Cordilleran metamorphic core complexes, the subject of this volume, are a group of generally domal or archlike, isolated uplifts of anomalously deformed, metamorphic and plutonic rocks overlain by a tectonically detached and distended unmetamorphosed cover. The features are scattered in a sinuous string along the axis of the eastern two-thirds of the North American Cordillera from southern Canada to northwestern Mexico. To date, more than 25 of them are known, and it is significant that more than half of them have been recognized only since 1970. They are, without question, the newest and most controversial addition to the recognized architecture of the eastern two-thirds of the Cordillera since the discovery, in the early 1960s, of the Tertiary calderas and their associated vast ignimbrite sheets. The first hint of the existence and potential significance of these complexes was certainly from the early work of Peter Misch and his army of students who invaded the Great Basin in the 1950s. Misch (1960) discovered and emphasized scattered occurrences of a major subhorizontal dislocation plane or “decollement,” as it was called, separating the unmetamorphosed Paleozoic miogeoclinal cover from a generally metamorphosed substratum. He found this relation repeatedly in many ranges scattered along, and west of, the Utah-Nevada border almost 200 km west of the already known low-angle, east-verging Mesozoic thrust faults in central Utah. What is most puzzling about this regional context is that the thrust faults in central Utah are typical “older-on-younger” faults similar to those found in foreland thrust belts throughout the. . .

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.

Metamorphic core complexes in southern Arizona may be subdivided into four elements: core, metamorphic carapace, decollement, and cover. Cores consist chiefly of mylonitic augen gneiss that is, for the most part, derived from Precambrian and Phanerozoic plutonic rocks of granitic composition. Foliation in the cores is characteristically low dipping and commonly defines large upright, doubly plunging foliation arches. The mylonitic augen gneisses everywhere display low-dipping mineral lineation of a cataclastic nature. Lineation within any given metamorphic core complex is generally remarkably systematic in orientation. In southeastern Arizona the lineation trends northeast to east-northeast; in south-central Arizona, it trends north-south. Ductile normal faults are locally abundant in the core rocks and always are oriented perpendicular to lineation. Core rocks in places are clearly transitional laterally or downward into undeformed protolith. The metamorphic carapace consists of penetratively deformed younger Precambrian and Phanerozoic strata metamorphosed to greenschist-amphibolite grade. It forms a crudely tabular layer that locally overlies the crystalline rocks of the core. The contact is commonly so tight that the rocks of the carapace appear to be plated onto the crystalline rocks of the core. Within the metamorphic carapace, overturned to recumbent folds are ubiquitous, transposition is rampant, and boudins and pinch-and-swell features are commonplace. In spite of spectacular deformation, individual formations within the metamorphic carapace are arranged in normal stratigraphic order. A decollement, marked by brittle low-angle faulting and shearing, separates carapace and cover or, where carapace is absent, core and cover. The surface thus separates rocks of remarkably contrasting deformational styles. Where the surface “caps” core rocks, a decollement zone is formed beneath the decollement and consists of a distinctive crudely tabular zone of crushed and granulated but strongly indurated mylonite, mylonitic gneiss, microbreccia, and chlorite breccia. Striking “younger-on-older” fault relations characterize the decollement. Decollements in this area typically separate orthogneisses, which were derived in part from Precambrian rocks, from unmetamorphosed upper Paleozoic, Mesozoic, or Tertiary strata. Overturned asymmetric folds, detached isoclinal folds, and unbroken cascades of recumbent folds are abundant in many of the cover sheets. The metamorphic core complexes in southern Arizona are interpreted to be products of high-temperature extensional deformation, regional in extent, superseded by moderately ductile to moderately brittle tectonic denudation. Rocks in the augen gneissic core and metamorphic carapace were affected by profound ductile through brittle extension and flow in the direction of mineral lineation. Evolution of the decollement zones largely postdated the development of foliation and lineation. Mechanics of strain are interpreted in the context of megaboudinage. Cores are pictured as parts of megaboudins imposed on heterogeneous crustal rocks. Profound thinning of younger Precambrian and Paleozoic sediments through flow during deformation had the effect of significantly decreasing the stratigraphic separation between individual Phanerozoic formations and the Precambrian basement. Intrafolial folds developed in the carapace as products of passive flow. The relatively brittle, massive crystalline basement responded to extension and flattening by ductile normal faulting and the development of penetrative foliation and lineation. The high-temperature extensional disturbance, which began in early Tertiary time and ended about 25 m.y. ago(?), was accompanied by moderately ductile to moderately brittle tectonic denudation and gravity-induced folding of cover rocks. Major listric normal faulting postdated the development of tectonite, shaped the internal fabric of the decollement zones, and effected final movements of the covers.

Field studies in the Whipple Mountains, southeastern California, and in the Buckskin and Rawhide Mountains, western Arizona, have defined the existence of an O1igocene(?) to middle Miocene gravity slide complex that is at least 100 km across in the direction of its transport (N50° ± 10°E). The regionally developed complex is underlain by a subhorizontal detachment fault, named the Whipple detachment fault in western areas and the Rawhide detachment fault in eastern areas. The fault, which was warped and domed after its formation, separates a lower-plate assemblage of Precambrian to Mesozoic or lower Cenozoic igneous and metamorphic rocks and their deeper, mylonitic equivalents from an allochthonous, lithologically varied upper plate. Most lower-plate crystalline rocks were subjected to regional Late Cretaceous and / or early Tertiary mylonitization and metamorphism. The abrupt (3- to 30-m-wide) upper limit of mylonitization, the Whipple “mylonitic front,” is a mappable zone of high strain and, presumably, high thermal gradient. In parts of the Whipple Mountains, mylonitization was accompanied by the intrusion of subhorizontal sheets or sills of adamellite to tonalite up to a few hundreds of metres thick, although elsewhere thick sections of mylonitic rocks are devoid of such sills. The sills include both peraluminous and metaluminous varieties and are compositionally distinct relative to plutons in the overlying upper plate, being richer in Al, Mg, Ca, Na, and Sr and depleted in K and Rb. The compositions of most of the minerals in the mylonitized sills and their country rock gneisses did not reequilibrate during metamorphism. However, reequilibrated phases do occur in the ultrafine-grained mylonitic matrix and in tension gashes developed perpendicularly to mylonitic lineation. As a result of incomplete reequilibration, bimodal compositional ranges exist for plagioclase, epidote, celadonitic muscovite, and biotite. The minimum depth for intrusion and mylonitization is estimated to be 9.6 km from consideration of the interaction of compositionally corrected curves of muscovite stability and the adamellite solidus. Metamorphic mineral assemblages and feldspar thermometry indicate that mylonitization occurred from solidus temperatures of the plutonic sills down to middle greenschist grade. Allochthonous (upper-plate) units in the detachment complex include Precambrian to Mesozoic crystalline rocks, Paleozoic and Mesozoic metasedimentary rocks, Mesozoic metavolcanic rocks, and Tertiary volcanic and sedimentary rocks. The oldest Tertiary rocks are debris flows (some containing mylonitic rocks), fanglomerates, lacustrine sediments, and volcanic rocks, all of the Oligocene(?) to lower Miocene Gene Canyon Formation. Red beds and volcanic rocks of the Copper Basin Formation overlie Gene Canyon rocks unconformably and are tilted less steeply than the older Tertiary rocks along northeast-dipping listric normal faults that occur widely within the upper plate. In the Whipple Wash area of the eastern Whipple Mountains, volcanic rocks of the Copper Basin Formation sit unconformably on brecciated lower-plate mylonitic rocks in a channel cut ~70 m below the Whipple detachment fault. These volcanic rocks were themselves involved in renewed detachment faulting along that fault. Collectively, these stratigraphic-structural relations indicate that detachment faulting occurred during Tertiary sedimentation over a significant period of time, and was therefore of growth-fault rather than catastrophic nature. Upper Miocene valley-fill sediments and alkali basalts unconformably overlie upper-plate structures and tilted strata, thus providing an upper age limit for the detachment faulting. Northeastward movement of the thin (<5 km) upper plate is believed to have occurred under the influence of gravity, although the Whipple-Rawhide detachment fault could not have originally dipped more than a few degrees. The head, or breakaway zone, of the crustal slide is apparently defined by northeast-dipping normal faults in the Mopah Range, just west of the Whipple Mountains. Central areas of the slide complex in the vicinity of the Colorado River (Whipple and Buckskin Mountains) are characterized by extreme distension of the detached slab along northwest-striking, northeast-dipping, listric normal faults. There is telescoping of allochthonous units in distal, or toe, portions of the slide complex in the Rawhide and Artillery Mountains of western Arizona, where thrust faulting of older rocks over rocks as young as middle Miocene is common. Northeastward displacements of allochthonous units in excess of several tens of kilometres are indicated by field relations in the Buckskin and Rawhide Mountains.

Reconnaissance mapping indicates that parts of nine mountain ranges previously considered to be Precambrian basement are instead variations of Tertiary metamorphic core complexes. From southeast to northwest, these ranges include the Pinaleno, Picacho, South Mountains, parts of the Buckeye, White Tank, Harquahala, Harcuvar, Buckskin, and Rawhide Mountains. Together with the already recognized Santa Catalina–Rincon–Tortolita complex, these ranges define a broad northwest-trending belt through Arizona. The northeast-trending Buckskin-Harcuvar-Harquahala Mountains are transverse foliation arches, the latest expression of a huge, northwest-elongated metamorphic area herein named the Harcuvar metamorphic core complex. This Tertiary phenomenon is superimposed on an ill-defined center of late Mesozoic metamorphism. Traverses into the complex from its unmetamorphosed southwestern margin reveal progressive Cretaceous conversion of Mesozoic sedimentary rocks into migmatites. Metamorphism just preceded intrusion of the Tank Pass batholith, an Upper Cretaceous pluton which itself became foliated and involved in early Tertiary migmatization and intrusion in the Harcuvar Mountains. A marginal zone of penetrative mylonitization, capped by a more brittlely deformed dislocation surface, flanks the Harcuvar complex on its upper and broadly arcuate northeastern margins. Resting on this tectonic surface are highly tilted, unmetamorphosed, layered rocks (Paleozoic to Tertiary). Geochronologic and geologic data place the time of mylonitization as Tertiary, perhaps as recently as 25 to 20 m.y. B.P. This deformation (flattening and northeast-southeast extension) was closely followed by development of chlorite breccia, the dislocation surface, thick wedges of coarse clastic sediment, and listric faulting. Finally, the core complex was arched and uplifted. A model for this sequence of events is predicated on mobile northeast-directed extension of a flat upper-crustal layer facilitated by intense mid-Tertiary plutonism in an actively tensile stress field. Tectonism in a brittle, surficial upper plate is governed by listric faulting and detachment as the plate fragments and extends “piggyback” style upon subjacent, ductilely stretched layer.

Rocks in the South Mountains of central Arizona are representative of rocks found in metamorphic core complexes elsewhere in Arizona. These core-complex terranes are in part characterized by low-angle mylonitic foliation that contains penetrative northeast-trending mineral lineations and pervasive smearing out of mineral grains. In the South Mountains, mylonitic rocks form a doubly plunging, northeast-trending foliation arch and have been derived from Precambrian amphibolite gneiss and a composite mid-Tertiary pluton. The pluton is undeformed in the core of the arch, but shows a progressive increase in pervasiveness of mylonitic fabric up structural section. Mylonitic plutonic rocks are exposed as a carapace overlying their less-deformed equivalents. Mylonitically foliated Precambrian amphibolite gneiss is restricted to a zone underlain and overlain by nonmylonitic (crystalloblastic) gneisses that are lithologically identical and largely retentive of their Precambrian foliation. Fabrics in mylonitic rocks indicate extension parallel to the east-northeast–trending lineation and flattening perpendicular to the gently dipping foliation. The fabric is well dated as late Oligocene to early Miocene (25 to 20 m.y. B.P.). Mylonitic deformation was followed by more brittle deformation which produced a chloritic breccia that overlies mylonitic plutonic rocks in the northeast half of the arch. The chloritic breccia is probably related to normal faulting along a low-angle dislocation surface.

An elongate northwest-trending zone of batholith-size metamorphic core complexes extends some 130 km from the Rincon Mountains to the Picacho Mountains in southeastern Arizona. The complexes are characterized by undeformed to gneissic granitic intrusions, gneissic to phyllonitic xenoliths and wall rocks derived mainly from Precambrian granitic rock, shallow-dipping foliation, and remarkably uniform directions of lineation. Parts of this zone have been recognized and studied extensively for more than 30 yr, but there remains a divergence of opinion about the age, depth of emplacement, and origin of the complexes. Field relations indicate that host rocks as young as or younger than Mesozoic were involved in cataclasis. K-Ar and fission-track ages indicate that the complexes were at temperatures uniformly in excess of400 °C in the middle Tertiary (20 to 30 m.y. ago) and that the bedrock between and very near the complexes was not thermally affected. Tertiary plutons characteristically associated with the high-grade metamorphic rocks are also cataclastically deformed, and stratigraphic depths to the top of metamorphic terranes were no more than 6 km and possibly less than 3 km. These and other data suggest to some that the complexes developed during intrusion of composite batholiths at shallow depth in an anisotropic stress field during the middle Tertiary. On the other hand, Rb-Sr and U-Th-Pb techniques yielded older ages (≥44 m.y.) for some samples. These and additional data suggest to others that major development of cataclasis preceded the middle Tertiary and included regional thrusting.

Recent field work and accumulated Rb-Sr studies, when combined with previous U-Th-Pb and K-Ar investigations, allow a new synthesis of the crystalline terrane within the Santa Catalina–Rincon–Tortolita crystalline complex. When all the available data are integrated, it is apparent that the crystalline core is mainly a composite batholith that has been deformed by variable amounts of cataclasis. The batholith was formed by three episodes of geologically, mineralogically, geochemically, and geochronologically distinct plutons. The first episode (75 to 60 m.y. B.P.) consisted of at least two (and probably three) calc-alkalic, epidote-bearing biotite granodiorite plutons (Leatherwood suite). The Leatherwood suite is intruded by distinctive leucocratic muscovite-bearing peraluminous granitic plutons (Wilderness suite), which are 44 to 50 m.y. old. At least three Wilderness suite plutons are known, and their origin has been much debated. Leatherwood and Wilderness plutons are intruded by a third suite of four biotite quartz monzonite to granite plutons (Catalina suite) that mark the final consolidation of the batholith 29 to 25 m.y. ago. Much of the mylonitic (cataclastic) deformation of the plutonic rocks and recrystallization of the enclosing host rocks may be related to intrusion of the various plutons. At least three episodes of mylonitization (cataclasis) may be delineated by observing relations between mylonitic and nonmylonitic crosscutting plutons. The southern part of the Leatherwood pluton bears a moderate to strong mylonitic foliation that is cut by undeformed leucogranites and pegmatite phases of the Wilderness pluton. Elsewhere in the Santa Catalina–Rincon–Tortolita crystalline core, Wilderness suite plutons contain penetrative mylonitic foliation. Foliated Wilderness suite plutons are intruded by an undeformed portion of a Catalina suite pluton. In the Tortolita Mountains, however, intrusions of the Catalina suite themselves contain evidence for at least two events of mylonitic deformation. The most significant of these events is clearly constrained to the Catalina intrusive episode because it formed during or after the emplacement of Tortolita quartz monzonite (about 27 m.y. B.P.) but before the intrusion of postfoliation dikes (about 24 m.y. B.P.). All three episodes of mylonitization contain the distinctive and much discussed east-northeast–trending lineations. All events of mylonitization are constrained to a 50-m.y. interval of time from 70 to 20 m.y. ago. Although continuous mylonitization from 70 to 20 m.y. ago cannot be unequivocally disproved, the strong association of mylonitization with the three plutonic episodes suggests that deformation in the Santa Catalina-Rincon-Tortolita crystalline core, like intrusion, was episodic.

Characteristically lineated and foliated rocks of middle to late Mesozoic(?) age crop out in ranges in north-central Sonora throughout an area of 15,000 km 2 , between lat 31° 307′N and 30° 30′N. The terrane consists predominantly of layered sedimentary, volcanic, and volcaniclastic units, commonly metamorphosed to greenschist facies. Associated plutons, which are unambiguously intrusive, consist mainly of biotite- or biotite-muscovite–bearing granite. The layered suite and intrusive rocks are distinguished by subhorizontal, penetrative lineation, commonly defined by smeared mineral grains, which consistently trend northeast. Foliation is also predominantly low dipping. South of lat 30° 30′N, sporadic occurrences of lineated granitic and metamorphic rocks suggest the existence of this deformational fabric at least to Sierra Mazatan, which lies east of Hermosillo near lat 29°00′N. Cogenetic suites of zircon from one undeformed and three deformed plutons yield U-Pb ages from 75 to 55 m.y. These intrusive bodies are elements of a widespread, time-transgressive, late Mesozoic magmatic suite. They are not known to be affected by folds and faults commonly related to the Laramide orogeny. The apparent ages of the plutons are interpreted to be crystallization ages and therefore indicate that lineation and foliation formed, at least locally, later than 55 m.y. ago. Outcrops of distinctively deformed rocks appear to crudely define a north-trending belt. Rocks outside of this general zone are composed of sedimentary, volcanic, and volcaniclastic rocks of Precambrian and Mesozoic age which locally have been strongly folded and metamorphosed to greenschist or higher facies. However, postdeformational pegmatites and intrusive rocks indicate pre-Tertiary minimum ages for related episodes of deformation and metamorphism.

In the northern Ruby Mountains, Nevada, the transition from high-grade, migmatitic infrastructure to the nonmetamorphosed but allochthonous rocks of the suprastructure is a zone of intense ductile deformation and high strain. This zone, which is disharmonic to the terranes both structurally above and below, is characterized by amphibolite-facies metamorphism, polyphase deformation, mylonitic textures, a west-northwest lineation, and tabular masses of orthogneiss. Major map-scale folds are overturned westward opposite the common direction of overturning in the infrastructure. Many of the large folds in the transition zone ( Abscherungszone after the concepts of Haller, 1956 Abscherungszone after the concepts of Haller, 1971) have evolved into ductile faults (tectonic slides) and form complex braided systems. Tectonic slices of low-grade metasedimentary rocks (greenschist facies?) mark the boundary between the top of the Abscherungszone and an overlying low-angle fault complex of unmetamorphosed upper Paleozoic and locally Tertiary rocks. These upper Paleozoic rocks represent attenuated fragments of the suprastructure which was structurally above the metamorphic complex during middle Mesozoic regional metamorphism and deformation. The Tertiary sedimentary and volcanic rocks are Miocene (K-Ar age on sanidine from a rhyolite flow), and conglomeratic facies contain clasts derived from the underlying metamorphic complex. Therefore, the low-angle fault complex is a Neogene brittle phenomenon apparently superimposed on an older Mesozoic “stockwerk” terrane. Despite these relations, a tectonic continuum involving a diachronistic transition from ductile to brittle deformation is an alternative explanation; however, such a tectonic regime must have spanned an interval from middle Mesozoic to Miocene.

The metamorphic complex of the northern Ruby Mountains in northeastern Nevada exposes Paleozoic strata that are metamorphosed to sillimanite grade, migmatized, and recumbently folded. Nappes are variously overturned to the east, north, south, and west. The deeper part of this metamorphic infrastructure is a migmatitic zone pervaded by pegmatitic two-mica granite. A structurally higher transition zone underwent extreme tectonic flattening and some thrusting as the mobile infrastructure rose buoyantly against more rigid suprastructure. Relief by flow and stretching to the west-northwest and east-southeast resulted in a regionally constant lineation in this transition zone. The age of metamorphism is uncertain but may be Jurassic.

A gneiss complex in the Grouse Creek Mountains, northwestern Utah, consists of 2.5-b.y.-old adamellite that was remobilized and intruded younger Precambrian and Paleozoic cover rocks 25 m.y. ago during an extended period of synkinematic metamorphism. One or more structural domes formed in the region during metamorphism. The remains of a middle Tertiary culmination in the central Grouse Creek Mountains is an asymmetrical welt of schistose Precambrian adamellite that protrudes into greatly attenuated autochthonous and allochthonous cover rocks. Fine- to medium-grained gneiss and schist of the upper 200 m of the welt, or dome, grade upward to retrograde phengite-quartz-albite schist that intruded the lowermost beds of upper Precambrian(?) quartzite. The age of the schistose Precambrian adamellite and its geographic continuity with less-metamorphosed Precambrian adamellite lying unconformably beneath the quartzite indicate that remobilization of the former was post-Precambrian. Simultaneously, cover rocks were thinned to one-fifth their original thickness by metamorphic flattening, extension, and low-angle faulting. A 25-m.y.-old pluton underlies the area of remobilization. Its outer shell bears metamorphic folds related to a second deformation. These folds also predominate in a metasomatic aureole in the Precambrian adamellite surrounding the Tertiary pluton and in the upper, schistose part of the gneiss dome. Thus, development of the dome was closely related to the second deformation and to the intrusion 25 m.y. ago. Late-metamorphic folds and low-angle faults chiefly affected allochthonous cover rocks, and postmetamorphic movements carried parts of these rocks over 1 l-m.y.-old sedimentary rocks in the adjoining basin. Isotopic data on autochthonous rocks suggest that metamorphic temperatures persisted into Miocene time and provided the potential for continuing uplift and basinward shedding of sheets which ended after 11 m.y. ago.

Solid-state flow during metamorphism was studied in a 300-km 2 exposure where Elba Quartzite forms the upper part of an autochthon and is overlain by two major allochthonous sheets. The quartzite is very locally thrown into north-verging recumbent folds, the largest having a wavelength of 1 km. Fold axes and elongate metamorphic grains trend approximately east, and foliation is subhorizontal. A vertical sequence of samples was collected from the large fold, and a second sequence was collected from unfolded quartzite 4 km away. Axes of strain ellipsoids measured in five quartzites from the unfolded sequence are close to 0.5:1.1:1.7 (assuming an original sphere of rádius one). Five samples from the fold have ellipsoid axes ranging from 0.4:1.1:2.5 at the top of the sequence to 0.2:1.4:4 near the base. Most quartz grains deformed plastically without recrystallizing and developed strong c -axis fabrics. The degree of orientation of both quartz c axes and muscovite plates increased with increasing strain. Most of the quartz fabrics have orthorhombic symmetry and cross-girdle patterns like fabrics produced experimentally by J. Tullis and computer-simulated by G. S. Lister and coworkers, for quartzites extended in plane strain. The results thus indicate a gradual east-west extension and consequent flattening by the force of gravity. Perhaps the fold formed concurrently where material flowing laterally under a broad dome was arrested locally and thus forced to buckle. Dating of nearby granite bodies indicates that the deformation began in Eocene time and continued until Miocene time.

The northernmost of a chain of four or five domes in the Albion Mountains was studied in order to understand its origin and deformational history. The dome is part of a metamorphic terrane that is composed of Precambrian W basement gneiss that is overlain by a Precambrian(?) and Paleozoic metasedimentary rock succession that was metamorphosed to the amphibolite facies in the Albion Mountains. Four phases of ductile deformation are recognized in minor structures. The first two phases of deformation followed extensive low-angle faulting of younger-on-older type and resulted in the formation of overturned and flattened folds in the weaker rock units and penetrative lineations and foliations in the thick quartzite units. Vergence of minorfolds that formed during D 1 and D 2 indicates that shear occurred along planes approximately parallel to bedding such that strata moved to the northwest (D 1 ) and the northeast (D 2 ) relative to the basement. Third-phase deformation locally created overturned folds and small bedding-plane faults with geometries that indicate that higher strata moved outward from the apex of the present dome relative to the underlying strata, which suggests that these structures formed during the rise of the dome. The fourth phase of deformation is typified by kink folds whose axes trend N15° E and are horizontal. Movement on a low-angle fault that separates Pennsylvanian sedimentary rocks from underlying metamorphic rocks followed the second phase of deformation and may have been concurrent with late ductile deformation in the more deeply buried rocks. Timing of the metamorphism and intense deformation in the Albion Mountains is elusive, but it apparently occurred between Late Triassic and Early or middle Cretaceous times; numerous Tertiary K-Ar and Rb-Sr ages from metamorphic rocks indicate that less intense deformation and moderate temperatures continued in the more deeply buried rocks until the Miocene. The structural history of the Albion metamorphic terrane is thus interpreted as an early to middle Mesozoic period of extensive younger-on-older faulting and ductile shearing on nearly horizontal planes over a little-deformed basement, and a late Mesozoic to early Cenozoic period of less intense deformation and metamorphism. This history is similar to that of other metamorphic terranes in the Great Basin.

The Bitterroot dome-Sapphire tectonic block appears to be a well-developed example of a plutonic-core gneiss-dome complex or infrastructure separated from the adjacent suprastructure by a gently dipping zone of mylonitic shearing or an “Abscherungszone.” The suprastructural Sapphire block, on the order of 15 km thick, 100 km long, and 70 km wide, apparently moved eastward about 60 km, bulldozing rocks of the eastern Flint Creek Range ahead of it. Movement of the block must have occurred about 75 or 80 m.y. ago during late stages of consolidation of the Idaho batholith which, along with sillimanite-zone regional metamorphic rocks, makes up the infrastructure under the mylonitic detachment zone. Timing of movement in the Sapphire block matches that in the Bitterroot dome. The Bitterroot dome must have risen after off-loading of the Sapphire block, because the shear foliation and lineation that formed during movement completely cross the dome; this indicates that the block must have moved eastward across the whole of the area now occupied by the dome rather than radially down the flanks of an existing dome. The shear lineation maintains its eastward trend even at the south end of the dome where the foliation dips southward. The lineation and shear foliation are strongest along the eastern flank of the dome, over which the greatest thickness of the block would have passed.

The southern Canoe River area (lat 52° N, long 118°W), situated adjacent to the northeast flank of the Shuswap complex of British Columbia, was mapped in detail in conjunction with mineralogic and petrologic studies. This work is the basis for a section some 50 km long which illustrates the transition from the Rocky Mountain fold and thrust belt on the east to the metamorphic core zone of the Columbian orogen on the west. A 5-km-thick succession of Proterozoic and Cambrian clastic rocks have been metamorphosed from biotite to sillimanite grade. Three sets of major structures are superimposed in the core zone, although only one major set of buckle folds and thrusts (which formed during the last two deformation phases) is predominant in the Rockies. The Malton Gneiss, a sheet of reworked basement, emerges in the northern part of the area. Mylonite marks the cover-basement decollement that was active early in the orogeny. Southwest-verging isoclinal recumbent nappes sheared off from the Malton Gneiss are refolded by two sets of northeast-verging folds. Isogradic surfaces were imposed late in the second deformation phase with a morphology related to heat flow. These surfaces were subsequently deformed and folded in a third deformation phase. In the core zone, staurolite disappears within the kyanite zone, and trondhjemite migmatite appears concommitantly. In the Rocky Mountains, stauro-lite persists to the appearance of fibrolite, and no migmatite is found. Geobarometric studies suggest metamorphic pressures in the core zone some 2-kb higher than in the western part of the Rockies and indicate that the vertical motion on the Purcell thrust was about 7 km. Contrasting stratigraphic thicknesses and facies as well as contrasting structural styles and histories across the fault suggest substantial horizontal shortening. Temperatures and pressures estimated from element fractionation between coexisting minerals are consistent with the structural and isograd pattern.

The Kettle River Range in Ferry County, Washington, is underlain by sillimanite-grade rocks of the Tenas Mary Creek sequence. Two >800-m-thick sheets of augen gneiss occur above and below feldspathic quartzite, biotitic gneiss, and minor marble. Polyphase deformation (including mylonites and east-trending lineations) and slightly uraniferous aplitic to pegmatitic bodies are common. Cataclasis is common, and rocks of the Tenas Mary Creek sequence appear to be in tectonic contact with overlying upper Paleozoic phyllitic rocks. Fine-grained biotitic metasedimentary rocks occurring locally between the phyllitic rocks and rocks of the Tenas Mary Creek probably are older than the late Paleozoic phyllitic rocks. Foliation and contacts in the Tenas Mary Creek sequence rarely dip >25° and define the flat-topped Kettle dome (which is >65 km long north-south, 27 km wide, and has about 3 km of structural relief). The Okanogan dome west of the Kettle dome consists primarily of orthogneisses and granitic plutons of Mesozoic(?) age. Rocks in the flat-topped Spokane dome along the Washington-Idaho border are lithologically similar to those of the Tenas Mary Creek and may be pre-Beltian. The Sanpoil syncline between the Kettle and Okanogan domes and a syncline on the northeastern margin of the Kettle dome contain Eocene sedimentary and volcanic rocks. Because the axes and structural reliefs of the Okanogan dome, the Sanpoil syncline, and the Kettle dome are similar, the present structural relief (as opposed to the internal structure and high-grade metamorphism) of the Kettle dome probably is due to post-Eocene folding. The gently synformal Tertiary Newport fault straddling the Washington-Idaho border may be a related structural feature. Four other low-angle faults, three of which cut Tertiary rocks, occur between the domes. The low-angle faults commonly are marked by cataclastic zones more than 100 m thick. Cataclasis occurred as the basement of batholiths and pre-Beltian(?) metamorphic rocks became decoupled from overlying Precambrian to Tertiary layered rocks. Whether this decoupling represents one or more zones of Tertiary decollement of regional extent is not yet known. Owing to post-Eocene (possibly late Miocene or younger) folding, the cataclastic zones crop out on the margins of the domes.

To attempt to summarize the series of papers that constitute this volume in a few pages, much less a few paragraphs, would require temerity indeed. Nevertheless, I will take this opportunity to point out a few of the geologic relations described that appear to be of unusual interest for someone attempting to unravel the origin of these fascinating geologic terranes. SALIENT CONCLUSIONS Clearly the most important conclusion regarding these complexes, particularly those south of the Snake River Plain to which the bulk of this volume is devoted, is that they are startlingly young and that they show consistent evidence of horizontal extension and vertical attenuation. It is primarily the latter processes that have operated to create the array of features by which these structures are characterized (Davis, this volume). In the core, ductile deformation took place at depths and temperatures sufficient to cause quartz to behave plastically producing extensive foliated and lineated mylonitic fabrics. In the cover, horizontal extension and vertical attenuation was accommodated by low-angle, younger-over-older faults and by listric normal faults that commonly affected rocks of Tertiary age. In some areas, however, evidence suggests that these two processes operated together or sequentially over a considerable period of time, in places beginning in the Mesozoic. TECTONIC SETTING As Coney (this volume) and many others have pointed out, most of the characteristic metamorphic core complexes lie in the midst of the Cordilleran miogeocline between the zone of imbricate thrusts that define the leading edge of the Cordilleran thrust and fold. . .