an expandable fracture ball seat assembly for use in wellbore zone fracturing operations functions to permit passage therethrough and exit therefrom of fracture ball plugs of only diameters less than a predetermined magnitude . in a representative form , the seat assembly includes a ring stack disposed within a tubular member and formed from a first expandable ring coaxially sandwiched between a setting ring and a second expandable ring . when an oversized fracture ball plug is forced into the seat assembly it axially compresses the ring stack and reduces the diameter of the first expandable ring and telescopes it into the second expandable ring , with the first expandable ring and the setting ring blocking passage through and exit from the seat . a reverse passage of a suitably large diameter fracture ball plug through the seat assembly axially returns the setting ring and first expandable ring to their original positions .