There is at least one interesting abstract from the latest LPSC on this subject. At this point, there are three different theories as to what causes Io's puzzling regions of blocky mountains. The first is that, as existing surface crustal layers are gradually buried by new lava flows and thus shoved downwards towards Io's center, their horizontal circumference naturally also shrinks, and the result is that they frequently buckle and wrinkle in some place to shove mountains upwards:
http://www.sciencemag.org/cgi/content/full/279/5356/1514
http://adsabs.harvard.edu/abs/2001JGR...10633201S
However, this theory by itself doesn't make it clear why Io's mountainous areas seem to be concentrated in two regions 90 degrees separate from its two antipodal regions where its volcanoes are concentrated. The second theory tries to explain this on the grounds that the mountainous regions are places where Io's convective mantle currents (which upwell in the two volcanic regions) sink down again and squeeze toward each other in the process, sliding sections of rigid upper crust along to collide with each other and wrinkle up:
http://adsabs.harvard.edu/abs/2001JGR...10633201S
http://adsabs.harvard.edu/abs/2001JGR...10633175G
http://www.agu.org/pubs/crossref/2003/2002JE001946.shtml
Finally, the third theory calls for volcanic outbreaks to the surface to serve as "heat vents" allowing subsurface mantle heat to be locally released into space and allow the crust near the volcanoes to actually cool off somewhat -- whereas, in regions where volcanic eruptions are not occurring, the local buildup of heat trapped underneath the solid crust causes it to heat up and expand, thus buckling locally for that reason:
http://www.gsajournals.org/gsaonline/?requ...MF%3E2.0.CO%3B2
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/2030.pdf
These three effects, of course, may well be working simultaneously. Anyway, Bill McKinnon and M.R. Kirchoff -- advocates of the third theory, who have already concluded that it would cause the same 90-degree anticorrelation of volcano concentrations and block-mountain concentrations as the second theory -- have a new abstract concluding that the effects of such thermal expansion will be even greater when one keeps in mind the extent to which Io's basalt crust is laced with and weakened by easily meltable sulfur, and that the buildup of trapped mantle heat underneath the crust in volcano-free regions will melt its lower parts and cause the solid crustal layer to become thinner, and thus to buckle and wrinkle horizontally more easily in reponse to its horizontal thermal expansion:
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2120.pdf