Major Dextral Transcurrent Displacements Along the Northern Rocky Mountain Trench and Related Lineaments in North-Central British Columbia

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Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia

H. GABRIELSE Geological Survey of Canada, 100 West Pender Street, Vancouver, B.C. V6B 1R8, Canada

ABSTRACT aments, across which lithology, stratigraphic geographic elements (Figs. 1, 2, 3). In general sequence, grade of metamorphism, and struc- terms, the region includes the western part of the The Northern Rocky Mountain Trench tural style change abruptly. Preservation of Cordilleran miogeocline, represented by the and a number of other prominent lineaments, Paleocene-Eocene nonmarine sedimentary and Foreland Fold and Thrust Belt east of the along and east of the eastern margin of the volcanic rocks in grabens or half-grabens along N.R.M.T. The region also includes the Omineca Intermontane Belt, mark faults along which the major lineaments suggests late normal fault- Crystalline Belt, which at this latitude is con- dextral transcurrent movements have been ing, but offsets of a variety of geologic elements sidered herein to be a dextrally offset slice of the dominant. Offsets of shelf to off-shelf facies indicate earlier and probably in part contempo- miogeocline, west of the N.R.M.T. Alloch- boundaries in lower Paleozoic rocks indicate raneous dextral transcurrent faulting. thonous oceanic and island-arc terranes locally a cumulative displacement of at least 750 km, The faults described herein form an anas- overlie the miogeoclinal rocks west of the and probably >900 km, within the system of tomosing network between the Northern Rocky N.R.M.T.; to the west, they are in fault contact faults related to those in the Northern Rocky Mountain Trench (N.R.M.T.) on the east and with oceanic and island-arc terranes of the Mountain and the Tintina Trenches. Farther the Cordilleran Intermontane Belt on the west. Intermontane Belt. west, another system of faults appears to To the east of the N.R.M.T., prominent line- The northern Rocky Mountains are underlain offset plutons and stratigraphic assemblages aments in the Rocky Mountains appear to have by upper Proterozoic rocks lying on basement along the eastern margin of the Intermontane resulted from erosion of relatively soft rocks granitoid gneiss (Evenchick, 1982; Gabrielse, Belt by as much as 300 km. These faults, in- parallel with regional fold structures. To the 1975; Gabrielse and others, 1977) and lower cluding the Kutcho and the Pinchi, connect in west, the distribution of strata in the Bowser and Paleozoic strata. The latter show a well-defined, part with the Teslin Suture Zone in Yukon the Sustut basins of western British Columbia fairly narrow transition from carbonate and Territory and probably with the Fraser River- precludes significant transcurrent faulting there clean sandstone of stable-platform to subsiding- Straight Creek fault zone in southern British since Middle Jurassic time. shelf facies on the east, to shale, siltstone, and Columbia. Although dextral transcurrent local slide breccias of off-shelf facies on the west faulting may have taken place between the PHYSIOGRAPHIC EXPRESSION (Figs. 1,2). These rocks are overlain by westerly Middle Jurassic and early Cenozoic, the most OF FAULTS derived shale and turbiditic sandstone, of Late convincing evidence points to middle Cre- Devonian to Mississippian age, with local taceous and particularly to early Cenozoic Most of the regional faults in north-central Lower Mississippian limestone. Except for a (Eocene?) displacements. The Eocene(?) British Columbia lie along conspicuous line- narrow, complexly deformed zone flanking the movements were temporally related to plu- aments tens to hundreds of kilometres long. N.R.M.T., the northern Rocky Mountains are tonism, volcanism, lamprophyre dike em- Some of the lineaments are deeply incised, flat- characterized by easterly directed, fairly continu- placement, high heat flow, sedimentation in bottomed, steep-walled valleys, whereas others ous thrust faults and folds with westerly dipping grabens, and rapid uplift of northwesterly comprise aligned, narrow notches on ridge axial surfaces (Gabrielse, 1962a; Gabrielse and trending elongate ranges. Climactic episodes crests. Faults, along which major movements others, 1977). of granite emplacement, particularly in and may have ceased before Late Cretaceous time, On its west side, the N.R.M.T. acutely trun- near the northern Omineca Crystalline Belt, are not everywhere coincident with obvious cates a number of northwesterly trending, fault- at -100 m.y., 70 m.y., and 50 m.y. ago may topographic lineaments. In general, faults, bounded panels characterized by distinctive have been facilitated by changes from domi- along which there have been significant facies, thickness and sequence of strata, grade of nantly compressional to dominantly transcur- Cenozoic displacements, show marked topo- metamorphism, the presence or absence of vol- rent and related tensional strain. graphic expression. canic and granitic rocks, and structural style. In the Cassiar Mountains, lower Paleozoic strata INTRODUCTION SUMMARY OF exhibit a westward change in facies from plat- REGIONAL GEOLOGY form to subsiding shelf (transitional) to off shelf. Regional, mainly northwest-striking faults A narrow panel northeast of Burnt Rose fault are conspicuous elements in the geology of A brief review of the stratigraphy and (Fig. 1) is exceptional, for the region southwest north-central British Columbia (Fig. 1). They structure flanking the major faults emphasizes of the N.R.M.T., in having graptolitic Orodo- are generally marked by prominent line- the degree to which they have disrupted paleo- vician and Silurian shale and siltstone of an off-

Geological Society of America Bulletin, v. 96, p. 1-14, 12 figs., 1 table, January 1985.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/1/1/3430183/i0016-7606-96-1-1.pdf by guest on 02 October 2021 Figure 1. Distribution of the main stratigraphie assemblages and major faults in north-central British Columbia. Faults are shown by heavy dash symbol. Barbs are on upper plates of thrust faults. Arrows indicate the relative sense of transeurrent displacements.

LEGEND * • /iiiiiiiiiiTTT^^^^Xb^ '"^"""/VVVVVVW UNr^V ^^ I Lower Cenozoic, mainfy: coarse clastic sediments" minor volcanics E Mid Cretaceous: granite pa^Ji —Lower Jurassic & Upper Triassic:

M ywacke, slate, conglomerate; andljsitic volcanics Early Jurassic: mainly granodiorite & quartz dioritej Lvvvvi Upper Paleozoic to Middle Jurassic arc facies; v.^^v andesitic volcanics, sediments |-"-~2>i Upper Paleozoic: oceanic facies ; mafic volcanics, ultrcimafics, chert, limestone, shale. A, strongly X ' "V' V v"" J deformed (Cache Creek) B, less deformed ¿¿i »'ngen;C (Sylvester) 4hutadeL^ Lower to Mid Paleozoic: transitional facies; platformal Lower & Middle Devonian strata; graptolitic Ordovician & Silurian strata :offshelf facies: mainly fine-grained clastics; includes some narrow belts of carbonate : platformal & shelf facies; carbonate & sandstone; includes Ordovician graptolitic shale in Cassiar Mountains lUpper Proterozoic; sedimentary & metamorphic rocks Granite gneiss; basement Facies boundary Middle Silurian

SCALE LEGEND 1000 EI3 SHALE F^H VOLCANICS 3000 ES3 SILTSTONE IZ3 CHERT 2000 EDSANDSTONE.QUARTZITE [T^N LIMESTONE BRECCIA 500 EZ3 CONGLOMERATE FH LIMESTONE 1000 m~i HIATUS DOLOMITE L.MISS.

U.DEV METRES FEET L. MISS. U.DEV. M. DEV. T- - L.MISS. / / \ U.DEV. / / , S x M.DEV. rtz rt: \ N L .DEV. U.DEV. M.DEV. ^ N U.SIL. — ZZUlii / L .DE V. L. DEV. •v TTAND" M / — // DEV. SIL. SIL.ORD. SIL. ^ÎL ennr U.CAMB. / / U.CAMB. ORD. AND L.ORD. U.CAMB. rlïz L.ORD. 1 - -1— U.CAMB. L.ORD. L.ORD. M .CAMB. L.CAMB. L.AND.M CAMB. L.CAMB. L.CAMB. L. CAMB.I OFF-SHELF HADRYNIAN HADRYNIAN L.CAMB. ? HADRYNIAN HADRYNIAN HELIKIAN (?) TRANSITIONAL PLATFORM MAINLY OFF- OFF-SHELF SHELF PLATFORM TO SHELF SHELF

Figure 2. Generalized stratigraphie columns in Cassiar and northern Rocky Mountains showing thickness and facies changes. See Figure 1 for locations.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/1/1/3430183/i0016-7606-96-1-1.pdf by guest on 02 October 2021 Figure 3. Summary of structures and structural styles near major faults in northern Rocky, Cassiar, and Omineca Mountains.

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shelf fades overlain unconformably by Lower stone, graywacke, and conglomerate of Early Paleocene-Eocene sediments in the N.R.M.T. Mississippian limestone (Figs. 1, 2; Gabrielse, Jurassic age. In some areas, an Upper Triassic are gently to tightly folded, faulted, and locally 1962b). Overlying the miogeoclinal strata, there limestone formation is present between the vol- brecciated. Numerous clasts in conglomerate is an allochthon of oceanic, mainly Mississip- canic and turbidite units. have been systematically fractured and, although pian to Permian rocks (Sylvester Terrane). Middle Cretaceous granitic plutons are wide- only local studies have been made, it appears Northeast of a zone of structural divergence spread in areas believed to be underlain by that the strain is in accord with the orientation of along the Kechika fault (Fig. 1), most structures continental crust. Early Jurassic plutons are folds which trend more westerly than does the are eastward verging. Structures with an oppo- closely associated with volcanic rocks of the arc N.R.M.T. (see area near lat. 58° in Fig. 3). Tight site sense of vergence occur southwest of the terranes. folds in Paleozoic strata near the mouth of the Kechika fault, southeast of Rapid River (Fig. 3). Structurally complex, high-grade metamor- Turnagain River have steeply plunging axes, but Structures west of the N.R.M.T. commonly phic rocks that yield Eocene K-Ar ages overlie similar steep plunges have not been noted trend more easterly than do those in the Rocky granitoid basement gneiss northeast of the Spi nel elsewhere. Mountains; in some places they trend almost fault; a structurally complex area containing Along the Kechika and Spinel faults, there are east-west and are much less continuous. augen gneiss lies east of Horseranch fault spectacular folds, ranging from crinkles to those The Omineca Mountains are underlain (Gabrielse, 1963). A body of fresh, undeformed with amplitudes of > 100 m, whose hinges trend mainly by a thick succession of variably meta- ultrabasic rock of probable Eocene age Lies more westerly than the regional fault aid fold morphosed, miogeoclinal upper Proterozoic to along the east side of the Horseranch fault. axes (Eisbacher, 1972). The hinges of tight, Lower Ordovician strata assigned, for the most Areally restricted but tectonically significant chevron-crinkle folds in lower Paleozoic rocks part, to the Omineca Crystalline Belt (Mansy rocks occur in and along the fault zones. These in the Spinel fault zone are parallel with the and Gabrielse, 1978). Most regional structures include Upper Cretaceous, Paleocene and/or hinges of tight folds in Paleogene clastics. verge westward and are locally characterized by Eocene nonmarine conglomerate, sandstone, Granitic rocks along the Kutcho, Cassiar, nappes with gently dipping axial surfaces (C. A. siltstone, and shale, locally with coal. They are and Thudaka faults have been penetratively Evenchick, 1982, personal commun.). Along the most extensively developed in the N.R.M.T. sheared and foliated over widths of as much as west side of the Cassiar and Omineca Moun- near Sifton Pass (Fig. 1), but they are also pres- 2 km. Commonly, the deformation culminates tains, there are Mesozoic arc terranes thrust ent locally along the Kechika, Spinel, and Burnt in zones of ultramylonite <1 metre: wide. northeastward (on the Klinkit, Hottah, and Rose faults (Gabrielse, 1962a, 1963). A flora Prominent, horizontal to gently plunging line- Swannell faults, for example) over older mio- near the Turnagain River along the Kechika ations in the shear-zone rocks are evident. geoclinal rocks or juxtaposed against them along fault has been tentatively assigned a Late Cre- Middle Cretaceous K-Ar ages from muscovites transcurrent faulis (Gabrielse and Dodds, 1982). taceous (Santonian-Campanian) age by W. A. generated in the fault zones suggest that foliation Faulted against western portions of the arc ter- Bell of the Geological Survey of Canada. Ande- developed during or not long after emplacement ranes along the Thibert and Pinchi faults in the sitic volcanics of Eocene age (Wanless and oth- of the middle Cretaceous granitic rocks. Strongly northwestern and southern parts of the region, ers, 1978) are exposed near the junction of the foliated granitic rock in the N.R.M.T. east of the respectively, are oceanic terranes in part com- N.R.M.T. and Spinel faults and near Sifton Pass. mouth of the Ingenika River locally contains posed of tectonic mélange. They are bounded on Eocene rhyolitic volcanics underlie an area of spectacular cross-cutting veins of ultracata- the southwest by thrust faults (for example, King -35 km2 along the east side of Kechika fault clasite (Evenchick, 1982). Salmon, Nahlin, and Vital), along which they just north of the Turnagain River (Stevens and have been emplaced southwestward onto others, 1982). Finally, a swarm of northeasterly Structures in Panels between Faults another belt of arc rocks of late Paleozoic and and northwesterly striking Eocene lamprophyre In the Omineca Mountains northeast of the early Mesozoic ages (Gabrielse and others, dikes cuts metamorphic and granitic rocks Swannell fault, a late penetrative crinkle line- 1978, 1980; see also Fig. 1). All of the Mesozoic northeast of the Spinel fault and locally cuts ation trends frm almost east-west to —115 de- and Paleozoic arc and oceanic terranes are Paleogene rocks in the Spinel fault zone. grees. Similar trends are observed in folds and assigned to the Intermontane Belt. In terrane The distributions of sedimentary facies and of thrust faults in the panels northeast of the north- terminology, the arc rocks northeast of the domains of similar structural style (Figs. 1,2,3), ern part of the Thudaka fault (Fig. 3). On the Kutcho and Pinchi faults are referred to as outlined above, show the degree to which the northeast side of the Kechika fault, westerly Quesnellia; the oceanic rocks, to Cache Creek; original paleogeographic and tectonic elements trending folds plunge moderately toward the and the southwestern belt of arc rocks, to have been disrupted by faults. The same criteria fault. Stikinia. demand that the faults be transcurrent and of The orientations of structures noted a'sove are The assemblages of rocks between the Thibert regional importance. No evidence indicating compatible with stress related to dextral trans- and King Salmon faults and between the Pinchi that the facies distributions are the result of low- current movements on the major faults, evidence and Takla faulis are of particular importance angle thrust faulting followed by dip-slip dis- for which is presented below. Indeed, the con- because they are not only similar but also placements has been recognized in the region. trasting structural styles of the northern Rocky unique. They comprise structurally complex Mountains, characterized by low-plunging, uni- sequences of dominantly upper Paleozoic and STRUCTURES RELATED TO FAULTS formly trending fold axes and relatively contin- possibly minor early Mesozoic chert, argillite, uous thrust faults, and the Cassiar-Omineca limestone, volcanic, and ultramafic rock that is Structures in Fault Zones Mountains, characterized by local steeply plung- commonly in fault contact with, but locally ing and variably trending fold axes, ma y be the overlain stratigraphically by, a distinctive Upper A wide variety of structures occurs in and consequence of an overprint west of the Triassic volcanic unit characterized by dacitic- along the fault zones. Sedimentary rocks along N.R.M.T. of structures related to dextral trans- quartz porphyry. Overlying the porphyry unit, the Kechika and Horseranch faults are intensely current faulting on earlier formed structures there is a thick sequence of turbiditic shale, silt- brecciated over widths of tens of metres. The similar to those east of the trench (Fig. 3).

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DISPLACEMENTS OF tions is evident from an analysis of sedimentary water facies to the west as far as the N.R.M.T. REGIONAL FAULTS facies and depositional polarity in lower Paleo- The facies boundaries can be followed north- zoic rocks (Figs. 4, 5). In the northern Rocky ward for hundreds of kilometres and are inter- The best criteria for estimating displacements Mountains, all lower Paleozoic rocks change preted as marking the western limit of the broad on regional faults are those that are least affected facies westward (Fig. 2). In general, well-bedded carbonate platform of the lower Paleozoic con- by depth of erosion. These include offsets of carbonates of Middle Ordovician to Middle tinental shelf. This simple, general paleogeog- geologic units with considerable vertical extent, Devonian age to the east reflect stable platform raphy is complicated locally by narrow, for example, batholiths, distinctive cylindrical or subsiding-shelf environments of deposition. northwest-trending belts of Middle Cambrian folds that involve thick stratigraphie sequences To the west, the carbonates grade across well- and Devonian reefoidal carbonates, generally and, particularly, paleogeographic elements, defined, narrow transition zones into shale and < 100 km long, that may have been deposited on such as strand lines and shelf-to-basin transition siltstone, with local slide breccias derived from uplifted blocks bounded by rifts in the off-shelf zones. In the Northern Rocky, Cassiar, and the carbonate banks. Clean, Lower Cambrian region (Gabrielse, 1981; Mclntyre, 1981). Facies Omineca Mountains, the criteria mentioned sandstone defines a narrow facies belt along the boundaries associated with these blocks are above occur singly or in combination and pro- eastern margin of the subsiding shelf. To the commonly restricted to rocks representing a vide information on displacements for several of west, siltstone, shale, impure sandstone, and relatively short time span and easily separated the major faults. local members of limestone are typical. Lower from the much more fundamental boundaries Devonian clean sandstone and dolomitic sand- noted above. Lower Cambrian clastic facies are Northern Rocky Mountain stone of the eastern platform facies are equiva- not affected by the fault blocks. Trench Fault Zone lents of a mixed carbonate sandstone and Similar stratigraphy in the Cassiar and breccia facies of the subsiding shelf which, in Omineca Mountains show the same kinds of The N.R.M.T. is the physiographic expression turn, are correlative with off-shelf shale, silt- facies changes in a westward direction as do of a major structural discontinuity, across which stone, and debris flows to the west. those in the northern Rocky Mountains. More- there are abrupt changes in stratigraphy and The distribution of all units described above over, current directions indicate easterly source paleogeographic trends. That the changes are clearly indicates that the rocks, in general, are of areas for shallow-water Lower Cambrian and not simply the result of original basin configura- shallow-water facies to the east and deeper- Lower Devonian sandstones (Figs. 4, 5). Clearly, the clastic rocks could not have been carried through the off-shelf environment east of 136° N.R.M.T. It appears, therefore, that lower Paleozoic strata have been displaced from their former position adjacent to platform and subsiding-shelf facies now exposed in the northern Rocky Mountains (Figs. 6A, 6B). The magnitude of displacement is suggested by the location of the intersections of facies boundaries with the Tintina and Rocky Moun- tain Trenches. Offset of the Lower Cambrian clean-quartzite-facies belt is difficult to deter- mine accurately. Intersection of the belt with the east side of the N.R.M.T. must be near, or south of, latitude 56°N. On the west side, it apparently lies near latitude 61°32'N (Read, 1980; Tempelman-Kluit and others, 1976), suggesting a minimum dextral offset of 750 km. Location of the lower to middle Paleozoic carbonate to shale boundary can be determined more pre- cisely. The boundary is closely fixed east of the LEGEND N.R.M.T. at about latitude 56°N. There, distinc- Inner detrital belt: dominantly tive Silurian and Devonian carbonate strata, m sandstone with diagnostic faunas including Stringocephalus Middle carbonate belt: carbonate, sp. of late Middle Devonian (Givetian) age siltstone, shale (Taylor and Mackenzie, 1970), are similar to Mixed carbonate and inner detrital rocks just west of the Tintina Trench, near lati- facies: sandstone, carbonate, shale tude 61°15'N—about 700 km to the northwest Outer detrital belt: shale, siltstone, (Fig. 5). The latter are found in a northwesterly minor carbonate trending belt as far north as latitude 62°N, but 0 300 Tempelman-Kluit (1977) has suggested that i= .. J they were deposited on a peninsula with off- shelf facies between them and the Tintina Figure 4. Facies distributions, paleocurrent directions (closed arrows), and fining directions Trench (Fig. 5). Much greater apparent (open arrows) of clastic Lower Cambrian strata in part of northern Cordillera. displacement—more than 900 km—is indicated

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Figure 5. Faciès distributions, Stringocephalus sp. localities, and paleocurrent directions in Upper Silurian, Lower and Middle Devonian strata.

LEGEND rri Platform and shelf ' 1 dolomite, sandstone rrri Offshelf shale, siltstone, I 1 fine-grained quartzite & Strinqocepholus sp. localities Western boundary of carbonate with Stringocephalus sp.

if, alternatively, the off-shelf fades between the Tintina Trench and the platform facies west of Ross River is a faulted slice and has also been displaced dextrally (Fig. 5). Studies of sedimen- Cenozoic offset along the N.R.M.T. is sug- tary transport in clastic rocks of strata west of gested by the match of an Eocene structural Tintina Trench would provide constraints on culmination, with granitic gneiss in its core, east these options. of the Ingenika River and a structural culmina- Not all of the displacement discussed above tion, cored by Eocene granite and basement took place along the N.R.M.T. but was dis- gneiss, west of the N.R.M.T.—125 km to the tributed between the Trench and several major northwest (Fig. 1). Moreover, related, probable faults farther west that connect with faults in the early Cenozoic erosion surfaces are also offset; Tintina and N.R.M. Trenches. these flank the N.R.M.T., and seem to have been

OI:F-SHELF \ --/ SHELF

PEACE RIVER ARCH

0 500 km

Figure 6. Interpretation of Cassiar Platform as a displaced part of the Peace River Arch (A, B) based on Figure 5. A possible explanation for the discrepancy in offsets, suggested by pre-Late Cretaceous contractional structures and early Paleozoic facies boundaries, is shown in 6C (regional contraction) and 6D (subsequent further dextral displacement). C.P. = Cassiar Platform; C.P. + A. = Cassiar Platform with al- lnchthonous teiranes. See text for further explanation.

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upwarped on the culminations. The surfaces west by Thudaka fault. It is foliated, muscovite- Horseranch and Deadwood Faults now slope to the north and south, away from the bearing, and has been dated by the K-Ar method structural highs. It is possible, however, that the at between 88 and 100 m.y. B.P. Similar rocks The most obvious component of movement culminations represent independent Eocene up- of the Whudzi pluton, probably displaced on these faults is vertical, and the intervening lifts, thus providing no information on the dextrally from the Thudaka pluton, lie along the Proterozoic rocks are strongly uplifted. Strati- amount of transcurrent displacement. east side of Finlay fault to the south. It is clear graphic throws probably exceed 2 km. The from the size of clasts that the boulders could not orientation of these faults suggests a tensional Spinel Fault Zone have been transported far, and it is improbable origin related to dextral movement on the that they traveled the distance required by the Kechika fault. Lower Cenozoic conglomerate, deposited on present distribution of the conglomerate and the erosion surface west of the mouth of Fox source rocks. Most likely, the terrane west of PeUy Fault River, includes boulders of foliated, megacrystic, Spinel fault was displaced to the northwest as muscovite-bearing granite as much as 2 m in much as 85 km, following deposition of the North of the Ingenika River, the Pelly fault is diameter (Fig. 7). Muscovite from the boulders conglomerate. marked by a straight lineament trending parallel was dated by the K-Ar method at ~97 and with the N.R.M.T. A significant vertical com- 106 m.y. B.P. Granitic rocks nearby to the north Kechika and Thudaka Faults ponent of movement is shown by the presence are of markedly different character and have of upper Proterozoic strata to the west and been dated by K-Ar method at ~42 m.y. B.P. West of Kechika fault near Rapid River, the Cambrian-Ordovician strata to the east. At its The obvious source for the boulders is the mid- southwesterly directed structures typical of the south end, this fault intersects two northeasterly dle Cretaceous Thudaka pluton, bounded on the southern Cassiar Mountains and Omineca striking faults, along which apparent transcur- Mountains are replaced transitionally along rent displacement has been sinistral. They offset structural trend to the northwest by northeasterly the axes of two regional anticlinoria as much as directed structures. These structures are typical 15 km (Fig. 3). If they are conjugate faults re- of the northern Cassiar Mountains and the area lated to the Pelly fault, the compatible move- northeast of Kechika fault. Restoration along the ment on the Pelly fault would be dextral. Kechika and Thudaka faults in the order of Farther south, the Pelly lineament along the 60 km and a further minimum of 110 km along Mesilinka River is crossed by a regional syncline, Kechika fault southeast of its intersection with precluding the possibility of significant transcur- Thudaka fault juxtaposes terranes of similar rent faulting. structural style (Fig. 8). Thudaka, Finlay, Ingenika, and Kutcho Faults

The distribution of middle Cretaceous granite in the region between the Turnagain and Finlay Rivers strongly suggests that the southeast end of the Cassiar batholith has been offset dextrally by the Thudaka and Finlay faults (Figs. 1, 9A). Displacements on the Thudaka fault appear to have been in the order of 60 km and, judging from K-Ar ages on muscovite developed in the foliated granitic rocks, took place during late middle Cretaceous or early Late Cretaceous time. Displacement on the Finlay fault, indicated by offset of granitic rocks, was a minimum of 15 km and a maximum of 50 km. The latter amount is compatible with offsets along the Finlay fault farther south. Further indication that considerable dextral movement took place on the Thudaka, Finlay, and Ingenika faults is given by the apparent displacement of —125 km of an upper Paleozoic and Upper Triassic terrane in the hanging-walls of the Hottah and Swannell thrust faults mi (Figs. 9A, 9B). Similar dextral offset of 0 10 — 110 km is suggested by the disposition of two km belts of Upper Triassic(?) zoned intrusive ultramafic bodies (Figs. 9A, 9B). This amount may have been distributed about equally along the Thudaka fault and the Finlay fault, on the east Figure 7. Diagram showing proposed dextral offset on Spinel fault zone following deposi- side of the Thudaka pluton. tion of lower Paleogene conglomerate. The Cassiar, Klinkit, Thibert, Nahlin, King

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Salmon, and Hottah faults and bounding ter- Pitman and Related Faults istrally -4 km (Gabrielse and Dodds, 1982). ranes are truncated by the Kutcho fault (Figs. 1, There, however, striae and crinkle crenulations 9A). Monger and others (1978) suggested the About 3 km of sinistral displacement, sug- associated with the easterly striking fault indi- possible dextral offset of 200 km to account for gested by offsets of the Kutcho and Thudaka cate that important displacement has resulted in the distribution of Cache Creek oceanic rocks in faults, is apparent on the east-northeast-trending relative uplift of rocks to the south. The sinistral the Omineca and Cassiar Mountains. Restora- Pitman fault, but the amount, if any, of vertical movement and orientation of the fault, east of tion of dextral offset of —300 km along the movement is difficult to assess. The sense of ap- Dease Lake and the Pitman fault, is compatible Kutcho, Finlay, Ingenika, and Takla faults, parent vertical movement along the fault is indi- with their origin as antithetic shears related to however, juxtaposes the Nahlin and the Vital cated along its western part, where Upper the dominant northwest-striking dextral fault. thrust faults and the unique stratigraphic as- Triassic volcanics in its southern wall are juxta- semblages associated with these structures posed against Middle Jurassic sediments and Klinkit and Hottah Faults (Fig. 9C, 9D; P,Hereon, 1977). It also brings volcanics to the north. East of the north end of together a complex batholith (Hogem) of Juras- Dease Lake, a fault of similar trend, one of a set These thrust faults place younger reeks of sic and Cretaceo as granitic plutons east of the of subparallel faults, offsets the Kutcho fault sin- the upper Paleozoic to Middle Jurassic arc Takla fault with an equally complex batholith southwest of the Kutcho fault and east of Dease Lake. Indeed, the two batholiths and a similar one straddling the Pitman fault may have been one body originally. Displacement along the Kutcho fault of ~ 100 km is indicated by offset of the Klinkit and Hottah faults, which bound similar Upper Triassic and related granitic rocks. If this is the case, the remaining 200 km of displacement could have taken place along the Thibert and the Finlay-Thudaka fault systems, possibly —75 km on the former and 125 km on the latter. Movement of this order of magnitude is consistent with suggested offsets described above for the Finlay, Thudaka, and Kechika faults. Also implied is that the Kutcho and Pinchi faults, two of the most important structures in northern British Columbia, are segments of a formerly continuous fault, offset by the Finlay fault zone, along the northeast side of the Cache Domains of east verging Creek Terrane (Fig. 9). structures p^-.-j Domains of west verging Because the orientations of the Thibert, Kutcho, Thudaka, and Finlay faults are signifi- cantly different from one another, even the simplest restorations involve some rotation of geologic units. Fewer problems occur if it is as- sumed that all of the movement on the Finlay and Thudaka faults postdated movements on the Kutcho and Pinchi faults (Fig. 9B). It appears, however, that some movements on the Kutcho and Thudaka faults were contemporaneous (see below), and if restoration of the traces of the Nahlin and Vital faults is valid, then the total strain involved entails a complex geometry of dextral displacements coupled with rotations (Fig. 9D).

Cassiar Fault

Although stretching lineations in mylonitic Figure 8. Apparent dextral offsets on Kechika and Thudaka faults showing displacements of rocks in the Cassiar fault zone (Fig. 1) suggest distinctive structural domains. 8A. Present distribution of structural domains. 8B. Distribution transcurrent movement (Gabrielse, 1969; Poole, after restoration on Kechika and Thudaka faults of -50 km. Further restoration of 1956), no correlations across the zone have been >110 km on Kechika fault is required to juxtapose east-verging structural domains along the made. fault southeast of Thudaka fault.

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terrane onto metamorphosed miogeoclinal strata TABLE 1. SUMMARY OF AMOUNTS AND AGES OF DEXTRAL TRANSCURRENT DISPLACEMENTS (Gabrielse and Dodds, 1982). In some places, ON SELECTED FAULTS IN THE NORTH-CENTRAL CANADIAN CORDILLERA overlap of the terranes is probably > 10 km. As Fault Displacement Age Criteria noted above, the two structures may represent offset segments, along the Kutcho fault, of a Northern Rocky Mountain and 700 to > 900 km Pre-middle Cretaceous(?) to Offsets of lower Paleozoic continental-shelf formerly continuous thrust fault. Tintina Trenches fault zone Eocene or Oligocene fades boundaries Northern Rocky Mountain 125 km Late Eocene to Oligocene Offset of Eocene structural culmination and erosion Trench surface

Thibert Fault Kechika fault >170 km Middle Cretaceous to Oligocene Offset of pre-middle Cretaceous structures; generation of middle Cretaceous mica in associated Thudaka fault; fault zone contains Santonian-Campanian to Where best exposed near Dease Lake, the Eocene rocks Thibert fault appears to be steeply southwest- Spinel fault 85 km Late Eocene to Oligocene Displaces Paleogene conglomerate from source area dipping, its southwest side up. Its trace to the Thudaka fault 60 km Middle Cretaceous Offsets middle Cretaceous granitic rocks with generation at middle Cretaceous micas in fault zone northwest is truncated by an unfoliated, high- Finlay fault, north 50 km Middle Cretaceous and (?) Offsets middle Cretaceous granite level, middle to Late Cretaceous granite pluton. of Thudaka fault younger

If offsets postulated for displacements on the Finlay fault, total 110 km Middle Cretaceous and (?) Offsets Kutcho and Pinchi faults, Hottah and younger Swannell faults, belt of Late Triassic ultrabasic Kutcho and Finlay faults are correct, then as intrusions; continuous with Thudaka and Finlay faults much as 75 km of dextral movement must have Kutcho fault 100 km Pre-Late Cretaceous Offset of Klinkit and Hottah faults; fault zone involves taken place along the Thibert fault. middle Cretaceous granite; trace cut by Late Cretaceous granite

Thibert fault 75 km Pre-Late Cretaceous Linked to Kutcho and Finlay fault displacements King Salmon and Nahlin Faults

These are southwestward-directed thrust evidence thus support the contention of large dike swarms are compatible with dextral strain faults >300 km long. Distributions of rock dextral strain between the Intermontane Belt on bounding faults. Finally, the possible offset, units and structures within and near the fault and the North American craton. along the N.R.M.T., of Eocene structural culmi- zones show that the dominant movements along nations and related erosion surfaces and of a easterly striking segments of the faults have been AGES OF TRANSCURRENT postulated source area for Paleocene-Eocene parallel with the dip. Along northwesterly DISPLACEMENTS conglomerate along the Spinel fault, shows the striking segments, however, the faults are com- importance of Cenozoic movements—possibly monly steeply dipping, and local steep, minor- Data that constrain postulated ages or spans as young as late Eocene or Oligocene. fold axes suggest a component of transcurrent of ages for transcurrent displacements are In summary, several kinds of criteria indicate displacement. meager. Although some faults appear to have that major dextral transcurrent displacements The geometrical relationship between the been active as early as middle Cretaceous time, took place along a system of faults along and Kutcho, King Salmon, and Nahlin faults is it is not known whether the entire system was west of the N.R.M.T. in an interval between compatible with related dextral transcurrent initiated that early. Furthermore, the relative middle Cretaceous and late Eocene or Oligo- strain on the Kutcho fault and with southerly amounts of strain on various faults may have cene time. Most, if not all, of the movement on directed thrusting on the other faults. changed in response to alterations in directions the Thibert, Kutcho, and Pinchi faults may have of regional principal stress (see Fig. 12 below). been of pre-Late Cretaceous age. Indeed, the Cumulative Dextral Displacement On a regional scale, it is clear that pre-middle Kutcho fault may have been initiated during the Cretaceous structures (Figs. 1, 3, 7, 10) and Middle to Late Jurassic, the time of southerly Based on estimates of displacements along middle Cretaceous granitic plutons (Figs. 1, 9) thrusting along the King Salmon and Nahlin transcurrent faults described herein (Table 1), it have been offset. Middle Cretaceous K-Ar ages faults. The remaining faults may have had a his- seems probable that the cumulative dextral dis- have been obtained on muscovite generated in tory of displacements from middle Cretaceous to placement of terranes west of the faults relative fault zones where they cut granitic rocks along the Oligocene (Table 1). to the terrane east of the N.R.M.T. was the Cassiar, Kutcho, Thudaka, and Finlay faults. >1,000 km. Paleomagnetic data from Upper Where unstrained or little strained, the granitic REGIONAL IMPLICATIONS Triassic, Lower Jurassic, and Lower Cretaceous rocks contain biotite, hornblende, and no mus- rocks west of the Ingenika and Takla faults covite. Fault movements of Late Cretaceous age Regional Continuation of Faults (Monger and Irving, 1980) indicate northward are suggested by the presence of Santonian- displacement relative to cratonal North America Campanian(?) clastic rocks along the Kechika The faults described above are continuous in the order of 1,300 km. The distribution of fault, near the Turnagain River, and by Maas- with specific faults or fault zones north and Late Triassic (Karnian and Norian) faunas in the trichtian to Danian sediments in the southern south of the Cassiar-Omineca region (Figs. 10, Cordillera suggests that those faunas of warmer- part of the N.R.M.T. south of Williston Lake 11). Faults along or near the eastern margin of water habitats in the Intermontane Belt have (Rouse, 1967). The Thibert fault appears to be the Intermontane Belt, including the Kutcho and been displaced northerly, compared with those cut by a Late Cretaceous pluton. Many of the Thibert faults, connect northward with the of cooler-water habitats at comparable latitudes faults contain Paleocene and/or Eocene non- Teslin Suture Zone (Tempelman-Kluit, 1979). on the craton (Tozer, 1970). Early Jurassic marine clastic rocks, and Eocene volcanic rocks To the south, they are continuous with two main (Pliensbachian) faunas show the same anomaly are present locally along the Kechika, Spinel, faults, the northerly striking Takla fault and the in distribution (Tipper, 1981). Several lines of and N.R.M.T. faults. Northerly trends of Eocene northwesterly striking Pinchi fault, which fol-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/1/1/3430183/i0016-7606-96-1-1.pdf by guest on 02 October 2021 Figure 9. Proposed restoration of geologic elements along the Thudaka, Kutcho, Finlay, and Pinchi faults. See Figure 1 for legend. Solid black areas represent zoned, intrusive ultrabasic rocks of Late Triassic age. 9A. Generalized geology near faults. 9B. Restoration of 60 km on Thudaka fault and 50 km on Finlay and Ingenika faults. 9C. Further restoration of 100 km on Kutcho fault. 9D. Restoration juxtaposing Nahlin and Vitiil faults with displacements distributed on Thibert, Kutcho, Thudaka, Finlay, Ingenika, and Pinchi faults.

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Figure 9. (Continued).

0 50 100 h= I =d km

lows the northeast side of the Cache Creek Ter- the Liard Plain in the region straddling and N.R.M.T. faults (Fig. 6C), although they rane (Richards and others, 1976). Farther south, the British Columbia-Yukon Territory bound- could have been removed by erosion. This the Takla fault probably connects with the ary. The difference in estimates for total kind of evidence would be difficult to re- Fraser River-Straight Creek fault zone. The displacements along the Tintina Trench of-450 cognize in rocks that have undergone signi- relationship of the Takla, Finlay, and Thudaka km (Roddick, 1967; Tempelman-Kluit, 1979; ficant folding and thrusting. Nonetheless, faults to the more northwesterly trending struc- Gordey, 1981) and along the N.R.M. and Tin- the location of the Cassiar Platform of tures is remarkably similar in geometry and tina Trenches of—900 km or more, as suggested Tempelman-Kluit (1977), east of the Tintina scale to that of the Fraser River-Straight Creek in this paper, are difficult to reconcile. Criteria Trench, might be explained by this model fault zone and bordering structures (Kleinspehn, used for estimates of movements on the Tintina (Figs. 6C, 6D). Another possibility is that 1982). In each case, the northwesterly striking Trench involve middle to late Mesozoic struc- the structures preserving the Jurassic-Cretac- faults are offset by northerly striking faults. tures, whereas displacements indicated herein re- eous clastics (Roddick, 1967; Fig. 10) were The Thudaka, the northern part of the Finlay late to Paleozoic paleogeographic features. One dynamically related to transcurrent movement faults, and those faults farther east, including possibility is that the discrepancy results from on the Tintina fault and were not continuous the N.R.M.T. fault zone (Figs. 10, 11), appear significant dextral movements that occurred be- before decoupling of the terranes on either to be direct continuations of the Tintina and fore the major contractional deformation of the side of the fault. The magnitude of displace- possibly related faults in the Yukon Territory supracrustal rocks. If this had been the case, ment, suggested by matching allochthonous (Tempelman-Kluit, 1979). The connections transported traces of the early fault zone might terranes across the Tintina Trench (Tempelman- are obscured, however, by the drift cover of be expected in cover rocks east of the Tintina Kluit, 1977), is minimal because of reinterpre-

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I 1 JURASSIC-CRETACEOUS CLASTIC5 MID-CRE'ACEOUS GRANITIC PLUTONS

MARGIN OF ALLOCHTHONOUS TERRANE

~~ Ml DDLE DEVONIAN SHALE - CARBONATE FACIES BOUNDARY

Figure 10. Reconstruction for offsets along the Tintina Trench and N.R.M.T. Data from Roddick, 1967; Tempelman-Kluit, 1977; Beikman, 1980; and Gordey, 1981. Note that the restoration of —420 km, obtained by matching the Jurassic-Cretaceous clastics, still leaves the Middle Devonian facies Itoundaries 480 km apart.

tation of the geology on the northeast side by Alaska is a further problem. There, at least the the difficulty in tracing faults of the N.R .M.T. S. P. Gordey (1982, personal commun.). There, late dextral displacement must be expressed in system to the south. Some idea of variations in the northwest end of the allochthon is now contractional and transpressive structures or in principal stress directions in the Cordillera, from believed to lie 1£.0 km farther southeast than dextral strike-slip faults in the west-central part pre-Albian through Oligocene time, can tie ob- shown in Figure 10. In any event, the position of of the state. This region is characterized by tained from trends of regional fold axes and the intersection oi allochthon boundaries with southwesterly and westerly structural trends. orientations of transcurrent faults coupled with the Tintina Trench would be affected markedly If, as suggested above, the transcurrent faults their presumed ages (Fig. 12). For the north- by erosion because of their relatively shallow in the northern Cordillera have had a history of central Cordillera, at least, there appears to have dip. Of the various; possibilities suggested above, initiation and displacements during the interval been a marked change in general orientations of the hypothesis of dextral transcurrent displace- between middle Cretaceous (or earlier) and late principal stress from northeast in pre-Albian ments overlapping regional contractional de- Eocene or Oligocene, then it is possible that time to north or even north-northwest in the formation seems to best explain the disposition early-formed faults may have been complicated middle Cretaceous to north-northeast in Late of parameters used to estimate offsets along the or obscured by Cenozoic deformation. This is Cretaceous and/or early Cenozoic time. The Tintina and N.R.M.T. faults. particularly important in the southeastern part of most conspicuous expression of this evolution is The continuation of the Tintina fault in the Canadian Cordillera, perhaps accounting for the offset of the Kutcho and Pinchi faults

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I I Eocene Paleocene and younger • Mid to Late (?) Cretaceous [2] Pre Albian 0 km 250 0 250

Figure 12. Approximate regional stress orientations compatible with structures of pre- middle Cretaceous (JK), middle to Late(?) Cretaceous (mK), and early Cenozoic ages (P = Paleocene; E = Eocene and ^Olig- ocene). T.T. = Tintina Trench; N.R.M.T. and S.R.M.T. = northern and southern Rocky Mountain trenches. Faults: K = Kechika, Ku = Kutcho, T = Thibert, F = Finlay, P = Pinchi, M.L. = McLeod Lake, Y = Yalakom, FRFZ = Fraser River Fault Zone, Pa = Pasayten.

Figure 11. Regional continuation of faults in the Cordillera. S.C. = Shuswap Complex.

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along the Finlay, Ingenika, and Takla faults. cene compression was succeeded farther west by REFERENCES CITED The early-formed faults, representing zones of Eocene volcanism and plutonism. Perhaps the Reikman, H. M., 1980. Geologic map of Alaska: U.S. Geological Su vey, scale 1:2,500,000. weakness, may have facilitated some later trans- emplacement of plutons reflects the regional Eisbacher, G. H., 1972, Tectonic overprinting near Ware, northern Rocky current movements, except where principal change in stress: from stress producing the Mountain Trench: Canadian Journal of Earth Sciences, v. 9, no. 7, p. 903-913. stresses were at too high an angle to their trends. characteristic Cordilleran compressional struc- Evenchick, C. A., 1982, Stratigraphy, structure and metamorphism in Deserters Range, northern Rocky Mountains, British Columbia, in Current Re- In the southern part of the Canadian tural trends to one producing northwesterly search, Pari A: Geological Survey of Canada Paper 82-1A, p. 325-328. striking dextral transcurrent faults. In this view, Ewing, T. E., 1980, Paleogene tectonic evolution of the Pacific Northwest: Cordillera, the southern Rocky Mountain Journal of Geology, v. 88, no. 6, p. 619-638. Trench has about the same orientation as the the initiation of tensional environments associ- Gabrielse, H., 1962a, Kechika, British Columbia: Geological Survey >f Canada Map 42-1962. early-formed Pinchi, Yalakom, and Pasayten ated" with transcurrent faulting either (1) facili- 1962b, Rabbit River, British Columbia: Geological Survey of Canada tated the emplacement of plutons whose origin Map 46-1962. faults (Fig. 11). Ii. traverses a region, however, 1963, McDame map-area, Cassiar District, British Columbia: Geologi- that has undergone significant Paleocene com- was dependent upon preceding crustal thicken- cal Survey of Canada Memoir 319, 138 p. 1969. Geology of Jennings River map-area, British Columbia (104-0): pression directed almost at right angles to it. ing and heating or (2) led directly to melting of Geological Survey of Canada Paper 68-55. crust that may or may not have been previously 1975, Geology of Fort Grahame El/2 map-area, British Columbia: Considerable extension in the south-central part Geological Survey of Canada Paper 75-33. of the Canadian Cordillera took place in Eocene thickened. 1981, Stratigraphy and structure of Road River and associate i strata in Ware (west half) map-area, northern Rocky Mountains, Britiih Colum- time. These Cenozoic deformations may have bia, in Current Research, Part A: Geological Survey of Canada Paper 81-1A, p. 201-207. displaced the southerly continuation of faults CONCLUSIONS Gabrielse, H„ and Dodds, C. J., 1982, Faulting and plutonism in northwestern that had pre-Cenozoic displacements. Perhaps Cry Lake and adjacent map areas, British Columbia, in Cirrent Re- search, Part A: Geological Survey of Canada Paper 82-1A, p. 321-323. the graben structure of the southern Rocky Dextral transcurrent faults with cumulative Gabrielse. H„ Dodds, C. J., Mansy, J. L„ and Eisbacher, G. H„ 1977. Geology of Toodoggone River (94E) and Ware (west-half) (94E1/2) map-areas: Mountain Trench has been superimposed on offset of possibly more than 1,000 km along the Geological Survey of Canada Open-File 483. cover rocks thrust over the locus of an earlier Gabrielse, H., Anderson, R. G., Learning, S. F., Mansy, J. L., Monger, J.W.H., N.R.M.T. and several prominent lineaments Thorstad, L„ and Tipper, H. W„ 1978, Geology of Cry Lake map-area: transcurrent fault continuous with the N.R.M.T. farther west in north-central British Columbia Geological Survey of Canada Open-File 610. Gabrielse, H., Anderson, R. G„ Learning, S. F., Monger, J.W.H., and Tipper, If not, the southerly continuation of the belong to a system of structures with a similar H. W., 1980, Dease Lake map-area: Geological Survey of Canada N.R.M.T. fault zone must lie in the structurally Open-File 707. sense of displacement that have markedly Gordey, S. P., 1981, Stratigraphy, structure and tectonic evolution ol southern complex region within or flanking the Shuswap disrupted the original distribution of paleo- Pelly Mountains in the Indigo Lake area, Yukon Territory: C eological Survey of Canada Bulletin 318, 44 p. Metamorphic Complex (Fig. 11). geographic elements in the northwestern Cordil- Kleinspehn, K. L., 1982, Cretaceous sedimentation and tectonics, T)aughton- lera. The history of displacements on the faults Methow basin, southwestern British Columbia [Ph.D. thesis]: Princeton, Related Phenomena New Jersey. Princeton University, 184 p. dates from the middle Cretaceous, or earlier, to Mansy, J. L., and Gabrielse, H„ 1978, Stratigraphy, terminology anil correla- tion of upper Proterozoic rocks in Omineca and Cassiar Mountains, Mention has been made of phenomena that late Eocene or Oligocene. Attempts to reconcile north-central British Columbia: Geological Survey of Canada Paper 77-19, 17 p. are temporally and spatially associated with dex- offsets of-450 km along the Tintina fault, based Mdntyre, D. G., 1981, Akie River project, in Geological field wo'k, 1980: on pre-Late Cretaceous contractional structures, British Columbia Ministry of Energy. Mines and Petroleum Resources tral transcurrent faulting in the north-central Paper 1981-1, p. 33-47. Cordillera. These include granitic plutonism dur- and offsets of >900 km along the Tintina and Monger, J.W.H., and Irving, E., 1980, Northward displacement of north- central British Columbia: Nature, v. 285, p. 289-294. ing the middle and Late Cretaceous (circa 100 N.R.M.T. faults, based on early Paleozoic shelf Monger, J.W.H., Richards, T.A.. and Paterson, I.A., 1978, The Hinteiland Belt to off-shelf facies boundaries, suggest the possi- of the Canadian Cordillera: New data from northern centrll British and 70 m.y. B.P.) and early Cenozoic (circa Columbia: Canadian Journal of Earth Sciences, v. 15 no. 5, 50 m.y. B.P.); possible westerly striking, north- bility of transcurrent displacements bracketing p. 823-830. Paterson. I. A., 1977, The geology and evolution of the Pinchi fault zone at erly directed thrusts and folds of middle the time of the major regional thrusting and fold- Pinchi Lake, central British Columbia: Canadian Journal of Earth ing. Cenozoic deformation in the southern Sciences, v. 14, p. 1324-1342. Cretaceous and younger age; early Cenozoic Poole, W. H„ 1956, Geology of the Cassiar Mountains in the vicin ty of the volcanism and lamprophyre dike emplacement; Cordillera greatly complicates attempts to trace Yukon-British Columbia boundary [Ph.D. thesis]: Princeton, sev, Jer- sey, Princeton University. 247 p. early Cenozoic high heat flow followed by rapid the N.R.M.T. fault zone to the south. Early dis- Price, R. A., 1979, Intracontinental ductile crustal spreading linking the Fraser placements may have occurred along the south- River and northern Rocky Mountain Trench transform fai It zones, uplift; and Late Cretaceous to Eocene nonmarine south-central British Columbia and northeast Washington: Geological sedimentation in restricted, fault-bounded ba- ern R.M.T., to be obscured later by overlying Society of America Abstracts with Programs, v. 11, no. 7, p. 499. Read, B. C., 1980. Lower Cambrian archaeocyothid build-ups, Pell / Moun- sins. Dynamic models have been proposed that thrust sheets. Subsequent and possibly minor tains, Yukon: Geological Survey of Canada Paper 78-18, 54 p. movements could have produced the present Richards, T. A„ Dodds, C. J.. Irvine, T. N„ Jeletzky, O. L, Mansy, J. L.. link many of these processes (Ewing, 1980; Monger, J.W.H., Tipper, H. W„ and Woodsworth. G.. 1976, McCon- Price, 1979), and they involve ductile spreading physiographic feature. The extent to which dex- nell Creek map-area: Geological Survey of Canada Open-File 342. Roddick, J. A., 1967, The Tintina Trench: Journal of Geology, v. 75, p. 23-33. and thinning of the deep crust accompanied by tral displacements were distributed west of the Rouse, G. E., 1967. A Late Cretaceous plant assemblage from cart-central southern R.M.T. is not known. If little or no British Columbia: I. Fossil leaves: Canadian Journal of Earth Sciences, high heat flow in a tensional and/or transcurrent v. 4, no. 6, p. 1185-1197. strain regime. Coeval volcanism and plutonism dextral displacement has taken place along the Stevens. R. D., Delabio. R. N., and Lachance, G. R„ 1982, in Age determinations and geological studies, K-Ar isotopic ages. Report 15: G »logical are, however, commonly related to subduction southern R.M.T., major dislocations must have Survey of Canada Paper 81-2, p. 5. occurred within, or flanking, the Shuswap Taylor, G. C., and Mackenzie, W. S., 1970, Devonian stratigraphy of north- zones (Ewing, 1980). The remarkable coinci- eastern British Columbia: Geological Survey of Canada Bulletin 186, dence of early Cenozoic plutonism, volcanism, Metamorphic Complex. 62 p. Tempelman-Kluit, D. J.. 1977, Stratigraphic and structural relations between high heat flow, and rapid uplift with structures the Selwyn Basin, Pelly-Cassiar Platform, and Yukon crystallin: terrane ACKNOWLEDGMENTS in the Pelly Mountains, Yukon, in Report of activities, Part A: Geologi- related to transcurrent faulting in the northern cal Survey of Canada Paper 77-1 A, p. 223-227. Cordillera raises the question as to whether these 1979, Transported cataclasite. ophiolite and granodiorite in Yukon: I am grateful for comments and suggestions Evidence of arc-continent collision: Geological Survey of Canaila Paper processes might have occurred in environments on earlier drafts of this paper by R. B. Campbell, 79-14, 27 p. Tempelman-Kluit, D. J., Gordey, S. P., and Read, B. C., 1976, Stratigraphic not directly associated with subduction. Thin- G. H. Eisbacher, C. A. Evenchick, S. P. Gordey, and structural studies in the Pelly Mountains, Yukon Territory in Cur- ning of continental crust, accompanied by a rent Research: Geological Survey of Canada Paper 76-1A, p. 97-106. and J. O. Wheeler. In particular, I am indebted Tipper, H. W., 1981, Offset of an upper Pliensbachian geographic zoration in rapid rise in geothermal gradient, may have re- the North American Cordillera by transcurrent movement: Canadian to C. J. Dodds for many discussions concerned Journal of Earth Sciences, v. 18, no. 12, p. 1788-1792. sulted in the plutonic and volcanic events. In the with problems of transcurrent faulting in the Tozer, E. T., 1970. Marine Triassic faunas, in Douglas, R.J.W., ed., Geology Cassiar and Omineca Mountains, the onset of and economic minerals of Canada: Geological Survey of Canada Eco- Cordillera. Additional data, critical for re- nomic Geology Report No. 1, 5th ed., p. 633-640. granitic plutonism on a large scale in the middle Wanless, R. K„ Stevens, R. D, Lachance, G. R, and Delabio, R. N„ 1978, in gional synthesis, were kindly provided b\i Age determinations and geological studies, K-Ar isotopic ages Report Cretaceous coincided with transcurrent displace- J.W.H. Monger, L. Struik, and H. W. Tipper. 13: Geological Survey of Canada Paper 77-1, p. 21. ments and clearly postdated a major episode of Critical reviews by G. A. Davis, H. L. Foster, MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 10, 1983 regional compression. In the southeast, Paleo- REVISED MANUSCRIPT RECEIVED MAY 21, 1984 and an anonymous reviewer were most helpful. MANUSCRIPT ACCEPTED JUNE 1, 1984 Printed in U.S.A

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