of talks on MoonZoo the other day and heard geomorphologists are in need. So far I'm 3 pages into this page and see little explanation of what might cause these features. My work concerns boulder clustering on patterned ground terrains on Mars.
With that said I'm very interested in boulder scale processes.
Craters with boulder barren centers are also common on Mars. Here are various hypotheses.
The reason for this is different depending on the age of the crater and the environment of the crater. For fresh craters, some amount of melt is generated in the course of an impact that takes diffusivity dependent cooling timescales for different depths of melt (which scales to crater size) (see Melosh, 1989). For highly degraded craters it is likely the regolith has all been broken up to pieces beyond the limit of resolution due to impact gardening (very small impactors break up boulders and generally resurface the moon at small scales) (sorry, don't have a reference for you at the moment). For craters in the middle there could be a wide array of things going on. My preferred hypothesis comes from previous studies I worked on regarding glacial moraines. Basically, the idea is that fine particles are more easily mobilized during any erosive process. Since most erosive processes are gravity driven and craters generate slopes you would expect small particles to migrate to the lows in slopes while leaving the large particles (boulders) behind (check out Putkonen, Connolly, and Orloff 200?).
I'm sure there are numerous other hypotheses but here's what I'm thinking of sitting in the hotel.