Plate boundary lon/lat data
I did a little poking around and came across a few projects that might be interesting.

Based on the work of Peter Bird (2003) there's a github project (fraxen/tectonicplates) that hosts georeferenced data of most major plate boundaries.

Seeing as you're interested more in minor plate dynamics, there's another project called GPlates that is an multi-OS open source visualizer for plate boundary dynamic reconstructions. While I haven't looked at the potential outputs available, I imagine you would be able to export data from this platform into a more traditional GIS data format (.shp, .gml) that could be read in GMT.