Richard J. Wardle

Roots of the Labradorian orogen in the Grenville Province in southeast Labrador: Evidence from marine, deep‐seismic reflection data

Marine, deep-seismic reflection data obtained as part of the Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT) Lithoprobe project from offshore southeast Labrador are evaluated using geodynamic and gravity models, and a tectonic interpretation is developed. The geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust. Middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore. As there is little evidence of subsequent severe structural modification (apart from strike-parallel Grenvillian dextral transpression, which does not alter the crustal geometry along the line of the seismic transect), the seismic reflectors are considered to represent Labradorian structures, inferred from onshore geochronological data to be mostly related to a ∼ 1665 Ma (Labradorian) accretionary event. The southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward Labradorian subduction. Coast-parallel gravity and magnetic anomalies are interpreted as an expression of a Neoproterozoic to early Phanerozoic extensional basin, the existence of which had little effect on the underlying crystalline basement, other than to downfault it relative to that exposed onshore.

The Makkovik Province: extension of the Ketilidian Mobile Belt in mainland North America

Abstract The Makkovik Province of Labrador represents the extension of the Ketilidian Mobile Belt of south Greenland into mainland North America; it exhibits a threefold division into a foreland region, a fold-and-thrust belt, and an interior magmatic zone. The Kaipokok Domain is dominated by Archaean basement rocks that form an extension of the North Atlantic Craton, but Proterozoic reworking is recorded by the reorientation of a c. 2230 Ma dyke swarm. Supracrustal rocks, consisting of shallow-marine sedimentary rocks overlain by greywackes and mafic volcanic rocks, rest unconformably upon Archaean basement, but towards the interior of the belt the basal unconformity is eradicated by northwest-directed thrusting. In the Aillik Domain, highgrade supracrustal rocks of similar aspect to those of the Kaipokok Domain are separated from the basement by mylonite zones, in a thick-skinned fold-and-thrust belt (Kaipokok Bay Structural Zone), believed to record significant northwest-directed translation. The Aillik Domain also contains abundant felsic volcanic rocks that lack typical arc-like geochemical signatures. The Cape Harrison Domain is dominated by plutonic rocks, including suites of 1840 Ma, 1800 Ma, 1720 Ma and 1650 Ma age, but gneissic inliers apparently represent ‘juvenile’ Proterozoic crust. The dominant 1800-1720 Ma plutonic suites are late-orogenic to post-orogenic, siliceous, potassic granitoid rocks, which resemble Phanerozoic post-collisional suites, rather than subduction-related arc batholiths. Nd isotopic variations position the eastern edge of the North Atlantic Craton close to the boundary between the Aillik and Cape Harrison Domains. The structural evolution of the Makkovik Province records a shift from pre- 1800 Ma northwest-directed thrusting to post-1800 Ma tight upright folding, also northwest-verging. However, there is also evidence for earlier (pre- 1890 Ma) events in the Kaipokok Domain. Major unresolved problems include the timing of early sub-horizontal deformation (perhaps related to collisional events), the age relations and setting of supracrustal sequences, the location of suture zones, the absence of clear arc-like magmatic assemblages, and the nature and antiquity of the eastern juvenile crustal block.

New reflections on the structure and evolution of the Makkovikian ‐ Ketilidian Orogen in Labrador and southern Greenland

Marine seismic reflection profiles across the Early Proterozoic Makkovikian - Ketilidian Orogen in the Labrador Sea region suggest that it is a doubly vergent, asymmetric orogenic belt, comparable in width to younger collisional orogens. A southeast dipping reflector package is correlated with on-land shear zones that mark the southeastern limit of exposed reworked Archean crust and is also associated with a cryptic isotopic boundary in granites, which documents a transition from “ancient” to “juvenile” basement. This boundary is interpreted as a suture, along which juvenile Proterozoic crust has been juxtaposed over (thrust over ?) the Archean craton. A major synorogenic to postorogenic plutonic terrane to the southeast has a poorly reflective upper crust but shows strong subhorizontal reflectivity in the lower crust and Moho regions. The southeastern part of the profile correlates with metasedimentary terranes of the Ketilidian Belt in Greenland and contains low-angle, northwest dipping, reflector packages suggestive of large-scale crustal imbrication by thrusting. At least two broad zones of reflectivity at mantle depths (up to 16 s) are also recognized. One dips northwest below the Archean craton, but the most widespread mantle reflectivity is southeast dipping in opposition to the dominant fabric in the overlying crust. These contrasting crustal and subcrustal reflectivity patterns define a geometric “focus” beneath the orogenic belt and may provide information about subduction polarity during its development. The doubly vergent reflectivity pattern resembles images from possibly correlative Precambrian orogenic belts such as the Penokean and Svecofennian and also younger “small collisional orogens” such as the Appalachian and Pyrenean belts. It also broadly resembles some results of geodynamic crustal deformation models based on detachment and underthrusting of mantle lithosphere following collision and the squeezing of “weak” zones between rigid bounding blocks. A speculative multistage model for the belt incorporates initial northward subduction beneath the craton, shifting to later southward subduction, followed by oblique accretion of a composite arc terrane and juvenile (?) continental block. The accreted, hotter, juvenile, Proterozoic crust behaved differently than the stable, cool, Archean crust and experienced subhorizontal shearing in lower crustal and Moho regions, associated with southeast directed imbrication of the middle and upper crust by thrusting. However, the present reflectivity pattern of the orogen may also include elements related to post collisional extensional collapse.

Three-dimensional structure of the Torngat Orogen (NE Canada) from active seismic tomography

The crustal velocity structure and the Moho depth of the Proterozoic Torngat Orogen, NE Canada, is determined by active seismic tomography using travel times of crustal turning rays and Moho reflections. The orogen developed during oblique convergence of the Archean Superior and Nain Provinces, which trapped an interior belt of Archean crust (Core Zone) between them, with the Torngat Orogen evolving between the Core Zone and the Nain Province. Beneath the central orogen a crustal root is found with a preserved depth of >52 km and a width of ∼100 km. To the north, the root shallows to <44 km and narrows to a width of ∼45 km. The root correlates with a set of major, late orogenic shear zones that accommodated oblique convergence of the Superior and Nain Provinces. This correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening. Absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root. The lack of late magmatism is probably related to the depleted and refractory nature of the Archean lithosphere underlying the orogen. Upper crustal velocities are lowest in the Core Zone (∼5.7 km/s at the surface) and are compatible with laboratory measurements carried out on gneissic rock samples from that area. Higher velocities in the Nain Province (∼5.9 km/s) correlate with felsic gneiss and anorthosite rock samples. A high-velocity region immediately to the north of the crustal root is associated with a Moho uplift (∼34 km). This is explained by extension along the Ungava transform fault, and possibly in Hudson Strait, at ∼55 Ma when rifting in the Labrador Sea was transferred into Baffin Bay.

The Palaeoproterozoic Southeastern Churchill Province of Labrador-Quebec, Canada: orogenic development as a consequence of oblique collision and indentation

Abstract Palaeoproterozoic orogenic development in the northeastern Canadian Shield was controlled by the successive, oblique collision of the Archaean Nain (North Atlantic) and Superior cratons with a southwards projecting promontory of the Archaean Rae Province (part of the northern Churchill Province hinterland). By this process the Rae Province became sutured to the Nain craton by the Torngat Orogen and to the Superior craton by the New Quebec Orogen, resulting in the formation of a 400 km wide orogenic belt known as the Southeastern Churchill Province (SECP). Initial rifting at 2.45–2.1 Ga, and early (arc?) magmatism and deformation at 2.3–1.9 Ga, were restricted to the Rae Province. They were followed by arc magmatism in the Torngat Orogen at c. 1.91–1.88 Ga and Rae/Nain collision 1.87–1.86 Ga, which resulted in the formation of an orogen with east- and west-verging structures. Arc magmatism in the New Quebec Orogen commenced at c. 1.845 Ga and was succeeded by Rae/Superior collision and widespread deformation across the SECP at c. 1.83 Ga. Deformation at this time was dominated by west-vergent thrusting in the New Quebec Orogen and Rae Province, and by renewed east-vergent thrusting in Torngat Orogen. Deformation was then progressively transferred to major sinistral (1.845–1.82 Ga in the eastern SECP) and dextral (1.83–1.80 Ga? in the western SECP) shear systems that accommodated continued northwards motion of the Nain and Superior cratons relative to the Rae Province. Juvenile crust expelled by thrusting effectively doubled crustal thickness in parts of the Rae Province and the northwestern edge of the Nain craton. Late stage development (1.8–1.71 Ga), was restricted to the margins of the SECP, where deformation was associated with renewed outward-directed overthrusting and transcurrent shear in conjunction with uplift of the orogenic core.

Relationship between Proterozoic dykes and associated volcanic sequences: evidence from the Harp Swarm and Seal Lake Group, Labrador, Canada

Abstract Geochemical studies of the Harp dyke swarm and extrusive/intrusive igneous rocks of the Seal Lake Group of Labrador, Canada show that each can be subdivided into three distinct chemical groups. Two of the chemical groups within the Seal Lake Group show strong similarities to those present within the Harp dykes, but with more restricted and generally more primitive compositions. Chemical variation within individual Harp dykes suggests a dominant role for phenocryst differentiation processes, with little or no influence by crustal contamination. However, fractional crystallisation processes cannot account for the variation observed within each of the Harp dyke chemical groups, which instead is likely to be dominated by in-situ crystallisation processes. The greater range of incompatible trace element concentrations within many Harp dykes compared to Seal Lake Group igneous rocks is most likely a result of such mechanisms, rather than by fractional crystallisation or crustal contamination processes. Although dykes that do not chemically correspond with sampled lavas may have fed flows which are now eroded, it is more likely that they were non-emergent. Hence the chemical similarities and differences between the dykes and lavas can be linked to progressive mantle melting processes. This is consistent with spatial and temporal evidence that the Harp and Seal Lake igneous rocks are not strictly coeval, and hence most Harp dykes did not act as feeders to Seal Lake Group lavas. A model of heterogeneous lithosphere extension with a progressively increasing pure shear component with time can satisfactorily explain the Harp-Seal Lake magma relationships.

Burwell domain of the Palaeoproterozoic Torngat Orogen, northeastern Canada: tilted cross-section of a magmatic are caught between a rock and a hard place

Abstract The northern segment of the Palaeoproterozoic Torngat Orogen is unique relative to the southern segment in that it is underlain by a 1910–1885 Ma suite of diorite-tonalite-granite rocks (DTG suite) emplaced into the margin of the Archaean Nain Province. Field and geochemical data indicate that it represents a subduction-generated Andean-type magmatic arc. Slivers of MORB-type, tholeiitic amphibolites and metasedimentary rocks within the DTG suite are interpreted as remnants of a backarc basin. Arc rocks are preserved in a tilted section from homogeneous, foliated tonalites at mid-amphibolite facies in the east, through gneissic tonalites, to massive orthopyroxene-bearing granitoid rocks of the Killinek charnockitic suite in the west. This assemblage is separated from the Archaean Rae Province, with which Nain Province collided, by a wide belt of turbiditic sedimentary rocks, now at granulite facies, known as the Tasiuyak gneiss. A younger suite (c. 1865 Ma) of mafic tonalites and megacrystic metagranites within the Nain arc suite represents either syn-collisional plutons or late additions to the continental magmatic arc. The southern segment of the orogen differs from the northern segment because it preserves no evidence of the DTG suite on Nain Province margin, but instead contains a widespread suite of calc-alkaline plutons across the Tasiuyak gneiss/Rae Province margin, dated as 1880 Ma. We develop two testable models to account for these along-strike variations and the presence of arc magmas on both sides of the suture. In Model A, a flip in subduction polarity from east-dipping at 1910–1885 Ma to west-dipping at c. 1885 Ma is invoked, in which the c. 1865 Ma suite of plutons in the northern segment are interpreted to represent syn-collisional magmas generated during accretion of the Burwell arc onto Nain Province. In Model B, the younger plutons in the northern segment may be interpreted as a late phase of the arc built on Nain Province, which we suggest may have been accomplished during a phase of double subduction immediately prior to collisional orogeny. The Nain-Rae collision at c. 1870–1860 Ma formed a doubly vergent thrust wedge, burying part of the arc in Burwell domain to depths of 35 km. During subsequent sinistral shear deformation at 1845–1822 Ma, Nain Province was buried obliquely beneath the still hot arc to depths of 40 km. At 1798–1780 Ma, crustal-scale disharmonic folding of the northern segment resulted in the simultaneous exhumation of the deeply-buried granulites of the arc and of the northwestern margin of the Nain Province across the Komaktorvik shear zone (KSZ). The KSZ was the site of significant translation between the Burwell domain and unreworked portions of the Nain Province, but it does not represent a fundamental plate boundary.