Rocky Mountain Thrust Belt Research Articles

Ductile thrusting versus channel flow in the southeastern Canadian Cordillera: evolution of a coherent crystalline thrust sheet

Abstract The Late Cretaceous Gwillim Creek shear zone (GCSZ) exposed in the core of the Valhalla complex, and located in the hinterland of the southern Canadian Rocky Mountain thrust belt, is a 5–7 km thick, easterly verging, ductile thrust zone. It was active after c. 90 Ma and during anatexis (800°C and 800 MPa), rose eastward in the direction of transport, and its base was refrigerated from below at c. 60 Ma by thrust translation onto a cold footwall. Extensional shear zones are younger than the GCSZ, and there is no evidence of channel flow or ductile extrusion. Instead, a 30 km thick, coherent sheet was translated on the GCSZ, which at depth was linked to the Foreland thrust belt such as to form a composite crystalline thrust sheet. Doming of the Valhalla complex may be related to Eocene thrusting beneath the complex during the last stage of shortening. A channel flow, proposed by others for the region north of the Valhalla complex, could have evolved within the crystalline sheet by activation of lateral transition zones and an upper detachment, but is no wider than 250 km, does not represent the dominant orogenic process and may represent a nascent channel.

Where are the missing faults in translated terranes?

A central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers, while the faults along which the terranes moved these great distances are not found. Relations long known from geology and from recent studies of modern transform-plate boundaries suggest that preserving strike-slip faults that record 1000-km-scale translations is unlikely, and therefore, in many cases, paleomagnetism may be our only reliable indicator of how far terranes translated. Four reasons are explored here for why large-offset strike-slip faults are likely to be ‘missing’ in translated terranes. (1) Strike-slip faults in continents commonly are partially or wholly destroyed, or reactivated. Only ∼400–500 km of over 1000 km of offset on Cretaceous–early Tertiary strike-slip faults in northern British Columbia are accounted for in southern British Columbia. The missing strike-slip faults probably continued south into the hinterland of the Rocky Mountain thrust belt as oblique-thrust faults, much like the modern Alpine–Marlborough fault system of New Zealand. The major strike-slip faults in southeastern British Columbia were obliterated by Eocene extensional faulting, uplift, and erosion. (2) Offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale, many of which are ‘left behind’ and not found in the translated terrane. The greater San Andreas plate boundary has accumulated ∼1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is a narrow ‘terrane’ in the offshore borderland of California that has moved ∼1200 km relative to North America, but no single fault has nearly that much offset. (3) Major faults in narrow oceans formed by oblique-divergent plate boundaries have a low potential for preservation. For example, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the oblique rift lie, has a low potential for preservation. In contrast, Baja California is currently a continental terrane that has translated ∼300 km relative to North America along the plate boundary in the Gulf of California. Because of the relative buoyancy of continental crust, Baja California is likely to be preserved in any future accretion event. (4) Faults in the margins of oblique-divergent plate boundaries are dominantly normal faults and not strike-slip faults. There is little strike-slip faulting in central Baja California, even though the plate boundary has a rift angle ( α) of only 20°. Recent field data and modeling show that at α=0–20° strike-slip faults will dominate the secondary faults. Surprisingly, at α=∼20 to ∼35°, there are both strike-slip and normal faults, and at α>∼35–45°, there are few or no strike-slip faults, even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion.

The control of the smectite–illite transition on passive-roof duplex formation: Canadian Rockies Foothills, Alberta

The roof décollement to the passive-roof duplex and triangle zone in the central Foothills, Rocky Mountain thrust belt is located within a sequence of siliciclastic sediments dominated by shale/mudstone. It is unclear why the décollement is found at a certain level in the stratigraphy, when the whole package appears to have similar lithilogical and hence mechanical properties. Investigation of the mineralogy of the mudstones reveals that the clay fraction preserves a smectite-to-illite transition, which formed during maximum burial conditions. The top of the transition is associated with the top of the roof décollement in the region. In the Gulf of Mexico, the transition correlates with both a zone of overpressure, favorable for detachment, and a corresponding drop in density (rise in porosity). In the Foothills, the transition also records a drop in density and a concomitant drop in sonic velocity. By analogy with the Gulf of Mexico, the drop in density may record a paleo-overpressured horizon that was favorable for detachment and formation of a roof décollement in the Foothills.

Geology of metasedimentary rocks and Late Cretaceous deformation history in the northern Valhalla complex, British Columbia

The Valhalla complex, a Cordilleran metamorphic core complex, is a domal culmination made up of gently dipping interlayered sheets of igneous and supracrustal rocks that were deformed and metamorphosed in the Middle Jurassic and Late Cretaceous, and exhumed by extensional faults in the Eocene. Mapping, fabric, and metamorphic studies of predominantly metasedimentary rocks in Valhalla and Passmore domes in the northern part of the complex, together with published geochronological data, reveal a significant Late Cretaceous tectonic history. This includes extensive magmatism, the culmination of upper amphibolite facies metamorphism (approx. 800°C and 8 GPa), migmatization, development of a dominant penetrative transposition foliation, and localization of strain on ductile thrust faults termed the Gwillim Creek shear zones. The Valhalla assemblage, a package of metasedimentary rocks in Valhalla and Passmore domes, comprises a heterogeneous sequence of pelitic schist, marble, calc-silicate gneiss, psammitic gneiss, metaconglomerate, quartzite, amphibolite gneiss, and ultramafic rocks. Based on the presence of distinct laterally continuous marker units and similar lithologic ordering, we propose that the Valhalla assemblage is correlative with part of the Palaeozoic North American stratigraphic succession. If this is correct, then the Valhalla assemblage represents an inverted sequence of strata that has been thinned by as much as 60%; thinning may have occurred during Late Cretaceous transposition foliation development. The Gwillim Creek shear zones, originally mapped in a restricted locality in Gwillim Creek, were found to merge into one broad, ductile shear zone beneath Valhalla dome and extend throughout the entire Valhalla complex. The general style and timing of Late Cretaceous deformation in the Valhalla complex is characteristic of that found throughout the Shuswap complex in a belt of rocks that were at mid-crustal levels during the Cretaceous. This zone is thought to have accommodated Cretaceous - Early Tertiary shortening in the eastern Cordillera, and is the ductile equivalent of the higher level Rocky Mountain thrust belt to the east.

Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River complex, northwestern United States, and their implications for Cordilleran structure and Precambrian continent reconstructions

Precambrian basement rocks exposed within tectonic windows in the North American Cordillera help to define the Precambrian crustal structure of western North America and possible reconstructions of the Late Proterozoic supercontinent Rodinia. New geologic mapping and U-Pb dating in the infrastructure of the Priest River metamorphic complex, northern Idaho, documents the first Archean basement (2651 ± 20 Ma) north of the Snake River Plain in the North American Cordillera. The Archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying Hauser Lake gneiss, which is correlated with the Prichard Formation of the Belt Supergroup. A structurally higher sheet of augen gneiss interleaved with the Hauser Lake gneiss yields a U-Pb zircon crystallization age somewhat greater than 1577 Ma. The slivers of augen gneiss were tectonically interleaved with the surrounding Hauser Lake gneiss near the base of the Spokane dome mylonite zone, which arches across this part of the Priest River complex. We conclude that the Spokane dome mylonite zone lies above the Archean basement-cover contact and that it was, in part, equivalent to the basal décollement of the Rocky Mountain fold and thrust belt. New U-Pb dates on metamorphic monazite and xenotime reveal peak metamorphism at ca. 72 Ma, compatible with movement along the Spokane dome mylonite zone at that time. The Archean basement could be interpreted as the western extension of the Hearne province, or a new Archean basement terrane separated from the Hearne province by an Early Proterozoic suture. The unique assemblage of 2.65 Ga basement, ~1.58 Ga felsic intrusive rocks, and the Middle Proterozoic Belt Supergroup can be used as a piercing point for the identification of the conjugate margin to Laurentia. Our new dating supports previous correlations of Australia's Gawler craton (2.55-2.65 Ga) and its 1590 Ma plutons with the Priest River complex basement gneisses. The Priest River complex basement may be a piece of eastern Australia stranded during rifting of the supercontinent Rodina in the Late Proterozoic.

Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River comple×, northwestern United States, and their implications for Cordilleran structure and Precambrian continent reconstructions

Precambrian basement rocks exposed within tectonic windows in the North American Cordillera help to define the Precambrian crustal structure of western North America and possible reconstructions of the Late Proterozoic supercontinent Rodinia. New geologic mapping and U–Pb dating in the infrastructure of the Priest River metamorphic complex, northern Idaho, documents the first Archean basement (2651 ± 20 Ma) north of the Snake River Plain in the North American Cordillera. The Archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying Hauser Lake gneiss, which is correlated with the Prichard Formation of the Belt Supergroup. A structurally higher sheet of augen gneiss interleaved with the Hauser Lake gneiss yields a U–Pb zircon crystallization age somewhat greater than 1577 Ma. The slivers of augen gneiss were tectonically interleaved with the surrounding Hauser Lake gneiss near the base of the Spokane dome mylonite zone, which arches across this part of the Priest River complex. We conclude that the Spokane dome mylonite zone lies above the Archean basement–cover contact and that it was, in part, equivalent to the basal decollement of the Rocky Mountain fold and thrust belt. New U–Pb dates on metamorphic monazite and xenotime reveal peak metamorphism at ca. 72 Ma, compatible with movement along the Spokane dome mylonite zone at that time. The Archean basement could be interpreted as the western extension of the Hearne province, or a new Archean basement terrane separated from the Hearne province by an Early Proterozoic suture. The unique assemblage of 2.65 Ga basement, ~1.58 Ga felsic intrusive rocks, and the Middle Proterozoic Belt Supergroup can be used as a piercing point for the identification of the conjugate margin to Laurentia. Our new dating supports previous correlations of Australia’s Gawler craton (2.55–2.65 Ga) and its 1590 Ma plutons with the Priest River complex basement gneisses. The Priest River complex basement may be a piece of eastern Australia stranded during rifting of the supercontinent Rodina in the Late

Implied basement-tectonic control on deposition of Lower Carboniferous carbonate ramp, southern Cordillera, Canada

cambrian (Kanasevich et al., 1969) and preThe Mount Head embayment is a regional downwarp along the west coast of Lower Middle Devonian (Norris and Price, 1966; Carboniferous western Canada that developed by differential subsidence, which we recog- Benvenuto and Price, 1979). nize from lithostratigraphic and biostratigraphic patterns. Palinspastic restoration of the Mount Head embayment illustrates that subsidence domains are geometrically coincident with previously identified tectonic elements of the autochthonous basement. Thus, it ap- METHODS pears that basement-tectonic elements controlled sediment accommodation and accumu- Geologic observations were recorded on lation within the Mount Head embayment. The overall shape of this embayment is probably palinspastically restored maps and cross setinherited from the arcuate shape of the Late Proterozoic cratonic rift margin. Within the tions and to basement-tectonic Mount Head embayment, several northeast-southwest-trending Archean and Proterozoic domains defined on aeromagnetic and gravbasement-tectonic elements intersect the autochthonous cratonic margin at high angles. ity maps (Brandley, 1993). The palinspastic Carboniferous piano-key-like structural reactivation of these tectonic elements produced base map is modified from Gibson,s (1985) oriented trends of differential subsidence that partitioned the Mount Head embayment into map of displacement vectors for Jurassic two subbasins (Crowsnest and Kananaskis depocenters) separated by a more positive area rocks. The magnitudes of the vectors were (Highwood high). Moreover, our data imply that Precambrian basement structure noted by others under the Plains and Rocky Mountain fold and thrust belt continued across the altered to reflect displacements at the MisRocky Mountain trench to the west, and that tectonic control on sedimentation noted by sissippian level documented by Bally et al. others for Precambrian and pre-Middle Devonian rocks extends clearly into overlying (1966), Price (1981), and Price and Fermor Carboniferous deposits. (1985), university theses, and Geological Survey of Canada maps and sections across

Is Fluid Flow in Paleozoic Formations Affected by the Rocky Mountain Thrust Belt, West Central Alberta?: ABSTRACT

Is Fluid Flow in Paleozoic Formations Affected by the Rocky Mountain Thrust Belt, West Central Alberta?: ABSTRACT

Precambrian basement in the Canadian Cordillera: an introduction

s, 15: A4. ARMSTRONG, R. L., TAUBENECK, W. H., and HALES, P. 0.1977. Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington and Idaho. Geological Society of America Bulletin, 88: 39741 1. 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, 14: 337-381. BENNETT, V. C., and DEPAOLO, D. J. 1987. Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping. Geological Society of America Bulletin, 99: 674-685. BERRY, M. J., JACOBY, W. R., NIBLEIT, E. R., and STACEY, R. A. 1971. A review of geophysical studies in the Canadian Cordillera. Canadian Journal of Earth Sciences, 8: 788-801. BOWRING, S. A., and PODOSEK, F. P. 1989. Nd isotopic evidence from Wopmay Orogen for 2.0-2.4 Ga crust in western North America. Earth and Planetary Science Letters, 94: 217-230. BURWELL, A. D. M. 1975. Rb-Sr isotope geochemistry of lherzolites and their constituent minerals from Victoria, Australia. Earth and Planetary Science Letters, 28: 69-78. CAMPBELL, R. B. 1968. Canoe River, British Columbia. Geological Survey of Canada, Map 15-1967. CARR, S. D., and BROWN, R. L. 1990. Southern Cordilleran Lithoprobe transect lines 6 to 10: crustal structure and tectonic chronology. Southern Canadian Cordillera Transect Workshop, March 3 4 , University of Calgary, Calgary, Alta., pp. 10-18. COLLERSON, K. D., VAN SCHMUS, W. R., LEWRY, J. F., and BICKFORD, M. E. 1988. Buried Precambrian basement in south-central Saskatchewan: provisional results from Sm-Nd model ages and U-Pb zircon geochronology. In Summary of investigations 1988. Saskatchewan Geological Survey, Miscellaneous Report 88-4, pp. 142-150. COOK, F. A., SIMONY, P. S., COFLIN, K. C., GREEN, A. G., MILKEREIT, B., PRICE, R. A., PARRISH, R., PATENAUDE, C., GORDY, P. L., and BROWN, R. L. 1987. Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex. Geophysical Journal of the Royal Astronomical Society, 89: 91-

Paleography and Conodont Biostratigraphy of Devonian Carbonate Rocks of the Whitefish MacDonald Range, NW Montana and SE British Columbia

Five remote Upper Devonian carbonate sections measured within the Rocky Mountain fold and thrust belt of the Whitefish Range, northwestern Montana and contiguous MacDonald Range, southeastern British Columbia are bounded unconformably at the base by Middle Cambrian carbonate rocks and unconformably at the top by the Devonian-Mississippian Exshaw Formation. Conodont zonation provides an age of late Frasnian to middle Famennian. Standard pelagic conodont zones represented are Polygnathus asymmetricus, Ancyrognathus triangularis, Palmatolepis gigas, Pa. triangularis, Pa. crepida, P. rhomboidea, and lower Pa. marginifera zones. Strata are equivalent in age to the Jefferson and Three Forks formations in central Montana, the Jefferson Formation in east central Idaho, carbonate units of Limestone Hill in eastern Washington, the Devils Gate Limestone and pilot Shale in eastern Nevada, and the Fairholme Group, Alexo, and Palliser formations in British Columbia. Devonian carbonate lithologies examined in this study are dominantly subtidal and intertidal lime mudstone, wackestone, and bindstone interpreted to have been deposited above storm wave base on a wide carbonate platform, during the early, less dramatic movements of the Antler orogeny. This research will provide a data point that will fill a significant void in Devonian paleogeographic information in the northern United States and Southern Canadianmore » cordillera.« less

Provenance of post-Wapiabi sandstones and its implications for Campanian to Paleocene tectonic history of the southern Canadian Cordillera

Late Campanian to Early Paleocene sandstones of the Alberta Foothills were derived from three types of rocks: (i) andesitic–dacitic volcanic rocks that were presumably comagmatic with middle to late Mesozoic plutons in the Omineca Crystalline Belt; (ii) low-grade metamorphic rocks in the suprastructure of the Omineca Crystalline Belt; and (iii) sedimentary rocks in die Rocky Mountain Thrust Belt, principally pelitic rocks in the western Main Ranges and carbonates and chert-arenites in the eastern Main and Front ranges. A paucity of quartzo-feldspathic rocks fragments and potassium feldspar indicates that the core of the Omineca Crystalline Belt was not extensively exposed at that time.Vertical trends in composition of the sandstones reveal five petrographic stages. Stage I is dominated by volcanic rock fragments and plagioclase, suggesting that initial progradation of the sediment was largely a response to coeval volcanism or tectonic emplacement of older volcanic rocks. Stages III and V are characterized by a significant decrease in the relative proportion of metamorphic detritus and an increase in the proportion of carbonate and chert detritus. These stages may represent periods of thrusting in the eastern Main Ranges or Front Ranges. In contrast, stages II and IV display increases in metamorphic detritus and stage II shows a concomitant decrease in carbonate and chert detritus, trends that indicate wearing down of the eastern Main Ranges or Front Ranges thrust sheet(s) and reintegration of the Omineca Crystalline belt and the western Main Ranges into the drainage basin. The compositional stages indicative of thrust events are associated with coarse facies, including the Entrance and High Divide Ridge conglomerates, whereas those stages indicative of tectonic quiescence are associated with fine-grained facies including coal.

Geochemistry, geochronology, and tectonic implications of two quartz monzonite intrusions, Purcell Mountains, southeastern British Columbia

The Reade Lake and Kiakho stocks are posttectonic mesozonal quartz monzonite porphyries that intrude dominantly Middle Proterozoic Purcell Supergroup rocks in southeastern British Columbia. K–Ar dates of hornblende from the Reade Lake stock range from 103 to 143 Ma. However, a U–Pb date of 94 Ma from zircon concentrates is interpreted to be the age of emplacement of the stock, suggesting the range and older K–Ar dates are due to excess 40Ar. A K–Ar date of 122 Ma for the hornblende from the Kiakho stock is believed to be a more reliable intrusive age.Both stocks cut across and apparently seal two faults that have played roles in the tectonic evolution of the Purcell anticlinorium and Rocky Mountain thrust belt. The Reade Lake stock cuts the St. Mary fault, an east-trending reverse thrust that crosses the Rocky Mountain trench and links with thrusts in the Rocky Mountains; the Kiakho stock cuts the Cranbrook fault, an older east-trending normal fault. Hence, the 94 Ma date on the Reade Lake stock constrains the latest movement on the St. Mary fault to early Late Cretaceous; and the 122 Ma date on the Kiakho stock appears to limit latest movement on the Cranbrook fault to Early Cretaceous. These faults and the intrusions are part of an allochthonous package, displaced eastward by underlying thrust faults during formation of the Purcell anticlinorium and more eastern thrusts in the Rocky Mountains.

Lithoprobe southern Canadian Cordilleran transect: Rocky Mountain thrust belt to Valhalla gneiss complex

Summary. LITHOPROBE has acquired nearly 270 km of crustal seismic reflection data across the eastern portion of the southern Canadian Cordillera. These reflection profiles, obtained during the Fall of 1985, extend from the Rocky Mountain thrust and fold belt, across the Rocky Mountain Trench, Purcell anticlinorium, Kootenay Arc, Nelson batholith and Valhalla gneiss complex. North American basement and its overlying foreshortened miogeoclinal rocks can be traced westward to the Kootenay Arc. The Purcell anticlinorium is carried by a series of west dipping thrust faults which emerge east of the anticlinorium and converge downward and merge with a detachment surface above autochthonous North American basement. Proterozoic supracrustal rocks, thickened by folding and thrusting, occupy the core of the anticlinorium. Steeply dipping surface structures of the western Purcell anticlinorium and Kootenay Arc appear to be truncated at 3 - 4 s (9-12 km) by a gently east-dipping reflection that may delineate the upper boundary of an allochthonous wedge inserted between the near surface rocks and autochthonous basement below. Beneath the Kootenay Arc, at a travel time of 9-10 s (27-30 km), the North American basement seems to be truncated by the major east-dipping Slocan Lake fault zone, which can be traced from its surface exposure at the east edge of the Valhalla gneiss complex eastward to near the base of the crust. A high amplitude, west-dipping reflection underlies the Valhalla complex and may be related to a major compressional shear zone.

Cocorp deep seismic reflection traverse of the interior of the North American Cordillera, Washington and Idaho: Implications for orogenic evolution

Deep seismic reflection data collected by the Consortium for Continental Reflection Profiling along a transect of the interior of the northwestern U. S. Cordillera indicate the presence of crustpenetrating faults; these faults developed during an orogenic cycle consisting of Jurassic to Paleocene accretion‐related crustal thickening followed by Eocene crustal thinning. The reflection data also suggest that the Moho is a young, dynamic feature which developed a relatively smooth, flat geometry beneath a complexly deformed crust, probably during an Eocene episode of crustal extension. The profiles reveal prominent west dipping reflection sequences (and associated diffractions) interpreted as a Mesozoic thrust system of crustal dimensions that linked accretion‐related shortening in the western Cordillera with the thrust belt in the eastern Cordillera, east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated Eocene crustal extension, and essentially flat Moho reflections at 33–35 km beneath complexly deformed crust. These features are consistent with a sequence of (1) late Mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the North American passive continental margin along a Jurassic thrust (2) propagation of crust‐penetrating thrusts to shallower levels in the east, where they probably formed the sole of the Rocky Mountain Thrust Belt (3) Eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and (4) development of a flat Moho geometry during Cenozoic extension and magmatism. These data provide new information about the structure of the entire crust in the Cordilleran interior and offer a working model, both for evolution of Cordilleran core complexes and possibly for other collisional orogens.

Anomalous paleomagnetism of the Crowsnest Formation of the Rocky Mountains

The volcanic Crowsnest Formation of Albian age (late Early Cretaceous) from the Rocky Mountain fold and thrust belt of Alberta has a stable remanent magnetization with a mean direction of 349°, 59 °(α95 = 5°) and paleopole at 78°N, 108°E(dm = 7°, dp = 5°). The inclination is lower than, and the declination clockwise of, the expected mid-Cretaceous paleogeomagnetic field for cratonic North America. Taken at face value the result indicates that the Crowsnest Formation and the thrust sheet in which it occurs have been transported from the south relative to cratonic North America by 17 ± 6 °(about 1800 km) and rotated 24 ± 10° clockwise. It is also possible that flattening of inclination is caused by magnetic anistropy, but tests show this to be unlikely. A third possibility is that the magnetization is secondary and of latest Cretaceous age, but there are good reasons for believing this is not so. Lastly, it is possible that the unit could have been formed close to its present position relative to the craton but was deposited so quickly that the paleosecular variation was not adequately sampled, and the result is only a "spot" reading of the paleofield. The last is our preferred interpretation of the flattened inclination, but the clockwise deflection of the declination could reflect rotation. Other paleomagnetic data from the fold and thrust belt are generally consistent with the third interpretation.

Paleomagnetic results for Eocene volcanic rocks from northeastern Washington and the tertiary tectonics of the Pacific Northwest

The direction of remanent magnetization for 102 sites in Eocene volcanic and volcaniclastic rocks of the O'Brien Creek Formation, Sanpoil Volcanics, and Klondike Mountain Formation suggests approximately 25° of clockwise rotation of a 100 by 200 km area in northeastern Washington. The volcanic rocks consist chiefly of rhyodacite and quartz latite flows, with intercalated ash flow tuff and volcaniclastic layers. These rocks have been sampled at 102 sites distributed among five volcanotectonic depressions: the Toroda Creek, Republic, Keller, and First Thought grabens and the Spokane‐Enterprise lineament. The volcanic rocks probably range in age from 55 m.y. to about 48 m.y., and the 50‐ to 48‐m.y.‐old volcanic rocks within this suite appear to be rotated as much as the older rocks. Previous investigators have shown that 40‐m.y.‐old and younger plutonic rocks of northwestern Washington are not rotated; hence we infer that the north‐central Washington rocks were rotated to their present declination between 48 and 40 m.y. B.P. (during the middle and/or late Eocene). During early Eocene time this region was extended in a westward direction through crustal necking, gneiss‐doming, diking, and graben formation. Internal deformation of the region related to this crustal extension was extreme, but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation; hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area. Faults active during Paleogene time appear to define boundaries of a triangular crustal block (the Sanpoil block), encompassing much of northeastern Washington, northern Idaho, northwestern Montana, and adjacent parts of British Columbia. The faults include the Laramide thrusts of the Rocky Mountain thrust belt, the strike‐slip faults of the Lewis and Clark line, and strike‐slip faults of the Straight Creek‐Fraser zone. We suggest that during early Eocene time the Sanpoil block was extended westward through crustal necking and dilation and then during the middle Eocene was rotated clockwise and thrust over the craton in a final stage of Laramide thrusting. The “motor” driving these deformations presumably was interaction of North America with oceanic lithosphere off its western margin; such interaction probably involved right‐oblique underthrusting and dextral shear.

Libby Thrust Belt and Adjacent Structures--New Factors to Consider in Thrust Tectonics of Northwestern Montana: ABSTRACT

About 40 mi (65 km) west of the Rocky Mountain trench and at least 9 mi (15 km) above the sole detachment of the Rocky Mountain thrust belt is a zone of Cretaceous-Tertiary thrust faults up to 25 mi (40 km) wide in middle Proterozoic and Cambrian rocks. This zone (the Libby thrust belt) extends northward from the Lewis and Clark line to the northwest corner of Montana. Within the Libby thrust belt is a series of complex ramps, horsts, splays, and folds that accommodate a tectonic shortening End_Page 850------------------------------ of about 6.2 mi (10 km). Backsliding has occurred on some listric thrust faults, and middle Tertiary(?) extensional horst-and-graben faults offset or join most thrust faults. On the east, the lead thrust ramps up onto the broad open Purcell anticlinorium. On the west, the Libby thrust belt is overridden in the north by the lead thrust of the Yaak plate (whose central part is the broad, open Sylvanite anticline), and in the south, it is overridden by the Moyie thrust (which trends northwest and also overrides the west edge of the Yaak plate). An essentially continuous section, 46,000 ft (14,021 m) thick, of Belt rocks is displayed on the south-plunging Sylvanite anticline. The base is not exposed, and the top is eroded. A section of similar thickness exists on the west flank of the Purcell anticlinorium, where the Belt Supergroup is overlain by about 3,000 ft (914 m) of Cambrian rock. The Cambrian occurs in the broad synclinal Libby trough that is paired with the Purcell anticlinorium, and these Cambrian strata are also caught up in the Libby thrust belt. Geologic cross sections suggest that the Belt rocks have overridden the Cambrian at shallow depths only and that Cambrian and younger Phanerozoic strata probably do not occur at greater depths beneath and west of the Purcell anticlinorium. This interpretation differs significantly from interpretations that suggest intercalation of major wedges of Paleozoic and Belt rocks at depth in this same area. End_of_Article - Last_Page 851------------

Structural evolution of the Thor-Odin gneiss dome

The Thor-Odin dome is a large structural culmination along the eastern margin of the Shuswap terrain. Previous workers have presented divergent models for the origin of the dome. Some have proposed that it is a product of diapiric uprise of material from the lower crust; alternatively, the dome is interpreted to have formed from the interference of later folding. This study presents the results of a detailed analysis of new data on the structure, pattern of strain variation and metamorphic fabric of the dome. These data together with new geochronological and metamorphic phase assemblage information provide the basis for a revised interpretation of the evolution of the Thor-Odin gneiss dome. The pre-doming evolution of the Thor-Odin gneiss dome was characterized by the formation of large-scale nappe structures and the imbrication of Archean basement rocks with the Cambrian and younger cover rock sequence. The first period of deformation (Phase One) consisted of large-scale infolding of the cover rock sequence into the basement rocks. The Pingston fold in the core of the dome is a product of this event. The second period of deformation (Phase Two) was marked by the forcing of wedges of basement into the cores of northerly-directed nappes. These structures had no observable effect on the central core zone, which is here termed the Autochthonous Core Gneiss Domain. The third period of deformation (Phase Three) was co-axial with Phase Two and consisted of imbrication and refolding of the upper levels of the stack of Phase Two nappes. The actual domal shape was formed by fold interference of Phases Four, Five and Six, which were upright, chevron-style, flexural-slip folds with northwest, southwest and northerly-trending axial planes. The “doming” events occurred at upper crustal levels, probably during the Tertiary. Analysis of the strain pattern suggests that little of the total deformational strain is related to these dome-forming deformations. This outline of the events that formed the dome is consistent with the new geochronological results and interpretation of metamorphic phase assemblages. Cordierite-gedrite-sillimanite assemblages, previously interpreted as indicating 1000 MPa pressure, probably formed under reduced water fugacities and 500–700 MPa total pressure. The Thor-Odin gneiss dome thus provides no evidence for lower crustal upwelling related either to dome formation or to the formation of the Rocky Mountain thrust belt.

Seismicity in the Mica Reservoir (McNaughton Lake) area: 1973–1978

Impoundment of McNaughton Lake was initiated on March 29, 1973 and full load of 25 × 109 m3 and maximum depth of 191 m were first achieved in August 1976. The reservoir lies principally in the Rocky Mountain Trench, a major structural boundary between the Rocky Mountain Thrust Belt to the east and the Eastern Metamorphic Belt to the west. Although the regional seismicity is moderate, with an average of about one earthquake with Richter magnitude ML ≥ 3.0 every 2 years in the period 1963–1972, a magnitude 6 earthquake occurred adjacent to the present reservoir site in 1918.A monitoring program initiated in January 1973 has located 212 earthquakes with M ≥ 1.5; the largest ML = 4.8. Most of the seismicity was confined to four sequences. The first swarm started prior to initiation of impoundment. The next two sequences occurred in the same area 17 km into the Rocky Mountains from the central section of the reservoir. The final sequence, consisting of the ML = 4.8 earthquake and its aftershocks, had a preferred epicentre 4 km east of the Rocky Mountain Trench near the northern section of the reservoir. Comparison of the characteristics of the earthquake sequences with data from reservoirs where seismicity has been induced indicates that the observed seismicity at McNaughton Lake is unlikely to have been induced.The regional seismicity pattern from 1973 to 1978 was similar to that of the historical record: distributed earthquakes in the Rocky Mountains and in the adjacent Monashee and Cariboo Mountains of the Eastern Metamorphic Belt, no earthquakes within the Rocky Mountain Trench, and the Selkirk Mountains continuing to be almost aseismic. Both the regional seismicity and swarms yield b values near 0.65. Inclusion of the historical data with the regional seismicity gives b = 0.74 ± 0.08.

Upper Cretaceous-Paleocene Synorogenic Conglomerates of Southwestern Montana

The sequence of pebble types and pattern of sediment deposition observed in the thick (10,000+ ft) Late Cretaceous and Paleocene Beaverhead and Monida Formations in the Lima area of southwestern Montana permit determination of source areas, approximate carrying capacity and slope of streams transporting debris to the depositional site, and character of the depositional environments. These data lead, in turn, to an accurate reconstruction of the paleogeography in the Lima-Monida area during the Late Cretaceous and early Tertiary. Directional sedimentary structures and variations in maximum boulder size and pebble-cobble composition suggest the presence of two source areas. Both source areas, the Blacktail-Snowcrest uplift and ancestral Beaverhead Range, were domal uplifts and/or high-angle fault blocks, as indicated by the simple inverted sequence of pebble-cobble lithologic types, sandstone lithologic types, and heavy-mineral suites. Uplift of the source areas preceded emplacement of Laramide low-angle thrust plates, which probably originated as late-stage gravity slides off the ancestral Beaverhead Range. This sequence of Laramide events conflicts in part with the sequences suggested by workers in other areas of the Rocky Mountain thrust belt where synorogenic conglomeratic rocks are present. The coarseness, graded bedding, channeling, imbrication, bed geometry, and bedding characteristics indicate that the conglomerates of the Beaverhead Formation were deposited as alluvial-fan debris. The cross-bedded sandstone of the Monida Formation on the southeast may represent deposition on the distal parts of alluvial fans or possibly on adjacent fan aprons or floodplains.