Simplified maps of the western Cordillera, indicating distribution of metamorphic facies and times of principal recrystallization have been compiled. Despite rearrangement by transcurrent shuffling, general trends are recognizable. Narrow belts of Phanerozoic high-P blueschist, and serpentinized periodotile lie outboard from broad, penecontemporaneous high-T metamorphosed calcalkaline magma-intruded belts bordering the Pacific Ocean, and both are disposed approximately parallel to the present-day North American margin; they represent recrystallized products of mid-Paleozoic and younger subduction zones, Andean-type margins, and exotic island arcs. In contrast, dominant metamorphic mineral parageneses within the continental interior are spatially associated with chiefly mesozonal granitic plutons. Glaucophane schists, eclogites, and large tracts of ophiolitic ultramafics are absent, in part due to selective overprinting of old accretionary margin lithotectonic assemblages by subsequent orogenies, high-T recrystallization, and anatexis. For example, at depth within the California Coast Ranges, high-P phases are currently being destroyed; farther inland, a general paucity of oceanic petrotectonic assemblages is evident, possibly reflecting the differential sinking of dense basaltic/peridotitic units during thermal softening. Preserved metamorphic field gradients reflect the magnitude of subsequent uplift and erosion as well as original PT conditions.

Late Mesozoic to mid-Cenozoic recrystallization accompanied earlier crustal thickening+ductile deformation, succeeded by later extension+brittle faulting+mylonitization; Mesozoic-Cenozoic metamorphism involved cratonal rocks in the Mojave-Sonoran Desert, but in Nevada and to the north, eugeoclinal sections were recrystallized. Metamorphic intensity decreases eastward toward the Phanerozoic hinge line. Basement rocks in the Rockies display the effects of shallow level crustal deformation but much less widespread evidence of post-Proterozoic plutonism and regional metamorphism. Initial Pb, Nd, and Sr isotopic compositions and crystallization ages of igneous rocks, in comparison with times of intrusion, recrystallization, and metamorphic facies distributions are compatible with a geologic scenario involving gradual, proximal growth of the ancient continent chiefly southward from the Archean Wyoming craton during early and mid-Proterozoic time, followed by late Proterozoic-Phanerozoic westward development. Accretion involved the incorporation of some exotic, mostly oceanic, outboard terranes.

However, continental growth in the western U.S. Cordillera during both Proterozoic and Phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo-Pacific lithospheric plates, combined with metamorphic (+sedimentary) reworking in the forearc+trench+back arc setting, rather than resulting from the amalgamation of preexisting ancient continental fragments. ¿ American Geophysical Union 1988