John F. Lewry

Anatomy of North America: thematic geologic portrayals of the continent

Abstract Six thematic tectonic maps are used to analyse the makeup of the North American continent. Themes are: 1. (1) major tectonic elements of the continent 2. (2) time of last major deformation 3. (3) time of first major deformation 4. (4) miogeoclines and terranes by kindred 5. (5) suture zones and terrane boundaries by age, and 6. (6) time of accretion. Features illustrated include distribution of orogenic belts and their extensions beneath cover sequences to the continental edge, contrast between juvenile and reworked crust in orogenic belts, geometry of ancient continental margins, distribution and classification of accreted terranes, geometry of suture zones and courses of ancient oceans, and how the continent evolved from an assemblage of Archean minicontinents to its present configuration. It is suggested that essentially similar plate tectonic processes controlled continental breakup and assembly from the Archean onwards, albeit with gradual increase in size of continental lithospheric plates and quantitative change in other parameters such as heatflow and character of the mantle.

Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada

Abstract An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.

Thermotectonic evolution of the churchill province in Northern Saskatchewan

Abstract The paper describes the lithological, structural and metamorphic features of the main crustal elements of the western part of the Precambrian Shield in Saskatchewan. Junction relationships, lithological, tectonic and metamorphic correlations are examined in developing a model for the thermotectonic evolution of these crustal divisions. Previously identified “linear belts” are revealed as integral elements of a more extensive ensialic Hudsonian mobile belt, rather than sites of independent fold belts and/or prior Aphebian depositional troughs. Distinction between the various domains results from variation in the degree and character of basement—cover remobilization and the “linear” Wollaston and Virgin River domains appear less highly remobilized than the intervening Mudjatik domain. The proposed model for Hudsonian thermotectonic evolution is similar to that applied by Wegmann (1935) in the East Greenland Caledonides, and may have more general application to much of the western Churchill Province. The Hudsonian belt in Saskatchewan resembles more recent orogenic belts. The eastern-southeastern margin of the Wollaston domain coincides approximately with major changes in Aphebian crustal conditions and depositional history and with changes in subsequent character of Hudsonian thermotectonism and plutonism. Evidence for proposed major crustal collisional suture at this margin is evaluated and found inadequate to explain simply observed relationships.

Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada

Abstract The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of decollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10–15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.

Seismic reflection images of high‐angle faults and linked detachments in the Trans‐Hudson Orogen

Postcollisional (1.8–1.7 Ga) intracontinental deformation in the Trans-Hudson Orogen (Canada) produced a series of orogen-parallel high-angle faults and folds. In seismic reflection profiles, the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4–8 s (12–24 km). The folded and faulted upper part of the crust is underlain by laterally coherent shallow-dipping reflections that are locally bounded by discrete, highly reflective zones. These zones are interpreted as detachments (shear zones) and can be traced from the upper to lower crust, where some of them appear to pass into laterally continuous reflections that define the Moho. Two distinct regimes of postcollisional crustal deformation are inferred from the seismic images: high-angle faulting and lateral block extrusion in the upper crust and low-angle ductile shearing in the mid/lower crust. The surface geology indicates that the faults resulted in southwest (orogen-parallel) extrusion of the orogens internal zone relative to the bounding Archean Hearne and Superior cratons. Faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction. The seismic images suggest that the high-angle fold/fault structures are kinematically linked to low-angle detachments represented by laterally coherent, highly reflective zones. The detachment shear zones are inferred to have a top-to-the-southwest sense of shear associated with a subhorizontal, northeast-southwest extension direction, parallel to those observed for 1.83–1.80 Ga collisional shear zones exposed in major postcollisional fold culminations. Long-lived orogen-parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the Superior and Hearne margins.

Crustal history of the Rae and Hearne provinces, southwestern Canadian Shield, Saskatchewan: constraints from geochronologic and isotopic data

Abstract The Cree Lake Zone (Hearne Province) and Western Granulite domain (Rae Province), in northern Saskatchewan, are contrasting parts of the variably reworked, (?) upper plate western hinterland of the Paleoproterozoic Trans-Hudson Orogen. Their junction is defined by the Virgin River Shear Zone, a part of the more extensive Snowbird “tectonic zone”. Subsurface extension of the ∼ 2.0 Ga Thelon Orogen lies west of the Western Granulite domain. We present UPb zircon ages, SmNd and RbSr isotopic data from the Cree Lake Zone and Western Granulite domain, and from the Junction Granite which intrudes the Virgin River Shear Zone. Data from the Western Granulite domain indicate that prevalent granulite facies metamorphism occurred between ∼ 2.3 and 2.0 Ga, but is imposed on Archean protoliths ∼ 2.8−3.0 Ga old. Low amphibolite facies retrogression, which increases toward the Virgin River Shear Zone, probably postdates 2.1 Ga. Data from the Cree Lake Zone also yield Archean protolith ages as old as 3.0 Ga, and some rocks yield interpreted high-grade metamorphic overprint ages of ∼ 2.3−2.0 Ga . However, two samples yield data indicating “Hudsonian” high-grade metamorphic overprint ∼ 1.87 to 1.81 Ga . The Junction Granite has a UPb zircon age of 1.82 ± 0.30 Ga , but relict zircons and SmNd data show that it was derived by melting of Archean crust. High-grade mid-crustal thermal resetting in the Western Granulite domain may be coeval with the 1.9–2.0 Ga Thelon Orogen, although the data do not preclude an earlier event or events. Similar, ∼ 2.3−2.1 Ga reworking extends into the Cree Lake Zone, but high-grade “Hudsonian” (1.87-1.84 Ga) reworking is also evident in parts of this zone well to the west of the Wollaston domain, and may extend entirely across the zone to the Virgin River Shear Zone. Relative importance of the “Thelon” and “Hudsonian” overprints in Cree Lake Zone is unknown. Data from the Junction Granite and Western Granulite domain suggest that all recorded high-strain events in the Virgin River Shear Zone postdate ∼ 2.3 Ga and are thus unrelated to deep-crustal Archean granulite facies mylonites in the Tantato-Black Lake segment of the Snowbird tectonic zone, north of the Athabasca basin.

SmNd, UPb, and RbSr geochronology and lithostructural relationships in the southwestern Rae province: constraints on crustal assembly in the western Canadian shield

The Rae province is an area of Archean crust bounded by the Early Proterozoic Thelon orogen to the northwest and mylonites of the Snowbird Tectonic Zone to the southeast. The southern part of the province in Saskatchewan is a highgrade complex of ultramafic, mafic and felsic orthogneisses with interlayered supracrustal units. The oldest rocks in the area were derived from layered mafic plutonic complexes and supracrustal units. The earliest deformation (D1) resulted in development of a layer-parallel foliation preserved in small lensoid pods of anorthosite, gabbro, and pyroxenite. A pyroxenite yields a SmNd TDM age of ∼ 3.5 Ga and is isotopically similar to a mafic granulite (metabasalt?) associated with early supracrustal units. These rocks were intruded by a sequence of mafic and tonalitic orthogneisses that yield a SmNd isochron age of 3.32±2.87 and TDM ages of ∼3.2−3.1 Ga. The mafic-tonalitic gneisses were subsequently migmatized producing anatectic net veins of tonalitic to granitic pegmatite. These were deformed during D2 deformation. The terrane was then intruded by minor granodioritic to tonalitic plutonic sheets and a swarm of mafic dikes that were deformed by the last major period of inhomogeneous deformation (D3) which ended at ∼2.0 Ga. The Clearwater Complex, a large 24×8 km long anorthosite massif, intruded the gneiss complex following D3, and yields a SmNd mineral-wholerock isochron age of 1.85 Ga. SmNd, RbSr, and UPb SHRIMP data record multiple thermotectonic events and provide constraints on crustal evolution of the Rae province. UPb SHRIMP data for metamorphic zircons separated from an early anorthosite associated with units that give pre-3.2 Ga Nd model ages show considerable variation in Th/U ratio. Zircons yield virtually concordant data that record Archean (2.9 Ga) and several Early Proterozoic (between 2.3 and 1.85) thermotectonic events. SmNd data are generally poorly correlated possibly as a result either of slight remobilization of REEs during granulite facies metamorphism or that rocks with distinct origins were compared. RbSr whole-rock and mineral data show substantial re-equilibration in mafic-tonalitic gneisses at ∼2.8 Ga, and in supracrustal units at ∼2.3 Ga. Also, a discordant granitic dike and biotite separated from a metapelite yield RbSr TDM ages of ∼1.9 Ga. These metamorphic ages are interpreted to reflect thermotectonism associated with formation and assembly of juvenile Early Proterozoic crust in the Talston-Thelon plutonic zone to the west, and with terminal collision in the Trans-Hudson orogen to the east.

Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada

Lithoprobe acquired a 190-km-long deep seismic reflection profile across the Superior boundary zone at the eastern margin of the Paleoproterozoic Trans-Hudson orogen. The profile is located ∼250 km south of a major promontory along the ancient margin of the Superior craton and complements earlier profiles closer to the promontory. The new data image the remnants of a craton-verging thrust belt, younger than 1864 Ma, within the Superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement. Across the surface suture zone, a dip reversal to east-dipping reflections occurs within the adjacent Reindeer zone, where a crustal-scale imbricate stack is imaged. Reindeer zone lower crust and upper mantle extend eastward beneath the Superior craton margin for 40–100 km. Preservation of a foreland thrust belt within the Superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the Thompson promontory.