Torngat Orogen — a Palaeoproterozoic example of a narrow doubly vergent collisional orogen

Three widely spaced transects across northern Torngat Orogen demonstrate the broadly synclinorial structure and axial location of the monocyclic, supracrustal Tasiuyak gneiss complex in the centre of a doubly vergent orogen, with east-verging thrusting onto the Nain craton and west- or southwest-verging thrusting onto the Southeast Rae craton. On the basis of structural evidence, ages of detrital zircons and ɛ Nd signature, the sedimentary protolith of the Tasiuyak gneiss is inferred to be unrelated to the autochthonous platformal sequences on the adjacent Nain and Southeast Rae cratons, and an origin as an allochthonous accretionary thrust wedge is proposed. Structural studies of the Tasiuyak gneiss indicate the presence of early recumbent isoclinal folds (F 1 ) and associated stretching lineations (L 1 ), interpreted to have developed during thrust imbrication, that are refolded by later upright coaxial F 2 structures with opposing vergences towards the marginal cratons. The upright fabrics in the centre of the orogen can be traced without metamorphic break into mylonitic fabrics of the crustal-scale, sinistral transcurrent Abloviak shear zone, implying that the latter was initiated late in the structural history, but during peak metamorphic conditions. The emplacement of the primary, mantle-derived, but variably crustally contaminated (+3>ɛ Nd <−3), calc-alkaline diorite-tonalite-granite intrusions between 1910–1870 Ma into the northwestern margin of the Nain craton suggests that subduction was easterly-directed, under the Nain craton. Association of these intrusions with crustally-derived (or severely crustally contaminated) charnockites (ɛ Nd c. -6.7) of similar age (1895–1886 Ma) implies that substantial melting of Nain crust occurred at this time. Crustal thickening during the collisional stage of Torngat Orogeny resulted in extensive reworking of the margins of the Southeast Rae and Nain cratons and widespread in situ generation of leucogranites at c. 1860 Ma in the Tasiuyak gneiss, and was coeval with emplacement of charnockite intrusions into the Tasiuyak gneiss between c. 1860–1840 Ma. Metamorphic grade varies from greenschist facies near Torngat Front in the east of the orogen, to uppermost amphibolite and granulite facies in Tasiuyak gneiss in Abloviak shear zone in the axis of the orogen. Within Komaktorvik shear zone, developed in reworked Nain gneisses along the east side of the orogen, metamorphic grade also attained granulite facies. Estimates of the peak P-T conditions along the present erosion surface are in the range 10–12 kb and 850°C in both Abloviak and Komaktorvik zones, implying that double crustal thicknesses were attained during Torngat Orogeny. Syntectonic uplift and retrogression in Abloviak shear zone down to 6–7 kb/750–650°C did not involve significant incursion of fluids, whereas in Komaktorvik shear zone syntectonic uplift and retrogression down to 4–6 kb/550–500°C involved significant hydration of the granulite facies assemblages. Available age determinations suggest that peak metamorphism was synchronous along the length of the orogen at c. 1870–1840 Ma, with transcurrent shearing occurring between c. 1840–1820 Ma in Abloviak shear zone. Syntectonic uplift, exhumation and retrogression, and movement on Komaktorvik shear zone continued until closure of geochronologic and geothermobarometric systems between c. 1790–1720 Ma. A new tectonic model compatible with these constraints and inspired by the numerical modelling experiments of Willett et al . (1993, Geology , 21 , 371–374) is presented. The apparent symmetry of the doubly vergent orogen is shown to be a result of a fundamentally asymmetric process.