Geometric test for Late Cretaceous-Paleogene ... - GeoScienceWorld

Geometric test for Late Cretaceous-Paleogene intracontinental transform faulting in the Canadian Cordillera Raymond A. Price Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Dugald M. Carmichael Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada

ABSTRACT The Tintina trench-northern Rocky Mountain trench (TT-NRMT) fault zone and the Fraser River-Straight Creek (FR-SC) fault zone are separate, en echelon, concentric, smallcircle fault segments of a composite intracontinental transform fault zone more than 2 500 km long that cuts diagonally across the Canadian Cordillera, from the outboard part of a tectonic collage of accreted foreign terranes in the south into the North American preaccretionary continental margin in the north. Most of the 4S0 km of right-hand slip on the TT-NRMT fault zone was transformed southward during the Late Cretaceous and Paleocene into oblique convergence in the southern Canadian Rockies; the remainder, probably com prising less than 100 km, was transformed southwestward during early and middle Eocene time via a zone of distributed shear and east-west crustal stretching into right-hand slip on the en echelon FR-SC fault zone. These interpretations, based on regional systematic mapping of geologic structures, are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post-mid-Cretaceous, right-hand displacement along the general locus of the TT-NRMT fault zone, involving foreign terranes and the parts of North America to which they had been accreted. This paradox must be resolved.

Figure 1. Regional tectonic setting of Tintina trench (TT)-northern Rocky Mountain trench ( N R M T ) and Fraser River (FR)-Straight Creek (SC) righthand, strike-slip faults. Stipple = major granitic batholiths; diagonal-line pattern = area of Eocene crustal stretching. Other major right-hand, strike-slip faults Identified are Kaltag (KL), Denali (DE), Shakwak (SH), Fairweather (FA), Chatham Strait (CS), and Queen Charlotte (QC). Offset of two arrows pointing against opposite sides of fault shows 450 km of right-hand strike separation along T T N R M T fault. Right-hand strike separation of 80 to 110 km along FR-SC fault is shown by offset of Yalakom fault (YA) relative to Ross Lake fault (RL). Pinchi fault (PI) separating C a c h e Creek and Quesnellia terranes extends between F R - S C and T T - N R M T fault zones, without offset. At north end of TT fault, right-hand strike slip appears to have been transformed into northwestsoutheast crustal stretching on northeast side of TT fault beneath topographically low area (L) occupied by Yukon Flats basin, and into northeastsoutheast crustal shortening on southwest side of T T fault beneath structurally and topographically high area (H) occupied by YukonTanana Plateau. Dashed line shows outer limit of Cordilleran folding and faulting. (Adapted with modifications from King, 1969.)

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INTRODUCTION The Tintina trench-northern Rocky Mountain trench (TT-NRMT) fault zone and the Fraser River -Straight Creek (FR-SC) fault zone, both of which are reported to have been the locus of large (¡S100 km) right-hand strike slip during Paleogene and(?) Late Cretaceous time (Roddick, 1967; Tempelman-Kluit, 1979; Gabrielse and Dodds, 1977; Misch, 1966; Tipper, 1977; Monger, 1985; Gabrielse, 1985; Kleinspehn, 1985), are aligned, en echelon, to form an arcuate composite fault zone that extends along the Cordillera for more than 2500 km, from east-central Alaska to northwestern Washington (Fig. 1). On a regional scale, this composite fault zone is subparallel with the tectonic fabric of the Canadian segment of the Cordillera (Davis et al, 1978; Monger and Price, 1979; Monger, 1985); however, in the south it cuts diagonally across the tectonic collage of allochthonous terranes that compose the interior of the Cordillera, and in the north it offsets the Paleozoic-Proterozoic continental terrace wedge (miogeocline) of western North America (Ternpelman-Kluit, 1979; Gabrielse, 1985). The composite TT-NRMT-FR-SC fault zone has the earmarks of an intracontinental transform fault system, but it is enigmatic. The suggestion has been made (Davis et al., 1978; Monger and Price, 1979) that it records the counterclockwise rotation, relative to North America, of a slice of Cordilleran lithosphere that became partly coupled to the Kula (or Farallon?) plate, in much the same way that the Neogene and Holocene right-hand transform faults that branch off the Queen Charlotte fault in the Gulf of Alaska are associated with the relative counterclockwise rotation of the Pacific plate (Atwater, 1970; Monger and Price, 1979; Stout and Chase, 1980); however, unlike these latter faults the TT-NRMT-FR-SC fault zone cannot be shown unequivocally to be a splay that extends northward from an oceanic plate boundary into a continental margin zone of plate convergence. The southward extension of the Straight Creek fault is concealed by overlapping younger Tertiary strata, and the Tintina fault, at its northward end, abuts the eastnortheast-trending Kaltag fault (Fig. 1). Furthermore, the TT-NRMT-FR-SC fault zone is GEOLOGY, v. 14, p. 468-471, June 1986

not a single through-going crustal rupture, nor has it been established that the two major components, the TT-NRMT fault and the FR-SC fault, have the appropriate shape and orientation to have been mechanically coupled in a single composite right-hand transform fault zone. Whatever their regional tectonic significance may be, the TT-NRMT and FR-SC faults are large structures. Along the FR-SC fault zone, older faults that cut strata of early Late Cretaceous age show 80 to 110 km of right-hand strike separation (Misch, 1966; Tipper, 1977; Kleinspehn, 1985; Monger, 1985). Along the TT-NRMT fault zone, mid-Cretaceous rocks show a net right-hand strike separation of 450 km (Roddick, 1967; Tempelman-Kluit, 1979). Gabrielse (1985) has shown that matching early Paleozoic facies boundaries may indicate more than 900 km of displacement, but this was distributed between the TT-NRMT fault zone and several major faults farther west, and it may include displacements prior to midCretaceous time. GEOMETRIC CONSTRAINTS If the TT-NRMT and FR-SC fault zones were indeed the loci of large strike slip, both must have intersected the surface of the earth along circular arcs because, on a spherical Earth, strike slip is, by definition, parallel to the surface of a sphere and therefore must involve relative rotations about an axis through the center of the sphere. Furthermore, if the TT-NRMT and FR-SC fault zones were mechanically coupled, en echelon, to form a single composite regional strike-slip fault zone, then the circular traces of both faults should be centered on the same axis (pole) of rotation. Our purpose in this paper is to report on the results and implications of some tests for these geometric relationships. Best-fitting small-circle arcs have been established for the TT-NRMT and FR-SC fault zones. The locus of the TT-NRMT fault zone was digitized by reading off the latitude and longitude on the l:5000000-scale Tectonic Map of North America (King, 1969), every 50 km or so, over the interval from the south side of the Yukon Flats basin near the projected intersection of the Tintina Trench and Kaltag faults (lat 66.10°N, long 147.80°W) to the southern Rocky Mountain Trench near the Big Bend of the Columbia River (lat 52.25°N, long 122.65°W). The locus of the FR-SC fault zone was digitized in the same way, over the interval from Williams Lake, British Columbia (lat 52.25°N, long 122.65°W), to the south end of the Northern Cascade Mountains of Washington (lat 47.85°N, long 121.25°W). The small circle of best fit and the corresponding pole (axis) of rotation for this array of points were established by treating each point as the locus GEOLOGY, June 1986

of a unit vector from the center of Earth and by minimizing the sums of the squares of the deviations of these unit vectors from a circular cone surface with its apex at the center of Earth (Ramsay, 1967, p. 20-21). An APL computer program, written for the purpose, calculates the half apical angle of the cone surface of best fit and the latitude and longitude of the position of its axis. Also, for each point in the data array, the program computes the angular distance between that point and the cone surface, as well as the coordinates, in a stereographic projection centered on the cone axis, for that point. The use of a stereographic projection centered on the axis of the best-fitting cone facilitates the graphic display and analysis of the data because the cone axis corresponds to the axis of rotation for the strike slip, which must follow circular arcs that are centered on the axis of rotation. Any significant deviation from this condition

will be readily apparent in the stereographic projection. It is obvious from even a cursory inspection of regional tectonic maps (King, 1969; Fig. 1 here) that south of approximately lat 56°N the TT-NRMT fault zone diverges southeastward from the apparently uniform southwesterly concave circular arc that characterizes the rest of its trace. Accordingly, it should come as no surprise to learn that the closeness of the fit of the array of points to a small circle is improved substantially when points on the TT-NRMT fault zone south of lat 56°N are omitted from the array. The best-fitting small circle calculated in this way for the TT-NRMT fault zone, exclusive of the segment south of 56°N, has an apical half angle of 31.2°, with the pole of the cone axis at lat 35.4°N, long 158.7°W. The remarkably good fit to a small-circle curve at the surface of the earth (Fig. 2) demonstrates

Figure 2. Tintina trench (TT)-northern Rocky Mountain trench (NRMT) fault zone and Fraser River (FR)-Straight Creek (SC) fault zone in stereographic projection centered on axis of bestfitting, small-circle curve for TT-NRMT fault zone. Numbered ticks along fault trace are points along TT-NRMT and FR-SC fault zones. Long dashed curve is best-fitting small-circle curve for points 1 to 34; its axis is at lat 35.4°N, long 158.7°W, and its radius is 3430 km (apical half angle—31.2°). Short dashed curve is drawn about same axis, but with a radius of 3350 km (apical half angle—30.5°), which corresponds to mean difference between points 48 to 57 on FRSC fault zone and best-fitting small-circle through points 1 to 34 on TT-NRMT fault zone.

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that this part of the TT-NRMT fault zone meets the geometric requirements of an ideal strike-slip fault; furthermore, it indicates that the 450 km of right-hand strike separation of mid-Cretaceous and older rocks along this small-circle fault trace, as established in the Yukon Territory (Roddick, 1967; TempelmanKluit, 1979), is a measure of the relative rotation (7.82°) since mid-Cretaceous time along the TT-NRMT fault zone between the North American plate and the smaller plate of continental lithosphere that lies between the North American plate and the Pacific Ocean basin. The easterly deviation from the small-circle curve that occurs south of lat 56°N implies that this part of the TT-NRMT fault system is a zone of right-hand reverse slip (a right-hand transpressive fault zone). This transformation of strike slip into oblique convergence provides an elegantly simple explanation for the otherwise enigmatic observation that the Rocky Mountain foreland thrust and fold belt becomes wider and topographically higher and involves more than 200 km of horizontal shortening in the region south of lat 52°N (Bally et al., 1966; Price and Mountjoy, 1970; Price, 1981; Price and Fermor, 1985) but less than 100 km in the region north of lat 52°N (Gabrielse and Taylor, 1981; Thompson, 1981). In south-central British Columbia, the Pinchi fault zone (Fig. 1), which separates the upper Paleozoic and Triassic ocean-floor rocks of the Cache Creek terrane on the west from the Triassic and Jurassic volcanic-arc assemblage of the Quesnellia terrane, extends without offset across the gap between the en echelon northerly and southerly terminations of the FR-SC fault zone and the TT-NRMT fault zone, respectively (Monger and Price, 1979; Tipper et al., 1981); this very effectively precludes any direct connection between these two fault zones. The FR-SC fault zone has a shorter radius of curvature; moreover, it overlaps, en echelon, the southern end of the TT-NRMT fault zone. The dashed small-circle curve that follows the FR-SC fault zone shown in Figure 2 is a segment of a small circle, the apical angle of which has been established by subtracting from the apical angle of the best-fitting small circle for the TT-NRMT fault zone north of 52°N the mean angular deviation from that best-fitting small circle of points lying along the FR-SC fault zone (-0.712° ±0.134). REGIONAL TECTONIC IMPLICATIONS The shapes and relative orientations of the FR-SC fault zone and the segment of the TTNRMT fault zone that lies north of lat 56°N are consistent with the hypothesis that they are two en echelon segments of an intracontinental 470

transform fault system along which a slice of Cordilleran continental lithosphere haji rotated counterclockwise relative to the rest of North America. However, there is a conspicuous discrepancy between the post-mid-Cretaoeous right-hand strike slip of about 450 km on the TT-NRMT fault zone and the posl.-midCretaceous right-hand strike slip of 80 to 1100 km on the en echelon FR-SC fault zone. This discrepancy, together with the eastward deviation of the TT-NRMT fault zone south of lat 56°N, implies that most of the strike slip on the TT-NRMT fault zone was transformed southward into the oblique, right-hand convergence that accounts for the southward increase in the amount of Late Cretaceous-early Tertiary horizontal compression across the Rocky Mountain fold and thrust belt. The right-hand en echelon offset between the TT-NRMT fault zone and the FR-SC fault zone implies that in the zone of en echelon overlap between them, the strike slip was transformed, via distributed shear, into west-northwest to east-southeast horizontal stretching (Price, 1979). This stretchin g is expressed as (1) early and middle Eocene low-angle normal faults on the flanks of north-northeast-trending upwarps of the Cordilleran metamorphic infrastructure (Price, 1979; Price et al., 1981; Ewing, 1980,1981; Tempelman-Kluit, 1984; Parrish, 1984; Parkinson, 1985); (2) Eocene K-Ar cooling (quenching) ages; and (3) north-northeast-trer ding Eocene dikes (Monger, 1968; Price et al., 1981). The early Eocene change from transpression to transtension in the southern Canadian Cordillera may be a result of a small change in regional plate kinematics and/or the initiation of the FR-SC fault zone. It is important to note that these regional tectonic interpretations based on geologic mapping are in conflict with regional tectonic interpretations of measurements of paleomagnetism in mid-Cretaceous rocks that occur within a large area west of both the FR-SC and NRMT fault zones between lat 48° and 56°N (Monger and Irving, 1980; Irving et al., 1985; Armstrong et al., 1985). On the basis of these paleoinagnetic measurements, it has been inferred that the allochthonous and suspect terranes of central British Columbia, and the parts of Noith America to which they had become attached by midCretaceous time, subsequently were displaced northward relative to the rest of North America by more than 1100 km and perhaps a;; much as 2400 km. The TT-NRMT fault zone clearly will not accommodate displacements of this magnitude after mid-Cretaceous time. If faults marking the loci of these very large displacements lie west of the Rocky Mountain trench, they have yet to be identified. Speculation by Chamberlain and Lambert (1985) thai the

middle Proterozoic Belt-Purcell Supergroup rocks lying sistride the southern Rocky Mountain trench in southeastern British Columbia and the adjacent United States are part of a composite allochthonous terrane, "Cordilleria," which has been displaced 1500 km northward relative to adjacent North America, tacitly ignores the many published arguments supporting the conclusion that there has been no significant right-hand strike slip along the southern Rocky Mountain trench or in the adjacent Rocky Mountains. In the region south of lat 50°N, major transverse, northeast-trending faults and thickness and lithofacies changes have been correlated in detail from the Rocky Mountains westward across the Rocky Mountain trench into the central Purcell Mountains without any significant right-hand offset (Leech, 196:2; Price et al., 1972; Harrison, 1972; Harrison et al., 1974; Benvenuto and Price, 1979; Price, 1981; McMechan, 1981; McMechan and Price, 1982). Furthermore, conspicuous transverse, northeast-trending magnetic and gravity anomalies, which have been linked to major fault structures in the Precambrian crystalline basement beneath the foreland basin east of the Cordilleran orogenic belt, have been traced across the Rocky Mountains and the southern Rocky Mountain trench into the southern Purcell Mountains (Kanasevich et al., 1969). These relationships have been cited as evidence that the Precambrian crystalline basement extends, without fault offset, from the interior of the craton to beneath the Purcell Mountains (Price, 1981). CONCLUSIONS The conflict between what is inferred from measurements of paleomagnetism and what is known about the geologic structure and stratigraphic evolution of the southeastern Canadian Cordillera remains to be resolved. However, our analysis of the TT-NRMT fault zone and the FR-SC fault zone, as they are known now from systematic geologic mapping of the Cordillera, leads; us to the conclusion that since mid-Cretaceous time the total north and northwest displacement of the accreted allochthonous and suspect terranes and attached North American rocks on this fault system has been less than 500 km, that most of this displacement was transformed southward into right-hand oblique convergence that produced the large-scale thrusting and folding in the southern Canadian Rockies during Late Cretaceous and Paleocene time, and that part of the displacement was transformed southward during early and middle Eocene time, via a large area of distributed shear and westnorthwest to east-southeast crustal stretching, to the en echelon FR-SC fault zone. GEOLOGY, June 1986 469

REFERENCES CITED Armstrong, R.L., Monger, J.W.H., and Irving, E., 1985, Age of magnetization of the Axelgold Gabbro, north-central British Columbia: Canadian Journal of Earth Sciences, v. 22, p. 1217-1221. Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America: Geological Society of America Bulletin, v. 81, p. 3 5 1 3 - 3 5 3 6 . Bally, A.W., Gordy, P.L., and Stewart, G.A., 1966, Structure, seismic data, and orogenic evolution of southern Canadian Rockies: Bulletin of Canadian Petroleum Geology, v. 14, p. 3 3 7 - 3 8 1 . Benvenuto, G.L., and Price, R.A., 1979, Structural evolution of the Hosmer thrust sheet, southeastern British Columbia: Bulletin of Canadian Petroleum Geology, v. 27, p. 3 6 0 - 3 9 4 . Chamberlain, V.E., and Lambert, R.St.J., 1985, Cordilleria, a newly defined Canadian microcontinent: Nature, v. 314, p. 7 0 7 - 7 1 3 . Davis, G.A., Monger, J.W.H., and Burchfiel, B.C., 1978, Mesozoic construction of the Cordilleran "collage," central British Columbia to central California, in Howell, D.G., and McDougall, K.A., eds., Mesozoic paleogeography of the western United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeography Symposium 2, p. 1-32. Ewing, T.E., 1980, Paleogene tectonic evolution of the Pacific Northwest: Journal of Geology, v. 88, p. 6 1 9 - 6 3 8 . 1981, Regional stratigraphy and structural setting of the Kamloops Group, south-central British Columbia: Canadian Journal of Earth Sciences, v. 18, p. 1464-1477. Gabrielse, H., 1985, Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia: Geological Society of America Bulletin, v. 96, p. 1-14. Gabrielse, H., and Dodds, C.J., 1977, The structural significance of the northern Rocky Mountain Trench and related lineaments in north-central British Columbia: Geological Association of Canada Programs with Abstracts, v. 2, p. 19. Gabrielse, H„ and Taylor, G.S., 1981, Geological maps and cross-sections of the northern Canadian Cordillera from southwest of Fort Nelson, British Columbia to Gravina Island, southeastern Alaska: Geological Survey of Canada Open-File Report 864, 1 sheet, 2 p. text. Harrison, J.E., 1972, Precambrian belt basin of the northwestern United States: Its geometry, sedimentation, and copper occurrences: Geological Society of America Bulletin, v. 83, p. 1215-1240. Harrison, J.E., Griggs, A.B., and Wells, J.D., 1974, Tectonic features of the Precambrian Belt basin and their influence on post-belt structures: U.S. Geological Survey Professional Paper 866, 15 p. Irving, E.T., Woodsworth, G.V., Wynne, J., and Morrison, A., 1985, Paleomagnetic evidence for displacements to the north of the Coast Plutonic Complexes: Canadian Journal of Earth Sciences, v. 22, p. 5 8 4 - 5 9 8 . Kanasevich, E.R., Clowes, R.M., and McCloughan, C.H., 1969, A buried Precambrian rift in western Canada: Tectonophysics, v. 8, p. 5 1 3 - 5 2 7 . King, P.B., compiler, 1969, Tectonic map of North America: Washington, D.C., U.S. Geological Survey, scale 1:5,000,000.

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Kleinspehn, K.L., 1985, Cretaceous sedimentation and tectonics, Tyaughton-Methow Basin, southwestern British Columbia: Canadian Journal of Earth Sciences, v. 22, p. 154-174. Leech, G.B., 1962, Structure of the Bull River Valley near latitude 49°35': Alberta Society of Petroleum Geologists Journal, v. 10, p. 3 9 6 - 4 0 7 . McMechan, M.E., 1981, The middle Proterozoic Purcell Supergroup in the southwestern Rocky and southeastern Purcell Mountains, British Columbia, and the initiation of the Cordilleran miogeocline, southern Canada and adjacent United States: Bulletin of Canadian Petroleum Geology, v. 29, p. 5 8 3 - 6 2 1 . McMechan, M.E., and Price, R.A., 1982, Superimposed low-grade metamorphism in the Mount Fisher area, southeastern British Columbia— Implication for the East Kootenay orogeny: Canadian Journal of Earth Sciences, v. 19, p. 4 7 6 - 4 8 9 . Misch, P., 1966, Tectonic evolution of the northern Cascades of Washington State, in Gunning, H.C., ed., A symposium on the tectonic history and mineral deposits of the western Cordillera in British Columbia and neighboring parts of the United States: Canadian Institute of Mining and Metallurgy Special Volume 8, p. 101-148. Monger, J.W.H., 1968, Early Tertiary stratified rocks, Greenwood map-area ( 8 2 E / 2 ) , British Columbia: Geological Survey of Canada Paper 6 7 - 4 2 , 3 9 p. 1985, Structural evolution of the southwestern Intermontane Belt, Ashcroft and Hope map areas, British Columbia, in Current research: Geological Survey of Canada Paper 851A, p. 3 4 9 - 3 5 8 . Monger, J.W.H., and Irving, E., 1980, Northward displacement of north-central British Columbia: Nature, v. 285, p. 2 8 9 - 2 9 3 . Monger, J.W.H., and Price, R.A., 1979, Geodynamic evolution of the Canadian Cordillera—Progress and problems: Canadian Journal of Earth Sciences, v. 16, p. 7 7 0 - 7 9 1 . Parkinson, D., 1985, Geochronology of the western side of the Okanagan metamorphic core complex, southern British Columbia: Geological Society of America Abstracts with Programs, v. 17, p. 399. Parrish, R.R., 1984, Slocan Lake fault: A low angle fault zone bounding the Valhalla Gneiss Complex, Nelson map area, southern British Columbia: Geological Survey of Canada Paper 84-1 A, p. 3 2 3 - 3 3 0 . Price, R.A., 1979, Intracontinental ductile crustal spreading linking the Fraser River and northern Rocky Mountain Trench transform fault zones, south-central British Columbia and northeast Washington: Geological Society of America Abstracts with Programs, v. 11, p. 499. 1981, The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Moun-

tains, in McClay, K.R., and Price, N.V., eds., Thrust and nappe tectonics: Geological Society of London Special Publication 9, p. 4 2 7 - 4 4 8 . Price, R.A., and Fermor, P.R., 1985, Structure section of the Cordilleran foreland thrust and fold belt west of Calgary, Alberta: Geological Survey of Canada Paper 84-17, 1 sheet. Price, R.A., and Mountjoy, E.W., 1970, Geologic structure of the Canadian Rocky Mountains between Bow and Athabasca Rivers—A progress report, in Wheeler, J.O., ed., Structure of the Southern Canadian Cordillera: Geological Association of Canada Special Paper 6, p. 7 - 2 5 . Price, R.A., Balkwill, H.R., Charlesworth, H.A.K., Cook, D.G., and Simony, P.S., 1972, The Canadian Rockies and tectonic evolution of the southwestern Canadian Cordillera: International Geological Congress, 24th, Montreal, Field Guidebook A C 15, 129 p. Price, R.A., Monger, J.W.H., and Muller, V.E., 1981, Cordilleran cross-section—Calgary to Victoria, in Thompson, R.I., and Cook, D.G., eds., Field guides to geology and mineral deposits: Geological Association of Canada/Mineralogical Association of Canada/Canadian Geophysical Union Meeting, Calgary, p. 2 6 1 - 3 3 4 . Ramsay, J.G., 1967, Folding and fracturing of rocks: New York, McGraw-Hill, 568 p. Roddick, J.A., 1967, Tintina Trench: Journal of Geology, v. 75, p. 2 3 - 3 3 . Stout, J.H., and Chase, C.G., 1980, Plate kinematics of the Denali fault system: Canadian Journal of Earth Sciences, v. 17, p. 1527-1537. Tempelman-Kluit, D.J., 1979, Transported cataclasite, ophiolite, and granodiorite in Yukon: Evidence of arc-continent collision: Geological Survey of Canada Paper 79-14, 27 p. 1984, Meteoric water model for gold veins in a detached terrane: Geological Society of America Abstracts with Programs, v. 16, p. 674. Thompson, R.I., 1981, The nature and significance of large 'blind' thrusts within the northern Rocky Mountains of Canada, in McClay, K.R., and Price, N.V., eds., Thrust and nappe tectonics: Geological Society of London Special Publication 9, p. 4 4 9 - 4 6 2 . Tipper, H.W., 1977, The Fraser River fault system of southwestern British Columbia: Geological Association of Canada Programs with Abstracts, v. 2, p. 52. Tipper, H.W., Woodsworth, G.V., and Gabrielse, H., 1981, Tectonic assemblage map of the Canadian Cordillera: Geological Survey of Canada Map 1505A, scale 1:2 0 0 0 000. Manuscript received November 6, 1985 Revised manuscript received February 3, 1986 Manuscript accepted February 25, 1986

Reviewer's comment An ideal paper for Geology because it contains an original plate-kinematic analysis of on-land faults, and it also emphasizes, up-front, a major problem in the reconciliation of paleomagnetic and geologic data. I feel this problem necessarily has implications for most of the western Cordillera, especially California. In effect the paleomag says large displacements have occurred (surprise!), but geologists can't find the structures responsible. Darrel Cowan

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