Comparison of thermal structure and predicted regions of metastability for a relatively younger and slower-subducting slab which approximates the Aleutian arc, and an older and faster-subducting slab which approximates the Tonga arc. A metastable wedge forms only for old and fast-subducting slabs which are sufficiently cold that kinetic hindrance prevents phase transformations from keeping pace with the descent rate. For the model parameters used, the metastable wedge is bounded on its sides and bottom by the 600°C isotherm, in those slabs cold enough for it to form. Deep earthquakes are presumed to occur by transformational faulting in the metastable wedges.