Shuswap Metamorphic Complex Research Articles

Protracted (>100 m.y.) deep crustal orogenesis revealed by in situ monazite petrochronology in the Shuswap Metamorphic Complex, British Columbia, Canada

The Shuswap Metamorphic Complex in southeastern British Columbia, Canada, exposes penetratively deformed rocks exhumed from deep crustal levels (∼25−30 km depth) of the Canadian Cordillera. Existing models describing its tectonometamorphic evolution are not directly linked to absolute age constraints acquired through modern petrochronological methods and, therefore, remain ambiguous. To differentiate between proposed models, here we apply U-Th-Pb monazite petrochronology, petrological modeling, and microstructural analysis to quantify conditions and timing of deformation across a transect of the Shuswap Metamorphic Complex. Our results demonstrate that metamorphism decreases in age with increased structural depth associated with progressive localization of NE-directed shearing toward the base of the Shuswap Metamorphic Complex. Monazite U-Th-Pb age data from the structural level exposed in the study area are consistent with protracted northeast-directed compression from ca. 167 Ma to ca. 59 Ma, after which west-directed ductile extension continued until at least ca. 49 Ma and was progressively localized along the Okanagan Valley fault system. New data presented are consistent with a model of basal accretion in front of a foreland-propagating ductile thrust system, followed by exhumation of the Shuswap Metamorphic Complex facilitated in large part by crustal-scale extension.

Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera

The distribution of tectonic superstructure across the Shuswap metamorphic complex of southern British Columbia is explained by east-west– trending corrugations of the Okanagan Valley shear zone detachment. Geological mapping along the southern Okanagan Valley shear zone has identified 100-m-scale to kilometer-scale corrugations parallel to the extension direction, where synformal troughs hosting upper-plate units are juxtaposed between antiformal ridges of crystalline lower-plate rocks. Analysis of available structural data and published geological maps of the Okanagan Valley shear zone confirms the presence of ≤40-km-wavelength corrugations, which strongly influence the surface trace of the detachment system, forming spatially extensive salients and reentrants. The largest reentrant is a semicontinuous belt of late Paleozoic to Mesozoic upper-plate rocks that link stratigraphy on either side of the Shuswap metamorphic complex. Previously, these belts were considered by some to be autochthonous, implying minimal motion on the Okanagan Valley shear zone (≤12 km); conversely, our results suggest that they are allochthonous (with as much as 30–90 km displacement). Corrugations extend the Okanagan Valley shear zone much farther east than previously recognized and allow for hitherto separate gneiss domes and detachments to be reconstructed together to form a single, areally extensive Okanagan Valley shear zone across the Shuswap metamorphic complex. If this correlation is correct, the Okanagan Valley shear zone may have enveloped the entire Shuswap metamorphic complex as far east as the east-vergent Columbia River–Slocan Lake fault zones.

New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone

The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an ∼1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64–89 km of WNW-directed horizontal extension based on an original shear zone angle of ∼15°. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic–Mesozoic layered sequence of sedimentary rocks.

A reappraisal of the tectonic significance of early Tertiary low-angle shear zones exposed in the Vernon map area (82 L), Shuswap metamorphic complex, southeastern Canadian Cordillera

Detailed geological mapping across the Shuswap metamorphic complex between latitudes 50°00′N and 50°45′N reveals that superstructure forms a semicontinuous carapace across the complex, with minimal evidence of internal thinning. Near the western margin of the complex, superstructure and infrastructure are juxtaposed across low-angle, ~2 km thick, ductile shear zones spatially associated with Paleocene to Early Eocene syn-kinematic granitic rocks. The shear zones, which yield upper plate to the west shear-sense indicators, are interpreted as the northern extension of the Okanagan Valley fault. Farther east, near the north–south axis of the complex, superstructure and infrastructure are separated by an attenuated metamorphic section, but evidence of noncoaxial strain is lacking. Discrete detachments were not found. Steeply dipping normal faults cut low-angle shear zones and do not merge with them at depth. Middle Eocene volcanic and sedimentary rocks rest unconformably on metamorphic basement. The continuity of superstructure indicates that infrastructure was not exhumed by crustal-scale detachments. The results provide the basis for a complete reinterpretation of the tectonic significance of low-angle shear zones exposed in the Vernon area. It is proposed that Late Cretaceous to early Tertiary partial melting of the middle crust resulted in the development of a zone of channel flow. As the channel was underthrust by a crustal-scale ramp in underlying, more competent Paleo proterozoic basement, it was exhumed from depths of 20–30 km and thinned vertically. Shear zones between infrastructure and superstructure are interpreted as being a transient rheological interface at the upper boundary of the channel.

U–Pb constraints on the thermotectonic evolution of the Vernon antiform and the age of the Aberdeen gneiss complex, southeastern Canadian Cordillera

The Aberdeen gneiss complex is composed of complexly deformed migmatitic orthogneiss and paragneiss situated within the core of the Vernon antiform, a structure defined by a series of subparallel reflectors visible at upper to middle crustal depths (6–18 km) in seismic reflection data from the Vernon area of the Shuswap metamorphic complex. The Vernon antiform and the Aberdeen gneiss complex lie within the footwall of the gently west dipping (top to the west) Kalamalka Lake shear zone. Migmatitic gneiss exposed within the antiform records evidence (recorded as age domains in complexly zoned zircon grains) of three metamorphic events, occurring at 155–150, 90, and 66–51 Ma. The timing of magmatic events within the antiform includes emplacement of diorite at ~232 Ma, tonalite at ~151 Ma, granodiorite at 102 Ma, and monzonite at 52 Ma. Middle to Late Jurassic metamorphism resulted in widespread migmatization. Early Tertiary metamorphism (66–51 Ma) was coeval with the emplacement of granitic rocks and exhumation typical of other areas of the Shuswap metamorphic complex. Highly deformed orthogneiss situated within the hanging wall of the Kalamalka Lake shear zone, comprising the superstructure, was emplaced at ~171 Ma. Ductile deformation had ceased by 162 Ma. The complex metamorphic and magmatic evolution of the Vernon antiform, which is similar to other areas of the southern Canadian Cordillera including the Nicola horst, Mount Lytton – Eagle plutonic complex, Cariboo Mountains, and Mica Creek area, may reflect episodic tectonic activity at the plate margin.

Possibility of channel flow in the southern Canadian Cordillera: a new approach to explain existing data

Abstract Existing structural, metamorphic and geochronological data in and close to the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model. Four general structural levels (domains) can be distinguished in the region, based on the orientation and vergence of folds. In the lowest three levels folds are mostly recumbent, whereas in the uppermost level they are upright. The lowest three levels are interpreted as a channel flow zone. NE-verging folds of the lowest level (Domain 1, e.g. the Monashee Complex) formed during top-to-the-NE detachment flow and/or in the lower part of a channel flow zone. When detachment flow changed to channel flow, the sense of shear changed in the upper part of the channel flow zone, resulting in overprinting of NE-verging folds by SW-verging folds (Domain 2, e.g. most parts of the Shuswap Metamorphic Complex to the west of the Monashee Complex). Temperature was probably increasing, weakening a progressively larger portion of the crust, and the crustal shear zone therefore widened. Thus, in the highest structural levels within the channel flow zone, SW-verging folds developed in areas where no NE-verging folds originally formed (Domain 3, e.g. the Cariboo Mountains). The channel flow model as presented here is compatible with many of the ductile structures and accommodates existing metamorphic and geochronological data in the part of the southern Canadian Cordillera described.

Zoltaiite, a new barium-vanadium nesosubsilicate mineral from British Columbia: Description and crystal structure

Zoltaiite, ideal formula BaV24+V123+ Si2O27, space group P3̅, a = 7.601(1), c = 9.219(1) Å, V = 461.34(1) Å3, Z = 1, is a new mineral found on the eastern edge of the Shuswap metamorphic complex of British Columbia, Canada. It is a metamorphic mineral formed under greenschist-facies P-T conditions as part of an assemblage that includes quartz, celsian, apatite, sphalerite, pyrrhotite, galena, and pyrite. Zoltaiite has a Mohs hardness of 6.7, no cleavage, an anhedral to semi-prismatic habit, and a calculated density of 4.83 g/cm3. It is opaque with reflectance and color similar to those of sphalerite. The strongest eight lines of the X-ray powder diffraction pattern [d in Å (I) (hkl)] are 3.103(78)(021), 2.934(89)(21̅2), 2.785(67)(013), 2.679(48)(022), 2.403(50)(211), 2.190(100)(212), 1.934(53)(213), and 1.438(63)(140). The empirical formula, derived from electron-microprobe analysis and the crystal structure, is Ba1.05(Ti1.31V4+0.69)Σ2.00(V3+11.06Fe3+0.49Cr0.34)Σ11.89Si2.06O27 based on O = 27. The crystal structure was solved by direct methods and refined on the basis of F02 using all unique reflections measured with MoKα Xradiation on a CCD-equipped diffractometer. The final R factor was 3.2%, calculated using 659 unique observed reflections. The unit cell contains four layers of two types parallel to (001): X, an octahedral and tetrahedral sheet, and Y, an octahedral plus barium sheet; both layers are doubled through inversion centers resulting in the sequence XXYY... Two consecutive equivalent layers are interconnected through shared octahedral edges, whereas consecutive non-equivalent layers are linked through shared corners. The high calculated density is consistent with the dense packing of the structure.

Reconstructing paleoelevation in eroded orogens

Hydrogen isotope and 40Ar/39Ar geochronological data are presented from muscovite within a crustal-scale extensional detachment of the Shuswap Metamorphic Complex, North American Cordillera. The hydrogen isotope compositions (δDms) of precisely dated muscovite attain values as low as −156‰ in the detachment mylonite, whereas footwall quartzite has a δDms value of −81‰. The very low δDms values in the detachment are best explained by infiltration of meteoric water, with maximum δD values of −135‰ ± 3‰, during extensional unroofing of the orogen at 49.0–47.9 Ma. On the basis of the empirically determined relationship between elevation and isotopic composition of precipitation, the reconstructed early Eocene paleoelevations of the orogen are 4060 ± 250 m to 4320 ± 250 m, at least 1000 m higher than the highest present-day peaks. We propose that the isotopic composition of surface-derived waters in extensional detachments represents a newly recognized method to estimate maximum paleoelevations attained immediately preceding extensional orogenic collapse.

Early Cretaceous kyanite-sillimanite metamorphism and Paleocene sillimanite overprint near Mount Cheadle, southeastern British Columbia: geometry, geochronology, and metamorphic implications

Mapping of isograds related to regional amphibolite-facies metamorphism constrains a three-dimensional model of isogradic surfaces near Mount Cheadle in the northern Shuswap metamorphic complex (lat. 52°20'N, long. 119°05'W). Kyanite and sillimanite coexist in a lens-shaped zone, bounded by the kyanite-out and sillimanite-in isogradic surfaces, that is 50 km long, up to 10 km thick, and up to 20 km wide. Textural equilibrium, simple regular geometry of isogradic surfaces, and simple mineral assemblages suggest that metamorphism occurred at P-T conditions near those of the kyanite-sillimanite equilibrium curve. Reconstruction of isotherms in the kyanite + sillimanite zone suggests that the metamorphic field gradient was about 14°C·km-1. A 5 km thick, staurolite-free kyanite zone adjacent to the sillimanite-in isograd suggests a pressure range of about 1.5 kbar (1 kbar = 100 MPa) for Bathozone 5 of D.M. Carmichael. Regional metamorphism was Early Cretaceous (monazite U-Pb geochronology) with quenching in the Late Cretaceous, possibly caused by motion on the basal thrust beneath the Malton complex. A younger generation of sillimanite grew in discrete outcrop-scale ductile shear zones, veins, and pods in a north-south-oriented belt (50 km by 20 km). U-Pb dates on zircon, monazite, and titanite indicate an age of the sillimanite overprint of 65-59 Ma. It may have resulted from the influx of hot fluids associated with widespread Late Cretaceous and Paleocene leucogranite emplacement concomitant with extensional faulting.

Metamorphism in the Clachnacudainn terrane and implications for tectonic setting in the southern Omineca Belt, Canadian Cordillera

New and previously published metamorphic data suggest that the Clachnacudainn terrane of the southern Omineca Belt has tectonic affinities with the overlying Selkirk allochthon, rather than the underlying Shuswap metamorphic complex. This interpretation is based on relationships between metamorphic minerals and deformation phases, plutons, and the upper boundary of the terrane, the Standfast Creek fault. Regional kyanite and staurolite zones in the structurally lowest part of the terrane are overlain by a garnet zone that is continuous upward across the Standfast Creek fault into the Selkirk allochthon. This metamorphism is inferred to be Jurassic age based mainly on the continuity of these zones with those of known age in the allochthon. Textural relationships show that metamorphism occurred at different times relative to deformation across the terrane. Thermobarometry and a petrogenetic grid indicate that the terrane attained lower to middle amphibolitc facies conditions. Sillimanite and andalusite zones in the contact aureoles of posttectonic mid-Cretaceous plutons overprint the regional metamorphic zones and the Standfast Creek fault. Comparison of estimated pressures shows that approximately 5–10 km of exhumation occurred between regional and contact metamorphism. These metamorphic data are interpreted to indicate that the Standfast Creek fault had minor displacement after regional metamorphism and negligible displacement after contact metamorphism. Therefore, the fault cannot be an Eocene ductile to ductile–brittle shear zone that appressed or omitted metamorphic isograds and rapidly exhumed the Clachnacudainn terrane in its footwall, as was previously proposed.

Three crustal zones in the Thor–Odin – Pinnacles area, southern Omineca Belt, British Columbia

The present crustal architecture of the southern Omineca Belt in the Canadian Cordillera is a product of Eocene extension and crustal thinning superimposed on a crust that was thickened and deformed during Paleozoic and Jurassic to Late Paleocene compression. Amphibolite-facies rocks exposed as gneiss complexes within the Shuswap Metamorphic Complex, in the southern Omineca Belt, were buried during compression and were exhumed in the lower plates of low- to moderate-angle plastic–brittle Eocene extensional faults.In the Thor–Odin – Pinnacles area three crustal zones, which have experienced different deformation and thermal histories, and intervening shear zones can be correlated with Lithoprobe seismic reflection data. The Basement Zone, which comprises crystalline basement and overlying supracrustal gneisses, is bounded above by the Monashee décollement, a deep-seated northeasterly directed Mesozoic–Paleocene thrust fault. In the hanging wall of the décollement, polydeformed gneisses and schists of the Middle Crustal Zone are characterized by Late Cretaceous–early Tertiary ductile strain, plutonism, and thermal quenching. They are bounded at the top by crustal-scale Eocene normal faults that juxtapose Upper Crustal Zone rocks characterized by Jurassic and older structures and a Jura-Cretaceous cooling history.Middle Crustal Zone rocks of the Thor–Odin – Pinnacles area are correlative with part of the Late Proterozoic Horsethief Creek Group and Cambrian to Jurassic strata and host extensive plutons, stocks, and sheets of the syntectonic and posttectonic Late Paleocene – Early Eocene Ladybird granite suite. Field mapping and geochronology indicate that (i) a substantial part of the penetrative compressional polydeformation history and the thermal peak of metamorphism within the Middle Crustal Zone occurred in the Late Cretaceous–Paleocene; (ii) thrusting on the Monashee décollement had ended by 58 Ma; (iii) the onset of extensional deformation either overlapped or closely followed the compressional regime; (iv) Middle Crustal Zone metamorphic and igneous rocks were hot in the Paleocene and cooled rapidly in the early Tertiary because of extensional denudation.

Mise en évidence d'un cisaillement ductile dextre d'âge crétacé moyen dans la région de Tête Jaune Cache (nord-est du complexe métamorphique Shuswap, Colombie-Britannique)

The boundary between the external zones (Rocky Mountains) and the internal zones of the eastern Canadian Cordillera is marked by a Tertiary half graben, the Rocky Mountains Trench (RMT). In the south Cordillera, east of the Shuswap metamorphic complex, the fault limiting the trench is superimposed on an early major thrust, the Late Jurassic Purceli thrust. On approaching this discontinuity, the ductile deformation of the Miette Group, a detrital Precambrian suite, is characterized by a subvertical foliation and a subhorizontal stretching lineation parallel to the fold axes. The deformation intensity, its noncoaxial characters, and its geographic extension are interpreted as resulting from a dextral crustal shear, parallel to the mapped trace of the Purcell thrust and RMT. The dextral slip is deduced from a microtectonic analysis of the observed rotational criteria and is consistent with the small angle occurring between the directions of the linear structure (stretching lineations and fold axes) and those of adjacent discontinuities. The Middle Cretaceous age (100–78 Ma) attributed to this deformation is based on the age of syn- to late-tectonic metamorphic minerals as dated by the 39Ar–40Ar method. A kinematic model involving vectorial decomposition of an oblique convergence is proposed, suggesting the simultaneous occurrence, in the Middle Cretaceous, of two suborthogonal conjugated movement directions, respectively parallel and normal to the general Cordilleran trend. [Journal Translation]

Seismic reflection geometry of the Columbia River Fault Zone and East Margin of the Shuswap Metamorphic Complex in the Canadian Cordillera

Vibroseis (CONOCO, Incorporated) seismic reflection data, recorded near the transition between the Cordilleran fold and thrust belt and the east margin of the Shuswap metamorphic complex of the southern Canadian Cordillera, outline an east tapering wedge geometry in the upper crust. The wedge comprises part of the Shuswap metamorphic complex and may be the result of either a foreland‐dipping duplex or tectonic wedging. The geometric wedge separates the overlying miogeoclinal rocks of the Selkirk fan structure from the crystalline basement (?) rocks below, and delineates the eastern limit of the Monashee complex. Brittle motion on the Columbia River fault zone produced an east dipping, listric normal fault which may have offset the upper portion of the Monashee complex. The Columbia River fault zone is imaged over the entire 15 km of the profile and can be traced to a depth of at least 10 km. Reflections from the lower crust are generally subhorizontal and discontinuous, and they may be a manifestation of crustal extension. The crust‐mantle transition is probably represented as reflections with travel times of 11.0–12.0 s.

The role of the Shuswap Metamorphic Complex in Cordilleran tectonism: a review

The Shuswap Metamorphic Complex consists of three parts, each with unique stratigraphy and orogenic evolution, separated by major faults of diverse nature and having in common only a post-late Mesozoic tectonic history. The first part, the Monashee Complex, is a possible extension of the Precambrian Shield that contains limited evidence of Mesozoic orogenesis and that was rapidly uplifted during the Cretaceous to Paleogene. The Monashee Décollement, a warped mylonite zone interpreted as a regional thrust fault active through the Middle Jurassic, separates this complex from the second part, which contains rocks correlative with Hadrynian to late Paleozoic strata of the pericratonic prism. The third part, the Okanagan Complex, straddling the 49th Parallel from the Okanagan Valley to Kootenay Lake, contains the probable exhumed roots of a Mesozoic magmatic arc built upon possible North American continental and transitional crust and includes late Paleozoic and early Mesozoic suspect terranes.The Columbian Orogen formed during westward drift of the craton into a continent of accreted elements. Response of the craton, attenuated during at least two episodes of Proterozoic rifting, and its overlying sedimentary prism to underthrusting from the east and simultaneous collision with an accreting collage from the west took place in two stages. First, attenuated crust was telescoped and thickened to its approximate original configuration while westernmost parts of the bordering prism were deformed and metamorphosed (the Jura-Cretaceous Columbian Orogeny that affected the Okanagan Complex and strata above the Monashee Décollement). Second, the thickened crust, the deformed prism, and platformal strata were thrust eastward (the Late Cretaceous – Paleocene Laramide Orogeny that formed the Rocky Mountains Thrust Belt). Waning convergent tectonism led to ascendancy of crustal extension (primarily in the Okanagan Complex) and final uplift of cratonic massifs.

Brittle deformation in the Columbia River fault zone near Revelstoke, southeastern British Columbia

The brittle Columbia River fault zone forms part of the eastern margin of the Shuswap Metamorphic Complex, and for much of its 230 km length it coincides with the eastern exposure of the ductile Monashee décollement. The Eocene brittle fracturing and displacement are broadly parallel with, but cut and disrupt the middle Jurassic ductile mylonite zone. Excavations for a hydroelectric dam and a highway have facilitated a detailed analysis of fracture patterns at several localities along the fractured zone.Regionally, the brittle fault zone strikes 350° (locally 310–030°) and dips shallowly eastward. Over most of the studied part of the zone, the distributions of subsidiary fractures and displacement vectors demonstrate a normal dip-slip displacement consistent with subhorizontal east–west extension within the fractured zone, irrespective of the local trend of the zone. At Revelstoke damsite, where the zone trends 030° for 2 km, motion was towards the southeast, possibly indicating a localized clockwise rotation postdating dip-slip displacement.Fractures preserving both early dip-slip vectors and later strike-slip vectors demonstrate that minor north–south, strike-slip motion was superimposed on all parts of the zone, though no throughgoing strike-slip fault formed.In the damsite excavation, the fractured zone is largely confined to mylonites derived from footwall terrane. The Tertiary brittle offset was minor by comparison with the Jurassic ductile displacement.

The Triassic unconformity of south-central British Columbia: Discussion

Read and Okulitch's excellent dissertation on the Triassic unconformity is most enjoyable reading. I feel it necessary, however, to comment on few points with respect to the region with which I am most familiar. Read and Okulitch (1977, p. 624) state that a cross section of Triassic and subjacent rocks across southern British Columbia shows westward younging of the base from earliest Triassic in the Rocky Mountains to Late Triassic (late Karnian) west of the Shuswap Metamorphic Complex. I believe this to be gross oversimplification of the facts, and their statement (p. 625) that Triassic sediments young westward from Early Triassic at Kelly Hill . . . is incorrect. Some years ago I had the pleasure of visiting that rattlesnake-infested locality with R. G. Yates, under the guidance of J. W. Mills. The fossils, which are of the Smithian stage of the Lower Triassic (Scythian) occur in shaly limestone that is at the top of greywacke assemblage, the Churchill Formation, in which, lower in the section, are fusulina-bearing, limestones of probably Middle, possibly Upper, Permian age (Bowman 1950) that do not resemble the Lower Triassic limestone. In other words, the Triassic beds there are not basal, but apparently represent the top of the Permian succession, as was stated by Kuenzi (1961,1965, p. 365). A similar situation may exist in the Pavilion Group in the Lillooet-Big Bar area (Trettin 1961, pp. 26-34; Campbell and Tipper 1971, pp. 29,30). In Greenwood area (Little and Thorpe 1965), fossils obtained from the Brooklyn limestone have been identified by Tozer (1967, p. 83) as of Middle Triassic, probably lower Ladinian, age. The limestone is underlain by chert sharpstone conglomerate, near the base of which the Rawhide Formation contains fossils that were identified by Tozer as probably of Middle or Upper Triassic age. The Rawhide siltstones lie near the base of the sharpstone sequence (Little and Thorpe 1965), or at the base, according to Seraphim (1956), so that they and underlying(?) sharpstone conglomerate must be at least as old as lower Ladinian. The rocks underlying the sharpstone conglomerate are the Knob Hill Group and associated limestone and argillite units on Mt. Attwood, (the Attwood 'Series' of Daly (1912)), all of which units contain fossils identified by E. W. Bamber as being of Carboniferous or Permian age. At Paterson, about 8 km south of Rossland, the Mount Roberts Formation is overlain unconformably by flow breccias of the basal Rossland Group, (Little 1960, p. 48; 1963, p. 2), which flows rest upon different members of the Mount Roberts Formation and contain fragments of the underlying rocks. Fossil collections made in 1906, 1949 and 1962 were identified as being of PermoCarboniferous age, Pennsylvanian age being favoured, but with one exception. A collection containing corals was regarded by E. W. Bamber (Little 1963, p. 2) initially as being probably Mississippian in age, but he subsequently revised his identifications in the light of new information. His report was unfortunately submitted after the publication of Paper 6313, so I can hopefully here place in print that revision which is as follows:

The Selkirk fan structure of the southeastern Canadian Cordillera

The Selkirk fan structure evolved by superposition of two distinct phases of deformation upon strata previously involved in nappe formation (phase I). Phase II folds are dominant on the southwestern flank of the Selkirks where the folds are strongly overturned toward the southwest. Phase III folds are dominant on the northeastern flank where the strata are overturned toward the northeast. The fan structure is located where northeastward-dipping phase II axial surfaces are overprinted and transposed by steeply dipping to vertical phase III axial surfaces. Granodioritic plutonism and the main growth of metamorphic porphyroblasts occurred after phase II and before the onset of phase III. Depths of burial compatible with the peak of regional metamorphism were maintained at least until the later stages of phase III deformation. The evolution of the fan structure may be explained in terms of initial underthrusting of the southwestern flank of the Selkirk terrane by the Shuswap metamorphic complex followed by underthrusting of the northeastern flank by basement rocks of the Rocky Mountain foreland.

Rb–Sr dates for granodiorite intrusions on the northeast margin of the Shuswap Metamorphic Complex, Cariboo Mountains, British Columbia

Whole rock Rb–Sr dates of 138 ± 12 Ma (all five whole rocks) and 163 ± 7 Ma were obtained for granodiorite stocks in Wells Gray Provincial Park, Cariboo Mountains, British Columbia. These dates bracket the biotite K–Ar date of 143 ± 14 Ma determined previously by the Geological Survey of Canada. Two biotite – whole rock ± hornblende dates of 119 ± 11 Ma and 77 ± 20 Ma indicate isotopic resetting. Initial 87Sr–86Sr ratios vary from 0.7061 ± 0.0001 to 0.7103 ± 0.0002 for rock and mineral dates.When combined with field relations, these dates restrict regional deformation and metamorphism in this area to the time interval between Upper Triassic and Upper Jurassic. The resetting event was probably Eocene, as shown in other areas along regional strike to the north and south.

The Triassic unconformity of south-central British Columbia

At five localities investigated in south-central British Columbia, Upper Triassic rocks are observed or inferred to unconformably overlie upper Paleozoic and older rocks. Paleozoic rocks beneath the unconformity show polyphase deformation and low-grade regional metamorphism which are absent in overlying rocks. Data from these and other localities define a regional angular unconformity of Late Permian or Early Triassic age on the western and southern margins of the Shuswap Metamorphic Complex. Permian and Triassic rocks preserve evidence of structural, sedimentary, and metamorphic events which permits separation of Triassic rocks into three fault-bounded tectonostratigraphic belts. The Eastern Belt contains the transition from miogeoclinal sedimentation throughout Triassic time in the Canadian Rockies to island arc volcanism in the Late Triassic to the west. Basal beds of the Triassic sequence become younger southwest-ward from the axis of the Early to Middle Triassic depocentre lying west of the Rockies. Rocks preserving Early Triassic deformation and metamorphism are restricted to the southwest corner of the belt and are truncated by the Pasayten Fault. The Central Belt, dominated by the products of Late Triassic volcanism in northern and central British Columbia, consists mainly of Middle (?) and Upper Triassic sediments in the south. Meagre evidence indicates that widespread deformation and low-grade regional metamorphism occurred just prior to the Late Triassic. Evidence for these events is not found beyond the faulted margins of the Central Belt. In the Western Belt, an Upper Triassic sequence of tholeiitic basalt and overlying calcareous sediments disconformably overlies Permian rocks. In the western Cordillera, low-grade regional metamorphism and minor plutonism characterize Triassic orogenies. Early Triassic orogenesis in the southwestern corner of the Eastern Belt is coeval with the Sonoma Orogeny and the Middle–Late Triassic orogenesis of the Central Belt represents the Tahltanian Orogeny.

Erratum: Stratigraphic revisions of the Nicola, Cache Creek, and Mount Ida Groups, based on conodont collections from the western margin of the Shuswap Metamorphic Complex, south-central British Columbia

Conodonts have been recovered from highly deformed limestone and calcareous argillite in Palaeozoic and Mesozoic successions near the western border of the Shuswap Metamorphic Complex. Presently kn...