Regions with active tectonic activity like the IANGCZ are often delineated in global plate models as "deforming zones" (Kreemer et al., 2014) or "orogens" (Bird, 2003), for which crustal motion data are excluded from the determination of the otherwise rigid plate motion. Even as early as the 1970s, however, it was recognized that such areas of deformation are often themselves usefully described in terms of discrete "microplates" (e.g., Alvarez et al., 1974;Dewey et al., 1973), relatively small-scale blocks with little internal deformation except near