Structural evolution of the Southeast Canadian Cordillera: A new hypothesis

Tec~ono~hysics, 48 (1978) 133-161 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

133

STRUCTURAL EVOLUTION OF THE SOUTHEAST CANADIAN CORDILLERA: A NEW HYPOTHESIS

RICHARD L. BROWN department

of Geology, Carleton University, Ottawa, Ontario {Canada)

(Submitted August 8, 1977, revised version received November 9, 1977)

ABSTRACT Brown, R.L., 1978. Structural evolution of the Southeast Canadian Cordillera: a new hypothesis. Tectonophysics, 48: 133-151. The Shuswap metamorphic core complex of the southeast Canadian Cordillera, which is alloehthonous with respect to a fixed reference frame beneath the Plains of the North American craton, evolved and became consolidated prior to major development of thrust faulting in the Rocky Mountains. Polyphase deformation of the infrastructure of the complex beneath a weakly compressed suprastructure is probably due to shear strains generated by underthrusting of marginal lithosphere rather #an to gravitational upwelling and lateral spreading. Pre-metamorphic to early metamorphic strain of the complex may be related to eastward underth~sting from the west. Late metamorphic strain may be related to westward underthrusting of the North American craton from the east. Continued underthrusting of the craton led to uplift and northeastward translation of the consolidated metamorphic complex together with development of thrust faults in the Rocky Mountains.

INTRODUCTION

The structural evolution of the Canadian Rocky Mountain Thrust and Fold Belt has been traditionally referred to as “the Laramide orogeny” of Early Tertiary times, and considered to be younger than the major erogenic event that gave rise to the metamorphic core complexes that characterize the Columbia Mountains (Fig. 1). This view has been effectively challenged by the elegant model proposed by Price and Mountjoy (1970) involving a gravitational cause and effect relationship between an upwell~g-lastly spreading core complex, and development of thrusts and folds in cover rocks of the Rocky Mountains. The model assumes that cratonic rocks beneath the Rocky Mountains remained rigid as far west as the Rocky Mountain Trench; the corollary of contraction of the cover rocks over a passive basement implies an allochthonous origin for the rocks of the metamorphic core complex. That deformation in the core complex is in some way related to the evolu-

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tion of the Rocky Mountain Thrust and Fold Belt appears to be generally accepted, but the exact nature of this interrelationship is not yet clear, Campbell (1973) has argued that the model of decollement th~sting on a passive crystalline basement may only hold for the Foothills and Front Ranges while beneath the Main Ranges high angle basement faults may be present. He suggests that the restricted decollement thrusting may result from westward unde~h~sting of the craton, and takes the view that the Shuswap metamorphic core complex of the Columbia Mountains is an autochthon that has been internally deformed together with the basement, and vertically uplifted. The purpose of this paper is to examine the a~umptions of the tectonic models proposed by Price and Mountjoy (1970) and Campbell (1973), and to focus on recent results that contribute to our further understanding of the relationship between deformation in the metamorphic core complex and

135

events in the Rocky Mountain Thrust and Fold Belt; a structural evolution of the southeast Canadian Cordillera is proposed that, in the author’s view, better accounts for the presently known geological constraints. Place names and geological features referred to in the text may be located by reference to either Figs. 1 or 2. THE ALLOCHTHONOUS

MODEL

The most complete statement of this model is presented in Price and Mountjoy (1970), and the basic ideas are also to be found in Price (1973). The interpretation has been derived from the results of the Geological Survey of Canada’s operation Bow-Athabasca (Price and Mountjoy, 1966; Price, 1967; Wheeler, 1970), and a wealth of data accumulated over the years in the Canadian Rocky Mountains by many workers such as McConnell (1887), Douglas (1950), Shaw (1963), Bally et al. (1966), Keating (1966) and Dahlstrom (1970), together with the influence of gravitational flow models as presented by Bucher (1956), Van Bemmelen (1960), and Ramberg (1967). The following statements and quotations will serve to outline the main assumptions of the model, but the reader is referred to the full text of Price and Mountjoy (1970). Deformation in the Rocky Mountains is restricted to the cover rocks; this mass moved at least 200 km northeastward over a rigid crystalline basement. Deformation of the metamorphic core complex of the Columbia Mountains is associated in time (Late Jurassic to Eocene) with development of the Rocky Mountain Thrust and Fold Belt. Price and Mountjoy (1970, p. 7): “Thrusting in the Rocky Mountains is a high-level manifestation, along the foreland-margin, of the same deformation that is expressed at depth to the southwest of the diapiric northeastward rise of an infrastructure of tongues of hot mobile, gneissic rocks”. And on their page 18: “Upwelling and large-scale lateral flow in a mobile infrastructure of hot gneissic and granite rocks, beneath a relatively passive suprastructure of colder, mainly younger rocks within the western Cordillera, probably is fundamentally a buoyancy phenomenon, reflecting unstable density distributions and contrasts in ductility, many of which were transient. The lateral spreading within the entire mass, comprising the mobile metamorphic complex beneath its less intensely deformed suprastructure of drastically foreshortened sedimentary strata in the Rocky Mountains is primarily a gravitational effect”. Their interpretive cross-section which is drawn between latitudes 51” and 53” North is reproduced in simplified form in Fig. 2A. THE AUTOCHTHONOUS

MODEL

Campbell draws on his extensive experience in the Columbia Mountains, (Campbell, 1968, 1970, 1973; Campbell et al., 1973), but also refers to the

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RANGES

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See Fig. 1 for location

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(1973).

MAIN

MOUNTAINS

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COLUMBIA

MOUNTAINS

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FIG 2A

COLUMBIA

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MOUNTAtNS

137

same data base as that employed by Price and Mountjoy (1970) together with the recognition of a change in structural style of the Rocky Mountains that is noted north of about latitude 55”, and has led to suggestions of basement involvement (Taylor, 1971; Stott and Taylor, 1972). It is again suggested that the reader refer to the text of Campbell (1973) for a complete statement of the model, but the following will serve to highlight the essential differences between this and the model proposed by Price and Mountjoy (1970). The Shuswap metamorphic core complex is considered to be an autochthon that has been internally deformed and uniformly uplifted approximately 11 km. Deformation of the metamorphic complex has involved the underlying basement, and this basement deformation extends to beneath the Main Ranges of the Rocky Mountains Thrust and Fold Belt. Decollement thrusting above rigid crystalline basement is restricted to the Front Ranges and Foothills of the Rocky Mountains. Decollement thrusting in the Front Ranges and Foothills may result from westward underthrusting of the craton. Campbell (1973, p. 1614): “The writer believes that the core zone, probably including the Main Ranges of the Rocky Mountains, has not moved eastward by an amount equivalent to the shortening in the thrust belt of the eastern Rocky Mountains, but that it is the site of mainly vertical movements, and that a transition from plastic to brittle structures involves the basement outward from the axis of the core zone toward each flank of the erogenic belt. The core zone is thus regarded as autochthonous on a broad scale within the context of the erogenic belt . . . . a general vertical basement uplift is modified in the metamorphic core by diapirism and the development of gneiss tongues as in the East Greenland Caledonides (Haller, 1971)“. The structural cross section drawn by Campbell (1973) is reproduced in simplified form in Fig. 2B. DISCUSSION

The problem

of basement

involvement

A model of erogenic belt development involving decollement thrusting of foreland rocks over an underlying rigid basement can obviously only be extended to a limited degree into the internal part of the orogen. The location and nature of the transition from rigid to deformed basement most assuredly varies to some degree along the length of the erogenic belt. In the southern Canadian Rocky Mountains there is no room to doubt the validity of decollement thrusting beneath the Foothills and Front Ranges (Bally et al., 1966). The possibility of basement strain beneath the Main Ranges cannot be ruled out, but the surface geology does not require it. Probable candidates for deformed basement rocks first appear at the Rocky

13X

Mountain Trench within the metamorphic rocks which mark the eastern margin of the metamorphic core complex (Campbell, 1968, 1973; Giovanella, 1968). It would be useful to know exactly where basement involvement begins, but this alone would not resolve the question of the amount of translation of the core complex. Faults may penetrate the crystalline basement at low angles, or ductile strain of basement rocks may involve major low angle shear displacements. Similarly evidence of major uplift of the core complex relative to the external zones does not argue for or against a horizontal component to the total displacement. The problem

of a reference

frame

Price and Mountjoy (1970) refer their model to a fixed frame of reference in the North American craton, at the eastern edge of the Rocky Mountain Thrust and Fold Belt, above which the cover rocks are undisturbed. From this point of reference, decollement thrust slices have moved from the west in a northeasterly direction up and over the rigid underlying basement. If the thrust slices are assumed to be rigid, then to the west of the belt a complimentary denudation of cover rocks must have occurred. Price and Mountjoy (1970) have proposed that the horizontal component of contraction of the Thrust and Fold Belt is compensated for by a horizontal component of extension in the upwelling and laterally spreading infrastructure of the metamorphic core complex. Whether or not there has been significant spreading of the infrastructure has yet to be determined. However, where the overlying suprastructure is preserved on the northern flanks of the Shuswap metamorphic complex it exhibits evidence of horizontal contraction rather than extension (Campbell, 1973). We are forced to conclude that, irrespective of possible spreading of infrastructure, the overlaying suprastructure has been carried northeastward together with the strata of the Rocky Mountain Thrust and Fold Belt in order to maintain the stratigraphic coherence that is demonstrated in Campbell’s cross section (Fig. 2B). The amount of horizontal northeasterly displacement required of the metamorphic complex as a whole is open to question, but must be at least equivalent to the measured contraction of the cover over the known rigid basement beneath the Front Ranges and Foothills of the Rocky Mountains. To this must be added the contraction of the cover rocks of the Main Ranges. This additional amount cannot be estimated without assuming the degree and nature of basement involvement, but it will only be small if the basement faults are nearly vertical. Campbell (1973) decided to place his fixed reference frame within the Columbia Mountains and argued that contraction of cover rocks over rigid basement of the Front Ranges and Foothills is due to a southwestward underthrusting of the craton and peeling off of cover along ddcollement thrust planes (see also Charlesworth, 1959; Bally et al., 1966). However, the choice

139

of a reference frame is arbitrary, and all movements can only be stated in a relative sense. From this discussion we conclude that both the Price-Mountjoy model and the Campbell model require the metamorphic core complex of the Columbia Mountains to be allochthonous with respect to a fixed reference frame beneath the Plains of the North American craton; what is being contended is the amount of horizontal displacement required. Whether the reader prefers a rigid basement beneath only the Front Ranges and Foothills or all the way to the Rocky Mountain Trench, does not change the fact that the cover rocks of the metamorphic core complex have been displaced northeastward relative to the North American craton. The Price-Mountjoy model requires some 200 km of displacement while Campbell’s model reduces this to approximately 35 km. To further pursue the question of the relationship between development of the Rocky Mountain Thrust and Fold Belt and the metamorphic core complex, some aspects of the structural history of the western Main Ranges and Western Ranges to the east of the Rocky Mountain Trench will be reviewed. Relationships on the eastern margins of the Shuswap metamorphic complex will then be outlined by reference to the role of the Purcell fault and discussion of the structural and metamorphic history of the adjacent ranges in the Columbia Mountains. MAIN

RANGES

AND WESTERN

RANGES

Decollement thrusting and flexural-slip folding which characterize much of the southeastern Canadian Rockies, give way westward to a more penetrative deformation in the western Rockies. In the Main Ranges the transition to penetrative deformation is controlled by the Cambrian facies change from the eastern, dominantly carbonate facies to the western, dominantly shale facies (Cook, 1970). Between latitudes 50” and 52”N the structure of the Main and Western Ranges is dominated by the fan shaped Porcupine Creek Anticlinorium (see Fig. 2A for location) and flanking synclinoria (Leech, 1959; Balkwill, 1968, 1972; Price and Mountjoy, 1970; Mielliez, 1972; Craw, 1977). “Stratigraphic and structural continuity is maintained through the western Main Ranges, Western Ranges and Rocky Mountain Trench to the Purcell thrust” (Balkwill, 1972, p. 631; see Fig. 2A for locations). Balkwill (1972) has concluded that shortening of the exposed cover rocks was accomplished principally by penetrative strain involving folding and development of pervasive axial-plane cleavage with thrust faults being relatively subordinate structures. In the Western Ranges, structures which are part of the southwestern limb of the Porcupine Creek anticlinorium, are overturned towards the southwest. Balkwill (1972) suggests that the overturned structures were rotated late in the deformation history, and that this is related to northeasterly flow and decollement of the cover rocks above a southwestward dipping rigid crystalline basement.

i

II!

Craw (1977) has mapped the northeastern extension of’ the Porcupine Creek anticlinorium in the vicinity of t,he Big Bend of the (olumbia River (Fig. l), where deeper stratigraphic and structural levels are exposed. In agreement with Balkwill (1972) and others, he recognized two phases of folding associated with the development of the Porcupine structure, hut the major folding event is attributed to the second phase. At this deeper strut tural level metamorphism rises t,o kyanite grade of regional Barrovian metamorphism. The metamorphic isograds outline a thermal antiform whose western limb is truncated by the Purcell t,hrust. These isograds were folded and faulted during the second phase of deformation. ROCKY

MOUNTAIN

TRENCH

AND PIJRCELI,

FAULT

In the southeastern Canadian Cordillera it is fairly well documented that major strike slip faulting has not occurred along this portion of the Rocky Mountain Trench (Leech, 1959; Bally et al., 1966; Simony and Wind, 1970: Balkwill, 1972; Campbell, 1973; Price, 1977). It has had a varying history along its length, but between latitudes 50” and 53” N it is an erosional feature that roughly parallels the trace of a system of major dip slip faults (Wheeler, 1963). East of the Dogtooth Range the t,rench is a valley, eroded int,o the weak McKay and Canyon Creek strata of the Western Ranges along the line of their truncation by the Purcell fault (Evans, 1933; North and Henderson. 1954; Wheeler, 1963; Simony and Wind, 1970). The Purcell fault truncates both hanging wall and footwall structures indicating that its movement outlasted the formation of folds and faults within the western and Dogtooth Ranges (Simony and Wind, 1970). In the vicinity of the Dogtooth Range, Simony and Wind (1970) estimate the dip of the Purcell fault to be 35” SW and in their cross sections it is assumed to be a major thrust fault that flattens with depth. The fault has been extrapolated by Price and Simony (1971) down to meet t,he inferred decollement with rigid crystalline basement. The Purcell fault cuts northwestward from its boundary with the western margin of the Purcell anticlinorium to bring rocks of the metamorphic core complex in the Selkirk and Monashee Mountains into juxtaposition with structures of the western Main Ranges. It was previously pointed out that in the vicinity of the Big Bend of the Columbia River, metamorphic isograds within rocks of the western Main Ranges are t,runcated by the Purcell fault (Craw, 1977). Craw’s work extended west of the fault into the Selkirk and Monashee terrane, and from phase equilibria studies he concluded that rocks to the west of the Purcell fault were metamorphosed at pressures 2-3 kbar higher than the 5 kbar that is estimated for the peak of metamorphism to the east. These results imply a post metamorphic vertical component of displacement on the Purcell fault of at least 7 km.

141

In the McBride area 200 km northwest of the Big Bend of the Columbia River, Campbell (1973) has estimated on stratigraphic and structural grounds a vertical component on the equivalent fault system of approximately 6 km. The cross sections drawn by Simony and Wind (1970) for the Dogtooth Range restrict the vertical component to about 3.5 km. Campbell (1973) and Craw (1977) suggest steep westerly dips for the Purcell fault (70” or greater), while to the southeast Simony and Wind propose shallow dips (35” and less with depth). BASEMENT

ROCKS WEST OF THE ROCKY

MOUNTAIN

TRENCH

West of the Rocky Mountain Trench, within the metamorphic core complex (Shuswap terrane), several occurrences of gneissic rocks have been interpreted to be remobilized crystalline basement (Ross, 1968; Campbell, 1968, 1970, Wanless and Reesor, 1975), but unequivocal evidence of derivation from cratonic rocks of the Churchill Province (Stockwell, 1969) has not been obtained. A large mass of paragneiss, schist, amphibolite and gneissic granite, the Malton gneiss, lies along the Rocky Mountain Trench northwest of the Big Bend of the Columbia River (Campbell, 1968; Giovanella, 1968). This complex has mylonite zones along its boundaries, and layering within both the body and surrounding Windermere metasediments is truncated at the tectonic contacts (Campbell, 1968). The gneissic rocks are only poorly dated but are probably at least pre-Windermere in age and may prove to be remobilized Purcell strata or older crystalline basement (for discussion see Campbell, 1973). Price and Mountjoy (1970, p. 13) have proposed that the Malton gneiss represents “a tongue of hot mobile rock, from a deeply buried infrastructure, that has risen diapirically, along the Purcell thrust, into the cooler less mobile, sedimentary rocks of the suprastructure”. Geophysical evidence suggests that the Rocky Mountain Trench between latitudes 50” N and 56” N marks the present edge of the Precambrian craton (Berry et al., 1971). Bally et al. (1966) extend the cratonic basement beneath the Columbia Mountains but this is based on northward extrapolation from seismic studies adjacent to the 49th parallel that appears to contradict the gravity data (Berry et al., 1971). Deformation of rocks deeper in the pile than the Proterozoic Windermere seems to be certain, and if the cratonic margin extends west of the trench, these rocks most certainly would be deformed beneath the high grade terrane where melting temperatures have been approached and locally achieved (Campbell, 1973). STRUCTURE

OF THE NORTHERN

SELKIRK

MOUNTAINS

The mountains of the northern Selkirks, within the Big Bend of the Columbia River, are carved into Proterozoic Winder-mere and Lower Paleozoic

strata, that expose infrast~~ture of the me~morphi~ core complex (Fig. I). The region was mapped on reconnaissance scale by Wheeler (1963, 1965), and several detailed studies have been carried out to the south adjacent to the Trans-Canada Highway (Poulton, 1970; Simony and Wind, 1970; Zwanzig, 1973; Gilman, 1972; Thompson, 1972). The northern Selkirks have been studied recently by the author together with graduate students (Franzen, 1974; Tippett, 1976; Van der Leeden, 1976; Brown et al., 1977) and structurally reinterpreted by Brown and Tippett (in press). Structural fanning across the Porcupine Creek anticlinorium that dominates the western Main Ranges and Western Ranges east of the Rocky Mountain Trench, is repeated to the west along the structural and metamorphic culmination of the northern Selkirks. Folds on the no~heastern flank are overturned northeastward toward the foreland while those on the southwestern flank are primarily overturned southwestward toward t,he gneissic terrane of the Shuswap metamorphic complex. The Selkirk structural fan is attributed by Wheeler (1963, 1966) to late stage pinching of northeasterly directed folds between the gneisses of the Shuswap terrane and the edge of the craton resulting in backfolding and westerly rotation of the earlier structures. Price and Mountjoy (1970, p. 15) interpret the fan structure and the metamorphic culmination associated with it as “a zone in which tongues of hot ductile rock from the infrastructure have risen diapirically northeastward and have spread laterally under gravity”. They suggested that the northe~terly diapiric rise of the rnet~o~hi~ infrast~cture was a continuing process in which each local pulse deformed structures that were formed earlier, and produced a local surge in thrusting to the east within the Rocky Mountain Thrust and Fold Belt. Brown and Tippett (in press) have demonstrated that, in the vicinity of the Big Bend of the Columbia River, the Selkirk fan structure evolved by superposition of two distinct phases of deformation, upon strata previously involved in nappe formation (Phase I). They tentatively correlate the early nappe phase with structures known to have deformed Lardeau Group rocks of the Kootenay Arc in pre-Middle Mississippian times (Read and Wheeler, 1975; Read, 1975, 1976) and suggest that the event is related to the Late Devonian to Early Mississippian Caribooan or Antler orogeny (Wheeler et al., 1972). The Selkirk fan structure evolved in Middle Jurassic times (Wheeler, 1965; Wheeler et al., 1972). The Phase II folds which are dominant on the southwestern flank of the Selkirks are strongly overturned toward the southwest; Phase III folds are dominant on the northeastern flank where they are overturned towards the northeast. The structural fan is located where northeasterly dipping Phase II axial surfaces are overprinted and transposed by steeply dipping to vertical Phase III axial surfaces (Brown and Tippet% in press). Granodioritic plutonism and the main growth of metamorphic porphyroblasts occurred after Phase II and before the onset of Phase III in the Big Bend region. Recrystallization and some new mineral growth continued

143

through Phase III, and depths of burial compatible with the peak of regional metamorphism were maintained at least until the later stages of Phase III deformation (Brown and Tippett, in press). Brown and Tippett (in press) have proposed that the overturning of Phase II structures towards the southwest reflects northeastward under-thrusting or flow of the infrastructure of the Shuswap metamorphic complex beneath the Selkirk terrane. This deformation is considered to predate any significant folding or thrusting in the Rocky Mountain Belt. The late metamorphic Phase III folds increase in intensity toward the Rocky Mountain Trench where they are overturned towards the craton and stacked up in the hanging wall of the Purcell Fault. These relationships have led Brown and Tippett (in press) to suggest that the Phase III folds developed in response to underthrusting of the northeastern flank by basement rocks of the Rocky Mountain Foreland. Timing of emplacement of the Malton gneiss to the northwest of the Big Bend area is yet to be established, but according to J. Wagner (personal communication to D. Craw, 1977) the gneiss has been deformed together with the surrounding Windermere metasediments, and he also reports that the metamorphic grade of the metasediments locally drops towards the gneiss. Although more work is required to unravel the true nature of the Malton gneiss it does appear to have been emplaced early in the structural history, and consequently did not act as a heat source during the regional metamorphism. Within the Big Bend area ductile deformation was essentially completed while the metamorphic rocks remained at depths compatible with the alumino-silicate triple point. There is no evidence of synmetamorphic upwelling or lateral spreading of the infrastructure of the Selkirk terrane as proposed by Price and Mountjoy (1970). Uplift may have been initiated late in Phase III deformation during the waning stages of metamorphism, but the major uplift must have occurred along faults active in post Phase III times. The results of Brown and Tippett (in press) fit in very well with conclusions drawn by Craw (1977) for the Park Ranges northeast of the Purcell Fault, discussed earlier in this paper. He relates the development of the Porcupine Creek anticlinorium to an early premetamorphic to synmetamorphic deformation that is followed by a second phase of deformation that occurred late in the metamorphic history. He suggests that a pressure of approximately 5 kbar prevailed during the metamorphism and associated ductile strain, and that uplift occurred later on faults that offset the metamorphic isograds. Furthermore he concludes that the pressure in the Selkirk and Monashee Mountains west of the Purcell Fault was 2-3 kbar higher than in the Park Ranges. To summarize

from the above observations

The penetrative strain in the western Main Ranges and western Ranges of the Rocky Mountains, and the Selkirk terrane of the Columbia Mountains

was initiated before the onset of regional Barrovian me~morphism, and was completed during its waning stages. The Porcupine Creek anticlinorium and the Selkirk fan structure developed at different depths and have been brought into juxtaposition by late thrust faulting on the Purcell and associated faults. Northeastward spreading and upwelling of tongues of hot gneissic rock beneath the Selkirk fan structure has not been demonstrated. Rather it appears that southwestward verging Phase II folds developed in the Selkirks before major heating of this part of the infrast~~tur~, and northeastw~d verging Phase III folds developed late in the thermal history of the region. The geometry and orientation of these folds imply horizontal contraction and vertical thickening across the Selkirk terrane (Brown and Tippett, in press). These penetrative events that are closely associated with regional heating extend eastward into the Main Ranges (Campbell, 1968; Price and Mountjoy, 1970; Craw, 1977). Movements that carried the Selkirk terrane upwards and northeastward were initiated in the waning stages of penetrative strain and met~o~hism. Neither the kinematics nor the timing of the ductile strain within the infrastructure of the Selkirk terrane are compatible with spreading tongues of hot gneiss. The essentially post metamorphic northeastward and upward transport of the Selkirk terrane took place by movement on the Purcell and related thrust faults. SHUSWAP TERRANE

Central to the metamorphic core complex of southeast Canadian Cordillera, the Shuswap terrane, an elongate zone of high grade metamorphic rocks, is marked on its eastern flank by exposures of several large domal structures at about 80 km intervals, which are cored by migmatitic granitoid gneiss (Wheeler, 1965; Reesor, 1970; see Fig. 1). Although it is generally accepted that much of the Shuswap terrane is underlain by deformed and migmatized Proterozoic Windermere and younger rocks (Wheeier, 1965; Reesor, 1970), there is evidence that o~ho~eiss in the cores of the gneiss domes was derived from older Precambrian basement. Wanless and Reesor (1974) have obtained an age of 1960 +35/-45 m.y. from zircons in granodioritic gneiss of the Thor-Odin dome, and have suggested that similar gneissic rocks in the cores of other domes in the complex may also be this old. Wanless and -Reesor argue for an igneous origin for the granodioritic rocks, however, the possibility that the gneisses were derived from sediments containing detrital Precambrian zircons can not be ruled out. The timing of events within the complex is difficult to assess and more work is required. Mention has already been made of evidence for Late Devonian to Early Mississippian nappe formation in the Kootenay Arc and Selkirk Mountains, and this event presumably involved the adjacent Shuswap terrane. Okulitch et al. (1975) have obtained Devonian ages from zircons in granitoid gneisses that have intruded the western flank of the complex, and

145

this event may prove to be a manifestation of Caribooan orogeny in the Shuswaps. Read and Okulitch (1977) have defined a regional angular unconformity of Late Permian or Early Triassic age on the western and southern margins of the Shuswap terrane, and they suggest that widespread deformation and low grade regional metamorphism occurred just prior to the Late Triassic. Until this early history of the metamorphic core complex is better understood, tectonic models for evolution of the region must be restricted in scope. There is, however, general agreement that high grade metamorphism and associated deformation, including emplacement of the gneiss domes, occurred during the climatic Columbian orogeny in Middle Jurassic times (see Wheeler and Gabrielse, 1972, p. 49). Since stratigraphic control is yet to be established over much of the Shuswap terrane, knowledge of the regional structure remains obscure despite several excellent detailed studies in the vicinity of the gneiss domes and elsewhere in the complex (Reesor, 1965, 1970; Fyles, 1970; Fyson, 1970; McMillan, 1970; Reesor and Moore, 1971 and others). Major early folds have been recognized with westward and southwestward trending hinge lines that predate and appear to be unrelated to development of the gneiss domes. These early structures have been refolded on a regional scale, and also locally by structures associated with development of the gneiss domes (see Wheeler, 1965; Fyles, 1970). Much of the terrane is characterized by gentle dips of bedding and foliation (Jones, 1959), and it is clear that migmatitic gneisses have been flattened with attendant development of boudinage in competent units. However, until sufficient stratigraphic control is established to determine whether or not the stacking of recumbent nappes, involving initial crustal thickening with attendant limb attention, has occurred, the observation of flat foliation and boudinage cannot be used as evidence for finite horizontal extension of the infrastructural terrane. It is also important to recognize that westerly verging structures dominate parts of the western border of the Shuswaps (Campbell, 1970; Campbell and Okulitch, 1973; Okulitch, 1974). The infrastructure of high grade metamorphic rocks penetrates the northern Selkirks as a southeast trending culmination that crosses the western flank of the Big Bend of the Columbia River and follows the axial zone of the Selkirk fan structure (Wheeler et al., 1972; Brown and Tippett, in press). Since metamorphic isograds can be traced out of the Shuswaps into the Selkirks, phases of deformation in the two terranes that are closely associated in time with the metamorphic culmination must be related events. Early deformation is recognized in both the Shuswap and Selkirk terranes, and there is evidence of high grade metamorphism associated with these preColumbian events. As discussed earlier in this paper and elsewhere, the synmetamorphic deformation in the Shuswaps, Selkirks and western Main Ranges occurred in Middle Jurassic times. Brown and Tippett (in press) have demonstrated that upwelling and lateral spreading did not occur in the

exposed infrastructure of the Selkirks. Some upwelling in the Shuswaps is implied by gneiss dome development but this may not be significant on the scale of the whole of the metamorphic complex. To the northwest the infrastructure of the Shuswaps is overlain by suprastructure that has experienced crustal shortening; these rocks are at the same stratigraphic level as the high grade terrane that is exposed in the Big Bend area of the Selkirks (see Campbell, 1973, Campbell et al., 1973; Brown and Tippett, in press). These observations imply that major uplift of the Shuswap terrane occurred in association with the post metamorphic uplift of the Selkirks and that ductile strain in the metamorphic core complex predates the development of faults, such as the Purcell thrust, that carried the rocks of the complex northeastward and upward over the basement rocks of the craton. DISCUSSION

The suprastructure from the Rocky Mountain Belt westward across the infrastructure of the metamorphic core complex exhibits evidence of finite horizontal shortening. Beneath the simple folds and faults which characterize the suprastructure of the complex is exposed the high grade metamorphic terrane of complex polyphase deformation. The eastern part of the infrastructure across the Selkirk terrane is a zone of crustal thickening as evidenced by the stacking of overturned folds. In the central part of the complex in the vicinity of the gneiss domes some crustal thinning (spreading) may have occurred beneath the suprastructure, but this is yet to be documented. Price and Mountjoy (1970) and Price (1973) have proposed that horizontal compression of supracrustal rocks is related to lateral gravitational spreading of a topographically high core complex in the manner modeled by Bucher (1956). Geological evidence presented in this paper and elsewhere argues against the existence of this uplifted core complex at the time of early thrusting in the western Main Ranges. Brown and Tippett (in press) have proposed that Phase II deformation in the northern Selkirks is related to eastward underthrusting by rocks of the Shuswap terrane lying to the west and that these movements resulted in crustal thickening associated with onset of high grade metamorphism in the infrastructure. The effects of early underthrusting from the west may well be associated with ductile strain as far east as the western Main Ranges and could be reflected in the early development of the Porcupine Creek anticlinorium. Late metamorphic Phase III folds in the Selkirks may reflect a change in boundary conditions with initiation of folds that are overturned toward the Rocky Mountain Belt (Brown and Tippett, in press). It is at this time of waning metamorphism and consolidation that northeasterly and upward displacement of the metamorphic complex occurs. At this point the rising core complex is moved over the craton on faults such as the Purcell thrust that are shallow dipping at high levels, but probably steepen with depth to a zone of ductile strain. At

147

this deep level beneath the presently exposed rocks of the metamorphic core complex, metamorphism and ductile strain presumably continued as long as thrust faulting in the Rocky Mountain Belt penetrated this far westward. CONCLUSIONS

The met~orphic core complex must be a~ochthonous with respect to a fixed reference frame beneath the Plains of the North American craton, and the relative displacement of the complex is estimated to be between 200 km and 35 km. Significant early (pre-Mississippian) deformation occurred in the Columbia Mountains that is not reflected in the Rocky Mountain Fold and Thrust Belt, Ductile strain and regional metamorphism, in Jurassic times, of the infrastructure of the exposed core complex was essentially completed before major development of thrust faults that carried the complex northeastward and upward during the evolution of the Rocky Moun~in Thrust and Fold Belt. Polyphase deformation of the infrast~ctu~ beneath a weakly compressed suprastructure is probably due to shear strains generated by underthrusting of marginal lithosphere rather than to gravitational upwelling and lateral spreading. Gneiss domes on the eastern margin of the core complex probably owe their origin at least in part to gravitational upwelling of mobile centers within the metamorphic core, but major uplift and northeastward translation of the metamorphic complex post dates these synmetamorphic events. Pre-metamorphic to early metamorphic strain of the complex may be related to eastward unde~h~sting from the west. Late metamorphic strain may be related to westward unde~h~sting of the North American craton to the east. Continued underth~sting of the craton led to uplift and northeastward translation of the consolidated metamorphic complex together with development of thrust faults in the Rocky Mountains. A comparison of the structural evolution of this part of the North American Cordillera with other segments of the orogen and other mountain belts is beyond the scope of this paper. However, it is worth pointing out that although gravitational layer instability may be responsible for generation of gneiss domes in the metamorphic core complexes of erogenic belts, we should not assume that this mech~ism is responsible for development of regional deformation within the me~o~hic belt as a whole, or for deformation events in the foreland areas; tectonic thickening induced by plate convergence and underthrusting appears to have played a far more important role. ACKNOWLEDGEMENTS

The author is indebted to J.O. Wheeler, whose initial enthusiasm and encouragement prompted pursuit of the wonders of Cordilferan geology. Enjoyable discussions along the way with R. Campbell, L. Lane, A.V. Okulitch,

148

R.A. Price, P.B. Read, J.E. Reesor and P. Simony have contributed to formulation of ideas expressed in this paper. The research has been made possible by funding from Energy Mines and Resources (Research Agreement 1135-013-4-107) National Research Council (Operating Grant A2693), British Columbia Department of Mines and Petroleum Resources, and Carleton University. REFERENCES Balkwill, H.R., 1968. Structural analysis of the Western Ranges, Rocky Mountains, Golden, British Columbia. In: Report of Activities, Part A. May to October 1967. Geol. Surv. Can., Pap., 68-l: 193-196. Balkwill, H.R., 1972. Structural geology, Lower Kicking Horse River region, Rocky Mountains, British Columbia. Bull. Can. Pet. Geol., 20: 608-633. Bally, A.W., Gordy, P.L. and Stewart, G.A., 1966. Structure, seismic data, and erogenic evolution of the Southern Canadian Rocky Mountains. Bull. Can. Pet. Geol., 14: 337381. Berry, M.J., Jacoby, W.R., Niblett, E.R. and Stacey, R.A., 1971. A review of geophysical studies in the Canadian Cordillera. Can. J. Earth Sci., 8: 788~-801. Brown, R.L. and Tippett, C.R., in press. The Selkirk Fan Structure of the Southeastern Canadian Cordillera. Geol. Sot. Am. Bull. Brown, R.L., Perkins, M.J. and Tippett, C.R., 1977. Structure and stratigraphy of the Big Bend area, British Columbia. In: Report of Activities, Part A. Geol. Surv. Can., Pap., 77-1A: 273-248. Bucher, W.H., 1956. The role of gravity in orogenesis. Geol. Sot. Am. Bull., 67: 12951318. Campbell, R.B., 1968. Canoe River, British Columbia. Geol. Surv. Can., Map 15-1967. Campbell, R.B., 1970. Structural and metamorphic transitions from infrastructure, to suprastructure, Cariboo Mountains, British Columbia. In: J.O. Wheeler (Editor), Structure of the Southern Canadian Cordillera. Geol. Assoc. Can., Spec. Pap., 6: 67-72. Campbell, R.B., 1973. Structural cross-section and tectonite model of the Southeastern Canadian Cordillera. Can. J. Earth Sci., 10: 1607-1619. Campbell, R.B. and Okulitch, A.V., 1973. Stratigraphy and structure of the Mount Ida Group, British Columbia. In: Report of Activities, Part A. Geol. Surv. Can., Pap., 73-l: 21-23. Campbell, R.B., Mountjoy, E.W. and Young, F.G., 1973. Geology of McBride map-area, British Columbia. Geol. Surv. Can., Pap., 72-35: 104. Charlesworth, H.A.K., 1959. Some suggestions on the structural development of the Rocky Mountains in Canada. Alberta Sot. Pet. Geol. J., 7: 249-256. Cook, D.G., 1970. A Cambrian facies change and its effect on structure, Mount Stephen - Mount Denis Area, Alberta - British Columbia. In: J.O. Wheeler (Editor), Structure of the Southern Canadian Cordillera. Geol. Assoc. Can., Spec. Pap., 6: 27-39. Craw, D., 1977. Metamorphism, structure and stratigraphy of the Park Ranges, Western Rocky Mountains, British Columbia. Thesis, Univ. of Calgary, Alta. (unpublished). Dahlstrom, C.D.A., 1970. Structural geology in the eastern margin of the Canadian Rocky Mountains. Bull. Can. Pet. Geol., 18: 332-406. Douglas, R.J.W., 1950. Callum Creek, Langford Creek and Gap map areas, Alberta. Geol. Surv. Can., Mem., 255: 124. Evans, C.S., 1933. Brisco - Dogtooth map-area, British Columbia. Geol. Surv. Can., Pap. 67-61: 101.

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