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More than 25 distinctive, isolated metamorphic terranes extend in a narrow, sinuous belt from southern Canada into northwestern Mexico along the axis of the North American Cordillera. Appreciation of these terranes has evolved slowly, and more than half of them have been recognized only since 1970. Growing evidence shows that these metamorphic terranes and related features evolved in part during early to middle Tertiary time (55 to 15 m.y. B.P.), that is, after the Laramide orogeny but before basin-range faulting. These terranes have been termed "metamorphic core complexes."The complexes are characterized by a generally heterogeneous, older metamorphic-plutonic basement terrane overprinted by low-dipping lineated and foliated mylonitic and gneissic fabrics. An unmetamorphosed cover terrane is typically attenuated and sliced by numerous subhorizontal younger-on-older faults. Between the basement and the cover terranes is a zone of "decollement" and/or steep metamorphic gradient with much brecciation and kinematic structural relationships indicative of sliding and detachment. Plutonic rocks as young as early to middle Tertiary age are deformed in the basement terranes of many of the complexes, and some of the deformed cover includes continental sedimentary and volcanic rocks of early to middle Tertiary age.Some complexes exhibit evidence of prolonged deformation and metamorphism extending back into Mesozoic and even Paleozoic time. All the complexes, however, reveal an early to middle Tertiary deformational and metamorphic overprint that is interpreted to be mainly of extensional origin. The extension coincided with a vast plutonic-volcanic flare-up of magmatic arc affinity mainly during Eocene time in the Pacific Northwest and mainly during late Eocene-Oligocene to middle Miocene time south of the Snake River Plain. The exact tectonic significance of the complexes remains obscure. Their extensional aspect clearly postdates, and seems unrelated to, Cretaceous and early Tertiary Sevier and Laramide compressional tectonics, but predates the more obvious late Tertiary basin-range extension and rifting. 7